Chapter 1
Theoretical Evidences
1.1 Rusting of Iron
Iron, in its various forms, when exposed to the different facets of en...
leading to use without protection and can also extend paint life by decreasing the
amount of corrosion underneath the pain...
protective film such as a corrosion product or when a coating breaks down.
Metallurgical factors that can affect corrosion ...
1.2.2 Weathering Steel
Weathering steels comprise a group of high strength, low alloy steels containing
alloying elements ...
tendencies and corrosion products [14, 15]. During atmospheric exposure, steel
gets a reddish brown corrosion product cons...
The oxidation of Fe2+
ions to green rust transformed to c FeOOH in well-
aerated systems and in turn is transformed to Fe3...
further corrosion. With time, a part of the FeOOH transforms to magnetic oxides
of iron, which are much more protective th...
0.5 % Cu during drying cycle and attributed this to the formation of a dense
corrosion product on Cu containing iron. Kish...
According to Yamamoto et al. [36], the amorphous and the crystalline con-
stituents were intermingled in the inner rust la...
surface from corrosion attack. High Si content in weathering steel also gives the
similar protective effect [47].
1.3.3 Ef...
Cu, Cr, Ni, Si, P, etc. are added in carbon steel to achieve compact, adherent and
pore free rust layer which in turn prov...
atmospheric pollutants like SO2 and showed that corrosion starts in non-contam-
inated atmospheres only at a relative humi...
Effect of Sulphates. Sulphur dioxide comes from combustion of fuels and is
identified as one of the most important air poll...
Effect of Chlorides. Marine environments have high percentage of relative
humidity and airborne salt. Studies have found t...
1.4.2 Organic Coatings
Organic coatings used on steel provide an effective barrier protection by isolating
steel from the ...
The level of dissolved salts of the exposure environment had an effect on the
properties of coatings [85].
Mayne [86] show...
interface can form a concentrated salt solution and that acts to draw water through
the coating, which behaves as a semipe...
required for the process to proceed are water, oxygen and free electrons. The
electrons may be generated by either an anod...
electrochemical impedance spectroscopy (EIS) can provide reliable data on per-
formance in a short time. Capacitance and e...
1.6.2 Electrochemical Methods
Since kinetics and mechanism of corrosion is controlled by electrochemical
principles, the t...
Where, ba and bc are anodic and cathodic Tafel slopes (mV/decade), Icorr is
corrosion current density (A/cm2
) and Rp is p...
kinetic and mechanistic information on electrochemical systems such as corrosion
processes. The slow electrode kinetics, s...
frequency impedance. The corrosion product accumulation at the coating–metal
interface can induce coating defects and ther...
Itagaki et al. [120] used EIS to investigate the electrochemical properties of the
rust film membrane formed on low alloy s...
addition of alloying elements increases the diameter of the capacitive semicircle.
The result meant the low permeation rat...
treatments with rust converters to rusted steel. Mild steel with mill scale were
prerusted for 2 years in a rural atmosphe...
EIS data are analysed by fitting them to an equivalent electrical circuit model
consisting of resistors, capacitors, and in...
weathering steel [128]. This is due to the presence of the nanophase oxides whose
diffraction lines are very broad and are...
showed that the corrosion rate of Cu–Mn weathering steel was lower than that of
Mn–steel, due to the formation of a denser...
Table1.2ImportantBandsofCorrosionProducts
OxidesDescriptionWavenos(cm-1
)PublishedWavenos(cm-1
)[R]
aFeOOH
Goethite
Givesr...
composed of c FeOOH while the inner layer was comprised mainly of densely
packed nanoparticles of a FeOOH. Further, a FeOO...
References
1. Graham, J.: Passivity of iron. Electrochem. Soc. Proc. 13, 112–115 (2002)
2. Evans, U.R.: Mechanism of atmos...
28. Oh, S.J., Cook, D.C., Townsend, H.E.: Characterisation of iron oxides commonly formed as
corrosion products on steel. ...
59. Rendón, J.L., Valencia, A.: Kinetics of structural rust transformation in environments
containing chloride and SO2. Re...
88. Rosales, B.M., Sarli, A.R.D., Rincón, O.D., Rincón, A., Elsner, C.I., Marchisio, B.: An
evaluation of coil coating for...
115. Ke, W.: Chinese Corrosion Survey Report, 1st Edn. Chemical Industry Press, Beijing, ISBN
7-5025-4792-4/TQx1816, (2003...
147. Jaén, J.A., Muñóz, A., Justavino, J., Hernández, C.: Characterization of initial atmospheric
corrosion of conventiona...
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Corrosion of constructional steels in marine and industrial environment

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Corrosion of constructional steels in marine and industrial environment

  1. 1. Chapter 1 Theoretical Evidences 1.1 Rusting of Iron Iron, in its various forms, when exposed to the different facets of environment it tends to be highly reactive owing to its natural tendency to form iron oxide. This degradation of iron is known as corrosion, more particularly rusting, when oxi- dation occurs in presence of moisture. However, if a thin film of iron oxide develops on its surface which is impervious and tenacious, it protects iron from further oxidation loss and it is called protective oxide film. This spontaneous formation of protective oxides which forms only on certain type of alloy steels is known as passivation. This hard nonreactive surface film (1–4 nm) inhibits further corrosion. Corrosion is an electrochemical phenomenon leads to the generation of very low electric currents. A mathematical relationship is available between the oxi- dation rate and the electrical properties. Good resistance to oxidation may gen- erally be expected when the electrical resistance of the oxide formed is high. J.C. Hudson worked on ferrous metals and established its relative resistance in different atmospheric conditions [1]. 1.2 Corrosion of Steel Steel corrodes when exposed to myriad conditions including outdoor atmosphere. It is noteworthy that all types of steel including the low alloy type are prone to rust in moist atmosphere. Rusting is an electrochemical process characterised by exchange of electrons. In some cases, the additions of 0.3 % copper to carbon steel can reduce the rate of rusting to a greater extent. The elements Cu, P, Cr and Ni have all been shown to improve resistance to atmospheric corrosion. Formation of a dense, tightly adhering rust scale is responsible in lowering the corrosion rate J. K. Saha, Corrosion of Constructional Steels in Marine and Industrial Environment, Engineering Materials, DOI: 10.1007/978-81-322-0720-7_1, Ó Springer India 2013 1
  2. 2. leading to use without protection and can also extend paint life by decreasing the amount of corrosion underneath the paint. The rate of rusting is usually higher in the first year of exposure to atmosphere than in subsequent years, and increase significantly with the degree of pollution and moisture in the air. Alloying ele- ments contribute to a more compact and less porous corrosion product as surface film. Adherent, protective films on these steels seal the surface against further penetration of water, which does not easily wet the oxide surface. Compact surface oxide films develop more rapidly in industrial atmospheres containing SO2, which is probably involved in film formation in presence of moisture by forming sulphurous and sulphuric acids which are very corrosive. This theory was based largely on the observation that the corrosion products formed on steel when exposed to industrial atmosphere were usually rich in sulphates. However, the corrosion rates of weathering steels are not reduced in industrial atmospheres to levels lower than those in non-corrosive rural or semi rural atmosphere. Periodic drying is required for the surface film to develop its protective properties [2]. In acidic solution, it involves an oxidation reaction (anodic reaction) where the metal gets into an ionic state by dissolution and releases electrons. Simultaneously, a reduction reaction (cathodic reaction) consumes the free electrons released by the anodic reaction and either a metal gets deposited or more usually the cathodic reaction. In order to continue the corrosion process in steel, formation of distinct anodic and cathodic areas is a prerequisite, which are electrically connected. These anodic and cathodic reactions occur in presence of an electrolyte, which can be a common aqueous solution, acidic medium or a thin film of moisture present on the surface, pores and crevices. At higher humidity, corrosion increases due to con- densation of moisture film on the metallic surface leading to formation of innu- merable galvanic cells. These cells are formed due to generation of electromotive force between surface film and trapped film in pores and crevices, which act as cathode and anode. Intrinsically, two important factors influence the corrosion phenomenon at a fundamental level. These are electromotive forces generated between the two electrodes and pH of the aqueous media. The electromotive force–pH relation was first proposed by M. Pourbaix [3] as shown in Fig 1.1 and these are useful in predicting zones of corrosion, passivity and immunity in metal– aqueous system. Pure iron exhibits formation of protective scale whereas carbon steel shows formation of incoherent layers of scale which easily flakes off to expose fresh areas for further attack. There are different forms of corrosion of which most important one is uniform corrosion which occurs over the majority of the surface of a metal at a steady and often predictable rate. Rusting can be slowed or stopped by using paint, controlling conductivity of solution, by applying current to metals and/or by stopping oxygen to reach the surface. Other forms of corrosion commonly encountered during service exposure of iron and steel is localised corrosion which is more severe than the uniform one as failure occurs without warning in a short period of use or exposure. Galvanic corrosion can occur when two different metals are placed in contact with each other and caused by the greater tendency of one of the metal to give up electrons than the other. Pitting corrosion occurs in materials that have a 2 1 Theoretical Evidences
