Corrosion and prevention basic principles-write-up
1 CORROSION & CORROSION PREVENTION Basic Principles Dr. T. K. G. Namboodhiri Consultant-Metallurgy & Corrosion, Tiruvalla, Kerala (Ex-Professor of Metallurgical Engineering, Banaras Hindu University)1. INTRODUCTION1.1 What is corrosion?Corrosion may be defined as the destruction or deterioration in properties of materials byinteraction with their environments. It is a natural phenomenon. Engineers generallyconsider corrosion when dealing with metallic materials. However, the process affects allsorts of materials, for example, ceramics, plastics, rubber etc. Rusting of iron and steel is themost common example of corrosion. Swelling in plastics, hardening of rubber, deteriorationof paint, and fluxing of the ceramic lining of a furnace are all incidences of corrosion in nonmetallic materials. Metallurgists may think of corrosion as reverse extractive metallurgy.Metals are extracted from their compounds occurring in nature through extractive metallurgyprocesses involving considerable expenditure of energy, natural resources, time, and manpower. Corrosion works to convert the metal I back into the same compounds.1.2. Why is corrosion important?Corrosion is a destructive natural process. It causes huge losses of money, material, andnatural resources. Estimated annual loss due to corrosion ranges from 3.5 to 5 % of the GNPof a nation. Any effort to reduce this huge loss will be of much advantage to the economy.Hence, it is advisable that all, particularly those involved with engineering materials, areaware of the basic principles of corrosion and corrosion prevention.1.3. Cost of corrosionLoss due to corrosion may be direct or indirect.Direct costs: cost of material lost, cost of repair and replacement of corroded parts, cost ofpainting & other protective measures, over-design to allow for corrosion, and inability to useotherwise suitable & cheaper materials.Indirect costs: May be economical or social in nature. These include, contamination ofproducts like food items or drugs, loss of valuable products from leaking tanks or pipes, lossof production due to shut downs, loss in appearance, as in automobiles or homes, and loss insafety reliability of structures, machines, pipelines, storage tanks etc.As per 2004 estimates, the annual direct loss due to corrosion in India was Rs. 36,000 crores,while in the USA the loss was $364 billion. If the indirect costs are also taken into account,this figure will be several times more. The higher the state of industrialization of a country,
2the higher will be the loss due to corrosion. India with its hot and humid tropical climate willexperience a larger proportion of corrosion loss than a temperate country like the U.K. orFrance. A developing country like India, where a considerable percentage of population livesunder the poverty line, can ill afford the loss of such huge amounts due to corrosion. Hence,corrosion should be dealt with all the seriousness it deserves.2. PRINCIPLES OF CORROSIONWhy do metals corrode?Every system in the universe tries to reduce its energy content so as to become stable. This istrue for all types of reactions we see in nature. A spontaneous chemical reaction will occuronly if it leads to a lowering of the total energy content of the system. In thermodynamics wesay that all spontaneous reactions are accompanied by a lowering of the free energy. Mostmetals and alloys, except the few noble metals, have higher free energies than those of theirchemical compounds. This is the reason they are not seen in nature as native metals.Metallurgists spend lot of energy to convert metal compounds into metals, which thusbecome unstable. As soon as they are put to service, they tend to get converted into theirmore stable compound form. This is the basis of metallic corrosion.Corrosion is an interdisciplinary phenomenon. It involves principles of thermodynamics,electrochemistry, metallurgy, and physics & chemistry.2.1 Thermodynamics of corrosionThermodynamics deals with the energy content of metals. The free energy of formation of acompound is a measure of its energy content and stability. A compound with a high negativefree energy is very stable and requires a high energy input to convert it to the metal. Thismetal will have a high tendency to be converted back to the compound, so as to reduce itsenergy content, and so a high tendency for corrosion. The free energy change associated withthe compound formation is thus indicative of the tendency for corrosion of the metal. As isdiscussed in the next section, the free energy change is used to calculate the electrodepotentials of metals and their corrosivity. Potential – pH diagrams or Pourbaix diagrams areused to predict the corrosion tendency of a metal electrolyte system at various conditions.2.2 Electrochemical nature of corrosionMetallic corrosion is essentially electrochemical in nature. An electrochemical reaction is achemical reaction where, in addition to mass transfer, electron transfer also takes place. Inorder to understand electrochemistry, we must understand what an electrode means. Ametallic conductor in contact with an ionic conductor (electrolyte) is called an electrode. Anyelectrode will have a stable electrode potential, which develops due to the electrochemicalreactions taking place at the interface, and is the difference in potential of the metal andelectrolyte surrounding it. The electrode potential developed under standard conditions, thestandard electrode potential, is a characteristic property of the electrode.
