Salt Spray Test


As I may have mentioned in an earlier blog, the topic of my thesis was the investigation of weak acidic chloride solutions for electrogalvanising a steel substrate, and examining the respective corrosion behaviours of zinc layers produced using different coating parameters. I also used different types of chromate treatments in an attempt to improve their corrosion resistance.

So far so good, but how to evaluate how well the zinc coating was performing ? Obviously there was the possibility of a visual examination, and cross sections could be made to see how the coating thickness varied across the sample. But most important of all was the sample’s corrosion behaviour, and this was evaluated using the laboratory’s salt spray test chamber.

The idea was that different samples with varied coatings and chromate treatments would be hung up in the test chamber until first white rust (i.e. corrosion of the zinc) or red rust (corrosion of the underlying steel substrate) occurred, or until a predetermined maximum time had elapsed. For some of the better chromate-treated samples the test duration could last as long as 320 hours (about 2 weeks).

Obviously, the test had to be interrupted at regular intervals in order to examine the samples, and here I must admit that I had my doubts whether I performed the test properly : you were supposed to wash the samples clean prior to re-inserting them in the chamber. I’m fairly certain I didn’t do this. I also don’t remember whether I discussed the test with my supervisor in order to make sure that I followed a standard way of operating the equipment.

I do remember having a discussion with my predecessor, who had performed similar tests using alkaline solutions, plus I had the benefit of being able to look up stuff in his thesis.

Still, the literature states that the salt spray test should not be used to predict actual corrosion behaviour in a natural environment, but instead compare the efficacy of different samples under controlled conditions. I suppose that’s what I did, and since I used the same methodology throughout, I suppose my data must have had some meaning, if only through its internal consistency, but comparison with other data sets might be more problematical.

To be honest, I never had any feedback from the company who supplied the chemicals for the weakly acidic solution or the chromate treatments, so not really sure how much practical use the outcome of my thesis had.

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