  3. 3. protective film such as a corrosion product or when a coating breaks down. Metallurgical factors that can affect corrosion in steel are crystal imperfections, grain size and shape, grain heterogeneity, impurity inclusions and residual stress. 1.2.1 Mild Steel In mild steels, passivation in the stricter sense is not possible. The passive region of iron is characterised by a thin film of cubic oxide of c Fe2O3/Fe3O4 in neutral solution. This type of film is formed by the reaction of clean iron with oxygen or dry air. The composition of the passive film depends on the type of electro- chemical reactions and the nature of solution to which it is subjected. In such a situation, Fe2+ in solution may anodically form on the surface to give an outer c FeOOH layer. Another passive film on iron is Fe(OH)2, which is a polymeric layered structure [1]. However, it is reported to change character on removal from the passivating medium and long-term drying, to a form more closely resembling to c Fe2O3. It is reported that with the exception of those formed at very low passivating potentials, passive films do not seem to undergo significant local structural changes upon drying in the air. It is also reported that the passive film on iron composed of small particle size of c Fe2O3/Fe3O4 [4]. Cahan and Chen [5] suggested that the passive film is not a semiconductor but a highly doped film with Fe2+ and Fe3+ as defects. The oxide film near the iron electrode contains Fe2+ and Fe3O4 on outer surface. Raman spectroscopy study of the passive film indicates that the film consists of a layered structure with at least two components. The inner layer is most likely Fe3O4 and the outer layer primarily Fe3+ species. X-ray dif- fraction data shows a spinel oxide (c Fe2O3, Fe3O4), which is inconsistent with other crystalline bulk oxides, hydroxides or oxyhydroxides [6]. Fig. 1.1 E-pH (Paurbaix Diagram) of iron in sulphate containing aqueous media 1.2 Corrosion of Steel 3
  4. 4. 1.2.2 Weathering Steel Weathering steels comprise a group of high strength, low alloy steels containing alloying elements to give an enhanced resistance to rusting compared with carbon steels. These steels have 1–2.5 % of alloying elements (Cr, Cu, Si, and P) and have a tendency to form rust at a rate depending on the access of oxygen in the presence of moisture and air. As the process progresses, the rust layer acts as a barrier to the ingress of oxygen and the rate of rust growth slows down. In mild steels, the rust layer becomes non-adherent and detaches after specific time to exposure condi- tions. In weathering steels, the rusting process is initiated in the same way, but the alloying elements help to produce less porous and more adherent rust film. This rust system develops with time, becomes protective by impeding further access of oxygen and moisture to the metal surface and hence reduces considerably the rate of rust growth. The rust colour and its characteristic of weathering steel depend upon the nature of the environment and exposure time. In an industrial atmosphere, the weathering process will generally be more rapid and the final colour becomes darker. In the rural atmosphere, the oxide formation is usually slower and the colour becomes lighter. The tightly adherent oxide usually forms over a period of 18 months to 3 years in industrial atmosphere. Weathering steel promotes for- mation of an adherent rust layer after about 8 years of service and retards the corrosion by 75 % compared to mild steel [7] and in presence of relatively high airborne sea salt (coastal environment) the protective layer cannot be formed. Weathering steel is not advised to be used in bare conditions involving severe marine and severe industrial environments [8]. Pourbaix [9] showed that the typical behaviour of weathering steel is due to passivation during drying and lack of activation during wetting. Rust reduction of weathering steel is slower than mild steel while not much difference has been found in chemical analysis of the rust films on MS and WS, the morphology is quite different [10, 11]. Rust formed on weathering steel is rather compact in comparison to the loose rust found on mild steel. However, favourable atmo- spheric conditions are required to get stable rust on weathering steel, like air borne chloride (0.5 mg/100 cm2 /day), average wetness time 60 %, industrial pollu- tants (SO2 2.1 mg/100 cm2 /day) [12, 13]. 1.3 Atmospheric Corrosion Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends on the deposition rates of the air pollutants and varies with the wetting conditions. The factors influencing the corrosivity of atmospheres are gases in the atmosphere, critical humidity and dust content. Two rural environments can differ widely in average yearly rainfall and temperature and can have different corrosive 4 1 Theoretical Evidences
  5. 5. tendencies and corrosion products [14, 15]. During atmospheric exposure, steel gets a reddish brown corrosion product consisting of different constituents. The electrochemical reactions at wet surface of steel as proposed by Evans [16] in neutral alkaline condition are: Anodic half-cell reaction Fe ! Fe2þ þ2eÀ ð1:1Þ Cathodic half-cell reaction H2O + 1/2 O2þ2eÀ ! 2OHÀ ð1:2Þ The anodic and cathodic reactions are only the first step in the process of creating rust. Several more stages must occur for rust to form: Fe2þ + 2OHÀ ! Fe OHð Þ2 ð1:3Þ Ferrous hydroxide [Fe(OH)2] and hydrated ferrous oxide (FeO.nH2O) is first diffusion barrier layer formed on the surface. As the pH of saturated Fe(OH)2 is about 9.5, the surface of steel corroding in aerated pure water is always alkaline. Due to incipient oxidation green coloured Fe(OH)2 is formed. Ferrous oxide is converted to hydrous ferric oxide or ferric hydroxide at the outer rust layer as dissolved oxygen is available by the following reaction. 2 Fe OHð Þ21=2 O2 ! Fe2O3:2H2O ð1:4Þ In weathering, steel rust formed on atmospheric corrosion in different envi- ronments is composed of crystalline compounds like haematite, magnetite and oxyhydroxides of iron like goethite, akaganeite, lepidocrocite and feroxyhite apart from amorphous ferric oxyhydroxide rust. These rust constituents transform to one another during wet–dry cycles of atmospheric exposure [17].Various phases of corrosion products formed in progressive exposure to atmosphere are given in Table 1.1 [6, 18]. The alloying elements play a major role in modifying the oxyhydroxide rust layer which inhibits the ingress of oxygen and iron cations. Orange to reddish brown in colour hydrated ferric oxide formed is called rust and available as non magnetic a Fe2O3 and magnetic c Fe2O3. Rust layers are not protective because they are permeable to air and water and steel continues to corrode even after rust has formed [1].The rusting of steel in the atmosphere is given by: 2 Fe + H2O + 3/2 O2 ! 2FeOOH ð1:5Þ FeOOH is the main component of the rust formed in presence of water on steel at room temperature. Misawa [10] has summarised these processes and according to him metal dissolution is the anodic reaction, while the dominant cathodic reaction is oxygen reduction. The presence of a thick electrolyte layer on the surface can limit oxygen reduction rate. In such situations, the following reduction reaction supports oxidation of steel. 8 FeOOH + Fe2þ + 2e- ! 3Fe3O4 + 4H2O ð1:6Þ 1.3 Atmospheric Corrosion 5
  6. 6. The oxidation of Fe2+ ions to green rust transformed to c FeOOH in well- aerated systems and in turn is transformed to Fe3O4 in oxygen depleted systems. The phases of change in rust with time in mild steel are c FeOOH transforms to the more stable a FeOOH. With increasing time, a FeOOH converts to either c Fe2O3 or a Fe2O3, while conversion to a Fe2O3 usually requires higher temperatures. The rust on weathering steel after 16 years of exposure in a rural environment was found to be composed of two layers, with the inner dull layer comprising nano-sized particles of a FeOOH and the outer bright layer, c FeOOH [19]. The rust on weathering steel after 25 years of exposure in an industrial environment exhibited similar characteristics [20]. 1.3.1 Corrosion Products Weathering steels develop a compact adherent protective oxide film that protects the surface against further corrosion with prolonged exposure to the atmosphere. The first oxyhydroxide form is c FeOOH and part of it begins to transform to a FeOOH. The remaining part at later stage is composed of both oxyhydroxides. These hydroxides are less protective against corrosion and they readily crack allowing for ingress of oxygen and moisture to reach the metal surface and cause Table 1.1 Phases of corrosion products in atmospheric exposure Phases Lattice Crystal system /Habits Density (gm/ cm3 ) Free energy (DG kJ/ mol) Features a b c a FeOOH Goethite 4.60 9.96 3.02 Orthorhombic/ Acicular 4.28 -490.4 Yellowish brown to dark brown, Scaly/fibrous c FeOOH Lepidocrocite 3.06 12.51 3.87 Orthorhombic/ Lath 4.09 -471.4 Polymorph of goethite, platy, orange colour, Red Rust d FeOOH Feroxyhite 2.94 4.49 3.8 Hexagonal/ Plates – – Thin rolled films a Fe2O3 Haematite 5.035 13.72 5.26 Hexagonal/ Plates 5.24 -742.4 Reddish brown to black flaky rust, characterised by red streak c Fe2O3 Maghemite 8.33 – 24.99 Cubic/Lath 4.69 -540.2 Black and similar to magnetite Fe3O4 Magnetite 8.396 – Inverse spinel/ Octahedra 5.18 -1014.2 Black colour, mill scale b FeOOH Akaganeite 10.48 10.48 3.02 Tetragonal/ Somatoids, 3.55 – Brown/white colour Contains Cl- ions a,b,c: Relative lengths of crystallographic axes DG : Free energy of adsorption 6 1 Theoretical Evidences
