Coatings continued performance evaluation

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... 

When the sucrose esters were evaluated as modifiers for cellulose acetate butyrate, the crystal-producing compounds gave brittle and sometimes hazy or opaque films as they continued to crystallize, even in the presence of the cellulose polymer (Table II). On the other hand, the liquid esters performed as plasticizers for cellulose acetate butyrate, producing soft and tacky films at 50% modification. Because of this softening action, sucrose esters, forming low-viscosity liquids, were eliminated from commercial consideration. It also was estimated that these esters could not compete costwise with the commonly used, coatings plasticizers. 

Continuous Exposure Tests Several tests are used to evaluate the performance of coated steel substrates that involve continuous exposure to... 

Salt-spray testing The salt-spray test, 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 [19 20]. However, extensive experience has shown that, although salt-spray tests yield results somewhat similar to those exposed in marine environments, they do not reproduce all the factors causing corrosion in marine service. Salt-spray tests should thus be considered to be arbitrary performance tests and their validity dependent on the extent to which a correlation has been established between the results of the test and the behavior under expected conditions or 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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