How to compare ASTM B117 Salt Spray and ASTM B368 CASS Test?

A. Hi Vishal. The buyer decides the testing requirements. It isn't completely clear to me where you fit into the supply chain, but it sounds like you are the buyer, which makes you the decider.

Your first task is to decide whether the service condition is mild, moderate, severe, or very severe before you can pick salt spray or CASS hours to simulate such exposure needs. The second issue is that salt spray and CASS documents tell you how to run the test, but not what constitutes passing vs. failing, so you need to decide that or quote another spec that does so.

I doubt that the PVD coating significantly improves the corrosion resistance of the nickel plating, but I suppose you could consider it similar to a chrome plating layer, and use nickel-chrome plating standards which are, according to "Quality Metal Finishing Guide" published by the Metal Finishers Supply Association (NASF.org):
mild ⇨ 0 CASS hours
moderate ⇨ 4 CASS hours
severe ⇨ 16 CASS hours
very severe ⇨ 22 CASS hours

As for neutral salt spray hours, buyers typically require 48 to 96 hours for high quality components

Electrophoretic lacquer is paint, albeit a quite thin layer. Why are you electrophoretic lacquering? Is it because the nickel is only thick enough to offer nice appearance but no significant corrosion resistance? You really need to decide exactly what you are trying to do, and toward that end, retaining a finishing consultant or establishing a genuine partnership with a competent finishing shop may be advisable.

Luck & Regards,


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

A. Some automotive fuel components have this type of plating and CASS testing. We use both an appearance criteria (5% red rust at 24 hours) and a functional requirement (a burst test at 72 hours).

The interpretation of CASS and any other corrosion test data is always subjective and open for debate. This sort of thing is usually agreed upon between a customer and supplier based on technical experience and judgement. I have not seen an industrial requirement that uses a microscopic analysis of pores or corrosion sites. It could be done, but would you want a manufacturing process that requires constant microscopic analysis of test parts? This type of analysis may be better suited to research or an initial validation.

I suggest you strike up this conversation with a nickel-chrome plating chemical supplier.

A. 1. The only way to accurately and reliably develop a correlation is to build one yourself. That is, put some parts in neutral salt spray and some in CASS and see what you get. Keep in mind that both tests have significant variability, so it'd actually take numerous tests, control panels or parts in both tests, multiple cabinets, etc.

People often ask these types of questions, especially for SWAAT and NSS. There is no simple formula or factor to convert one test to another. They all induce different corrosion mechanisms on different substrates.

BTW, my investigation found that CASS was developed for Ni-Cr plated parts in Detroit. I talked to a guy who worked on the test, and he told me that they eventually determined that 8-12 hours in CASS meant 1 year in Detroit's environment (in the 50s or 60s). If you're not doing Ni-Cr plated parts, technically you're not even using the right test. Then again, I've seen lots of people use CASS (or NSS or SWAAT for that matter) with no idea as to how the test was developed or what its original purpose was. I'm sure many can relate.

A. Hi Liu,

1. To amplify Christian's excellent presentation, recognize that if two different tests for the same quality (corrosion resistance in this case) actually tracked each other the way we'd like, such that 200 hours of ASTM B117 (neutral salt spray) was equal to xx hours of ASTM B368 (CASS), there would be no point to having different tests and we wouldn't :-)
Rather, all accelerated corrosion tests are inaccurate indicators of real life, but the hope is that some may be less misleading than others for certain substrates, coatings, situations, and conditions. For example, CASS seems to have been developed in the belief that it is a better indicator of nickel-chrome life than NSS (neutral salt spray)

2. Organic coatings are coatings that are based on organic chemicals (basically chemicals that contain carbon). Most lacquers, paints, and powder coatings would be organic coatings. Inorganic coatings are coatings that don't contain (significant amounts of) organic chemicals. Metallic coatings like galvanizing and nickel & chrome plating and PVD are inorganic. There are probably some blurry areas, as there almost always are in life. Good luck.

A. Hi Tom. If someone can suggest which of the two tests is better for your situation, or can propose another test, that would be great. But we appended your inquiry to an earlier thread which touches on the difficulties of correlation. Unfortunately, I think the answer is that you will not know the real world life of your cadmium substitute, nor how it compares to cadmium in the real world, except by real world testing. And only after that can you build a correlation to accelerated testing. While developing correlation can be useful, trying to develop a predictive correlation that doesn't require real world testing probably won't work. Many have tried.

Regards,

A. First a side note about CASS. A few years ago, I tracked down a gentlemen who was involved in the original development of CASS. According to him, CASS was used for chrome plated bumpers. 8-12 hours in CASS correlated to 1 year performance in Detroit in the 50s/60s. (He didn't say how they defined failure in the test or the field.)