  7. 7. further corrosion. With time, a part of the FeOOH transforms to magnetic oxides of iron, which are much more protective than these oxyhydroxides. In addition to a and c FeOOH, there is another oxyhydroxide of amorphous nature called d FeOOH. In mild steel, this does not form in a continuous manner and amorphous d FeOOH forms are not protective in nature for this reason. The formation of amorphous d FeOOH as a continuous layer next to the metal surface is catalysed by the presence of P, Cu and Cr in the steel. The presence of this amorphous layer was thought to be the reason for the excellent corrosion resistance of the weath- ering steels [21]. However, some findings [22, 23] show that the stable rust layer was not necessarily composed of amorphous rust but densely packed nano-size Cr substituted goethite. Cr substituted a FeOOH is very fine through which oxygen, water and corrosive substances are difficult to penetrate. Furthermore, chloride ions are also difficult to pass through this. However, for the formation of this protective rust layer, it is necessary that favourable atmospheric conditions exist for application of steel in bare condition. After extensive studies it has been found that Cr compounds are effective for obtaining the protective rust layer in a short period of time [24]. Cr substituted a FeOOH forms rapidly in presence of Cr2(SO4)3 solution. This accelerates the dissolution of steel and promotes the formation of goethite. On the other hand, Cr3+ forms fine particles of Cr substituted goethite and improves the protection ability of the rust layer. 1.3.2 Atmospheric Corrosion Mechanism Misawa et al. [10, 25] first investigated the mechanism of formation of constituents of atmospheric rust in aqueous solution and identified amorphous oxyhydroxide, FeOx(OH)3–2x, besides a FeOOH and c FeOOH in atmospheric rust. Again Mis- awa et al. [11] and Yamashita et al. [26] have reported that the c FeOOH forms at early stages of rusting and transforms into amorphous rust before converting to a FeOOH. Both Misawa [10] and Suzuki et al. [27] also concluded that the presence of Cu favours the formation of amorphous, crack-free uniform rust layer. It is reported that the formation of a FeOOH and c FeOOH results from water loss and crystallization of Fe(OH)3, the main corrosion product, with amorphous d FeOOH as an intermediate phase during drying cycle. In presence of high humidity, the reduction of c FeOOH results in the formation of Fe(OH)2 and finally Fe3O4. Under dry and oxidising conditions, when oxygen is easily able to penetrate into the rust layer, the ferrous layer/Fe3O4 is oxidised to unordered Fe(OH)3 and/or amorphous FeOOH which again transforms into crystalline a or c FeOOH by water loss and crystallization [28]. According to Larrabee et al. [22] Cu inhibits the formation of crystalline a FeOOH and c FeOOH and thus prevents microcracking in rust. This is attributable to crystallization of a FeOOH and c FeOOH during drying cycle of rust. Stratmann et al. [29] found significant differences between the rusting of iron and iron with 1.3 Atmospheric Corrosion 7
  8. 8. 0.5 % Cu during drying cycle and attributed this to the formation of a dense corrosion product on Cu containing iron. Kishikawa et al. [30] reported that weathering steel alloyed with Cr, Cu, P and Ni forms a non-amorphous densely packed nano-sized Cr substituted a FeOOH, which prevents the permeation of water, oxygen and corrosive substances. On the other hand, the c FeOOH mem- brane was found to possess anion selective property. The rust layer formed on the weathering steel has double-layered structure and protects steel from corrosion because of the formation of bipolar membrane which suppresses the cathodic reaction. Thus, the formation of a FeOOH in inner layer and c FeOOH in outer layer is important for protection ability against rust. The ratio of a FeOOH to c FeOOH in rust increases gradually as time passes. Corrosion rate decreases to almost zero when this ratio exceeds 1.4. A value of 2 is considered as rust stability index for maximum protection [31]. Yamashita et al. [32] observed that atmospheric rusts on weathering steels are composed of Cr substituted a FeOOH, c FeOOH and a small amount of c Fe2O3 and/or Fe3O4. The dark Cr substituted a FeOOH area was located in the inner layer while the bright c FeOOH area was in the outer layer. Thus, the innermost Cr substituted a FeOOH layer may be the final form of the protective rust layer which suppresses and prevents the transport of corrosive species through the rust layer to retard further corrosion. Study conducted in Taiwan by Wei [33] on carbon steels and weathering steel with high Phosphorous and exposed to rural, urban, coastal and coastal industrial environments concluded that the characteristics of the protective rust layer and the corrosion resistance of weathering steels depend on the environment and the test period. It was observed that c FeOOH formed in the inner rust layer along with some a FeOOH, a Fe2O3 and Fe3O4 in the initial exposure period and the amount of a FeOOH gradually increased in weathering steels. The enrichment of crack free and dense rust layer with Cr, Cu and P is attributed to the corrosion protection of the substrate steel. The rust layers on plain carbon and weathering steels exposed to coastal industrial environment in Japan for 17 years had been characterised by Asami et al. [34]. They found that the rust was composed of a FeOOH, b FeOOH, c FeOOH, Fe3O4 and amorphous rust. a FeOOH was predominant on all steels and appeared uniformly distributed throughout the rust layer. Concentration of a FeOOH was higher and c FeOOH was lower on weathering steels than on plain carbon steel. Amorphous rust was located at the bottom of the rust layer irre- spective of the steel types. Previous studies by authors on distribution of phases and alloying elements in the rust layers in weathering and plain carbon steels reported that the rust oxides consist of three layers: inner, outer and outermost [35]. The outermost layer was found to be about 3 lm thick and enriched with atmospheric deposits. The concentration of b FeOOH was reportedly higher on the skyward surface of both steels. In weathering steel, the alloying elements Cu and Cr enriched the inner layer of rust, while Si, P and Ni were not found to exhibit any characteristic distribution. 8 1 Theoretical Evidences
  9. 9. According to Yamamoto et al. [36], the amorphous and the crystalline con- stituents were intermingled in the inner rust layer of weathering steels exposed to rural environment for 35 years. Asami et al. [37] analysed the rust layers formed after 17 years on weathering steel bridge exposed to coastal industrial environment reported that c FeOOH and b FeOOH existed in outer layer while the amorphous rust and a FeOOH with enriched Cr, Ni and Cu were found widespread in the inner rust. Kihira et al. [38] referring to the work of Sakashita and Sato [39] tried to explain the difference in protectiveness of weathering steel rust by the phenom- enon of ion selectivity of the rust layer. Keiser et al. [40] found d FeOOH in the weathering steel rust along with about 10 % c FeOOH and a small amount of a FeOOH. Yamashita et al. [41] investigated the rust layers formed on weathering steels exposed for 17 years in Japan and reported that the protective properties of the rust layer on weathering steels containing alloying elements (Cr, Cu etc.) composed of a FeOOH. The amount of b FeOOH phases increased with the progressive increasing level of airborne salt. They also suggested that the pro- tective properties of the rust layer were related to the suppression of ion transport due to its densely packed structure. Most of the authors have indicated that long-term exposure results are impor- tant in terms of protective rust formation on weathering steels and also supported the view that the early stages of exposure determine the subsequent corrosion rate. Thus, corrosion rates during the early months of exposure are far more important than the ultimate rate in the context of a study of the mechanisms of protection [42]. It is proposed that during rust formation of steel in alkaline solutions the oxi- dation of Fe2+ proceeds via Fe(OH)2 and yields magnetite as the end product [43]. In another study of transformation of Fe(OH)2 at pH 11 and at temperature 65 °C, it was noticed that initially both Fe3O4 and a FeOOH form but a FeOOH formation takes place at an early stage of reaction. It was suggested that excess Fe+2 ions interacted with Fe+3 oxides, resulting in Fe3O4 formation. It is further reported that Cl- ions retard magnetite formation by binding the neighbouring OH- ions groups to form Fe–O–Fe linkage in alkaline pH 11. Sulphate has a tendency for a FeOOH formation of 0.1 M concentration of Fe(OH)2 [2]. Moreover, sulphate also plays role during transformation of green rust to oxides/oxy hydroxides. Subramanium [44] have shown that the presence of small amount of cations (Cu, Mn) in rusts may accelerate the oxidation of Fe+2 in solution due to their compound forming tendency with Fe+3 . Thus, magnetite is a major constituent in the weathering steel grades, which contains 0.3–0.4 % (wt.) of Cu. This corroborates the previous studies of Inouye et.al [45]. They indicated the strong magnetite promoting tendency of Cu during the formation of magnetite from Fe(OH)2. However, they have specified the maximum limit of Cu (3 % by wt.) above which it suppresses the magnetite formation. In another study [46], it was noticed that the presence of traces of Cr decreases the amount of Fe3O4 in oxide and increase unstable a FeOOH content in the corrosion product. It can be noted here is that the tendency of Cr ions to form fine particles of a FeOOH which may increase with passage of time and help in stabilizing the rust layer and protect the 1.3 Atmospheric Corrosion 9
  10. 10. surface from corrosion attack. High Si content in weathering steel also gives the similar protective effect [47]. 1.3.3 Effect of Acidity of Solution Acid solutions (low pH) are more corrosive than neutral or alkaline solutions. In ordinary iron or steel, the dividing line between rapid corrosion in acid solution and moderate or low corrosion is nearly neutral or alkaline solution at pH 7.5. In case of corrosion of iron or steel in aerated water, anodic reaction takes place at all pH values as per Eq. (1.1), but the corrosion rate varies due to changes in the cathodic reduction reaction as per Eq. (1.2). In the intermediate pH 4–10 ranges, loose, porous, ferrous oxide deposit shelters the surface and maintains the pH at about 9.5 beneath the deposit. The corrosion rate is nearly constant and is deter- mined by uniform diffusion of dissolved oxygen through deposit in this range of pH. In more acidic solutions (pH 4), the oxide is soluble and corrosion increases, due to availability of H+ ions for reduction by the equation 2Hþ + 2eÀ ! H2 ð1:7Þ The absence of the surface deposit also enhances access of dissolved oxygen, which, if present, further increases corrosion rate. Dissolved oxygen is cathodi- cally reduced in acid according to O2 + 4Hþ + 4eÀ ! 2H2O ð1:8Þ Reactions (1.7) and (1.8) occur simultaneously in acid solutions with dissolved oxygen. Diffusion of dissolved oxygen controls the corrosion rate at a constant level in the pH range 4–10. Thus, metallurgical variables affecting the anodic reaction [1] have no effect on the corrosion rate. Such is not the case for acid, where the cathodic reaction is under activation control. The carbide phase shows low overvoltage (higher rate) for reduction of H+ ions. Thus, high carbon steels have a higher corrosion rate in acid solution than that of the low carbon steels [48, 49]. 1.3.4 Effect of Alloying Elements Metallurgical factors affect metal loss and tend to corrode at a lower rate with higher alloy content. Atmospheric corrosion resistance of steel was improved by alloying with Cu, P or Cr to form passive oxide layer [50]. Studies have shown that these steels show superior corrosion resistance in particular during atmospheric exposure but not so much for immersed exposure as in seawater and close to the coastline in the presence of high chloride concentrations. Alloying elements like 10 1 Theoretical Evidences
