Letter 42078

Faraday effect in chroming yields "trouble areas"? [Denmark] 

August 30, 2006

Could anyone please confirm me, that when a chroming plant tells me, that because of the Faraday effect my product might suffer from weaker chrome plating in some trouble ereas, namely around place where two tubes are welded together.

I take it to understand, that because of the differently distributed mass of steel in the product, the conducting of electricity varies and consequently has less "plating effect" in the "trouble areas?

Thanks in advance.

Morten Hecquet
importer of Towel radiators. - Kolding, Denmark.

It is true that plating thickness is usually lower in low current density areas, and that chrome plating may be absent in those areas if careful attention is not paid. You probably should specify, and insist upon, full coverage with chrome and some minimum nickel plating thickness. Regarding the nickel plating thickness, it is common to specify the minimum thickness for any area that can be touched by a 3/4" diameter ball rather than debating whether it is possible to measure the thickness in a weld crevice and things like that.

 
Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

Only to add the mentioned effect is related to the shape of the part. The current sees the surface and prefers tips and edges refusing inside corners and cavities. Mass has nothing to do with it.

Guillermo Marrufo
Monterrey, NL, Mexico

Hexavalent chromium plating solutions are very inefficient and also require a (relatively) high current density. Consequently, when there is a wide current density distribution on an article, the chromium thickness will vary significantly. The combination of poor efficiency and high cd requiremenst makes the problem worse. As a rule of thumb, the highest current densities are on the edges of articles and the lowest is in the middle. Consequently, you will get chromium build-up on the edges and a much thinner deposit at the middle. If there is a variable conductivity on the substarte, this too will ahve an adverse effect on the thickness control. Thickness can be controlled by careful design of the product and by using robbers and/or secondary anodes to redirect the current to where it is needed.

Trevor Crichton R&D practical scientist - UK

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