Letter 3010

Corrosion testing 

-

Is salt spray testing an effective, industry standard, means for process control for a powder coating system? How often should salt spray tests be performed to verify the process; continuous, periodic?

Tom Clark
- Cleveland, OH

Salt spray testing is certainly effective and an industry standard, but I don't think I'd call it a process control.

For a parallel, think of health screenings where doctors do a risk assessment for certain diseases. If your parents had heart disease, and you're overweight and a smoker and a type-A personality, etc., etc., you're at high risk. If none of the risk factors apply, you're at low risk. But there is no guarantee either way that you'll get heart disease or you won't.

Salt spray hours don't correlate quantitatively with actual field performance; but if you get poor salt spray resistance it is highly unlikely that the finish will do well in the field, and if you get good salt spray performance it is unlikely that there is anything dramatically wrong with the finish.

Besides being only a statistical indicator, the other weakness of salt spray testing as a process control is the lack of useful real-time feedback: it can take 240 hours to run the test, or 500 or 1000. You can powder coat an awful lot of rejects in 1000 hours!

Periodic testing would seem to be adequate. Auto Technology, one of the major suppliers of salt spray chambers, is in your town and would probably be happy to talk to you about this.

Ted Mooney, P.E.  finishing.com Brick, NJ

I have to agree with Ted. Our experience with salt spray is that it is not very predictive as a measurement tool for most companies because the salt atmosphere corrosion mechanism is not what most things are exposed to day in- day out. For example, while salt spray may be valid for a mfr. of outdoor beach furniture, it is very much not valid for mfrs. of office furniture. Office furniture, for the most part spends its life in an air conditioned and humidity controlled environment.

The other thing to realize is that a coating "system" includes the pretatment and coating(s) used. There are plenty of excellent pretreatments that perform miserably in salt spray thanks to a weak coating or weak coating application. Likewise, there are terrible pretreatments that do great in salt spray thanks to a dynamite coating.

We have found that since the salt atmosphere corrosion mechanism is an aggressive one that salt spray is an effective "comparative" coating system evaluator. This means that it is a good way to compare changes in an existing coating system by varying the parameters, changing pretratments, sealers, process sequence, coatings, coating thickness, etc. This works well when compared to the current system. It is important to remember,as well, that other things such as adhesion and impact resistance should be tested for each variation.

Where to begin is with a good strong program toward analysis of field corrosion and paint failures. This will give you actual data (aka real world data) that will tell you how long your parts are lasting out in their real environments. Only then can you truly relate to your customer's concerns. You cannot do that through salt spray, because this is only one point on a curve(more later). What comes to mind is a good customer relations program that rewards them for bringing failures to your attention, with replacement parts or whatever. Do this under the auspices of a true continuous improvement program aimed at letting your customers help you make the product that the buy better. This will build customer loyalty. It is cheaper to keep the ones you've got than to find new ones.

But I digress... After you have this real world data, you may be able to say with salt spray that you have discovered a system which is much improved over the old system and implement it, then follow field failures again to assess the actual improvement. You will do this multiple times over the lifetime of a product, change paints, change coating systems, change cleaning systems, pretreatment suppliers, etc. These are the additional points on the curve that you are looking for the curve that relates salt spray to real world performance.

Only companies that have studied this for many years and have a very good idea of what environment their parts are stored in have a correlation between the two. If your parts are used in multiple environments, (i.e. inside, outside, seaside, etc.) this becomes multiple curves, one for each environment.

Ted is right about the predictive aspect of salt spray, as well. Not many coating systems operators like to wait for over a month to hear if the parts that they painted last month are coming back. Look at some other predictive methods like coating thickness, pencil hardness, impact resistance, tape adhesion, etc... for quick checks to see if something is grossly wrong. Use salt spray to help you improve the coating system and for periodic quality evaluations.

One last thing about specifications... Do create a spec. One that is comprehensive and takes into account what your cutomers expect from the product, not how your engineers think the product should perform. Don't make the salt spray goal be some unachievable number, just because it is an industry standard test. Do the real world correlation work as time allows. Look to continuously improve your product through whatever means are at your disposal including salt spray. Consider alternate cyclical failure analysis methods, UV exposure, outdoor weathering, cyclical humidity-salt-UV module testing, etc if they more closely match the environment that your customers use your parts in. As always, Good Luck.

Craig Burkart
- Naperville, Illinois

I want to post a question or inquiry of my own.

 

I want to answer or follow-up on this question publicly

 

I am a supporting advertiser and want to reach the inquirer privately.