  11. 11. Cu, Cr, Ni, Si, P, etc. are added in carbon steel to achieve compact, adherent and pore free rust layer which in turn provide good corrosion protection to the steel surface depending on the environment [27–30, 51, 52]. Weathering process of steel with Cu and P promote the formation of a tightly adherent, protective and stable rust layer to act as a barrier to electrochemical attack under wet–dry cycle. Cor- rosion of weathering steels containing Cu, P and Cr virtually ceases after 3 years of exposure. It is noteworthy that the formation of protective rust layer does not depend only on the alloying elements but also on the environment. It is observed that the formation of protective rust on the steel surfaces containing Cu is easier in industrial and rural atmospheres and difficult in marine atmospheres containing chloride ions. In their research work, Larrabee and Coburn [53] have revealed the effect of alloying elements on the corrosion resistance of steel where Cu and P additions are most beneficial in improving the resistance of steels. They have showed that the corrosion rate of plain carbon steels increases progressively with exposure time whereas same decreases in Cu containing steels in industrial atmosphere. Atmospheric attacks on steels have been studied on field exposed steel in industrial, rural and marine environments and found that P, Cu, Ni, Cr and Si improve the resistance to corrosion while Mn does not seem to affect it and S increases nucleation rate. The relative importance reported for marine atmosphere is P, Si, Cu (up to 0.3 %) and Cr, Ni, Cu (above 0.3 %) [52–54]. Horton et al. [55] observed that when steels containing Cu and Ni are exposed in industrial and marine atmospheres, the Cu and Ni appear in the rust layers both in the loose outer and adherent inner rust on skyward and ground ward surfaces. Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering steel appear in the rust layer and provides protection. Presence of chlorides in the atmosphere accelerates corrosion of steels leading to the formation of basic Fe2+ , Fe3+ chlorides and b FeOOH. Townsend et al. [56] conducted 8-year atmospheric corrosion tests on weathering steel in rural, industrial and marine environments with different heated conditions and indicated that heat treatments have no effect on the corrosion resistance/performance of weathering steels. 1.3.5 Environmental Factors The environmental factors that tend to accelerate metal loss include high humidity, high temperature and proximity to the ocean, extended periods of wetness and the presence of pollutants in the atmosphere. The small amount of carbon dioxide normally present in the air neither initiates nor accelerates corrosion. Vernon [57] was the first to study the corrosion rate of steel coupons in the presence of well- defined atmospheres. Atmospheric gases such as CO2, SO2, NO2, HCl, etc. after getting dissolved in the moisture layer on the metal surface, these gases result in a number of ions and ionic species like H+ , Cl- , CO3 2- , NO3 - , SO4 2- , etc. They measured corrosion rate was as a function of time, relative humidity and 1.3 Atmospheric Corrosion 11
  12. 12. atmospheric pollutants like SO2 and showed that corrosion starts in non-contam- inated atmospheres only at a relative humidity near to 100 %. Thus, they suggested that weather resistant steel is not to be used in uncoated condition in severe marine and severe industrial environments. Weathering steel is 5–8 times more corrosion resistant than plain carbon steel in industrial atmosphere and in marine atmo- sphere, superiority of the weather resistant steel depends on the salt content of the atmosphere [58, 59]. Effect of Temperature. Temperature plays an important role in atmospheric corrosion. There is normal increase in corrosion activity which can theoretically double for each 10° increase in temperature. As the ambient temperature drops during the evening, metallic surfaces tend to remain warmer than the humid air surrounding them and do not allow condensation until some time after the dew point has been reached. As the temperature begins to rise in the surrounding air, the lagging temperature of the metal structures will tend to make them act as condensers, maintaining a film of moisture on their surfaces [60–63]. Effect of Relative Humidity. Relative humidity is defined as the ratio of the quantity of water vapour present in the atmosphere to the saturation quantity at a given temperature. Atmospheric corrosion takes place in presence of a thin film electrolyte that can form on metallic surfaces when exposed to a critical level of humidity. While this film is almost invisible, the corrosive contaminants are known to reach relatively high concentrations, under conditions of alternate wetting and drying. The critical humidity level is a variable and depends on the nature of the corroding material, the tendency of corrosion products and surface deposits to absorb moisture, and the presence of atmospheric pollutants. It has been shown that, this critical humidity level is 60 % for iron if the environment is free of pollutants [60]. In the presence of thin film electrolytes, atmospheric corrosion proceeds by balanced anodic and cathodic reactions. The anodic oxi- dation reaction involves the corrosion attack of the metal, while the cathodic reaction is naturally the oxygen reduction reaction. The most important factor in atmospheric corrosion is moisture, either in the form of rain, dew, condensation or high relative humidity. In the absence of moisture, most contaminants would have little or no corrosive effect. Rain also may have a beneficial effect in washing away atmospheric pollutants that have settled on surfaces. This effect has been partic- ularly noticeable in marine atmospheres. On the other hand, if the rain collects in pockets or crevices, it may accelerate corrosion by supplying continued wetness in such areas. Continuous wetting and drying of the surface is required during atmospheric corrosion which makes distinctly different from the usual corrosion mechanism under immersed conditions [61].The atmospheric corrosion cannot be described as a simple oxidation reaction but associated with electrochemical reaction kinetics [62, 63]. Atmospheric corrosion experiences metal dissolution and the oxygen reduction during wet–dry cycles. In industrial atmosphere having sulphur bearing compounds, the corrosion product on steel is a basic iron sulphate. The Cu bearing steels form Cu compound which plugs the pores in the corrosion product, resulting in better corrosion resistance [64–66]. 12 1 Theoretical Evidences
  13. 13. Effect of Sulphates. Sulphur dioxide comes from combustion of fuels and is identified as one of the most important air pollutants which contribute to the corrosion of metals. In the presence of SO2, corrosion starts at 60 % relative humidity and the rates are considerably higher than in the absence of any con- taminate. There is a close relation between the uptake of SO2 and the corrosion rate measured during field exposure over a period and it depends considerably on the acid hydrolysis of SO2 in water and the fast oxidation of SO2 on iron surfaces. The cycle consists of two reactions, the oxidative hydrolysis of iron sulphate and the subsequent acid corrosion of iron [67]. 4FeSO4 + O2 + 6H2O ! 4FeOOH + 4H2SO4 ð1:9Þ 4H2SO4 + 4Fe + 2O2 ! 4FeSO4 + 4H2O ð1:10Þ The acid rain triggered atmospheric corrosion of steel and presence of SO2 accelerate corrosion rate due to formation of sulphuric acid resulting the deposi- tion of iron oxides in zones of decreased pH. Misawa [10] recognised the importance of amorphous and less crystalline iron oxide phases for the chemistry involved between iron dissolution and formation of stable FeOOH. Iron ions initially form green complexes, which are transformed into green rust, magnetite, FeOOH and finally after long exposure times transformed to a FeOOH. Hence, a large number of thermodynamically metastable phases exist which are transformed to stable oxides with the environmental conditions [68]. Stratmann [69] developed a number of experimental techniques to investigate the electrochemical reaction mechanism during the atmospheric corrosion even under wet/dry conditions. Studies indicate the adsorption of gaseous species into the corroding surface and subsequent chemical reactions occurring in the thin electrolyte layer [66]. Thus, in an industrial environment SO2 in presence of moisture, leads to the formation of a very thin oxide film composed of an inner layer of Fe3O4 covered by an outer layer FeOOH. The weak outer layer of FeOOH is penetrated by fissures and moisture enters into the pores in Fe3O4 layer in form of condensed water and dissolved O2, SO2 forms H2SO4 in the oxide pores. 2SO2 + 2H2O + O2 ! 2H2SO4 ð1:11Þ The oxide pores is to permit easier access for the electrolyte solution to the underlying surface. This H2SO4 partially dissolves the oxide, producing FeSO4 and thereby opens the oxide pores to permit easier access for the electrolyte to underlying surface. Fe3O4 + 4H2SO4 ! FeSO4 + Fe2 SO4ð Þ3 + 4H2O ð1:12Þ Fe2 SO4ð Þ3 + H2O ! FeSO4 + O2 + H2SO4 ð1:13Þ Also, FeSO4 is also hygroscopic in nature which enhances the atmospheric corrosion rate by attracting moisture from the atmosphere. 1.3 Atmospheric Corrosion 13
