Steel corrosion

Table of Content
1. Definitions.
2. Major Causes of Corrosion.
3. Other Causes of Corrosion.
4. Forms Of Corrosion.
5. How Does It Happen ?
6. The Process of Corrosion (Five facts).
7. Measurement of Corrosion.
8. Corrosion Rate.
9. Comparison between Different metals.
10. Corrosion Prevention.
11. Corrosion monitoring.
12. Side effects of Prevention Methods.
13. Conclusion.
Page 1 of 33

1. Definitions
What is Steel ?
Steel is an alloy of iron and other elements, primarily carbon, that is
widely used in construction and other applications because of its
high tensile strength and low cost.
What is Corrosion?
Corrosion is defined as the destruction or deterioration of a material
because of its reaction with environment. When metals revert to their
combined state, they corrode. Corrosion may affect one or more
properties of the metal, which need to be preserved.
Page 2 of 33

Figure 1. Minaus River Bridge collapse 1983, USA , the incident killed 3 people
and the repairs totaled $20 million USD (flickr.com)
Page 3 of 33

2. Major Causes of Corrosion
• Nature of the metal or alloy.
• Presence of inclusions or other foreign matter at the
surface.
• Homogeneity of the metallic structure.
• Nature of the corrosive environment.
• Incidental environmental factors such as variations in
the presence of dissolved oxygen, of temperature,
and in the velocity of movement either of the
environment or of the system itself.
Page 4 of 33

3. Other Causes of Corrosion
• Other factors such as stress (residual or applied, steady or
cyclic).
• Presence of deposits on surfaces.
• Fayed surfaces and the possibility of corrosion crevices.
• Incidental presence of stray electrical currents from
external sources.
Page 5 of 33

4. Forms Of Corrosion
Corrosion may be classified in
different ways:
• Wet / Aqueous corrosion & Dry
Corrosion
• Room Temperature/ High
Temperature Corrosion
CORROSION
WET
CORROSION
DRY
CORROSION
CORROSION
ROOM
TEMPERATURE
CORROSION
HIGH
TEMPERATURE
CORROSION
Page 6 of 33

4. Forms Of Corrosion (Con..)
• Wet / Aqueous corrosion is the
major form of corrosion which
occurs at or near room temperature
and in the presence of water.
• Dry / gaseous corrosion occurs
when there is no water or
moisture to aid the corrosion,
and the metal oxidises with the
atmosphere alone , and it is
significant mainly at high
temperatures.
Page 7 of 33
Figure 2. Forms of corrosion
(Corrosion.doctors.com).

(a) 1. Uniform Corrosion:
Generally occurs due to direct chemical
attacks.
(b) 2. Galvanic Corrosion:
An electrochemical action that occurs
between two dissimilar metals which are
in contact with other.
(c) 3. Crevice Corrosion:
It occurs when metals are in contact with
nonmetals.
(d) 4. Pitting Corrosion:
corrosion of an open metal surface,
confined to a point or small area, which
takes the form of small cavities.
Table 1. Classification of corrosion based on the appearance of the corroded metal (Edward , 2010)
Page 8 of 33

(e) 5. Intergranular Corrosion:
It occurs on grain boundaries of a metal
or alloy.
(f) 6. Dealloying Corrosion:
preferential dissolution of an alloying
element due to corrosion (aka parting
corrosion or selective leaching).
(g) 7. Erosion-corrosion:
a conjoint action involving corrosion and
erosion in the presence of a moving
corrosive fluid leading to the accelerated
loss of material.
(h) 8. Environmentally assisted cracking :
EAC is a general term that includes
corrosion fatigue and stress corrosion
cracking (SCC).
Figure 2. a-h shows different types of corrosion (chemistry.tutorvista.com)
Page 9 of 33

Table 2. 1970’s Industry Study of Failures (corrosion.doctors.com)
Method % of Failures
Corrosion (all types) 33%
Fatigue 18%
Brittle Fracture 9%
Mechanical Damage 14%
Fab./Welding Defects 16%
Other 10%
5. Forms Of Corrosion (Con…)
Page 10 of 33

6. How Does It Happen ?
• The corrosion product we see most commonly is the rust which forms
on the surface of steel and somehow:
Steel Rust (6.1)
• Two reaction : for this to happen the major component of steel, iron
(Fe) at the surface undergoes a number of reactions :
 Firstly, the iron atom can lose some electrons and become a
positively charged ion.
Fe 𝑭𝒆 𝒏+
+ n electrons (6.2)
Anodic reaction (corrosion)
Secondly, the other half of the reaction must involve water (H2O) and
oxygen (O2) something like this :
𝐎 𝟐+ 𝟐𝐇 𝟐 𝐎+ 4e- 4OH- (6.3)
Page 11 of 33

