RoHS Zinc Plating ASTM B633 Type III - are trivalent chromates ready for prime-time

A. Hi D. I spent a lot of time in dozens of zinc plating shops in my career which makes me suggest, as a first thought before I have any real data from you, that the problem may not be RoHS-compatible trivalent chromating vs. hexavalent chromating, but that many off the shelf fasteners don't have 5 microns of plating and are not plated in compliance with any spec ... they are plated as a commodity at the lowest possible price with just enough zinc (probably 3 microns average) to look okay at first glance. The top coat/sealer required with trivalent chromating might be skipped for cost savings.

Often these parts are ordered with 'clear zinc' being the totality of the specification; so the plating shop does the minimum they can get away with. Are the unsatisfactory parts you buy actually labeled as "ASTM F1941/F1941M-16 FE/ZN 5ANS"? -- because that specification seems to require passing 96 hours salt spray. Perhaps you could try dipping in water soluble lacquer (contact a plating process supplier), or switch to hot dipped galvanized, mechanically plated or zinc alloy (Fe-Zn or Fe-Ni) plated fasteners, or simply buy from a specialty hardware supplier who promises full ASTM F1941/F1941M-16 FE/ZN 5ANS compliance?
Luck & Regards,

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

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.

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.

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Hi, David. This is a site of international camaraderie where people help each other; Still, I know from dozens of similar postings over the years that an 800-pound gorilla just came into the room with you; so as the forum curator, I must introduce him: People don't like to hear that you're sending your business to their offshore competitors who work cheaper and don't know what they're doing ... and you'd therefore like the readers domestic shops to, for free, teach those lower-priced offshore competitors who "leave a lot to be desired" how to keep your business instead of it coming back to the USA :-)

A. Anyway, tin-zinc plating would give you better ductility for crimpability. All trivalent chromate conversion coatings are proprietary, and are based on varying technologies. If your job shop cannot get assistance from the process supplier to obtain these salt spray ratings, that shop must find another process supplier or you must find another jobshop, because the formulations are not generic information but proprietary info. Good luck!

Regards,

A. You request 400 Hours in salt spray.
I assume that you are referring to red rust and not white rust. No way 400 H to white rust.

Your hex chrome chromate -- was it a blue one or an iridescent one?

The replacement of the iridescent is a thick layer tri-chrome , while the replacement for blue is a thin one.

The iridescent and the thick ones are much more protective than the blue one.

Now - if you need corrosion resistance with a tri-chrome , a sealer on top of it is a MUST.

Last but not least, it is not only about chromate. In case you need resistance to red rust, 8 microns of think is the minimum thickness

? Dear Shay,
could you write your bath conditions and chemical analytical to try help you?
Best regards,

A. Hello Paul,

The equivalent is ASTM B633 Type V. You have the same thickness, you can have the same corrosion resistance. For clear blue chromates, trivalent are (in my opinion) far better than hexavalent ones. Take a try with a few in "lab scale" (2-3 gallons, a few parts) and have them tested for corrosion resistance.

For yellow chromates (ASTM B633 Type II), the equivalent is Type VI. But yellow hexavalent chromates are far better than yellow-colored trivalents, in my experience.

Well, I hope you can change to trivalent and improve your results (process cost, quality, and water treatment costs). Good luck!

A. The equivalent for yellow hexavalent chromate is a thick trivalent chromate, and the equivalent for clear hexavalent chromate is a thin trivalent chromate.

You can improve the corrosion resistance by applying a sealer on top of the trivalent chromate.

A. Perhaps my experience is dated. But I believe that a trivalent "chromate" is merely a decorative finish. Somebody show me a part with that on it, with no sealer, that will stand up to a salt spray, and I'll become a believer.

I question the environmental benefit of using some "eco/green" finish when corrosion resistance, and hence the useful life of the part is lowered.

A. Hi David. You are extremely knowledgable in metal finishing, but in this specific area I do think recent progress in trivalent chromates may be substantial. Although today's trivalent chromates are not the equal of hexavalent in ALL ways, they are not the trivalents of the old days, and I believe they are the equal of hexavalents in salt spray resistance. My understanding, subject to correction by someone more knowledgable, is that the thin film trivalents generally require a sealer but the thick film chromates do not.

Regards,

A. Hello Ted,
I fully agree with your view.I have been connected with this finish for auto parts for multi national companies for the last 6 years ever since the arrival of trivalent chromate. They have been so remarkable in Salt-spray testing -- one customer company has revised the hours for white corrosion from 96 to 140 hours now.

A. Hi Cathy. You and/or your plating shops need to spring for a recent edition of ASTM B633, rather than outdated versions. Type V is clear trivalent chromate ... but no one can certify to it, nor inspect for compliance with it, based on just that one fact; you need to comply with the whole spec.

Regards,