  14. 14. Effect of Chlorides. Marine environments have high percentage of relative humidity and airborne salt. Studies have found that the thickness of the adsorbed layer of water on zinc surface increases with % relative humidity and that cor- rosion rates increase with the thickness of the adsorbed layer. There also seems to be a finite thickness to the water layer that, when exceeded, can limit the corrosion reaction due to limited oxygen diffusion. However, when metallic surfaces become contaminated with hygroscopic salts their surface can be wetted at lower % RH. The presence of magnesium chloride on a metallic surface can make a surface apparently wet at 34 % RH while sodium chloride on the same surface requires 77 % RH to create the same effect. Corrosion rate in chloride environment can also be reduced in special grade of weathering steel as shown by the study. Maghemite as oxide phase formed on the carbon steel exposed in the marine environment is responsible for high corrosion rate [70, 71] although this rust phase is stable and protective in industrial environment. 1.4 Corrosion Protection by Coating The major protective coatings applied to structural steelwork are paints, metal coatings and combinations of both. The choice is partly governed by the actual environmental conditions and partly by economic considerations [72, 73]. The protection methods can be divided into three categories like applying organic or inorganic coatings, controlling electrode potential to make metal immune or passive by adding alloying elements in steels to promote formation of passive layer and addition of corrosion inhibitors to the environment [74]. 1.4.1 Passive Rust Coatings Passivity is due to the presence of a thin film which isolates the metal surface from a corrosive aqueous environment. A major impetus for work in neutral solutions came from the research of Nagayama and Cohen [75]. They considered that in the passive region, iron is covered by a thin film of cubic oxide of c Fe2O3/Fe3O4. Other compositions and structures were proposed for the passive film, some involving the inclusion of hydrogen or the presence of water [76, 77]. In fact, the composition of the passive film on iron depends on the type of electrochemical parameters during the formation of the film and the nature of solution in which it is formed. In passivity study, it is reported that the passive film on iron is composed of Fe(OH)2, c Fe2O3/Fe3O4. [78, 79]. 14 1 Theoretical Evidences
  15. 15. 1.4.2 Organic Coatings Organic coatings used on steel provide an effective barrier protection by isolating steel from the attacking species. Scientific investigations have determined that the limiting factor in the protective mechanisms of barrier coatings is their resistance to the flow of ionic current [80]. Coatings which contain large quantities of metallic zinc provide corrosion protection by galvanic action. The barrier prop- erties of the coating are improved by increased thickness, by the presence of pigments and fillers that increase the diffusion path for water and oxygen, and by the ability to resist degradation. Degradation allows access of reactants to the coating–substrate interface without the necessity for diffusion through the coating and it is an electrochemical process which follows the same principles as corrosion of uncoated steel. The total corrosion process comprises of components like transport of water, oxygen and ions through coating, development of aqueous phase at the coating–metal interface, activation of the metal surface for the anodic and cathodic reactions and deterioration of the coating–metal interfacial bond [81]. As coatings age in a corrosive environment, the interconnecting network of pores within the coating eventually become saturated with water, salts, etc., exposing the metal substrate to a corrosive environment. The saturation of the pores also creates paths of lower electrical resistance through the coating. Aged organic coating systems also possess dielectric properties, which cause them to act as capacitors to electrical current. Corrosion occurring at a metal surface has a polarisation resistance related to the corrosion rate, and an electric double layer that also behaves as a capacitor. An important property of a coating is its resistance to water penetration and two related properties are coating dielectric strength and coating resistance to ionic movement. Water penetration in coating decreases the dielectric strength, resis- tivity and makes the coating less insulative. Once corrosion has begun, the corro- sion products formed can cause undercutting and loss of adhesion of the coating. Water penetration may swell the coating and produce stresses that eventually lift the coating from the substrate. The presence of water increases coating deteriora- tion and substrate corrosion, since they can accumulate underneath the coating, cause delamination by blistering or accelerate corrosion of the substrate. Bacon, Smith and Rugg [82] determined a direct correlation between resistances and the ability of the coating to protect the underlying steel from corrosion. All coatings were found to exhibit an initial decrease in resistance, which varied in terms of rate and duration. For a good coating, this initial decrease was followed by an abrupt recovery to around the original value. They found that the resistance of a poor coating continued to decrease resulting in failure within 60 days. A coating that maintained a resistance of 108 ohm-cm2 provided good corrosion protection while those between 106 and 108 ohm-cm2 were fair and resistance less than 106 ohm-cm2 were poor performers [83, 84]. The dissolution of environmental water into the coating was more important than the uptake of salt from the solution by the coating. 1.4 Corrosion Protection by Coating 15
  16. 16. The level of dissolved salts of the exposure environment had an effect on the properties of coatings [85]. Mayne [86] showed that, upon immersion in an aqueous solution, most organic coatings acquire a negative charge and the acquisition of this charge has the effect of creating a selectively permeable membrane, which is preferentially permeable to cations; that is, a film that has gained a negative charge due to immersion. 1.5 Degradation of Organic Coatings There are several types of corrosion found beneath organic coatings and these are blistering, filliform corrosion, rusting, anodic undermining and cathodic delami- nation. Blistering is one of the first signs of breakdown in the protective nature of the coating. The blisters are local regions where the coating has lost adherence from the substrate and where water may accumulate and corrosion may begin. The blister formation occurs by volume expansion due to swelling, gas inclusion and gas formation [87, 88]. In all the cases, the blister provides a locale for collection of water at the coating–substrate interface. Oxygen penetrates through the coating and leaching of ionic materials from the interface. All the constituents are available for electrochemical corrosion and oxygen is necessary for the cathodic reaction: Again filliform corrosion is encountered on steel underneath organic coatings in a humid air environment. Corrosion initiates in the presence of soluble ionic species at defects in the coatings and propagates at the metal–coating interface as worm like filaments due to differential aeration oxygen concentration. Oxygen diffuses through the tail and leads to the separation of anodic and cathodic reaction zones. The primary cathodic region is near the back of the head (at the head–tail boundary) where oxygen is supplied and the primary anodic region is at the front edge of the head of the filament. Organic coatings slow the mass transport process of water, oxygen and ionic species which is necessary for corrosion. Non-sacrificial coatings show good barrier properties but corrosion phenomena can occur at corrosion defects where the steel substrate is not protected. Once corrosion starts on steel protected by an organic coating system, growing of blisters appear and a rapid deterioration occurs. This leads to the second type of protection that a coating can provide, sacrificial pro- tection. In addition, corrosion products from the sacrificial layers promote pore blockage in organic coatings preventing environmental intrusion [89]. 1.5.1 Delamination of Coatings Blistering and delamination are the most common forms of failure found in organic coatings. Factors affecting the performance of a system include surface prepara- tion, coating application, cure regime and film integrity. Soluble salts at the 16 1 Theoretical Evidences
  17. 17. interface can form a concentrated salt solution and that acts to draw water through the coating, which behaves as a semipermeable membrane from the exposure environment. Anodic blistering mode of failure was addressed by Koehler [90] who considered liquid filled blisters to be anodic in nature. Cathodic blistering is the result of an alkaline environment under the coating caused by the cathodic reaction, associated with corrosion that occurs at a damaged site of the film [91]. The fault may take the form of mechanical damage to the coating or may be inherent coating faults like pores/holidays. Anodic half-cell reaction: Fe ! Fe2þ + 2eÀ ð1:14Þ Fe2þ + O2 + H2O ! Fe2O3 + H2O ð1:15Þ Cathodic half-cell reaction: O2 + 4Hþ + 4eÀ ! 2H2O reduction in acidic solutionð Þ ð1:16Þ H2O + 1/ 2 O2 + 2eÀ ! 2OHÀ reduction in neutral/basic solutionð Þ ð1:17Þ Some pathways must exist through the film to allow the sodium ions to the interface in order to produce the alkaline environment. These pathways could be due to pores. However, an alternative theory is proposed by Leidheiser [92] suggests that beyond a given concentration, alkali cations may have a deleterious effect on the coating, which leads to morphological changes and introducing conductive pathways to the interface. 2Naþ + 2OHÀ ! 2NaOH in presence of alkaline solutionð Þ ð1:18Þ Similar to cathodic blistering, cathodic delamination is also the result of alkalinity at the interface. Again, this alkalinity is the result of cathodic activity under the coating. It is associated with faults, either inherent or induced, in the coating. Cathodic polarisation may be a consequence of either corrosion at the point of damage or the application of cathodic protection. Resulting from exper- iments carried out by Smith and Dickie [93] on primer failure, it has been shown that under impressed cathodic conditions, corrosion inhibitive pigments play no part in the reduction of disbonding. A number of explanations have been put forward for delamination mechanism whereby the alkaline environment under the film affects the integrity of the metal– polymer interface, or perhaps more properly the interface between the oxide and the polymer. Koehler [94] showed that this form of failure only occurs when there are alkali metal cations available in the environment to act as counter ions to the cathodically generated OH- ions. Considering a coated steel substrate, immersed in an electrolyte of neutral or near neutral pH, the half-cell reaction responsible for the delamination process is to be oxygen reduction. This reaction generates OH- ions at the cathodic site and is responsible for the alkaline environment at the delamination front. The elements 1.5 Degradation of Organic Coatings 17