6. How Does It Happen ? (Con..)
• This makes sense as we have a negatively charged material
that can combine with the iron and electrons, which are
produced in the first reaction are used up. We can, for clarity,
ignore the electrons and write :
2Fe + 𝐎 𝟐 + 𝟐𝐇 𝟐 𝐎 → 2Fe(OH)2 (6.4)
Iron + Water with oxygen dissolved in it → Iron Hydroxide
Page 12 of 33

• The Next Step, Oxygen dissolves quite readily in water and because
there is usually an excess of it, reacts with the iron hydroxide.
4Fe(OH)2 + 𝐎 𝟐 → 𝟐𝐇 𝟐 𝐎 + 2Fe2O3. 𝐇 𝟐 𝐎 (6.5)
Iron hydroxide + oxygen → water + Hydrated iron oxide
(brown rust)
Page 13 of 33

Figure 3. Rust, the Result of Corrosion of Metallic Iron
Page 14 of 33

7. The Process of Corrosion (Five facts)
This series of steps tells us a lot about the corrosion process:
1) Ions are involved and need a medium to move in (usually water)
2) Oxygen is involved and needs to be supplied.
3) The metal has to be willing to give up electrons to start the process
4) A new material is formed and this may react again or could be
protective of the original metal.
5) A series of simple steps are involved and a driving force is needed
to achieve them.
The most important fact is that interfering with the steps allows the
corrosion reaction to be stopped or slowed to a manageable rate.
Page 15 of 33

8. Measurement of Corrosion
During the process of corrosion, weight of material decreases and depth
of corrosion layer/pits increases. Further, the mechanical properties
such as yield (YS) and tensile strength (UTS) are decreased.
Corrosion effect Unit
Weight change g /m2 /year
Increase in corrosion depth µm /year; mpy (mil per year)
Corrosion current mA /cm2
Decrease in Y.S, UTS % / year
Table 3. The effects and measurement units of corrosion(unknown).
Page 16 of 33

9. Corrosion Rate
Status ipy mm/y
Completely Satisfactory <0.01 0.25
Use with caution <0.03 0.75
Use only for short exposure <0.06 0.15
Completely unsatisfactory >0.06 0.15
Table 4: Acceptable corrosion rate (Ukoba, O.Kingsley et. Al, 2013)
𝒄𝒐𝒓𝒓𝒐𝒔𝒊𝒐𝒏 𝒓𝒂𝒕𝒆 =
𝑾
𝑻𝑺𝑨 𝑿
𝑻
𝟑𝟔𝟓
Where: W = weight loss (gram)
TSA = total surface area (mm2)
T = time of exposure (days)
Page 17 of 33

10. Comparison between Different metals.
MASS LOSS g/m2
Corrosiveness categoryCarbon
Steel
Zinc Copper Aluminum
≤10 ≤0.7 ≤ 0.9 Negligible Very low C1
10–200 0.7–5 0.9–5 ≤0.6 Low C2
200–400 5–15 5–12 0.6–2 Medium C3
400–650 15–30 12–25 2–5 High C4
650–1,500 30–60 25–50 5–10 Very high C5
Table 5. Mass loss (g/m2 ) for one year field test exposure in the five corrosivity
classes C1–C5, the order being from the least to the most corrosive. (R.Landolfo,
2010)
Page 18 of 33

11. Corrosion Prevention
1) Conditioning the Metal: This can be sub-divided into two
main groups:
A. Coating the metal, in order to interpose a corrosion
resistant coating between metal and environment. The
coating may consist of (a) another metal, e.g. zinc (b) a
protective coating derived from the metal itself, e.g.
aluminium oxide (C ) organic coatings, such as resins,
plastics, paints.
B. Alloying the metal to produce a more corrosion resistant
alloy, e.g. stainless steel (alloyed with chromium and nickel)
Page 19 of 33

11. Corrosion Prevention (Con..)
2) Conditioning the Corrosive Environment :
A. Removal of Oxygn: By the removal of oxygen from water
systems in the pH range 6.5-8.5 one of the components
required for corrosion would be absent.
B. Corrosion Inhibitors: A corrosion inhibitor is a chemical
additive, which, when added to a corrosive aqueous
environment, reduces the rate of metal wastage. Such as
(a) Anodic inhibitors are chemical substances that form a
protective layer of oxide film on the surface of metal.
(B) Cathodic inhibitors slow the reaction at the cathode or
precipitate cathodic areas in order to increase the resistance
on the surface
Page 20 of 33

