What is the difference between salt spray and cyclic corrosion testing?

Salt spray and cycle testing are standardized test methods used to test the corrosion resistance of materials and surface coatings. The material to be tested is usually metallic and the surface coating is intended to provide some level of corrosion protection. These tests allow comparison of the relative corrosion resistance of coatings over accelerated or compressed time scales.

Salt spray testing (SST) is usually performed in accordance with the ASTM B117 test procedure. ASTM B-117 outlines procedures for conducting salt spray tests, salt spray tests, and salt spray tests. The samples to be tested are placed in a salt spray chamber at 35⁰C and exposed to dense fog generated by nebulization of a 5% sodium chloride solution with a pH between 6.5 and 7.2. The duration of exposure will vary by product and manufacturer's requirements. Most tests are performed in 24-hour increments. Common test durations range from 72 to 1,000 hours. Salt spray testing is an effective way to check for corrosion, but it does not correlate 100% to actual corrosion. Salt spray testing does not mimic the natural environment in which corrosion may occur. Regardless, many companies use the salt spray test to measure the corrosion resistance of non-precoated metal coatings without a galvanized precoat.

ASTM B117 Spray Test Accelerates False Corrosion Mechanisms on Galvanized Steel. Zinc is known for its ability to resist corrosion, but resistance depends on the formation of a zinc carbonate barrier on its surface. The constant spray during the salt spray test prevents the exposure to the atmosphere needed to form the zinc carbonate barrier. Without the zinc carbonate barrier, the salt spray interacts directly with the zinc metal and the parts corrode very quickly.

Cyclic Corrosion Testing (CCT) is a method of exposing a sample to a range of different environments in repeated cycles. There are no universally accepted international standard CCT criteria. CCT is mainly promoted within the automotive industry. As mentioned earlier, salt spray testing has nothing to do with real world corrosion, so the automotive industry needed a way to simulate the types of corrosion failures that occur naturally. Automotive companies have developed their own CCT methods to accelerate real-world corrosion failures under laboratory-controlled conditions by replicating naturally occurring environments. The tested sample will experience the same changing environment as in nature. These test methods allow manufacturers and suppliers to more accurately predict the expected useful life of their products. CCT methods vary by specific vehicle manufacturer and are industry specific.

Generally, the CCT method usually contains a salt spray phase, a drying phase, a condensing humidity phase and a controlled temperature humidity phase. The salt spray stage is similar to the salt spray test, only not as long. The drying phase uses ambient air or heated air to dry the samples, depending on the method. This stage can be done with or without relative humidity control. Samples should be visibly dry at the end of the phase. The condensing humidity or wetting phase is carried out at a high temperature and high humidity of 95% to 100%. This stage promotes condensation on the sample surface. The controlled humidity and temperature phase exposes the sample to a controlled temperature and controlled humidity climate. Climate can be constant or cycle between different levels. This list is not conclusive, as some car companies may require other climates to be included in the sequence.