Studies of a new accelerated evaluation method for coating corrosion resistance — thermal cycling testing

Abstract

A long time is often needed to differentiate the corrosion protection afforded by coatings systems, especially for high performance samples. Accelerating methods such as salt spray or Prohesion methods require as much as 2000 h exposure to qualitatively differentiate high performance samples. General room temperature immersion electrochemical measurements {electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM)} may require up to 10 weeks to differentiate high performance samples. For some high performance samples, EIS values changed very little over a 1 year immersion period. Thus, a faster method that can provide accurate quantitative results is needed for practical usage. Even when ENM and EIS are used with Prohesion™ for more quantitative results, the required time period of exposure can be extensive. In our previous studies, we found that thermal cycling methods can accelerate the corrosion of samples. We have been further examining the use of EIS in conjunction with thermal cycling exposure to give a possibly new, faster and quantitative method for coating corrosion resistance evaluation. The new thermal cycling EIS protocol has been developed in which the reversibility of the low frequency impedance modulus data is used as the index of the corrosion resistance of the samples. Two types of corrosion protective coatings systems, an aerospace electrodeposition-coat primer plus plasma polymer pretreatment and a SrCrO 4-pigmented epoxy-polyamide primer (PEPP) have been evaluated by the new method relative to the present standard coating system, chromate conversion pretreatment. For all samples, the corrosion resistance results can be differentiated during a week of testing period that included 3 days of thermal cycling in immersion followed by 3 days of room temperature immersion.