Pitting corrosion continued mechanism

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

While those authors have ruled out the effect of the acid produced by the bacteria on corrosion acceleration, they have suggested that in the presence of an aerobic hetertrophic bacterium, repassivation of pits does not happen but pit growth continues. They nominate pit propagation in the presence of bacteria as the main mechanism for observing the drop in carbon steel s open circuit potential (OCP) and polarisation resistance. 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

SCC starts by an electrochemical mechanism. A pit, scratch, or rupture in a protective film can act as the starting point for corrosion. Anodic and cathodic areas form on the metal surface, with the weakly film-covered region and the tip of the crack acting as an anode and the oxide-covered region acting as a cathode. Once corrosion starts, the stresses tend to concentrate at the tip of the crack, which remains active. At some critical stress value, deformation results in the formation of a fresh surface (at tips where all the stresses are relieved). The electrochemical mechanism takes over on the fresh surface, building up stress at the tip of the crack. This sequence of events repeats continuously. 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

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