Salt Spray Testing in the Industry.

The gradual degradation of metals by a chemical reaction is called Corrosion. In other words, a corrosive reaction would occur when a metal is exposed to an oxidant (oxygen) creating an electrochemical oxidation. Rusting of iron is one of the most common examples of corrosion that the general population is aware of. Since corrosion is common and can happen to most metals used commercially, theories and ways to protect metals from corrosion were developed. Applied coatings (paints, etc.) and reactive coatings (zinc, etc.) are some of the typical coatings used for the protection of metals from corrosion. Once the methods of protection from corrosion were developed, there was a need to test these processes to scale them on the benchmarks of time, cost effectiveness and overall performance.

Salt spray testing was identified and accepted as the standardized corrosion test methodology. The salt spray test helps determine the resistance of the metal and coatings against corrosion. With this test, a corrosive medium is produced to evaluate the coating used as the defensive finish for the metal. The salt spray test is cost effective, standardised and repeatable which makes it extremely popular in industry. It works on the principle of creating a corrosive environment and then observing and recording the time taken for corrosion products to appear on the test samples. Additionally, the comparison of actual test results v/s expected corrosion resistance of metals can be made. Thus, the salt spray test is quite often used for quality audit purposes in production processes for firms which deal in metallic products.

The salt spray test is performed using a test chamber that is closed, where a pure solution of salt water is atomised using spray nozzles. The salt water solution is typically comprised of five percent sodium Chloride (NaCl) mixed in deionized water. These tests are commonly performed in accordance with the national and international standards of ATSM B 117 and ISO 9227. These standards identify important parameters with which the tests should be conducted. Parameters such as salt solution pH, specific gravity, temperature, air pressure etc. are specified and recorded during the test. This requires continuous (usually daily) checks of the test parameters to ensure compliance with the standards. Also, the performance of one coating might differ with another during salt spray exposure, thus requiring different durations while testing different metals and coating combinations.

Typical types of coatings that are evaluated include: paint coatings, anodized aluminium, zinc on steel and a variety of other plating/base metal combinations.

Due to its widespread usage, it is regarded as a “universal” test. One of the greatest advantages of this test is that it can be performed on multiple materials. Thus, salt spray testing is a well-defined test and often is an integral part of the metallurgical engineering process. With the changing times, new techniques and other new methods have begun to supplement the standard salt spray test, however, the ASTM B117 test is still considered to be the standard salt spray test. Thus, the salt spray test has made its own place in the science of metallurgy and applied engineering and will continue to be relied upon for many years to come.

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