Salt spray test

The salt spray (or salt fog) test is a standardized and popular corrosion test method, used to check corrosion resistance of materials and surface coatings. Usually, the materials to be tested are metallic (although stone, ceramics, and polymers may also be tested) and finished with a surface coating which is intended to provide a degree of corrosion protection to the underlying metal. Salt spray testing is an accelerated corrosion test that produces a corrosive attack to coated samples in order to evaluate (mostly comparatively) the suitability of the coating for use as a protective finish. The appearance of corrosion products (rust) is evaluated after a pre-determined period of time. Test duration depends on the corrosion resistance of the coating; generally, the more corrosion resistant the coating is, the longer the period of testing before the appearance of corrosion/ rust. The salt spray test is one of the most widespread and long established corrosion tests. ASTM B117 was the first internationally recognized salt spray standard, originally published in 1939. Other important relevant standards are ISO9227, JIS Z 2371 and ASTM G85.

Salt spray testing is popular because it is relatively inexpensive, quick, well standardized, and reasonably repeatable. Although there may be a weak correlation between the duration in salt spray test and the expected life of a coating in certain coatings such as hot dip galvanized steel, this test has gained worldwide popularity due to low cost and quick results. Most Salt Spray Chambers today are being used NOT to predict the corrosion resistance of a coating, but to maintain coating processes such as pre-treatment and painting, electroplating, galvanizing, and the like, on a comparative basis. For example, pre-treated + painted components must pass 96 hours Neutral Salt Spray, to be accepted for production. Failure to meet this requirement implies instability in the chemical process of the pre-treatment, or the paint quality, which must be addressed immediately, so that the upcoming batches are of the desired quality. The longer the accelerated corrosion test, the longer the process remains out of control, and larger is the loss in the form of non-conforming batches. The principle application of the salt spray test is therefore enabling quick comparisons to be made between actual and expected corrosion resistance. Most commonly, the time taken for oxides to appear on the samples under test is compared to expectations, to determine whether the test is passed or failed. For this reason the salt spray test is most often deployed in a quality audit role, where, for example, it can be used to check the effectiveness of a production process, such as the surface coating of a metallic part. The salt spray test has little application in predicting how materials or surface coatings will resist corrosion in the real-world, because it does not create, replicate or accelerate real-world corrosive conditions. Cyclic corrosion testing is better suited to this.

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