3Electrochemical reactions take place in electrolytic cells, which consist of two differentelectrodes immersed in an electrolyteand connected electronically outsidethe cell, as in Fig. 1.Let us consider the corrosion of Znin hydrochloric acid. This processcan be represented by the electrolyticcell shown in Fig. 1. We have Znmetal as one electrode, and the inertpt as the second electrode. When Zncomes in contact with HCl, Zn atomsdissolve, as per the followingequations.DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,Zn + 2 HCl = ZnCl 2 + H 2 -------------------- -(1)Written in ionic form as,Zn + 2 H + + 2Cl − = Zn 2 + + 2Cl − + H 2 ----------------------(2)The net reaction being,Zn + 2 H + = Zn 2 + + H 2 ------------------------- (3)Equation (3) is the summation of two partial reactions,Zn → Zn 2* + 2e -----------------------------------------(4) and2 H + + 2e → H 2 ------------------------------------------(5)Equation (4) is the oxidation / anodic reaction andEquation (5) is the reduction / cathodic reactionThe Zn atoms get converted to Zn ions by the oxidation reaction (4) at the Zn electrode andget into the electrolyte, leaving behind two electrons/atom, which travel through the externalcircuit to the Pt electrode and reduce two hydrogen ions to a hydrogen molecule by thereaction (5). Thus one Zn atom dissolves while one molecule of hydrogen is liberated at the
4Pt electrode. The metal continues to dissolve liberating hydrogen molecules continuously.Such an electrochemical reaction leads not only to a mass transfer from the metal to theelectrolyte, but also electron transfer from one electrode to the other. He electrode on whichoxidation takes place is called the anode, while that on which reduction occurs is a cathode.Every electrochemical reaction thus has two components, one oxidation, and the secondreduction. These two reactions occur simultaneously at the same rate so that there is a chargebalance. The corrosion of a piece of Zn metal in HCl is shown schematically in Fig.2.Here both theanodic andcathodicreactions takeplace on thesame piece, atdifferentlocations. Themetal dissolvesas ions and theelectrons leftbehind move toanother point onthe metalsurface wherethey reducehydrogen ionsfrom theelectrolyte tohydrogenatoms.. Fig. 2 Corrosion of Zn in HClAs mentioned before, each electrode has a characteristic electrode potential, called the singleelectrode potential. This potential is related to the nature of the electrode reaction takingplace, and thus to the free energy change involved in the reaction. The relationship may begiven as, ∆G = -nFE --------------------------------------- (6)Where, ∆G is the free energy change in joules n is the number of electrons involved in the reaction E is the electrode potential in volts F is the Faraday constant, 96500 Coulombs/ g.equivalent.