Those in the business know that CASS is used for a bunch of things, yet you'd be hard pressed to find anyone who could tell you *why* the test is used in a specific instance. (You could probably say that about a lot of these tests.)

That said, you generally cannot correlate accelerated tests to tests such as NSS for numerous reasons. The corrosion mechanism may not (and usually is not) the same, and CASS (and even NSS) is not representative of what goes on in the field. Salt cabinets (assuming they're run properly, which is generally not true) also have a great deal of variation that makes any correlation, in my opinion, greatly suspect.

That said, I have seen some published R&D that correlates electrochemical tests (DC/EIS) to NSS for painted parts. Note, however, that you must develop the test yourself. You cannot simply pull a "method" off the shelf and say that an expected corrosion rate of x mm/yr in an electrochemical test corresponds to y hours in NSS. You must take the time to develop this correlation yourself because it depends on the paint, substrate, chamber, and test equipment.

A. CASS is not comparable to NSS, at least not in any generic way. It will give you a different corrosion mechanism.

If you want to use CASS as a proxy for NSS, you will have to very carefully develop your own correlation which will depend on the substrate, paint, even the cabinet(s) themselves. You'll also have to run periodic tests to verify that your correlation is still correct. This assumes, of course, that you're running the test purely as a quality assurance test to make sure your process is running the same sample to sample. If you're trying to mimic some field failure, you're talking completely different corrosion mechanisms....

A. Hi Kerem. The supplier of your salt spray cabinet can advise you on this, but if not, it would be best to get access to ASTM B368 because it specifies the equipment and procedure required. It's only 4-5 pages, and then you won't be guessing whether your procedure complies :-)

Regards,

? Hello Pankaj, which exact standards does your CASS test comply with?

A. ASTM B368

A. Pankaj,
I doubt the standard gets that specific, it is probably more a matter to discuss with your customer. If they only require that the non-knurled areas pass CASS, then you should be fine.

Are these parts stainless steel, or a non-ferrous alloy perhaps? If the knurling process is causing iron contamination, passivating the parts should help.

A. Hi Praveen. You appear to be acknowledging that there is no correlation, but then asking whether the correlation would be 9-15:1 if there was a correlation? :-)

I think it's true that CASS is more aggressive than NSS and will always be faster. Other than that, any correlation there may be would only be for a given plating on a given substrate ... because how much more aggressive the CASS solution is would depend on what you are trying to corrode with it.

The origin of your 9-15:1 ratio may be that copper-nickel-chrome coatings are sometimes spec'd to pass a few hours in CASS, but several times more hours in acidic salt spray or neutral salt spray. Good luck, but please accept the general thrust of this whole thread that any correlations between CASS, NSS, and real life are at best no better than the backup data for your parts that you have accumulated to demonstrate them :-)

Luck and Regards,

A. Hi Derrick. Back in 2002 Christian Restifo expressed his belief that "CASS was developed for Ni-Cr plated parts in Detroit . . . they eventually determined that 8-12 hours in CASS meant 1 year in Detroit's environment (in the 50s or 60s)."

But Christian also remarks that there is significant variability throughout, such that you need multiple tests, multiple panels in each test, cabinets from different manufacturers, etc.

And I need to repeat, so that your Q&A can stand on its own without being misquoted, that this correlation applies at best only to nickel-chrome decorative plated parts. Other materials and coatings corrode in fundamentally different ways such that CASS test results have essentially no bearing on their anticipated life. Good luck.

Regards,

Hi again Derrick. Way way back when I was chief engineer of the Equipment Division of Harshaw/Atotech and we built a large plating-on-plastic line for the company you work for. At the time, it was probably the biggest plating-on-plastics line in the country. It was a blast from the past hearing from you :-)

Regards,

A. Hi Ahmad. You can learn the details of CASS testing in ASTM B368 or ISO 9227. Still, such tests are designed for QA: that is, if the parts previously survives X number of CASS hours, and they no longer do, something has changed that will probably reduce the life of the component in the real world. But establishing an initial correlation between this QA test and real world life can be a challenge; and to assume that X hours of CASS equals Y months of real world exposure is highly questionable, and depends on the type of component, the exposure conditions, the substrate, etc. For example, does accelerating the corrosion with copper really have any relationship to the actual exposure? Good luck.

Regards,

A. Hi again. CASS is more severe than NSS, but as has been said before on this page, I don't think any further correlation is possible.

Do you passivate your components, or do you electropolish them. What alloy(s) are they? Thanks.

Something to remember about stainless steel when doing accelerated corrosion testing is that, because it is not a coating, porosity of the coating and life of the coating are rather meaningless -- such that once it passes 4 hours we shouldn't be shocked if it passes 400 hours. Similarly a stainless which fails in 2 hours may not be all that different from a stainless that lasts hundreds of hours. Accelerated testing of coatings is quantitative but testing of stainless is really more of a qualitative test.

Regards,