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Corrosion: The Frequently Asked Questions

Subject Index
 

  1 Is pitting caused by the environment?   2 How a pit once initiated can be re-pasisvated?
  3
 
What reaction occurs on the cathode in cathodic protection?   4
 
What reaction occurs on the anode in impressed current cathodic protection (ICCP)?
  5
 
Is there a difference between stress corrosion cracking and corrosion fatigue?   6
 
Is there a relation between the two forms of corrosion (SCC & CF)?
  7 What is the effect of stress on corrosion?   8 Does aluminum rust?
  9 Why does aluminum corrode?  10 What is stainless steel and who invented it?
11 Does stainless steel rust? 12 What causes the layer of chromium to be removed during heat treatment?
13 What is sensitization? 14 What is weld decay?
15 How to prevent weld decay? 16 What is knife line attack?
17 How to prevent knife line attack? 18 What is the difference between weld decay and knife line attack?
19 How to validate CO2 corrosion model prediction? 20 How to verify CO2 corrosion model prediction?
       
       

 


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1. Is pitting caused by the ENVIRONMENT?

Yes and No !

For a defect-free "perfect" material, pitting corrosion IS caused by the ENVIRONMENT that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks. The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (centre of the water droplet).

For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions (MnS is the major culprit for the initiation of pitting in steels) or defects. In most cases, both the environment and the material contribute to pit initiation.

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2. How a pit once initiated can be repassivated?

The ENVIRONMENT and the MATERIAL factors determine whether an existing pit can be repassivated or not.

Sufficient aeration (supply of oxygen to the reaction site) may enhance the formation of oxide at the pitting site and thus repassivate or heal the damaged passive film (oxide) - the pit is repassivated and no pitting occurs.

An existing pit can also be repassivated if the material contains sufficient amount of alloying elements such as Cr, Mo, Ti, W, N, etc.. These elements, particularly Mo, can significantly enhance the enrichment of Cr in the oxide and thus heals or repassivates the pit.

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3. What reaction occurs on the cathode in Cathodic Protection?

Reduction reaction occurs on the cathode. There are three possibilities:

  • pH=7: O2 + 2H2O + 4e = 4OH-
  • pH>7: O2 + 2H2O + 4e = 4OH-
  • pH<7: 2H+ + 2e = H2

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4. What reaction occurs on the Impressed Current Anode ?

In Impressed Current Cathodic Protection, the reaction on the anode depends on the nature of the anode material. For inert anode such as Platinized, Titanium Oxide anode, oxidation of the chemical species in the environment occurs. For example, in sea water,  the reaction is 2Cl- -2e ==> Cl2. For consumable anode, oxidation of the anode material itself constitutes the anodic reaction.

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5. Is there a difference between STRESS CORROSION CRACKING and CORROSION FATIGUE?

Yes, SCC is under constant load and CF is under cyclic load. This difference further leads to many other different characteristics of the two forms of failure e.g. transgranular or intergranular, local chemical environment inside the cracks etc.

"All metals which are subject to corrosion are subject to corrosion fatigue in any corrosive environment. Stress corrosion cracking, on the other hand, is suffered by alloys only in very specific environments and is normally investigated under conditions of static tensile stress rather than dynamic stress." (D. J. Duquette in "Corrosion Fatigue: Chemistry, Mechanics and Microstructure, eds: Devereux, McEvily and Staehle, NACE, Houston, 1972, p. 12).

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6. Is there a relation between these two forms of corrosion?

Yes, at least one common feature of the two forms of the failure is that they are all caused by the so-called "chemo-mechanical interaction" which is the enhanced electrochemical behavior under mechanical load. Also, localized dissolution due to rapture of oxide film is common to the two.

Stress corrosion during the tension half cycle is one of the mechanisms by which corrosion fatigue may occur, especially at low frequencies and, again, in specific environments.

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7. What is the effect of stress on corrosion?

Stress is a major influencing factor on the rate and form of corrosion. As discussed  in FAQ#5-6 above, stress can lead to cracking and fatigue depending the nature and magnitude of the stress present. If stress is below the threshold for cracking or fatigue, there would be no noticeable effect on corrosion. However, in unevenly stressed parts, the highly stressed area would act as anode and can suffer from preferential corrosion, for example, the nail head and tip in a nail and the bend section in a U-bend.

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8. Does aluminium rust?

No. Aluminum does not rust but it corrodes in the same way as other metals. Rust is a corrosion product (hydrated iron oxide, Fe(OH)3) formed on ferrous (iron-based) metals such as steels. The corrosion product on aluminum does not have the rusty appearance as observed on a steel.

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9. Why does aluminum corrode?

Aluminum and its alloys are normally protected by an ultra thin surface oxide film (known as the passive film) and are reasonably resistant to corrosion in environments having a pH of 4~8. In acidic or alkaline environment, the oxide film is no longer stable and aluminum corrodes actively, forming soluble corrosion product (Al3+ or AlO2-).

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10. What is stainless steel and who invented it?

Stainless steel refers to a family of iron-base alloys that contains at least 11% chromium. Chromium is an essential element that makes a steel "stainless" by forming a protective chromium oxide film on its surface. When you cut or scratch a stainless steel, the chrome in the steel will rapidly oxidize and repair the damaged oxide film - it is this self-healing property that maintains the stainless appearance of the steel.