5The single electrode potential of an electrode when all the reactants are at unit activity and at250 C is called the standard electrode potential (E0). These potentials are also referred to asredox potentials because they represent the equilibrium between an oxidation and a reductionreactions taking place at the electrode interface.EMF series & Galvanic series Standard electrode potentials of many common elements are tabulated as an EMF serieswhich is used by electrochemists to determine the possible direction of reactions. Corrosionengineers use another series, the galvanic series where metals are listed in the order of theirelectrode potentials measured under actual service conditions. Galvanic series predictscorrosion reactions more accurately.PolarizationWhen corrosion takes place on a metal, its electrode potential shifts away from the standardelectrode potential, according to the equation,E = E0 +2,303 RT/nF (product of activities of reactants/ product of activities of products)This shifting of the potential from the standard value is called polarization, which forms thebasis of the kinetics of corrosion reactions.Kinetics of corrosionWhile thermodynamics predict the possible direction of a reaction, it cannot predict the rateat which the reaction will occur. For this we require kinetics. The single electrode potentialof an electrode gets polarized when an electric current is flowing, ie, when corrosion takesplace. The rate at which corrosion occurs is determined by the Mixed Potential Theory ofcorrosion.Mixed Potential TheoryThe mixed potential theory of Wagnerand Traud helps us to determine thekinetic parameters of electrochemicalcorrosion. It consists of two simplehypotheses, 1) any electrochemicalreaction can be split into two or morepartial oxidation and reduction reactions,and 2) there can be no net accumulationof electrical charge during anelectrochemical reaction. Accordingly, acorroding metal cannot spontaneouslyaccumulate electrical charge. All the Fig. 3 Mixed potential theory
6electrons released by the anodic oxidation reaction must be consumed simultaneously by oneor more cathodic reduction reactions. The potential of a corroding metal will be determinedby the partial oxidation and reduction reactions involved in the process. This is schematicallyshown in Fig.3.2.3 Metallurgy of corrosionMetals and alloys are widely used in engineering applications and are the ones whichundergo corrosion in service. The nature and extent of corrosion strongly depend upon themetallurgical characteristics of the material. The tendency for corrosion of a metal dependsprimarily on its electrode potential and polarization behavior. The chemical composition ofthe material determines the electrode potential. Crystal structure of the metal may influencecorrosion. Metals are made up of tiny crystals called grains and many properties of thematerial depend upon the grain size. The grain boundary, the region between adjacent grains,is a defective region where impurities accumulate. Besides grains, engineering materialsgenerally contain different phases in their structure. These phases will have differentchemical compositions and hence different corrosion tendencies. Metals may also containdefects like dislocations, internal surfaces, inclusions and voids. All of these may affect thecorrosion behavior of the material. Mechanical properties like strength and ductility havemuch influence on certain forms of corrosion.2.4 Physics & Chemistry in corrosionMaterials undergo corrosion during service because they are exposed to corrosiveenvironments. Different environments have different corrosive properties. Hence, chemistryof the environment is a very important factor in corrosion. Physical properties like density,viscosity, melting point and boiling point, surface tension of the environment as well as thecorroding material may affect the corrosion behavior. Velocity of liquid media will have agreat influence on the extent of corrosion. Temperature and pressure are two other importantvariables in corrosion.3. FORMS OF CORROSIONThe process of corrosion may be classified in different ways. Some of these classificationsare given below. A) Wet corrosion and Dry corrosion: Wet corrosion is corrosion in presence of water or a liquid corrodent. Dry corrosion occurs in presence of gaseous atmospheres or in contact with solids. B) Room Temperature corrosion and High Temperature corrosion: Generally all wet corrosion takes place at or around room temperature. High temperature corrosion occurs generally much above the boiling point of water or other aqueous corrodents, and is a dry corrosion processes. C) Electrochemical corrosion and Chemical corrosion: Metallic corrosion is always electrochemical in nature. Dissolution of a metal in an acid was previously thought to be a simple chemical reaction and was called chemical corrosion. But now it is also seen as an electrochemical corrosion.
