Steel Corrosion
Table of Content1. 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.
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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.
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Figure 1. Minaus River Bridge collapse 1983, USA , the incident killed 3 people and the repairs totaled $20 million USD (flickr.com)
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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.
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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.
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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 TEMPERATURECORROSION
HIGH TEMPERATURECORROSION
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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.
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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.
5. Forms Of Corrosion (Con..)Table 1. Classification of corrosion based on the appearance of the corroded metal (Edward , 2010)
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(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)
5. Forms Of Corrosion (Con..)
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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…)
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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 → + n electrons (6.2) Anodic reaction (corrosion)
Secondly, the other half of the reaction must involve water (H2O) and oxygen (O2) something like this :
+ + 4e- → 4OH- (6.3)
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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 + + → 2Fe(OH)2 (6.4)
Iron + Water with oxygen dissolved in it → Iron Hydroxide
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6. How Does It Happen ? (Con..)• 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 + → + 2Fe2O3. (6.5)
Iron hydroxide + oxygen → water + Hydrated iron oxide (brown rust)
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6. How Does It Happen ? (Con..)
Figure 3. Rust, the Result of Corrosion of Metallic IronPage 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 process4) 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.
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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).
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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)
Where: W = weight loss (gram) TSA = total surface area (mm2) T = time of exposure (days)
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10. Comparison between Different metals.
MASS LOSS g/m2Corrosiveness category Carbon
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)
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11. Corrosion Prevention1) 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)
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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
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11. Corrosion Prevention (Con..)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.
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11. Corrosion Prevention (Con..)
Steel Tank Cathod
Mg Rod Anode
Hot Water
Current
Figure 4. sacrificial anodic protection method (nptel Lectures)Page 23 of 33
11. Corrosion Prevention (Con..)
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
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11. Corrosion Prevention (Con..)
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
HighVery low
LowMedium 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
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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.
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12.1 Corrosion Monitoring ActivitiesAn 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.
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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 testingThe selection of the suitable method as well as the inspection and monitoring of corrosion requires knowledgeable and experienced personnel.
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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.
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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)
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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.
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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.
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References1) 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.pdf6) http://www.corrosion-doctors.org/7) https://en.wikipedia.org/wiki/Corrosion_monitoring.
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