Pitting exposure environments

Local corrosion sites are typified by (1) local chemistries that are commonly only loosely related to the bulk exposure environment, (2) the separation of anodic and cathodic sites, and (3) the localization of corrosion damage sites (i.e., within pits, crevices, and cracks). Since, within a local corrosion site, the reactive surface area to available solution volume can be very large, extreme environments (in terms of concentration, concentration gradients, pH) are often encountered. For the same geometric reasons, these environments are difficult to characterize. Extremely high corrosion current densities can be sustained within the local site by the presence of much lower cathodic current densities over a much larger available surface area outside the corrosion site. Finally, the existence of ionic and concentration gradients between the local corrosion site and the external environment introduces complex transport scenarios. 

Anodising is a means to reduce or even prevent pitting corrosion, if the thickness is sufficient for the exposure environment (see Section C.5.5). 

Chromate conversion coatings perform poorly in environments containing acidified chloride. In salt/S02 spray tests the substrate metal is heavily pitted after three to four days of exposure. In this work, a new coating was developed which itiproved the corrosion resistance of the conventional chromate coating remarkably. 

Salt Spray Test Panels coated with the standard chromate conversion coating and CMT were compared with each other in their corrosion resistant properties in several ways. The conventional 5% NaCl/S02 fog chamber tests showed excessive corrosion and pitting within one week on chromate conversion coated (COC) 7075-T6 A1 alloy panels. The CMT coated panels were almost uncorroded and without any pits. The plates in Figure 1 show the conditions of the panels after 7 and 14 days exposure in this environment. Even after 14 days exposure the CMT panels were still far better than COC panels. 

Each practice indicates its limitations, and these are important to keep in mind. As with all accelerated tests, what is actually tested is the resistance of these alloys to IGA in the specific test environment. One must be very careful to be sure that these results correlate with longer term exposures to the field environment of interest. In addition, these tests are only useful for evaluating the susceptibility of these alloys to IGA, not to pitting or general corrosion or SCC. 

Initiation factors that are believed to contribute to pit initiation are (1) imperfections or defects in the metal oxide, passive, or protective layer between the metal substrate and its environment and (2) exposure of the metal substrate to an aggressive medium. 

Cruz, R.P., Nishikata A., Tsum T., Pitting corrosion mechanism of stainless steel under wet- dry exposure in chloride containing environments, corrosion science, 40, pp 125-139, 1998. 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

Fig. 1.9 Pits on 304 stainless steel after exposure to simulated marine environment (3.5 g/L NaCI) [27].

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