Improving CASS life of rims for motorcycles

Q. Dear sir ,

We are manufacturing silencers [mufflers] and rims with tri-nickel/chrome plating. We have a problem in rim CASS test life. Please guide us how can I increase CASS test life of my job. Recently we do semibright nickel 10 µm, tri nickel 2 µm, bright nickel 8 µm, chrome 1 µm.

A. Hello Yagnesh. The problem here is that accelerated corrosion testing is a QA measure that is employed to insure that your coatings are performing up to expectations for that finish. If you don't tell us what ASTM B368 CASS life you need and what CASS hours you are getting, we can't tell you whether the coating is performing to expectation and therefore whether need to specify a better one (thicker nickel) or whether there is a problem in the execution.

But the nickel sounds borderline thin to me: the MFSA Quality Metal Finishing Guide seems to suggests a total nickel thickness of 25 to 30 µm for duplex nickel, so 20 for triplex nickel may be adequate but is not generous.

Please get back to us with how many hours you expect and how many you get. Thanks!

TUTORIAL FOR NEW READERS:

Zinc anodes protect a ship by sacrificial protection, i.e., the steel is more "noble" than the zinc and steals electrons from it as needed so the zinc dissolves instead of the steel.

But when steel is nickel plated, things are reversed: nickel is more noble than steel, so as soon as there is a scratch or porosity, the steel effusively rusts to try to protect the nickel. A way of combatting this is to plate multiple layers of nickel with varying amounts of sulfur, so that an outer layer can sacrificially protect an inner layer and make corrosion spread laterally instead of penetrating through to the steel substrate. When two layers are applied it's called duplex nickel; when three are applied it's called tri-nickel.

CASS (copper accelerated salt spray) is an accelerated corrosion test protocol which is believed to be somewhat more indicative of real life expectations for nickel-chrome plating than standard salt spray tests.