ABSTRACT

Accelerated corrosion tests are widely used in the industry and Department of Defense to determine the environmental performance of materials and coatings. While some cyclic tests show good correlations between performance in the laboratory and operational environments, they are only valid for very specific sets of performance metrics and generalizations often cannot be made. Significant work has been performed to investigate in detail the relationship between environmental variables and corrosion failure modes of various material systems. Relative humidity was found to be the single most relevant factor in governing atmospheric corrosion. This paper focuses on the quantitative analysis of the corrosion morphology after accelerated testing and outdoor exposures of lapjoint test panels coupled with various fastener materials. Image analysis of 3-D microscope images was used to quantify average depth of attack and percent surface area damage. A curve fitting routine was developed to quantify corrosion damage within a fastener hole. The effect of relative humidity on the mode and degree of corrosion was evaluated. The effect of different primer systems was also compared.

INTRODUCTION

Accelerated corrosion tests are widely used in the industry and Department of Defense to determine the environmental performance of materials and coatings. The most widely recognized accelerated corrosion test is the ASTM B117 salt fog test1. This is a continuous test exposing the materials to a continuous salt fog. It has been shown, however, that this method has a very poor correlation to outdoor exposures2,3. As a result, more realistic cyclic environmental exposures have been developed4-6. While the cyclic tests were initially developed for coated steel, they have been used to evaluate other alloys, such as aluminum. While the correlations between the laboratory tests and operational environments improved, they are only valid for very specific sets of performance metrics and generalizations often cannot be made. The existing cyclic accelerated tests show especially poor correlation with the performance ranking of different chromate-free coating systems.

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