A professionally painted metal surface should never be corroded; the function of the paint is actually to protect the metal support from corrosion other than being an aesthetic factor. Nevertheless, painting cycles are sometimes performed on supports that have not been treated properly.
In this case, the symbiosis between paint and support becomes extremely vulnerable and a small grain of dust on a scarcely painted edge or any other cause is enough to start the corrosion process. For example, if a car is hit by a stone, its paint cracks: corrosion propagates under the paint layer with short-term destroying effects, on the sheet metal first then on the paint itself. The real enemy that traditional salt spray chambers fail to detect is this kind of corrosion that propagates under the paint film and whose effects may be either a scab corrosion or a filiform corrosion.
The ISO 7253 standard of 1984 at international level (ISO means International Standard Organisation) sets forth both the application modes of salt spray tests and their limits. This standard actually states that salt spray tests are not valid as corrosion tests (not involving a real exposure to environmental agents) but they are only a sort of behavioural test to check the quality of different lots of painted products as well as the quality of the painting processes.
From what has been said so far, it clearly emerges the need of laboratory tests aimed at detecting and highlighting the two forms of under-film corrosion (filiform and scab) in a fast, easily reproducible manner. This test already exists: it is the so-called Dry Corrosion Test.
The procedure is the same as that followed for salt spray tests, but the two salts used are different: sodium chloride and ammonium sulphate. Moreover, in dry corrosion tests, any wetting cycle is followed by a drying cycle with de-humidified hot air. A complete test cycle may last from 500 to 1000 hours depending on its severity.
For more information on Dry Corrosion Test Cabinets© contact Trevor Leake.
