Corrosion testing continued exposure methods

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... 

The salt spray test, for example, which was originally designed to test coatings on metals, has been widely used to evaluate the resistance of metals to corrosion in marine service or on exposed shore locations. Extensive experience has shown that, although salt spray tests yield results that are somewhat similar to those of exposure to marine environments, they do not reproduce aU the factors causing corrosion in marine service. Salt spray tests should thus be considered to be arbitrary performance tests and their vahdity dependent on the extent to which a correlation has been estabhshed between the results of the test and the behavior under expected conditions of service. Despite the current widespread use of continuous salt spray methods, their unrealistic simulation of outdoor environments is a serious shortcoming. 

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