  18. 18. required for the process to proceed are water, oxygen and free electrons. The electrons may be generated by either an anodic reaction or through the application of cathodic protection [95]. Cathodic delamination is a result of a damaged coating; there are two possible routes that the reactants for the cathodic reaction may take. The two alternatives are either through the coating or along the polymer–metal interface. An extensive review of the delamination process was carried out by Leidheiser et al. [96] and the results indicated that water was transported to the reaction zone through the coating. It was suggested that a certain fraction of this could be in the form of a cation as the cathodic nature and may favour the transmission of water associated with an ion possessing a positive charge. The supply of oxygen to the cathodic site was found to be largely through the coating, with a small contribution from interfacial transport, in the case of the epoxy coating studied. Adhesion plays an important role in the protective mechanism of coatings. When one considers the process of cathodic delamination it is clear that, once the paint has become detached from the substrate, the underlying metal is exposed to the environment and is no longer accorded any protection from the coating sys- tem. Whilst the loss of adhesion, resulting from the delamination process, effec- tively reduces the protection afforded by the coating, it is important to consider whether the original adhesion is the deciding factor in the delamination process. Gowers and Scantlebury [97] suggested that the beneficial role of the adhesion of a paint/coating is due to the impairment of the formation of a layer of electrolyte at the coating–substrate interface, preventing ionic current flow and the spread of corrosion over the surface. Gosselin [98] showed that good surface preparation is the key to good adhesion but the type, as well as the condition, of the substrate has been found to have a strong influence upon the initial dry, and the subsequent wet, adhesion of a metal/coating . 1.6 Corrosion Measurement and Analysis Laboratory corrosion testing and evaluation of uncoated and coated materials is an integral part of corrosion studies. This involves immersion, salt spray and elec- trochemical testing techniques. These simulative tests may prove to be very useful in generating data for estimation of corrosion performance and subsequent deg- radation. The testing must consider procedures which either reproduce a service environment or use an environment with higher severity. Emphasis is placed on coatings applied to steel surfaces and not many accelerated test methods are available for predicting reliably the service performance of paints. The permeability of organic coatings increases with time or the resistance to penetration decreases with time. The degradation is associated with corrosive ions and water penetration into the coating, transport of ions through the coating, and subsequent corrosion reactions at the coating–metal interface [99]. Standard coating immersion tests can take hundreds to thousands of hours, whereas 18 1 Theoretical Evidences
  19. 19. electrochemical impedance spectroscopy (EIS) can provide reliable data on per- formance in a short time. Capacitance and electrical properties of the coating are measured as a function of time. Since corrosion is an electrochemical process, it appears logical that the electrical resistance of a coating would be related to its protective ability. The DC resistance of the coating was essentially considered to be the internal resistance of the cell metal/coating/aqueous environment. For good coatings, the resistance changed slowly but for poor coatings the resistance dropped more rapidly. Rusting generally was not noted on the test panels until the DC resistance dropped 106 ohm-cm2 . Since it is difficult to accelerate evenly all the various factors involved, an accelerated method of detecting the deterioration, or the lack of continued protection, of a coating could be more useful and accurate than method of actually speeding up the deterioration or the corrosion process. This is one reason why electrical methods for detecting paint breakdown appear to show a comparatively high degree of correlation with actual breakdown in the same environment. The extension of electrical methods for measuring the degree of deterioration to coatings and uncoated steels exposed in atmospheric environ- ments may thus be promising [100, 101]. 1.6.1 Corrosion Rate Measurement Corrosion occurs at a rate determined by equilibrium between opposing electro- chemical reactions. The rate of any given electrochemical process depends on the rates of two conjugate reactions proceeding at the surface of the metal. Transfer of metal atoms from the lattice to the solution (anodic reaction) with the liberation of electrons and consumption of these electrons by some depolarisers (cathodic reaction). When these two reactions are in equilibrium, the flow of electrons from each reaction of balanced and no net electron flow (current) occurs. Various methods are available for the determination of dissolution rate of metals in cor- rosive environments but electrochemical methods employing polarisation tech- niques are by far most widely used. The corrosion rate (CR) is evaluated by mass loss method considering uniform corrosion. The Corrosion rate is determined by the following formula as per standard [102]. CR lm=yearð Þ¼ 87600W A Â T Â D ð1:19Þ Where, W is weight loss (mg), A is area of the specimen (cm2 ), D is density of the specimen (gm/cm3 ), T is exposure time (hours) and unit lm/year is micro- metre/year. Indirect methods of corrosion rate measurement involve anodic/ cathodic reaction, consideration of current potential relationship or polarisation resistance values. Tafel extrapolation method is the most popular laboratory methods for measuring corrosion rate of a metal from electrochemical data in a corrosive medium. 1.6 Corrosion Measurement and Analysis 19
  20. 20. 1.6.2 Electrochemical Methods Since kinetics and mechanism of corrosion is controlled by electrochemical principles, the technique based on electrochemical methods is used to determine the corrosion rate and understand the mechanism of corrosion process. The testing methods are based on principle of accelerating the corrosion process without changing the environment and the corrosion rates can be measured without removing the test specimens. These processes require anodes and cathodes in electrical contact and an ionic conduction path through an electrolyte. The electrochemical process includes electron flow between the anodic and cathodic areas; the rate of this flow corre- sponds to the rates of the redox reactions that occur at the surfaces. Monitoring this electron flow provides the capability of assessing the kinetics of the corrosion process. This also records the thermodynamic tendencies (potential) with the accumulated metal loss registered. This is used to manipulate potential of test specimen beyond its equilibrium value (OCP), a phenomenon called polarisation, to effect measurements and magnitude of polarisation is called the overvoltage or overpotential. It can have a plus or minus sign depending on whether it is above or below the equilibrium potential value. The test electrode polarisation can be accomplished by either DC/AC based polarisation measurements using a power supplying equipment called Potentiostat. Three electrode corrosion testing cell is employed with test electrolyte, specimen, counter and reference electrodes. The counter electrodes are usually conducting, noble materials such as graphite, plat- inum, etc. Reference electrode is used to measure and record the potential of the test electrode during the testing process. Normally, the more negative the potential, the higher the metal tendency to corrode [103]. Open Circuit Potential. Metal immersed in an aqueous solution develops an electric potential at its surface called open circuit potential (OCP) which is a characteristic of the metal solution system. The magnitude of OCP is measured with respect to reference electrode with the help of high impedance voltmeter and potentiostat is used to polarise or displace equilibrium potential of specimen in the negative (cathodic) or positive (anodic) direction with reference to OCP. This is manipulating the rates (ionic currents) of respective cathodic and anodic half-cell electrochemical reactions. The electrochemical potential of a metal in a certain solution is dependant on the type of the metal, the composition of the solution and its pH, oxygen content and temperature [104, 105]. Polarisation Test Method. This method is used to determine the corrosion rate. Polarisation resistance (Rp) is the resistance of specimen to oxidation during the application of an external potential in DC corrosion measurement methods. The CR and Icorr are related to Rp and can be calculated from equation given below and polarisation resistance is related to Icorr according to Stern Geary relation [106]. Rp ¼ babc 2:303Icorr ba þ bcð Þ ð1:20Þ 20 1 Theoretical Evidences
  21. 21. Where, ba and bc are anodic and cathodic Tafel slopes (mV/decade), Icorr is corrosion current density (A/cm2 ) and Rp is polarisation resistance (ohm-cm2 ). This involves a potential scan ± 250 mV of Ecorr at a scan rate of 0.1–1.0 mV/s. The technique is used to determine the equilibrium corrosion current, potential, Tafel constants and corrosion rates. The corrosion rate (CR) is determined from the Faraday’s law: CR ¼ 0:13IcorrðEWÞ q ð1:21Þ Where, CR is corrosion rate in mpy (1 mpy = 0.054 lm/yr), EW is equivalent weight, q is density of material in gm/cm3 and Icorr is corrosion current in A/cm2 . Tafel extrapolation is used to determine the equilibrium corrosion current, where linear extrapolations of anodic and cathodic branches of the plot beyond ±50 mV of OCP are made to intersect at OCP to measure the Icorr. This is a destructive technique as it can cause some degree of surface roughening on the test specimen [107]. General corrosion occurs in the active region, little or no corrosion occurs in the passive region and pitting corrosion can occur in the transpassive region [108]. Cyclic Polarisation. Cyclic polarisation curves are considered as an extension of potentiodynamic polarisation curves and used to measure the pitting tendencies. The potential scan begins at Ecorr (OCP) and continues in the positive (anodic) direction up to the transpassive region, where a large increase in current (corro- sion) occurs. At a threshold current density, the scan is reversed and continued in the negative (cathodic) direction back. The applied potential versus the log values of the measured current density are plotted. The cyclic polarisation plots can show positive hysteresis, negative hysteresis, repassivation or protection potential. Negative hysteresis occurs when reverse scan current density is less than that for the forward scan and positive hysteresis occurs when reverse scan current density is greater than that for the forward scan. A passive film is damaged when potential is raised into the transpassive region and pits can initiate when film damage is at discrete (localised) locations on the metal surface. Pits will continue to grow when protection potential (Epp) is greater than Ecorr and pits will not grow when Epp is less than Ecorr. In cyclic polarisation curve, hysteresis can provide information on pitting corrosion rates and how readily a passive film repairs itself. Positive hys- teresis occurs when passive film damage is not repaired and/or pits initiate; neg- ative hysteresis occurs when a damaged passive film repairs itself and pits do not initiate. Area of hysteresis is very important as more the area, more aggravated the corrosion is. Generally, the reverse scan is at a higher current level than the forward scan. The size of the pitting loop is a rough indication of pitting tendency; the larger the loop, the greater the tendency to pit [109, 110]. Electrochemical Impedance Spectroscopy. Electrochemical Impedance Spec- troscopy (EIS), a non-destructive investigative technique enables an insight into the corrosion process not obtained by DC techniques. EIS provides information on reaction parameters, corrosion rates, oxide characteristics and coating integrity, data on electrode interfacial capacitance and charge transfer resistance. It provides 1.6 Corrosion Measurement and Analysis 21
