Engineering Properties

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Corrosion Resistance Tests

Attempts to simulate long-term environmental resistance of most products are fraught with difficulties, no matter what materials they are made from.

However a simple test, devised by the DuPont Company in the 1980s to evaluate protective coatings used on threaded fasteners at chemical plants, has recently been discovered to give by far the best correlation with real life exposure situations for zinc alloy surfaces, both unfinished or coated. In this test parts are exposed to constant 100% relative humidity at 50oC, with weekly spritzing with a fine mist of an aqueous solution of 1% sodium chloride plus 1% sodium sulfate. What most differentiates the DuPont method from the various ASTM methods mentioned below is that it has been observed to correlate well with actual long-term outdoor exposure at coastal, industrial sites. As an approximate rule of thumb, one month in the DuPont test appears to equate, in extent and type of corrosion, to about one year at a chlorofluorocarbon plant site along the Texas Gulf Coast, or to about two years at an industrial site more inland, such as in Wilmington, Delaware or Louisville, Kentucky.

Salt spray, the most commonly used accelerated corrosion test, is now generally accepted not to be capable of correlation with real life situations. It also tends to yield results that are severely over pessimistic for zinc and zinc alloys when comparing them to other metals.However the warm salt spray test as specified in ASTM B117 is still the most commonly used accelerated corrosion test.

Cyclic corrosion tests similar to the ASTM G85, Annex A5, test which incorporate an element of salt spray, humidity and drying, are claimed to give better correlation to real life than the salt spray test, but they are still nowhere near as good as the DuPont test.

The salt spray and cyclic tests have the advantage that they are much shorter term than the DuPont test and they are hence more useful for quality control tests. However this is perhaps their best use as they are generally incapable of performing as reliable predictors of the relative performance of different coatings.

Products which will not see external service but which may be subject to very high humidity, like bathroom and kitchen fittings for instance, are perhaps best tested by a cyclic humidity test such as ASTM D1735.

When castings are to be exposed to unnatural environmental conditions it may be necessary to devise special tests to determine the corrosion resistance if one or more constituents of the environment is either known to be aggressive to zinc alloy or its effect has not been previously recorded and is not readily predictable.

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