Materials Evaluation Using Wet-Dry Mixed Salt-Spray Tests

Accelerated methods for corrosion testing of materials, based on the use of salt sprays, have been standardized since the early 1930s. However, despite the many significant advances in the mechanistic understanding of atmospheric corrosion phenomena since those days, most changes in accelerated test standards have been implemented to reduce the time of the tests, and thus their costs, by increasing their severity. Thus, although laboratory accelerated test methods have played an important role in the assessment of materials' performance, in general they have not been applied by virtue of their ability to simulate and enhance natural weathering. In particular, wet-dry cycles simulate the natural wetting and drying which occurs in practice and, under many conditions, can aid in the formation of naturally occurring corrosion product films, which frequently play a role in the kinetics of metal corrosion. In addition, the use of a relatively dilute mixed salt spray, in comparison with the more commonly used solutions based on 5% NaCl (e.g., ASTM B 117, Method for Salt-Spray Fog Testing) can result in corrosion morphologies and behavior which are more representative of natural conditions. This paper describes a wet-dry mixed salt-spray test using solutions containing mixtures of (NH₄)₂SO₄ and NaCl with a 1-h wet/1-h dry cycle. The basis and reasons for using the test are discussed in depth with the emphasis on illustrating the improvements in realism which are attained using this procedure rather than a standard continuous NaCl test. In addition, the results from accelerated corrosion testing of aluminum are presented to illustrate the main features of the test. Finally, possible developments and improvements in the salt-spray methods, including the use of solutions resembling artificial rainwater, are discussed.