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Correlation of salt spray fog test results versus long term atmospheric exposures  

June 12, 2008

I use salt spray fog chamber (according to ASTM B-117 [link is to spec at TechStreet]) to determine rust resistance of flat products coated with chrome, zinc (galvanization), tin and rust preventive oil. How can I evaluate the test results when taking long term atmospheric exposures into consideration? Is there a rating scale; area failed versus resistivity time.

Ferhat Parlak
employee of iron and steel co. - Turkey

Hi, Ferhat. This question comes up at least once a month, so it has been answered here dozens of times in case you want to use the search engine and read some other responses. But the answer is that there is absolutely no correlation and there wasn't intended to be one.

Salt spray testing is strictly a QA measure to help make sure your finishing process has stayed consistent and hasn't gone south. The corrosion mechanisms are very different in these accelerated tests than in real life, the finishes respond utterly differently than in real life, and there is no predictive value of one finish against another. Galvanizing, for example, easily lasts 50 years in real life where the finish can properly interact with carbon dioxide in the air to form very stable intermediate corrosion products, yet it is quickly destroyed in a salt fog chamber. Rust preventative oils that may vaporize in the hot sun in real life, might theoretically last longer in the "accelerated test" than in real life :-)

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Ted is quite correct in stating that ASTM B-117 [link is to spec at TechStreet] is a method used by the coating trade as a QA tool. It can also be used to evaluate a new coating vs the current material. It can also answer very quickly the question; "does the new additive show some improvement?). However, I was informed by the labs of Cranfield University (UK military test labs) that ASTM G85 [link is to spec at TechStreet] is now being evaluated by several labs around the world as a method that almost mirrors natural weathering. I would be interested if anyone else is evaluating this as the photo images did look comparable.

Terry Hickling Martex Paints Ltd    Birmingham, UK

It looks like G85 [link is to spec at TechStreet] contains five different modified salt spray tests, Terry; I suppose we'll have to see how it works out. It strikes me that developing an accelerated test that can the mimic the real life behavior of one type of coating ought to be doable.

But a test that properly compares the longevity of one coating to another may be a pipe dream. Most coatings develop some of their corrosion resistance or their propensity towards corrosion from intermediate corrosion products of specific composition and structure; and it is difficult to imagine a single test that would allow every different metal and every different coating to mimic real life at accelerated pace. But progress is certainly a good thing even if perfection seems a ways off.

Ted Mooney finishing.com Brick, New Jersey

hello,
salt spray test is a comparison and r&d test rather than Qc test,so it is easy to compare your current product with the past product , or new proccess with older one. i used it in comparision different coating supllier (painting) and to reach the best proccess parameters in hot dip galvanizing.

Haytham Kilany
- Egypt

Hi, Haytham. I guess we'll just have to disagree on this, but I maintain that you cannot use salt spray tests to compare one finish to another. The corrosion resistance of many finishes (like galvanizing) is largely due to the formation of tight, adherent, corrosion products that tend to isolate the substrate from the atmosphere. These tight adherent corrosion products require slow reactions with ingredients in the environment in order to form.

As an example, rain water certainly doesn't cause rapid failure of galvanizing on real roofs in service, yet the same roofing sheet will quickly get "wet storage stain" if it is wet and can't breathe properly. That is because exposure to the atmosphere allows the proper corrosion products to form whereas wet storage doesn't. Neither does the salt spray environment. Galvanizing is a very durable finish that performs poorly in salt spray.

Regards,

Ted Mooney finishing.com Brick, New Jersey