7the original Fontana classification of corrosion based on the appearance of the corrodedmetal. . Here we have 8 forms of room temperature or aqueous corrosion, and oxidationand corrosion under complex gaseous environments at high temperatures.Room Temperature or Aqueous CorrosionBased on the appearance of the corroded metal, wet corrosion may be classified as• Uniform or General• Galvanic or Two-metal• Crevice• PittingDealloying• For the purpose of this lecture, let us consider• Intergranular• Velocity-assisted• Environment-assisted crackingUNIFORM CORROSION• Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion.• Maximum metal loss by this form.• Not dangerous, rate can be measured in the laboratoryGALVANIC CORROSION• When two dissimilar metals are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junctionCREVICE CORROSION• Intensive localized corrosion within crevices & shielded areas on metal surfaces• Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap jointsPITTING
8• A form of extremely localized attack causing holes in the metal• Most destructive form• Autocatalytic nature• Difficult to detect and measure• MechanismFig.4 shows the mechanism of pitting. Fig. 4 Mechanism of Pitting.DEALLOYING• Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass.• Loss of structural stability and mechanical strengthINTERGRANULAR CORROSION• The grain boundaries in metals are more active than the grains because of segregation of impurities and depletion of protective elements. So preferential attack along grain boundaries occurs. e.g. weld decay in stainless steelsVELOCITY ASSISTED CORROSION• Fast moving corrosives cause• a) Erosion-Corrosion,• b) Impingement attack , and• c) Cavitation damage in metalsCAVITATION DAMAGE• Cavitation is a special case of Erosion-corrosion.• In high velocity systems, local pressure reductions create water vapour bubbles which get attached to the metal surface and burst at increased pressure, causing metal damageENVIRONMENT ASSISTED CRACKING• When a metal is subjected to a tensile stress and a corrosive medium, it may experience Environment Assisted Cracking. Four types:• Stress Corrosion Cracking• Hydrogen Embrittlement• Liquid Metal Embrittlement
9 • Corrosion Fatigue STRESS CORROSION CRACKING • Static tensile stress and specific environments produce cracking • Examples: • 1) Stainless steels in hot chloride • 2) Ti alloys in nitrogen tetroxide • 3) Brass in ammonia HYDROGEN EMBRITTLEMENT • High strength materials stressed in presence of hydrogen crack at reduced stress levels. • Hydrogen may be dissolved in the metal or present as a gas outside. • Only ppm levels of H needed LIQUID METAL EMBRITTLEMENT • Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc. • Molten metal atoms penetrate the grain boundaries and fracture the metal • • Fig 5 a). Tensile behavior under LME • Fig. 5 b). Brittle IG fracture in Al alloy by PbCORROSIONFATIGUE: S-N DIAGRAM
10Fig. 6a) gives schematic S-N curves for fatigue and corrosion-fatigue. Synergistic action of corrosion & cyclic stress. Both crack Stress Amplitude nucleation and propagation are accelerated by corrodent and the S-N diagram is Air shifted to the left Corrosion log (cycles to failure, Nf) Fig. 6a) S-N curves for fatigue and corrosion fatigueCRACK PROPAGATIONFig. 6b) shows schematic crack log (Crack Growth Rate, da/dN)propagation curves under fatigue as wellas corrosion-fatigue conditions. Crack propagation rate is increased by the corrosive action Log (Stress Intensity Factor Range, K
11 Fig. 6b) Crack propagation rates for fatigue and corrosion-fatigueHigh Temperature or Dry CorrosionOxidation under dry conditions, high temperature corrosion reactions in gaseousatmospheres, and hot corrosion come under this classification.OXIDATIONOxidation refers to the reaction between a metal and air or oxygen in the absence of water oran aqueous phase. Scaling, tarnishing and dry corrosion are other names for this process.Nearly all metallic materials react with oxygen at high temperatures. As the temperatureincreases, the oxidation resistance of materials decreases. As there are many applications ofmetals at high temperatures, like gas turbines, rocket engines, refineries, and furnaces, theimportance of high temperature oxidation is considerable.The oxidation resistance of a material may be related to the relative volumes of the metal andits oxide, through the Pilling-Bedworth ratio, R = Md / nmD, where, M is the molecularweight of the scale, D is the density of the scale, m is the atomic weight of the metal, d is thedensity of the metal, and n is the number of metal atoms in a molecular formula of the scale.R gives the volume of oxide formed from a unit volume of the metal. At a ratio of less than 1,the scale does not cover the metal completely, and the metal continues to get oxidized, whilea ratio much greater than one tends to produce too much oxide which introduces highcompressive stress and tendency for spalling of the scale. The ideal R value is close to one.Oxidation, like aqueous corrosion is an electrochemical process, and consists of two partialprocesses,M → M +2 + 2 e- ----------- Metal oxidation at metal-scale interface½ O2 + 2 e- → O2 --------- Oxygen reduction at scale-gas interface.