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Corrosion tests

 

I am a student in electromechanical engineering and I am doing my thesis about corrosion and especially about corrosion-tests. I have to make a matrix of different ferro-metals with common coatings involving how long these materials, combinations of both are resistant to corrosion. The main goal is to link corrosion-tests with reality.

For example: five years in a marine environment is the same as ? hours in a test chamber. My question is: is this done before? What is the best way to start with this? Which tests are most used to do this kind of research? Prof. Bogaerts of the University of Louvain told me that test results of ferro-metals with certain coatings, and their life expectation in a corrosive environment, exist in literature. He just doesn't know where to find them.

Could you help me out?

Matias Kerkhofs
- Louvain, Belgium

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I am afraid your endeavors are doomed for failure! There have been many attempts to directly correlate corrosion testing to real life exposure, but no such correlation exists between a laboratory test method and real life. This is especially true for the most common corrosion test method, salt spray via ASTM ASTM B117.

The main problem is that a reaction that takes one month is occurring via a different mechanism than one that takes 15 years. In general, the more you accelerate corrosion, the lower relevance it has on real life.

Industrial practice is to specify a number of hours of salt spray the part should last to white corrosion and/or red rust. Sometimes it is to the first sign, and sometimes until a certain percentage of the significant surface is corroded. More hours for parts that will see harsher environments. Corrosion testing is used for quality-control to pick out bad parts, and to compare different finishes. Predicting the life of a finish is generally made from real life observations and experience.

I work in the automotive industry, and GM, Chrysler, and Ford have developed durability tests that involve 6 months through a harsh daily schedule of exposure at a large proving ground facility. They claim these tests do correlate to real life exposure. However, these are not laboratory methods. I'm also a bit skeptical about how well these tests really correlate to real life.

If you want to know how long a certain finish should last, call a plater or plating chemical supplier and ask. There are a huge number of finishes and many variances on how they are applied. A full list does not exist, and you will need to compile it for the finishes you are interested in.

Tim Neveau
Rochester Hills, Michigan