Lacquer and zinc plating

A. It is not uncommon to lacquer the parts, Tejinder. For your non-demanding requirements I would be surprised if immersion in the simplest water soluble lacquer or wax wouldn't get you past this test. But your zinc plating plus chromating should easily pass this test without any lacquer coating, so you might concentrate on getting them right first.

You need an engineered salt spray cabinet to do an ISO or ASTM accelerated corrosion salt spray test. But if you want a simple test for internal use, you might do a search for "CAMRI testing". This is a repeatable accelerated corrosion test that can be done with very little investment, and has been used by very reputable organizations including the International Zinc Association. Good luck.

A. Salt spray is not an easy and inexpensive test. Obtain a copy of theASTM B117 specification, and you will see why. I suggest you find a few reliable testing companies and get a 2-3 quotes.

A. Hi, Mr. Senthilnathan. Lacquer of this type increases corrosion resistance the same way that paint would -- as barrier layer protection. For that reason it can be a good idea . . . if your customer agrees and understands. But it can also have a downside! If your customer relies on this finish for electrical conductivity, the lacquer will kill it; if he's planning to paint or powder coat the item, a lacquer or wax can destroy adhesion; if it's a threaded fastener, lacquer doesn't have the adhesion that plating offers (that's one reason fasteners are plated instead of painted).

A. Hi, Evan. No, the lacquer is not a substitute for chromate conversion coating . . . it's a topcoat over it. Lacquer might not even reliably stick directly to zinc.

You can use a proprietary trivalent chromate and a topcoat as a substitute for yellow chromate, but I don't think either the substitute or the original will hold up long on an offshore oil rig. I think you need galvanizing, or a heavy (25 µm) alloy zinc plating, or stainless steel construction. There are special standards for oil rig equipment.

Regards,

A. Hi Ravi. First, can you tell us what spec you are plating to which allows for a lacquer coating? Whether the white discoloration is due to the lacquer emulsifying to opacity, or whether it's due to white rust, may be a moot question because you can't take a spec which doesn't mention lacquer, then lacquer coat the parts, then claim you met the spec if you achieve the required salt spray testing time :-)

Trivalent chromating is a fairly high technology operation compared to the old hexavalent chromating, and I think you should always plate to a spec, and always use the topcoat specified by the trivalent chromate supplier, rather than applying a lacquer on top of it. If you are lacquering without your customer's understanding and permission you could find yourself in a contract dispute. Good luck.

Regards,

A. Hi again Ravi. I ducked your question the first time, and will have to duck it again because I don't know how a reader could know whether your verbally described discoloration is due to white rust or emulsification of a lacquer coating.

But I also don't know how we can say in the absence of a spec that 96 hours to white rust is satisfactory performance, and discoloration of a lacquer coating is to be ignored. A wiser man than me once told me: "If you have to talk yourself into the parts being good, they're not".

Surely the parts are supposed to be capable of withstanding 96 hours to white rust without any lacquer coating, so why not just test unlacquered parts and you'll know whether they get white rust. I'm betting that they will and the investigation into whether the whiteness is white rust or lacquer discoloration will become moot; but if unlacquered parts do not fail, you'll know that the lacquer is what is discoloring, and you'll be able to investigate whether a better lacquer will maintain clearness.

Regards,