Will Chromate Passivation Keep Brass Looking Natural and Protect the Finish.

A. Hi Luke. If we were speaking of stainless steel parts, "passivation" would have a very well defined meaning, accepted in formal specifications and referred to in tables of electrochemical potential. But for other materials like brass, 'passivate' is slang with no formal, accepted meaning. So one reader may not envision the same need as another.

If you are trying to maintain a bright shiny surface on brass near a salt environment without a lacquer or clearcoat you'll find it hopeless. The closest you can get is probably treating the brass with sodium benzotriazole preservative, then applying a 2-component automotive clearcoat -- but whether it will last a couple of weeks or a couple of years will depend on the environment.

I am not familiar with chromates that contain citric acid, but maybe another reader is. Most generic ones, I believe, are based on about 180 g/l chromic acid and a about 50 g/l sulphuric acid.

However, Biestek & Weber quotes a study showing that chromate solutions will not form on brass without halides present and seems to believe that the best solution is 180 g/l chromic acid, 10 g/l sodium chloride, 50 g/l sodium sulphate, and 6 ml/l conc. sulfuric acid.

Remember that hex chrome is not RoHS compatible and the whole world is trying to get away from its use.

Luck & Regards,

A. It will help, but it will not eliminate the problem. Consider having the unit titanium nitride (TiN) coated. The stuff that they put on the gold colored drill bits. They can plug the holes so they will not fill up.

A. Passivating the brass with a hex chrome type material will prolong the time until the brass discolors (oxidizes) but it will be a variable based upon the conditions of exposure. Sweaty hands are a difficult area to estimate since each individual has a somewhat different chemistry to the sweat - some being more corrosive than others.

Some trivalent passivations have been formulated for this use. The same caveats would apply.

Since you would be using potentially hazardous chemicals, the process should be performed by those who are experienced in the use, storage and disposal of chemical products.

A. Your object looks like ideal base for old shellac based golden yellow dyed lacquers. In old German books you can find many recipes for that process (G. Buchners book on metal colouring / Berlin 1922; good new book is Werkstattrezepte by H.Hilpke). Old clocks and microscopes are some examples of that finish. Hope it helps and good luck!

A. Hi Chris. Iridite #7 was or is a Macdermid offering, so you could inquire of them.

Brass parts can probably be bright dipped in a proprietary mix of sulphuric acid and hydrogen peroxide, and kept bright with sodium benzotriazole or, better yet, a chromate conversion coating.

Regards,

A. Hi Chris. Herewith a 5¢ tour of chromate conversion coating:

Many metals from aluminum, through brass, cadmium, copper, electroless nickel, and on through zinc may employ chromate conversion coatings to deter corrosion. Like with most other metal finishing processes, research regarding exact formulation and additives can be beneficial, so major suppliers did such research and they market their formulations as proprietaries: Iridite is Macdermid's tradename for their chromate conversion products, Alodine is Henkel's, Chromicoat and Gardobond are Chemetall tradenames, as a few examples.

The thing is, a formulation tailored for aluminum doesn't work as well on brass or zinc, and sometimes you want a clear finish and in other cases a yellow or black, so each vendor has a number of formulations. Whether Macdermid still has an Iridite #7 or some other product optimized for brass, I wouldn't know. And that still leaves you looking for a shop who offers it.

A further issue is that because of concern over 'hexavalent chromium' those older finishes may not conform to European RoHS requirements, automotive standards, etc., so you may want the coating limited to trivalent chromate.

It is possible to 'home brew' chromate conversion chemistry, and I believe it's often done in India and some other countries but rarely in the U.S. And, since RoHS days, home brew is more difficult because the trivalent replacement chemistries are more sophisticated than the older hexavalent formulations.

So you must determine whether you want to use hexavalent or trivalent chromates and find a shop which can do it on your brass parts.

Regards,