Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Physical Characteristics and Testing of
Part III Corrosion Resistance Testing
Well, I guess I have to start with salt spray testing. The oldest and most used accelerated environmental simulated testing method around. ASTM B-117, 5% neutral salt fog tests. How good is at predicting failure in real life in the environment? Consider the following: the U. S. Bureau of Standards prepared and sent to numerous laboratories doing B-117 salt fog testing samples to test and report the results. The bottom line was that there was no correlation between the various (and numerous) salt fog cabinets. That means there were many different answers. A test made comparing salt fog to other corrosion test also showed that “there is usually not a direct relation between salt spray resistance and resistance to corrosion in other media.” So, why use it? Well because a large number of less informed people require it.
There are numerous tests now floating around that are reported to be better than salt spray. For example, Chrysler Corp specifies Corrodkote. A paste painted on the surface to be tested, then exposed to humidity. Test conditions are according to ASTM B368
CASS (copper-acetic acid modified salt spray) this is much more corrosive than the ASTM B-117. Now specified by many of the Automakers. It is ASTM B368. It consists of 5% sodium chloride+ copper chloride.2H2O acidified with acetic acid to a pH of 3.2.
Out door exposure sites are scattered about the country for the purpose of evaluating industrial atmosphere exposure, these tests require years of observation.
“Controlled humidity test: there are fifteen ASTM standards relating different variations of creating and controlling fog and humidity in cabinets for corrosion testing of a broad spectrum of products, from decorative electrodeposited coatings to the evaluation of the corrosivity of solder fluxes for copper tubing systems. The basic humidity test is most commonly used to evaluate the corrosivity of materials or the effects of residual contaminants. Cyclic humidity tests are conducted to simulate exposure to high humidity and heat typical of tropical environments.”
A new corrosion test was developed by the Corrosion Task force of the auto/Steel Partnership, a consortium whose members includes the three major U.S. automakers and nearly all of the major steel producers. It is called “Cosmetic Corrosion Lab Test, SAE J2334. It is a rather simple test to perform and takes only 24 hours per test cycle and has three stages. It can be run manually or automatically. The tests were correlated with a series of real life tests. One of the tests consisted of mounting panels on pick up trucks driven in Montreal, Quebec and St. John’s Newfoundland, for five years; two of the most corrosive environments in North America. Similar tests were run in Michigan, Ohio and Pennsylvania. The tests showed the best correlation to the real world.
The last tests I will mention is anodic stripping and electrochemical anodic polarization that detects unseen pits and corrosion cell on the surface of the specimen. There are special lab instruments for these and are usually run by chemists or electrochemists. I am sure there are more tests that I don’t remember or simply don’t know.