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"Salt-spray comparison of RoHS compliant Steel finishes"

A discussion started in 2005 but continuing through 2017


Q. My company is looking for a replacement finish. The thread forming screws we use are currently non-compliant with RoHS. Does anyone know where I can find data on salt-spray life for various alternate steel finishes which are RoHS compliant?

Mark H. Stark
OEM valves - St. Louis, Missouri, USA


A. You didn't say what your current finish it, Mark, but I'll guess it's zinc plating with a chromate conversion coating. Most plating chemical suppliers and many plating shops now offer zinc plating with proprietary RoHS-compliant, non-hexavalent conversion coating systems. The new systems usually involve an additional step or two, perhaps a sealer, but will achieve similar salt spray results, although at a higher cost.

Zinc plating with conversion coating is probably the least expensive coating. There are alternative coatings that, depending on your needs, might be organic or dip spin, or platings of different metals, etc.; but in that case a comparison of salt spray life is meaningless because different coatings react differently to salt spray without implying anything at all about anticipated service life. Salt spray testing is a QA method to help insure that the process has not gone south, and has little other bearing on life expectancy. For example, no other finish even compares to galvanizing for real-world life in some environments (75 years and more outdoors with zero maintenance), but its salt spray performance is poor.

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


Q. Thanks for responding Ted,

Your guess was correct. We had to take a step backward years ago when cadmium plating became unusable. I'm just trying to make sure we don't reduce our corrosion protection further because of RoHS. Is this latest plating really comparable? Are there other alternatives?

Mark H. Stark [returning]
- St. Louis, Missouri


A. It is right to be concerned from the viewpoint that a more complicated solution introduces more opportunity for error and is therefore never as reliable as the simpler solution, but all of the world's auto manufacturers have now accepted zinc alloy plating with RoHS-compliant trivalent chromating as comparable in corrosion resistance. I don't think there's much need for worry about that part, but corrosion resistance was rarely the heart of why cadmium plating was done.

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

Salt fog expectations for electroless nickel plating, B4C, black oxide, Raydent coatings

August 17, 2017

Q. Hi! I want to learn salt fog test results for electroless nickel plating for steels (particularly 4340, nitrided).

I wanted to learn test results of salt fog tests for B4C, black oxide and Raydent coatings on steel. Can you help me? Thanks for your assistance.

Ekin Yildizhan
- Turkey

August 18, 2017

HI Ekin,

Rather than asking rather abstract questions could you frame your query in real life expectations? What are the products you want coated? What conditions of use will they see? What do you expect the salt spray results to tell you?

The one thing to remember about salt spray is that it does not reflect real time life in the field, the only thing that does that truly is weathering trials, which is some cases can take years.

As a general guide Electroless nickel, high phosphorus at 0.002" thick will pass a 72 hour salt spray. Black oxide coatings with a supplementary chromate treatment would be expected to last 1 1/2 hours, but to be honest these numbers mean nothing other than a guide to the quality of the process applying the coating.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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