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.

The End