The Application of Electrochemical Techniques to the Study of Corrosion of Automotive Trim Materials

De-icing salts are known to have a contributing effect on the corrosion of automotive trim materials. Various accelerated tests have been developed to evaluate the corrosion resistance of trim materials, but each test has its own limitations. Electrochemical techniques are a valuable supplement to these other methods, not only because they are fast and reproducible but also because they help in determining mechanisms of reactions. A potentiodynamic anodic polarization method was used to show the relative corrosion resistance of four different stainless steels and a chromium plated stainless steel used in trim applications, and the order of corrosion resistance compared favorably with that obtained by conventional chemical tests and by field exposure. Furthermore, the test was used to show distinct differences in the pitting susceptibility of stainless steels exposed to different concentrations of chloride ion. The thermographic test is commonly used to evaluate the corrosion resistance of stainless steels. The effect of specific ingredients in the solution on corrosion was determined by both chemical and electrochemical tests. Chemical tests showed that chloride and thiosulfate ions had the greatest effect on pit initiation, probably because the thiosulfate altered the redox potential of the solution. Potentiodynamic polarization measurements indicated that the chloride ion concentration was rate controlling after corrosion had begun, and the thiosulfate ion caused a shift in potential which placed the material in a corroding region.