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Purity of salt for salt spray testing

 

I have a question regarding the purity of salt for salt spray testing.

ASTM B117 requires salt with "no more than 0.3% total impurities", it does not indicate any minimum purity.

An auditor insists that this means the salt shall be 99.7% minimum pure.

The salt we use is U.S.P. / N.F. grade, which is pharmaceutical quality, the label indicates a purity of 99.0-101.0%, with total impurities listed as much less than 0.3% but some values are indicted by "passes test" rather than a numerical value.

The auditor says this is not acceptable since it does not indicate 99.7% minimum pure and that all our tests are invalid.

I have asked several chemists and the salt manufacturer, what exactly do these numbers mean? I have yet to get a clear answer. Any enlightenment would be appreciated.

Steve Peters
All Metals Processing - Stanton, California, USA

Steve - ASTM is in the process of revising ASTM B117 to clarify the description of the salt to be used. Since it presently states that the salt shall have "not more than .3% of TOTAL (my emphasis) impurities", a salt which is 99.7 to 100% meets this requirement. It actually sounds as if the salt you are using is a higher grade than most users.

Cynthia L. Meade
- Sylvania, Ohio, USA

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Steve Peters,

The simplest answer would be to get your Salt manufacturer to issue a report that says your Salt is 99.7% pure or to purchase another Salt that can produce a 99.7% pure lot analysis to satisfy your auditor.

Pharmaceutical quality Salt sounds like it should be good enough for ASTM B117, but you want to wary of any anti-caking agents or preservatives as they can act as corrosion inhibitors. Also copper contamination can act as a corrosion accelerator.

That said, I really think the Salt Manufacturer should be able provide you with a more detailed lot analysis that will satisfy your auditor.

Joshua B. Taylor, C.E.F.
- Baltimore, Maryland, USA

Keep in mind that 99.7% purity is only one of the requirements of the ASTM B117. The other main requirement is the requirement of < 0.1% sodium iodide. Unless you have a cert that specifically addresses this constituent and its actual % you're liable to have a finding in an audit.

Tim Martin
plating shop - Springfield, Massachusetts

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When I ran salt spray, I used to buy my salt from a local feed store. They have 20 lb bags of high-purity salt that work just fine. Perhaps there is a similar store near your lab.

Tim Neveau
Rochester Hills, Michigan