Metal finishing has two basic aims, first to treat metal to obtain a good quality highly decorative finish and, secondly to protect the
finished article from deterioration. Some applications may fall into either category or involve a combination of both. It is for this reason that some attention should be paid to the performance
and methods of testing for corrosion resistance.The only wholly reliable tests for establishing service performance are actual exposure to the natural environment encountered in service. It must
also be noted that the time results of exposure to natural environments may vary with climatic changes from season to season or from any one season or year to a subsequent season or year.
Finally, although
accelerated corrosion tests are an essential reinforcement of quality control procedures and give results reproducible within limits of their application as specified, their use to predict actual
environmental performance should generally be avoided since the degree of correlation is not known and cannot be determined.
Accelerated corrosion tests are unlikely to reproduce exactly the performance of
finishes in the environments normally encountered in the actual service of the substrate material. This is due to the large number of variable factors that directly influence the corrosion
reaction. At best it may be possible to correlate the results of the accelerated corrosion test.
At the completion of any type of corrosion test - natural or accelerated - it is necessary to
determine the nature and extent of the corrosion which has occurred. Since coated finishes for metals may be either protective or decorative or a combination of both, the way in which the corrosion
damage is assisted will vary considerably.
In the simplest instance where no corrosion of the substrate of any kind can be accepted the visual observation of the first signs of corrosion
may signify failure. Where the functional properties of the substrate are concerned ie. mechanical properties, appropriate mechanical or physical tests may be applied to determine the setting of an
acceptance (pass or fail) level.
