Zinc Chromate Paint Replacements

A. Larry,

The use of zinc chromate virtually disappeared from use because of the word chromate. Since hexavalent chrome is a known carcinogen, the world has gone the same way that it did when it found that if you gave a lab rat the equivalent of 500 cokes with saccharin,he would have a higher rate of cancer. Zinc chromate is still available to commercial users but not over the counter. Most painters do not want the hassle of chrome bearing waste disposal so went to substitutes.

If it were me, I would talk to at least three painting giants like Sherwin-Williams engineering departments. There are many better products than what you are using, but will cost more. Then, will your painter apply it?

A. FYI

Quote: "P&W -UTC concluded that TT-P-664 [link is to free spec at Defense Logistics Agency, dla.mil]D High Solids (Zinc Phosphate) Primer and TT-P-645B Zinc-Molybdate Primer are both acceptable alternatives to zinc dust, with TT-P-664D being preferred, for providing galvanic corrosion protection for inserts and fasteners used in aircraft engines manufactured at P&W -UTC."

Re."I have a serious galvanic corrosion problem where aluminum panels are riveted to steel plate."

A. 1. Either use all steel or all Aluminum. What you have is poor design. (Unless you are the vendor, it which case an automatically corrosive design designed for a short life is good).

2. Bolt sacrificial Cathodes (Zinc) onto the body. The cathode corrode and the body does not. Google "Cathodic Protection".

3. Do not waste your time with paint. It will never last in your environment. Too much wear on the paint for it to last.

 I hope that it gives someone pleasure to know, that while I was barreling down some salt laden interstate in the upper Midwest, drinking my saccharin free soft drink, and some rivets pop off of the oncoming truck, that I will never get cancer from the chromate it didn't use. (Don't he look natural?)

A. Phosphating treatment may be a promising alternative to zinc zinc dust er.

A. Colleagues:

Zinc chromate containing coatings are still readily available. However, with the drive to eliminate Cr⁺⁶ you may want to look for an alternative. General Motors specification GM7111M is a chrome-free product that performs exceptionally well against aluminum. Opel has another similar process as GME00255-B.

Brass Lacquers

on eBay or

Amazon

(affil links)

A. Hi, David. brass lacquer ⇦this on eBay or Amazon [affil links] would probably be the thing to use on copper to preserve its appearance. G.J. Nikolas [a finishing.com supporting advertiser] specializes in this, but it sounds like you just need one little can. Good luck.

Regards,

A. Hi David. Yes, it will deter rust. But as you can see, the question of whether it is 'suitable' is rather complex.

Regards,

 Hi, Neeraj.

For steel your choices include organic coatings such as paint and thermoset or thermoplastic powder coating on top of phosphatizing or electrocoating; and galvanized or electroplated inorganic coatings of some 20+ different metals, plus their alloys. For aluminum, eliminate the galvanizing but add chromating as an alternative to phosphating; and add anodizing in the inorganic category. For stainless, all of the alternatives for mild steel, plus passivation, electropolishing, or anodizing. And this list is just the beginning as there are hundreds of other specialty finishes.

All of these are valid alternatives and are widely used, and none can be eliminated in an abstract discussion. If you describe a particular piece part of a particular size & particular production volume, and add all of the additional constraints on the finish, the list may be shortened sufficiently that you can compare a few specific finishes. Otherwise, your question sounds like an excellent description for a 4-year degree program :-)
Good luck!

Regards,

A. A plethora of information is available on alternate primers.
Since there are so many experts all over the USA in the coatings field. I propose that we should have an annual get together at different sites, where we all can get together and exchange ideas, verbally in an informal setting, much like the Gordon Conferences in Chemistry.
Let me know what you all think.

Regards

A. Hi, Mohammed. I don't know if you are asking for a comparison between painting with a zinc phosphate based paint vs. painting with a zinc chromate based paint (if so, what is the material and condition of the substrate), or whether you are asking about phosphatizing vs. chromating zinc diecast parts or zinc plated parts, or whatever.. Please take the time to explain your situation, always using more words in your question than you want in the answer :-)

Thanks.

Regards,

A. Hi Pyotyr,

If I seem a little circumspect with my answer it is because I don't want to give you the impression that there is an easy answer.

Zinc chromates, indeed virtually all hexavalent chromates are considered to be toxic to some extent or another. As well as being skin sensitisers, respiratory sensitisers and generally nasty fellows, the main concern is the possibility of hexavalent chromium compounds to cause cancer.

Most chromates are considered to cause cancer by inhalation, in fact that is the usual classification, but there is quite strong evidence that they may cause cancer by ingestion.

Chromated primers are designed to slowly release the chromates over time to aid in corrosion prevention so there will more than likely be a slow release of the zinc chromate over time. This is where things get woolly, it comes down to the release rate of the chromate and how much of this is available to come into contact with humans, the concentrations involved and the form of release. We cannot as an anonymous group of engineers really advise you exactly how dangerous is the application as we do not have access to the parts in question to carry out a risk assessment.

If you are really worried I would suggest you contact your local EPA office and get them to carry out an assessment or contact the OSHA for a more definitive statement on the potential toxic hazard posed.

A. Chromate pigment is broken down by by a photocatalytic reaction. Example ZnO +hv → ZnO (h⁺ + e^- )
(1) Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ +7H₂O
(2) 2H₂O + 4h⁺ → O₂ + 4H⁺
(3) H₂O + h⁺ → OH + H⁺
(4) Cr(VI)+H2O2 + H⁺ →Cr(III)+H₂O+O₂

By adding a UV blocking pigment (like lamp black) you can slow down the process. For a long lasting protection, when exposed to light, you will need a UV resistant top coat. The best is to use a tested paint system which usually consists of a base coat for adhesion, a corrosion protective coat (zinc chromate) and a top coat for UV and mechanical protection. You can also get chrome free alternatives that are easier on the environment and your health. http://www.duncanaviation.aero/fieldguides/chromefreepaint/

? Hello Keem.

Are you talking about zinc zinc dust er PAINT, and thus asking for a comparison between painting vs. plating? Or are you talking about zinc plating with a chromate conversion coating? Zinc plating and galvanizing invariably receive a chromate conversion / chromate quench, so that question wouldn’t really make much sense since you're unlikely to find zinc plating or galvanizing (designed for final service) that doesn't include the chromate conversion coating :-)

Regards,