The first stainless steel (a martensitic Fe-Cr-C alloys) was invented by an Englishman, Harry Brearley, in 1912. The first commercial stainless steel cast was made in Sheffield, England in 1913. Brearley was granted an U.S. patent for his invention in 1916.

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11. Does stainless steel rust?

It is a common mis-conception that stainless steels do not rust. Occasionally, such deep mis-conception leads to disputes and law suits between the contractor and the facility owner. Stainless steels remain stainless only under certain conditions, for example, unpolluted atmosphere, moving or flowing fresh water and seawater. In humid marine atmosphere or under stagnant waters, type 304 stainless steel will rust, albeit in a localised manner often referred to pitting corrosion. Both the exposure environment and the chemical composition of the steel will determine if rust (pitting) will develop on a stainless steel.

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12. What causes the layer of chromium to be removed during heat treatment?

During heat treatment or welding, stainless steel experiences a temperature range of 550oC~850oC. Chromium and carbon react to precipitate chromium carbides along the grain boundaries. This process leads to the depletion of chromium in the adjacent narrow region along the grain boundaries. The chromium depleted region has poor resistance to corrosion compared with the normal grains where chromium content is not affected. A stainless steel is said to be sensitized when chromium carbides form in its structure. A sensitized steel is susceptible to intergranular corrosion or weld decay.

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13. What is sensitization?

Sensitisation refers to the precipitation of chromium carbides, usually at grain boundaries, on exposure to temperatures of about 550 to 850oC (about 1000 to 1550oF), leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding (oxidizing) medium.

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14. What is weld decay?

Weld decay refers to the intergranular corrosion, usually of stainless steels or certain nickel-base alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation.

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15. How to prevent weld decay?

Three methods are commonly used to deal with weld decay: (1) use low carbon (e.g. 304L, 316L) grade of stainless steels Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). (2) titanium and niobium are strong carbide-formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion. (3) Use post-weld heat treatment.

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16. What is knife line attack?

Knife-line attack refers to the intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range. The corrosive attack is restricted to extremely narrow line adjoining the fusion line. Attack appears razor-sharp and hence the name of "knife-line" attack.

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17. How to prevent knife line attack?

Knife-Line Attack can be prevented through heat treatment - heating the weld to 1065oC to re-stabilize the material.

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18. What is the difference between weld decay and knife line attack?

There are two major differences: (1) materials - weld decay occurs to normal, un-stabilized steels while knife line attack occurs to stabilized steels; (2) location of attack - weld decay occurs in the heat affected zone (HAZ) while knife line attack is located at the fusion line.

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19. How to validate CO2 corrosion model prediction results?

Commercial CO2 model software developers typically do not provide users with any validation details.  Validation of modeled results against lab or field data is often difficult as quality lab or field data under the prevailing operating conditions used in the CO2 corrosion prediction software are not readily available. This is particularly true at the design stage where the input parameters are often simulated or projected. Validation of modeled results against corrosion monitoring data in the field may not be applicable as the corrosion monitoring data is "spot" measurement at a specific location under some uncertain local operating conditions while the modeled results represent the "worst case" scenario in the whole system (not spot measurement) under the specific operating conditions. The only practical way to make sure that your modeled results are reasonably reliable is to validate the CO2 corrosion modeling software itself by using your own or any 3rd party's well-defined quality lab and/or field data before starting the modeling project. It is critical to use your own or any 3rd party's quality data, not the model developer's data (JIP or in-house), for the validation process. Any use of poor quality data would defeat the whole purpose of validation. WebCorr has developed a CO2 Corrosion Model Validation Matrix and Index Score system for objectively and quantitatively determine a CO2 corrosion model's accuracy of prediction. Please refer to the links below for details:

Pipeline and Gas Journal, March 2016, Vol.243, No.3

CO2 Corrosion Model Validation Matrix and Index Score System

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20. How to verify CO2 corrosion model prediction results?

Commercial CO2 model software developers typically do not provide users with any verification details.  Verification of modeled results against lab or field data is often difficult as quality lab or field data under the prevailing operating conditions used in the CO2 corrosion prediction software are not readily available. This is particularly true at the design stage where the input parameters are often simulated or projected. Verification of modeled results against corrosion monitoring data in the field may not be applicable as the corrosion monitoring data is "spot" measurement at a specific location under some uncertain local operating conditions while the modeled results represent the "worst case" scenario in the whole system (not spot measurement) under the specific operating conditions. The only practical way to make sure that your modeled results are reasonably reliable is to verify the CO2 corrosion modeling software itself by using your own or any 3rd party's well-defined quality lab and/or field data before starting the modeling project. It is critical to use your own or any 3rd party's quality data, not the model developer's data (JIP or in-house), for the verification process. Any use of poor quality data would defeat the whole purpose of verification. WebCorr has developed a CO2 Corrosion Model Validation Matrix and Index Score system for objectively and quantitatively determine a CO2 corrosion model's accuracy of prediction. Please refer to the links below for details:

Pipeline and Gas Journal, March 2016, Vol.243, No.3

CO2 Corrosion Model Validation Matrix and Index Score System

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Copyright 1995-2017. All rights reserved.