----------------------
12M + ½ O2 → MO --------------------Overall reactionThe oxide scale acts as the electrolyte through which ions and electrons move to make theabove reactions possible. The electronic and ionic conductivities of the scale thus determinethe rate of oxidation of the metal.Oxidation kineticsWhen a fresh metal isexposed to oxygen, athin surface layer of theoxide forms on themetal. As oxidationcontinues, the scalethickness increases andthe reaction ratedecreases dependingupon the scalecharacteristics. Manyempirical rate equationshave been developed tofit experimentaloxidation data. Some ofthese are linear,parabolic, logarithmicand cubic. These areshown schematically in Fig. 7. Fig.7 Oxidation Rate LawsOxidation-resistant alloysThe oxide characteristics determine the oxidation resistance of an alloy. Most oxides are non-stoichiometric compounds with structural defects. They may be n-type or p-typesemiconductors whose conductivities could be altered by alloy additions. This principle isused in developing high temperature oxidation resistant alloys like Fe-Cr, Fe-Cr-Al, and Ni-base alloys.Catastrophic oxidationMetals that follow linear oxidation kinetics at low temperatures may experience oxidation atcontinuously increasing rates at high temperatures. Metals like Mo, W, Os, Rh, and V whichhave volatile oxides may oxidize catastrophically. Alloys containing Mo and V may oxidizecatastrophically by the formation of low melting eutectic oxide mixtures. Combustion of fueloils with high V compounds produces vanadium oxides in the gas phase, and can lead tocatastrophic oxidation.Internal OxidationIn some alloys, one or more dilute components may form more stable oxides than the basemetal which get distributed below the metal-oxide interface. This is called internal oxidation
13because the oxide precipitate forms within the metal matrix. Dilute copper and silver basedalloys containing Al, Zn, Cd, Be show such a behavior.CORROSION IN OTHER GASEOUS ENVIRONMENTSSulfur compoundsHigh temperature degradation of metals occurs when exposed to sulfur compounds like H2S,SO2 and vaporized sulfur. This process is referred to as sulfidation. In reducing gasescontaining hydrogen, such as gasified coal, H2S is a major gaseous constituent. In oxidizinggases such as fossil fuel combustion products, considerable SO2 may exist. These sulfurbearing gaseous compounds can lead to rapid scaling and to internal precipitation of stablesulfides. Mechanical properties of high temperature alloys are seriously affected by theseprecipitates.Decarburization and hydrogen attack.When steels are exposed o hydrogen at high temperatures, the carbon present either indissolved form or s carbides, reacts with hydrogen to produce methane gas, as per thefollowing reaction.C (Fe) + 4 H→CH4This phenomenon is called hydrogen attack. Decarburization leads to a decrease in thestrength of the steel. The methane formed inside the steel may lead to cracking. Cr and Moadditions to steels improve their resistance to decarburization and cracking. A Nelsondiagram is used to predict safe working conditions of hydrogen partial pressure andtemperature for various steels.Hot CorrosionHot corrosion refers to the accelerated high temperature corrosion of materials under sulfurgaseous atmospheres and the presence fused sulphate compounds on the metal surface.4) CORROSION TESTING.Corrosion tests are of four types; 1. Laboratory tests 2. Pilot-plant tests 3. Plant or actual service tests 4. Field testsLaboratory tests use small specimens and small volumes of corrodents and actual conditionsare simulated as far as possible. These are most useful as screening tests to determine whichmaterial warrants further studies.Pilot plant or semi-works tests are made in a small-scale plant that essentially duplicates theintended large-scale operation. This type of tests generally gives the best results.
