Corrosion: The Frequently Asked Questions
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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 or create differential aeration. 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.
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.
1. What reaction occurs on the cathode in Cathodic Protection ?
Reduction reaction occurs on the cathode. There are three possibilities:
2. What reaction occurs on the Impressed Current Anode ?
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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 consumable anode, oxidation of the anode material itself constitutes the anodic reaction.
1. Is there a difference between STRESS CORROSION CRACKING and CORROSION FATIQUE ?
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).
2. Is there a relation between these two forms of corrosion ?
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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.
3. Does there exist reference material on one or both subjects ?
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There is a wealth of journal literature and conference proceedings on these topics. The following are two well known proceedings and a recent compilation: "Corrosion Fatigue: Chemistry, Mechanics and Microstructure," eds: . Devereux, McEvily and Staehle, NACE, (Houston, 1972). "Fundamental Aspects of Stress Corrosion Cracking," eds. Staehle, Forty and VanRooyen, NACE, (Houston, 1969). "Atlas of Stress Corrosion and Corrosion Fatigue Curves, A. J. McEvily, ed., ASM International, (Metals Park, Ohio, 1990). [Back to top]