  22. 22. kinetic and mechanistic information on electrochemical systems such as corrosion processes. The slow electrode kinetics, slow preceding chemical reactions and diffusion impede electron flow in electrochemical cells much in the same way as resistors, capacitors and inductors do in AC circuits. The working electrode interface undergoing an electrochemical reaction is analogous to an electronic circuit with a specific combination of resistors and capacitors and AC circuit theory can be used to characterise an electrochemical system in terms of equiv- alent circuit. The technique broadly involves subjecting an electrochemical system to a range of small magnitude AC polarising voltage frequencies and corroding the system response in the form of complex impedance plots. The complex impedance diagrams are correlated with an equivalent AC circuit model with unique values for circuit elements. These values can then be used to infer kinetic and mechanistic information about an electrochemical system [111–113]. The response of a cor- roding metal to small amplitude AC signal (10–20 mV) of widely varying fre- quency (0.001–100 kHz) can be analysed by EIS following the absorbance of electrical energy at a certain frequency at the metal solution interface. On appli- cation of a sinusoidal alternating potential signal of the form: V tð Þ ¼ V0Sin xt ð1:22Þ Time dependence current response of the form: I tð Þ ¼ I0Sin ðxt þ hÞ ð1:23Þ Where V(t) is applied potential,V0 is amplitude of applied potential, I0 is amplitude of generating current, electrode surface expressed as an angular fre- quency (x) and h is phase between V and I. Due to the applied potential frequency (x) dependent impedance Z(x) may be expressed as: Z xð Þ ¼ Ru þ Rp 1 þ x2R2 pCdl2 ! þ j xR2 P 1 þ x2R2 PCdl2 ¼ Zreal þ j Zimg ð1:24Þ Where Ru is the solution resistance, Rp is the polarisation resistance and Cdl is the double-layer capacitance. Various electrochemical phenomena at the metal solution interface causes a time lag and a measurable phase angle h. These pro- cesses will be simulated by resistive and/or capacitive electrical networks. The impedance behaviour of an electrode may be expressed in Nyquis plot of Zimg (imaginary part of impedance) as a function of Zreal (real part of impedance) or in Bode plots of mod Impedance and h versus frequency, where x = 2 pf. To evaluate a coating, along with Ru, Rp, Cdl and Wd (Warburg impedance) two additional circuit elements, namely coating capacitance (Cc) and resistance of coating pores (Rpo) come into account. The presences of Cdl or Cc can be idealised by a constant slope in Zimg versus frequencies plot and peaks in h versus fre- quencies plots. For uncoated sample, Zimg versus frequencies plot shows early low impedance at all frequencies. In coated specimen, Rpo measures the early deterioration at low 22 1 Theoretical Evidences
  23. 23. frequency impedance. The corrosion product accumulation at the coating–metal interface can induce coating defects and thereby reduces Rpo. The coating in these cases is applied to the surface, which is not completely derusted. Rp for corrosion beneath the coating is apparently quite high, as stated earlier and would require still lower frequency measurements, which are difficult and time consuming. In absence coating, the Zimg versus frequency plot measures the low value of Rp at low frequency resulting from the comparatively high corrosion rate. Electrified interfaces called electric double layers (Cdl) are set up at metal– electrolyte boundaries during electrochemical process. These interfaces are char- acterised by impedances to electron flow and ionic movement. The impedance of an electrified boundary manifests as interfacial capacitance and associated charge transfer resistance. The electrified interfaces are typified by time constants, which are given by product of magnitudes of associated capacitances and resistances. The time constants are noticeable in EIS spectra as semicircles in Nyquist plots, negative slopes in Bode magnitude plots and negative inflections in Bode phase plots. Mathematical regression of time constants in EIS spectra with equivalent electrical AC circuit models leads to quantification of associated resistances and capacitances. A higher charge transfer (ohmic) resistance implies greater polari- sation or corrosion resistance of the metal in a given aqueous environment. The capacitances themselves can be used to identify the corrosion, coating and diffu- sion processes with different time constants. Thus, the metal–electrolyte interface behaves and responds like an AC circuit with a specific combination of resistors and capacitors under the influence of the AC polarising voltage frequencies. Figs. 1.2, 1.3, 1.4, 1.5 show metal solution interface (single time constant system) where Ru is solution resistance, Rp is po- larisation resistance. The EIS spectra for coated metal–electrolyte systems are characterised by two time constants, two semicircles in Nyquist plots, two negative slopes in Bode magnitude plots and two negative inflections in Bode phase plots [114, 115]. Figs. 1.6, 1.7, 1.8, 1.9 show coated metal solution interface (two time constant system) and Cdl is double-layer capacitance. The coating time constants are smaller and manifest at higher frequency regions of the impedance spectra, whereas time constants corresponding to metal corrosion appear at lower frequency regions. An impedance plot obtained can be correlated with one or more equivalent pore resistance (Rpo), coating capacitance (Cc) and polarisation resistance (Rp). CPE is used in a model in place of a capacitor to compensate for non-homo- geneity in the system. A rough or porous surface can cause double-layer capaci- tance to appear as CPE and Warburg element [116, 117]. Kihira et al. [118] applied EIS to investigate the condition of the rust film formed on the weathering steel, and proposed new corrosion monitoring method based on rust film resistance. Nishimura et al. [119] measured the electrochemical impedance of a carbon steel covered with rust film formed in a wet/dry environ- ment containing chloride ions. They reported that the charge transfer resistance (Rp) increased with the wet–dry cycles of exposure. 1.6 Corrosion Measurement and Analysis 23
  24. 24. Itagaki et al. [120] used EIS to investigate the electrochemical properties of the rust film membrane formed on low alloy steels. The electrochemical impedance of the actual rust film membrane formed by wet–dry cycles showed the capacitive semicircle on Nyquist plot corresponding to a single time constant. The time constant of the capacitive semicircle was found composed of the rust film resis- tance and the film capacitance. The value of rust film resistance was shown to depend on the alloying elements in weathering steel and it was shown that the Fig. 1.2 Metal electrolyte interface of uncoated corroding steel Fig. 1.3 EIS spectra for single time Constant of bode magnitude plot Fig. 1.4 EIS spectra for single time constant of bode phase plot 24 1 Theoretical Evidences
  25. 25. addition of alloying elements increases the diameter of the capacitive semicircle. The result meant the low permeation rate of chloride ions in the rust films of these alloys. The Nyquist plots were found to diverge from a true semicircle due to the current distribution in the film [121]. Feliu et al. [122] have applied EIS to study the corrosion and electrochemical activity at the metal–rust interface in connection with the application of protective Fig. 1.5 EIS spectra for single time constant nyquist plot Fig. 1.6 Electrolyte interface of corroding coated steel Fig. 1.7 EIS spectra for two time constant of Bode magnitude plot 1.6 Corrosion Measurement and Analysis 25
  26. 26. treatments with rust converters to rusted steel. Mild steel with mill scale were prerusted for 2 years in a rural atmosphere before applying conversion treatments. They found that the shape of the low frequency areas of the Nyquist plots are markedly influenced by diffusion processes in the rust layer and/or by the porous nature of the rusted steel electrode itself. With alloying and increasing period of exposure to saline atmospheres, the magnitudes of rust pore resistances are expected to increase and rust capacitive reactance will decrease since capacitance is inversely proportional to AC imped- ance. It is also likely that the charge transfer resistance which is indicative of metal corrosion, itself will undergo an increase with alloying thereby signifying higher corrosion resistance for alloyed steels. Corrosion, coating and diffusion processes are not always associated with same frequency ranges. Corrosion resistances are observed at low frequencies, but coating pore resistances can also be observed at low frequencies, particularly, when a coating is saturated with electrolyte and metallic corrosion does not occur. Capacitance values can be used to guide interpretation as to what type of process is associated with each time constant. Corrosion time constants have capacitance values (1–20 lF/cm2 ), coating time constants have capacitance values (nF/cm2 ) and oxides have capacitance values (1000 lF/cm2 ). Capacitance values of the order of C100 lF/cm2 are found when surface adsorption occurs in conjunction with corrosion [103]. Fig. 1.8 EIS spectra for two time constant of Bode phase plot Fig. 1.9 EIS spectra for two time constant nyquist plot 26 1 Theoretical Evidences
