The self-regulating baths have a disadvantage common to all baths containing fluorides or complex fluorides, a tendency to corrode or etch recesses which do not cover with chromium. This tendency to etching is especially noted on steel, commercially, the ordinary baths with sulfate catalyst hate a strong tendency to etch copper and brass. This etching tendency is especially marked in over catalyzed bath, and can be overcome to a considerable extent by keeping the bath in balance, or operating at as low a catalyst concentration as possible. In higher ratio of baths, overcome this etching by prefilming the surface to be plated as cathode in a plain chromic acid solution.
Chromium Etching and pitting of the brass metal is sometimes encountered in hard chromium plating even in ordinary baths with only sulfate catalyst, if stray current is permitted to leave non plating area such as the exterior of cast iron diesel cylinder liners being plated on the inside only. This can be prevented by greater care in maintaining full insulation on racks and fixtures, or by insulating the surface to be protected.
By the immersion deposits of noble metals such as platinum and palladium on steel would prevent the etching that occurs in low current density recesses, particularly in baths containing fluoride catalyst. Such immersion deposits of noble metals create a low over voltage condition on the steel surface that favors hydrogen evolution and tends to prevent other reactions. They may even prevent chromium deposition at higher current densities.
Other articles:
No comments:
Post a Comment