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topic 8904

Salt Spray Hours vs. real life exposure

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A discussion started in 2001 & continuing through 2017

(2001)

Q. What do salt spray hours represent in days? In an hour a week, month, year? Please advise.

Craig Schmucker
distribution - Carrollton, Texas


(2001)

A. This is an often-asked question. The answer is that salt spray does not correlate to anything. Other corrosion tests do not correlate to real life or each other. Many have tried to study this and make a correlation, but nobody has done it yet. The problem is that the actual chemical reactions that make rust take years to occur. If you rust something in a few weeks, then you have obviously used a different chemical reaction!

Salt spray is only a predictor of real world results. This test is also good at detecting finish defects. The best one can do is to base salt spray requirements on prior history. If you want your part to last longer in the real world, look for finishes that will last longer in salt spray.

tim neveau
Tim Neveau
Rochester Hills, Michigan


(2001)

A. I really enjoyed the discussion of salt spray testing and what it means in the Aug. 2001 edition of Metal Finishing. That is the one question that always comes up when one teaches a corrosion class. I answer it with the following question : Which one of the eight types of corrosion are you trying to duplicate? Generally the person asking the question does not know the type of corrosion that is being duplicated. I mention that salt spray is a continuously wet test and in the past has been used mainly for process control checks as one of your respondents mentioned. The continuously wet test is probably some indication of corrosion rate in 5% salt solution or some form of general corrosion.

Cyclic testing, however, was initiated to duplicate the crevice corrosion observed on cars at scratches in the coating. Crevice corrosion requires a dry cycle to accelerate the effect of alkali formation at the cathode to produce the underfilm attack at the scratch. Aluminum has an Exco test to duplicate the exfoliation corrosion observed on high strength aluminum alloys. Anodic polarization testing originated as a test method for evaluating storage tanks for sulfuric acid and susceptibility to pitting corrosion. I don't want to bore you with more test methods, but as you can see there are reasons for tests other than salt spray. My references indicate that approximately 100 hours salt spray is similar to one year outdoors based on OEM testing in the US.

I've seen similar data for Europe. That's the closest I can come to based on published data I have seen. A 100 hours in the salt spray cabinet was equivalent to a month exposure in the Panama Canal Zone when I evaluated coatings for belt links in the late 1960's. For that reason, I call the salt spray a "rubber yardstick" in that you need to know what environment you intend to compare it with. Obviously, 100 hours in the salt spray could also be 15 years in the desert.

Some day someone will write a book on salt spray testing and start a discussion on effect of thickness on plated coatings, effect of pretreatment on paint coatings, effect of surface roughness on oil coatings, etc. That's what is really important to know. . . . . how all these variables effect not only the salt spray results, but the actual results achieved in service.

Joe Menke


January 5, 2010 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. How does 100 hrs of humidity testing (not Salt spray) in a controlled cabinet equate to real shelf life in hours, days, weeks, months?

Ginger Riley
Quality Engineer Supervisor - Moberly, Missouri


January 6, 2010

Sorry, Ginger, it doesn't. Salt Spray testing and Humidity testing are QA measures used to insure that the process has not deteriorated; they are not a predictor of real life in service or on the shelf.

If processed parts survived 100 hours in the past and now they don't, something has gone south that must be investigated. But a galvanized part that survives 100 hours may outlast by decades a painted part that survives 100 hours. The corrosion mechanism in salt spray cabinets is fundamentally different than the corrosion mechanisms of real life, and the corrosion prevention mechanisms of different coatings is fundamentally different one from the other as well: some are barrier layer and non conductive, some offer barrier layer protection but are cathodic to the substrate, some are anodic to the substrate so they offer cathodic protection, etc. Sorry.

I believe that humidity testing is primarily used on materials that are stipulated to not be resistant to salt, like the lower grades of stainless steel.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Salt spray test hours vs. humidity test hours

September 8, 2016

Q. I understand that there is no direct correlation between salt spray hours and hours in a real-life environment, but is there a known correlation between salt spray hours and humidity chamber hours? For instance, if I had a part spec'd to 240 hours no white rust in humidity, is there an equivalent salt spray duration I could use as an alternative test?

Dave Pawletki
- ITASCA Illinois


September 2016

A. As others have mentioned, there are different types of corrosion and they proceed differently on different substrates; there are different corrosion resistance mechanisms, ranging from barrier layer, to galvanically protective, to galvanically neutral, to galvanically destructive; sometimes the corrosion products are powdery and hygroscopic (iron rust) and accelerate the corrosion; sometimes the corrosion products are tight and glassy (galvanizing exposed to carbon dioxide over the years) and provide a relatively inert surface slowing corrosion to a crawl, etc. To expect accelerated testing corrosion rates to be proportional to anything against such a mix is a stretch, but, if you limit it to one specific finish on one specific substrate so you are substantially reducing the number of variables, maybe you can establish a correlation within some reasonable percentage of error ... maybe.

For example, you might be able to say if finish A of thickness X was found to last 48 hours in salt spray and 240 hours in humidity, and finish A in thickness 2X lasts 96 hours in salt spray, yes, I think it probably will last something like 480 hours in humidity testing. But this probably tells you nothing about the humidity chamber life or real life of finish B which lasted 48 hours in salt spray :-(

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


simultaneous September 12, 2016

Q. Thanks for your response, Ted. The variation in reactivity across coatings and substrates is a great point -- this is a subject I will need to research further.

To the point of humidity vs. salt spray, I agree it would take a pretty substantial amount of testing to try and draw any kind of correlation, even if coating and thickness were held constant. I would be curious to know if there is any kind of published study out there for zinc coatings on low carbon steel, but even if there were, wouldn't geometry of the test sample play a role in corrosion performance as well? I imagine it would be difficult to draw any kind of conclusion about the results when trying to apply to other configurations.

David Pawletki [returning]
- Itasca, Illinois


September 13, 2016

Dave,
If the spec requires something, you do what the spec requires. Don't go attempting to substitute something without the express permission of the purchaser or the "cognizant engineering organization" as some say.

The 240 hours in humidity was likely an arbitrarily chosen test and a somewhat arbitrarily chosen number of hours, but it's THEIR arbitrarily chosen test.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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