  27. 27. EIS data are analysed by fitting them to an equivalent electrical circuit model consisting of resistors, capacitors, and inductors. The working electrode interface undergoing an electrochemical reaction is analogous to an electronic circuit and can be characterised as an electrochemical system in terms of equivalent circuit. Typical circuits are shown in Figs. 1.10, 1.11, 1.12 and 1.13 where Yo is admit- tance (ohm-cm2 ),Cf is double-layer capacitance and a is the exponents [114]. (R.E: Reference Electrode and W.E: Working Electrode) 1.7 Rust Characterisation Several techniques are used in different stages of rust characterisation for steels and microscopy related techniques are useful in understanding the topological state of the corroded layers and in analysing their cross sections. 1.7.1 SEM and EDX The degree of corrosion, surface morphology, particle size and texture can be effectively studied by scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX). The optical microscope can be used for imaging the surface but it has limitations of resolution and depth of field at higher magnifications. SEM can be used for high-resolution imaging of the surface, with a large depth of focus. Atmospheric corrosion of weathering steel in the presence of NaCl and SO2 was investigated by A.Q. Qu found that NaCl can accelerate the corrosion [123]. The relationship between mass loss and amount of NaCl deposition follows the qua- dratic function both in SO2 free air and in air containing SO2. The combined effect of NaCl and SO2 on the corrosion of steel is greater than that caused by each single component [124]. SEM and EDX are used to characterise the corrosion products of steel. In the absence of SO2, a FeOOH, b FeOOH, c FeOOH, Fe3O4 and c Fe2O3 are the dominant corrosion products, while b FeOOH, c FeOOH, Fe3O4 and FeSO4.H2O dominate in the presence of SO2 [125]. 1.7.2 X-ray Diffraction X-ray diffraction (XRD) is used for identifying the oxides in rust and sometime provides incorrect identification of the composition of the rust formed on weath- ering and carbon steels [126, 127]. Separate identification of Fe3O4 and c Fe2O3 is not possible as both oxides have cubic structure and nearly identical lattice parameters at room temperature. Analysis of rust coatings by XRD significantly underestimates the goethite fraction in the corrosion products, especially for 1.6 Corrosion Measurement and Analysis 27
  28. 28. weathering steel [128]. This is due to the presence of the nanophase oxides whose diffraction lines are very broad and are frequently overlooked owing to their overlapping with sharper peaks for larger particles of the same oxide phases in the rust and are believed to be incorrectly referred to as amorphous. XRD measure- ments have lead to general conclusion that weathering steel forms a protective coating with ratio a FeOOH/c FeOOH [ 2 [129, 130]. In another study, corrosion rates of Mn–steel and Cu–Mn weathering steel in a simulated coastal environment were measured by wet–dry cyclic test. The rust layer was observed and analysed by SEM and XRD. The experimental results Fig. 1.10 Representative randle equivalent circuit Fig. 1.11 Representative CPE equivalent circuit Fig. 1.12 Representative CPE with diffusion equivalent circuit Fig. 1.13 Representative REAP equivalent circuit 28 1 Theoretical Evidences
  29. 29. showed that the corrosion rate of Cu–Mn weathering steel was lower than that of Mn–steel, due to the formation of a denser rust layer. The rusts on the two steels consisted of Fe3O4, a FeOOH, b FeOOH, c FeOOH and amorphous phases. The amount of a FeOOH and b FeOOH in the rust of Mn–steel was larger than that of Cu–Mn weathering steel. The addition of Cu increased the amount of Fe3O4, while the addition of Mn decreased the amount of c FeOOH in the rusts [131]. 1.7.3 Raman Spectroscopy Raman spectroscopy is used to study the internal structure of molecules and provides unique information about molecular patterns, spacing, and bonding. This is based on Raman Effect, which is the inelastic scattering of photons by molecules as every compound possesses a typical Raman spectrum. In order to be Raman active a molecular rotation or vibration must cause some change in any component of molecular polarisability. This is defined as the induced dipole moment set up in the molecule by applied electric field. Practically, stokes lines (high k, low t low t‹ ) are intense in the spectrum than anti stokes lines (low k, high t high t‹ ) and the shift are measured with respect to the reference Rayleigh lines (unshifted with same k, t t‹ ). Fine structure effects are not considered in practical situation as the corresponding effects are of less important. It is essentially an emission spec- troscopy. The source is a monochromatic (laser) and the instrumentation is simply a typical visible range (He–Ne) spectrometer. Raman Effect can take place for any frequency of the incident light which is simply a light scattering phenomenon. Spherical top molecules are completely Raman inactive whereas asymmetrical top one is Raman active [132–134]. This case is corresponding to rotation of Raman mode (some phases identified by XRD not by Raman due to inactive Raman). For vibrational Raman mode, symmetrical vibration always produce intense Raman lines whereas unsymmetrical ones are normally weak and sometimes unobservable [135, 136]. Table 1.2 provides the important bands of some common corrosion products of iron and [R] indicates the published references. Thibeau et al. [143]. have used Raman and infrared spectroscopy to investigate the structure of the inner rust layer formed on weathering steels exposed to an industrial environment for 4.5 and 8 years. The inner rust layer on weathering steel was composed primarily of d FeOOH with 10–20 %, c FeOOH and some a FeOOH irrespective of the exposure period. Dunnwald and Otto [137] found phase transformation of iron corrosion product to Fe(OH)3 in the atmosphere containing SO2 with humidity by Raman spec- troscopy. Subsequently, Fe(OH)3 gets transformed to crystalline FeOOH with amorphous FeOOH. It has been shown that the amorphous rust is the primary product of atmospheric corrosion, which later transforms to crystalline forms in the absence of copper. Yamashita et al. [144] studied the long-term growth of the protective rust layer formed on weathering steel under atmospheric corrosion in an industrial region involving an exposure for 26 years. The outer layer of rust was 1.7 Rust Characterisation 29
  30. 30. Table1.2ImportantBandsofCorrosionProducts OxidesDescriptionWavenos(cm-1 )PublishedWavenos(cm-1 )[R] aFeOOH Goethite Givesrelativelystrongpeaks205,247,300,386,418,481,549245,300,390,420,480,550,685 248,303,397,485,554,680,1002,1120 245,300,390,485,550,675 298,397,414,474,550 [138] [137] [139] [140] bFeOOH Akaganeite Characterised by4peaks 314,380,549,722310,386,497,538,723 310,385,415,480,535,615,675,725 [141] [139] cFeOOH Lepidocrocite Charactersed by7peaks 219,252,311,349,379,528,648255,380,528,654,1054,1307 252,380,660 [140] [142] dFeOOHGivesrelativelyweakpeaks297,392,666400,655 220,295,385,495,670 [141] [143] aFe2O3 Haematite Givesstrongestpeaks226,245,292,411,497,61227,245,293,298,414,501,612 225,245,295,415,500,615,1320 [143] [138] cFe2O3 Maghemite Characterisedby4peaks381,486,670,718265,300,345,395,515,645,670,715,1440 350,505,660,710,1425 [138] [141] Fe3O4 Magnetite Characterisedby2peaks532,667616,663 298,319,418,550,676,1322 [143] [137] BoldStrongestPeakinSpectrum,UnderlinedNextStrongestPeakinSpectrum 30 1 Theoretical Evidences
  31. 31. composed of c FeOOH while the inner layer was comprised mainly of densely packed nanoparticles of a FeOOH. Further, a FeOOH was found enriched with Cr and reported to be the stable and protective uniform rust layer. It was proposed that the c FeOOH, as an initial rust layer of weathering steel, formed after a few year of exposure, is transformed eventually into the final stable rust layer consisting of nano-size a FeOOH after decades with amorphous ferric oxyhydroxide as an intermediate transition product which is formed after several years of exposure in atmosphere. The mean diameter of the rust particles was found to be approxi- mately 0.5 lm in the outer loose layer aggregate whereas the inner layer was composed of densely packed fine particles within the larger secondary particles. In contrast, the corrosion product formed on mild steel contained number of voids and microcracks. Microscopic observation of weathering steel exposed outdoors during stable protective rust coating development, reveal two phases in layers parallel to the steel surface. The layer adjacent to the steel is grey and compact and the external is reddish and porous. The thickness of the inner phase increases up to outdoor exposure periods longer than 5 years, when it becomes the only component of the patina. It is responsible for the electrochemical potential increase and low corro- sion rate of steel, restricting oxygen and water access as a barrier to elements controlling further corrosion. In carbon steels corrosion products form also as two optically different phases, but they are mixed up. They experience lower increase of electrochemical potential during natural or simulated outdoor exposure. Their corrosion rate remains up to an order of magnitude above those determined for weathering steels in the respective atmospheres [145, 146]. 1.8 Rust Simulation Pourbaix diagram [3] maps out possible stable equilibrium phases of an aqueous electrochemical system and indicates that pure iron is passive at pH values from 9 to 12.5 to form iron hydroxide. Considering the interplay of atmospheric factors this diagram was used as guide to the steel dissolution process to form passivity on WS in laboratory. The passive films formed on pure iron are not so stable and consequently the passivation state of iron is not maintained for prolonged time periods [147, 148]. The rust layers of steels play a role as a barrier against cor- rosion, and their growth rate is decreased to a rate similar to that of the passive films, when suitable elements are added to the steel [149, 150]. Rust on weathering steel changes over time and the final protective rust has a fine a FeOOH and is dispersed as amorphous rust. It is reported that the addition of seed rust, which is a stage in rust formation, results in the preferential formation of homogeneous rust. This phenomenon suggested the possibility that protective rust will also form preferentially in atmospheric environments when protective rust is present [151, 152]. 1.7 Rust Characterisation 31
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