3) Electrochemical Control:
A. Cathodic protection (CP) is a technique used to control the
corrosion of a metal surface by making it the cathode of an
electrochemical cell. A simple method of protection connects the
metal to be protected to a more easily corroded "sacrificial metal" to
act as the anode.
B. Anodic protection (AP) is a technique to control the corrosion of a
metal surface by making it the anode of an electrochemical cell and
controlling the electrode potential in a zone where the metal is
passive.
Page 22 of 33

Steel Tank
Cathod
Mg Rod
Anode
Hot Water
Current
Figure 4. sacrificial anodic protection method (nptel Lectures)
Page 23 of 33

Power
Supply
BA
C
Figure 5. Anodic protection of inner surface of a steel acid storage tank (nptel Lectures)
A. Auxiliary cathode
B. B. Reference electrode
C. C. Anode connection to the tank
Page 24 of 33

Table 6. Comparison of Anodic and Cathodic protection (corrosiondoctor.com)
Anodic Cathodic
Applicability Active-passive
matals/alloys
All metals/alloys
Nature of corrosive
medium
Weak to aggressive Weak to medium
Cost: Installation
Maintenance
High
Very low
Low
Medium to high
Operating
conditions
Can be accurately
determine
Determined by
empirical testing
Significance of
applied current
Direct measure of
protected corrosion
rate
Complex to indicate
corrosion rate
Page 25 of 33

12. Corrosion monitoring
 Corrosion monitoring may be defined as the systematic
measurement of corrosion rate of equipment with the object of
diagnosis and controlling corrosion.
• It can also be used for monitoring efficiency of implementation of
corrosion control system.
• For reliable operation it is important to identify the location, rate,
and underlying causes of corrosion.
Page 26 of 33

12.1 Corrosion Monitoring Activities
An effective corrosion monitoring program includes a wide
range of activities:
• Identification of component alloy composition.
• Measurement of the location and extent of corrosion.
• Prediction of remaining life.
• Identification of failure mechanisms.
• Determination of fitness for service condition.
• Inspection scheduling.
• Development of recommendations for treatment and
correction of problems.
• Development of corrosion prevention strategies.
Page 27 of 33

12.2 Corrosion Monitoring Methods
Corrosion Monitoring uses a wide range of measurement techniques.
non-destructive testing (NDT) methods are the most effective and
broadly applied testing methods. Suitable NDT methods for the
monitoring of corrosion include:
• Ultrasonic testing
• Radiographic testing
• Guided wave testing
• Electromagnetic testing
The selection of the suitable method as well as the inspection and
monitoring of corrosion requires knowledgeable and experienced
personnel.
Page 28 of 33

13. Common methods of corrosion prevention and its
impacts on human and environmental health.
1) Painting:
Even though painting the metalic substance is a great method of
corrosion prevention, it also has negative impacts. When paints dry
up, they release VOC (Volatile Organic Compounds). The fumes of
VOC are very toxic and can cause problems when inhaled and may
play a role in pollution.
2) Galvanizing:
Galvanizing is more effective than painting the surfaces as it lasts
longer and doesn't have any harmful impacts on the environment
and human beings.
Page 29 of 33

13. Common methods of corrosion prevention and its impacts
on human and environmental health (Con..)
3) Cathodic protection:
A side effect of the cathodic protection was to increase marine growth.
Copper, when corroding, releases copper ions which have an anti-
fouling effect.
Figure 6. presents marine growth on offshore structure (offshorewind.biz)
Page 30 of 33

14. Conclusion
• Corrosion is defined as the destruction or
deterioration of a material because of its reaction
with environment.
• There are many Causes of Corrosion such as
Nature of the metal or alloy, and corrosive
environment.
• Corrosion may be classified in different ways for
example wet and dry corrosion.
Page 31 of 33

14. Conclusion
• The metal can be produced by many methods.
• For reliable operation it is important to identify the
location, rate, and underlying causes of corrosion.
• Non-destructive testing (NDT) methods are the
most effective and broadly applied testing methods
for corrosion monitroring.
Page 32 of 33

References
1) McCafferty, Edward. Introduction to corrosion science. Springer Science
& Business Media, 2010.
2) Ahmad, Zaki. Principles of corrosion engineering and corrosion control.
Butterworth-Heinemann, 2006.
3) Vandelinder, L. S. "Corrosion Basics-An Introduction." L. S. Vandelinder,
Ed. 364 pages, 8. 5 x 11 in.(22 x 28 cm), hard cloth. NACE, Houston,
Texas, 1984. Item 51020 (1984).
4) Olusunle, S. O. O., B. Ebiwonjumi, and R. O. Medupin. "Mathematical
Modeling: A Tool for Material Corrosion Prediction." (2011).
5) https://www.nrc.gov/docs/ML1122/ML11229A059.pdf
6) http://www.corrosion-doctors.org/
7) https://en.wikipedia.org/wiki/Corrosion_monitoring.

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