Salt Spray Vs. Cyclic Corrosion Tests

Subda

Q: What is the difference between salt spray and cyclic corrosion tests?

Salt spray and cyclic tests are standardized test methods used to test the corrosion resistance of materials and surface coatings. The materials to be tested are usually metallic with a surface coating that is intended to provide a degree of corrosion protection. The tests allow the comparison of the relative corrosion resistance of coatings in an accelerated or compressed time frame.

Salt spray testing (SST) is typically performed in accordance with the ASTM B117 testing procedure. ASTM B-117 outlines the procedures to conduct salt spray testing, salt mist testing and salt fog testing. Samples, to be tested, are placed in a salt fog chamber at 35⁰C and exposed to a dense fog created from atomizing a 5% sodium chloride solution with a pH between 6.5 and 7.2. The duration of the exposure will vary by product and manufacturer’s request. Most testing is conducted in 24-hour increments. The most common test durations are from 72 to 1,000 hours. A salt spray test is an efficient way to check corrosion but does not 100% correlate with actual real-world corrosion. Salt spray testing does not mimic the natural environments where corrosion can occur. Regardless, many companies use salt spray testing to measure the corrosion resistance of coatings on non-precoated metals with no galvanized precoating.

The spray test, ASTM B117, accelerates the wrong corrosion mechanism of galvanized steel. Zinc is well known for its ability to resist corrosion, but the resistance depends on the formation of a zinc carbonate barrier, zinc carbonate, on its surface. The constant spray during the salt spray test prevents exposure to the atmosphere which is needed to form the zinc carbonate barrier. Without the zinc carbonate barrier, the salt spray interacts directly with the zinc metal and the part corrodes very quickly.

Cyclic corrosion testing (CCT) is a method that exposes samples to a series of different environments in a repetitive cycle. There is not a generally accepted international CCT standard. CCT has been driven largely within the automotive industry. As stated before, salt spray testing does not correlate with real-world corrosion, so the automotive industry needed a method to mimic the type of corrosion failures that occur naturally. Automotive companies developed their own CCT methods to accelerate real-world corrosion failures, under laboratory-controlled conditions by duplicating natural occurring environments. Samples under test experience the same sort of changing environment that would be encountered in the natural world. These test methods allow manufacturers and suppliers to predict, more accurately, the service life expectancy of their products. CCT methods have evolved per specific vehicle manufacture and are industry-specific.

Generally, CCT methods usually contain a salt spray phase, drying phase, condensing humidity phase and a controlled temperature humidity phase. The salt spray phase is similar to salt spray testing, just not as long. The drying phase dries the samples using ambient or heated air, depending on the method. This phase can be done with or without controlling the relative humidity. The samples should be visibly dry at the end of the phase. The condensing humidity or wetting phase is conducted at an elevated temperature and high humidity, 95 to 100%. This phase promotes condensation on the surface of the samples. The controlled humidity and temperature phase exposes the samples to a controlled temperature and controlled humidity climate. The climate can either be constant or cycling between different levels. This list is not conclusive since some automotive companies may require other climates to be included in the sequence.

The completion of all the CCT phases is one cycle. The completion of a cycle takes 24 hours. The normal length of a CCT will range from 40 to 100 cycles, depending on product and manufacture.

Learn more: At ECOAT 2022, Joe Subda will be speaking along with Chad Andreae from TTX on the topic of ecoat oven fundamentals and the chemistry behind curing. The paper is titled “Electrocoat Oven: the Final Frontier.”