14Actual plant tests are done when an operating plant is available. The purpose is in evaluatingbetter or more economical materials or in studying corrosion behavior of existing materialsas process conditions are changed.Field tests are designed to obtain general corrosion information. Examples are atmosphericexposure of a large number of specimens in racks at one or more geographical locations andsimilar tests in soil or sea water.Corrosion tests are essential for the following purposes; 1. Evaluation and selection of materials for a specific environment or a given application. 2. Evaluation of materials as regards to their compatibility to various environments. The information generated helps in the selection of materials for a specific application. 3. Control of corrosion resistance of the material or corrosiveness of the environment. These are routine quality control tests. 4. Study of the mechanisms of corrosion or other research and development purposes. These are specialized tests involving precise measurements and close control.Corrosion test standards have been developed by many organizations like ASTM, NACE inUSA and similar ones in other countries. Corrosion data exists for a large proportion ofmaterials used in several environments which could be used by material selectors.Corrosion RateThe most basic corrosion property of a material is the rate at which material is lost due toexposure to an environment. This is expressed as the corrosion rate, which is expressed intwo ways; 1) weight of material lost per unit surface area per unit time, and 2) the rate ofpenetration or thinning down of a material. The common corrosion rate units are mdd(mg/dm2/day) in the first category, and mpy (mils/year) in the second category.Weight loss after immersion in the corrodent for a specific time is measured on a specimen ofknown surface area and then the corrosion rate can be calculated as R = KW/ATDWhere,K is a constant for a specific unit of RW is weight lost in gmA is the surface area in sq.cmT is time of exposure in hoursD is density in g/cu.cmThe constant K varies from unit to unit. For mdd, the value of K is 2.4 x 106D, and for mpythe K value is 3.45x 106.5) PROTECTION AGAINST CORROSIONNeed for corrosion prevention • The huge annual loss due to corrosion is a national waste and should be minimized • Materials already exist which, if properly used, can eliminate 80 % of corrosion loss
15 • Proper understanding of the basics of corrosion and incorporation in the initial design of metallic structures is essentialMethods • Material selection • Improvements in material • Design of structures • Alteration of environment • Cathodic & Anodic protection • CoatingsMaterial Selection • Most important method – select the appropriate metal or alloy. • “Natural” metal-corrosive combinations like • S. S.- Nitric acid, Ni & Ni alloys- Caustic • Monel- HF, Hastelloys- Hot HCl • Pb- Dil. Sulphuric acid, Sn- Distilled water • Al- Atmosphere, Ti- hot oxidizers • Ta- Ultimate resistanceImprovement of materials 1) Purification of metals- Al , Zr 2) Alloying with metals for: • Making more noble, e.g. Pt in Ti • Passivating, e.g. Cr in steel • Inhibiting, e.g. As & Sb in brass • Scavenging, e.g. Ti & Nb in S.S • Improving other propertiesDesign of Structures • Avoid sharp corners • Complete draining of vessels • No water retention • Avoid sudden changes in section • Avoid contact between dissimilar metals • Weld rather than rivet • Easy replacement of vulnerable parts • Avoid excessive mechanical stressAlteration of Environment
16 • Lower temperature and velocity • Remove oxygen/oxidizers • Change concentration • Add Inhibitors – Adsorption type, e.g. Organic amines, azoles – H evolution poisons, e.g. As & Sb – Scavengers, e.g. Sodium sulfite & hydrazine – Oxidizers, e.g. Chromates, nitrates, ferric saltsCathodic & Anodic Protection • Cathodic protection: Make the structure more cathodic by – Use of sacrificial anodes – Impressed currents Used extensively to protect marine structures, underground pipelines, water heatersand reinforcement bars in concrete • Anodic protection: Make Passivating metal structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plantsCoatings • Most popular method of corrosion protection • Coatings are of various types: – Metallic – Inorganic like glass, porcelain and concrete – Organic, paints, varnishes and lacquers • Many methods of coating: – Electro deposition – Flame spraying – Cladding – Hot dipping – Diffusion – Vapour deposition – Ion implantation – Laser glazingSurface EngineeringThe process of altering the surface characteristics of materials is known as surfaceengineering. Corrosion is a surface property and all the coating processes mentioned abovecome under surface engineering. Besides corrosion, wear, fretting, fatigue etc are alsodependent on the surface characteristics of materials.CONCLUSION
17 • Corrosion is a natural degenerative process affecting metals, nonmetals and even biological systems like the human body • Corrosion of engineering materials lead to significant losses • An understanding of the basic principles of corrosion and their application in the design and maintenance of engineering systems result in reducing losses considerablyREFERENCES 1. Corrosion Engineering, Mars G. Fontana, 3rd Ed. McGraw-Hill International, Singapore, 1987 2. Corrosion and Corrosion Control, Herbert H. Uhlig, 3rd Ed. John Wiley & Sons, New York, 1985 3. Metals Handbook, 9th ed. Volume 13, Corrosion, ASM International, Metals Park, Ohio, 1988