Automotive trim, anodized aluminum

Effect of Acid Rain on Exterior Anodized Aluminum Automotive Trim... 

Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion pro tec tion. > 2 Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. 

The combination of acid precipitation with road salt spray produced the worst effects on anodized aluminum. As the MIST test pH became more acidic and the amount of salt increased the time to achieve an equivalent milky white appearance was reduced significantly (Table I). At more neutral high chloride concentrations severe pitting occurred while at more acid high chloride concentrations blush and bloom predominated. The latter environment is similar to that existing in the Northeast USA and Canada and, therefore, the results can explain the problems of blush and bloom in these areas. Corrosion surveys by automotive companies and trim producers in these areas have shown that blush and bloom and pitting have become increasingly more severe over the last ten years.5 These problems have led to a shift away from anodized aluminum as an automotive trim material in recent years. 

An accelerated MIST test procedure has been developed which duplicates the mechanisms of corrosion of anodized aluminum trim in automotive environments. Blush and bloom of anodized aluminum automotive trim is more severe in environments with acid precipitation and this effect can be duplicated in an acidified MIST test procedure. Pitting of anodized aluminum is more prevelant in automotive environments with high chloride concentrations and this effect can be duplicated in a neutral chloride MIST test procedure. A change in the mechanism of corrosion of anodized aluminum trim from pitting to blush and bloom in chloride containing environments occurs in the pH range of 2 to 4. These results indicate that blush and bloom of anodized aluminum will become more severe as the acidity of precipitation increases. Thus more expensive trim materials such as bimetal are being used by the automotive industry. 

The section on metallic corrosion clearly indicates that the conventional method of classification of environments into marine, industrial, and rural no longer is adequate. More specific information is needed about the actual chemical components in the atmosphere as well as humidity and other factors. Specific environments also are addressed in the metallic corrosion section. For example, the automotive environment in the northeastern United States is particularly severe because of the combination of acid deposition and the use of road de-icing salts. These factors exert a synergistic effect on the corrosion behavior of auto-body steel and on exterior anodized aluminum automobile trim. 

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