Saturday, February 16, 2008

Chromium Alloy Plating

Chromium alloy plating can be considered a subheading under trivalent bath, there is almost no alloy plating possible from hexavalent solutions. There has been a great deal work, and some reviews are available, but nothing of commercial importance seems to have been developed. It appears likely that some fundamental problems of depositing chromium from trivalent baths remain to be solved, and that these are probably not made appreciably easier by introducing the added factor of alloy deposition.

One exception to the rule of no alloy plating from chromic acid baths is the work of Vagramyan and his collaborators. Alloys of up to 37% selenium, 15% manganese, 2% molybdenum, and 1 % rhenium were obtained with cold chromium deposits at about 20oC. These alloys are on the whole no longer obtained as the temperature is raised, so presumably they are alloys with the dull hexagonal hydride produced at low temperatures, and not with the bee deposits which form ordinary bright plate.

Much work has been done, mainly in France, to investigate claims of improved wear resistance of chromium-molybdenum alloys produced from chromic acid solutions. It appears that bright deposits generally contain less than 1% molybdenum, and this could possibly result from solution contamination of the deposit instead of alloying. Hard chromium plating baths are frequently a little low in catalyst due to too strict adherence to the 100:1 ratio of chromic acid to sulfate, and the improved wear resistance of deposits from solutions containing molybdenum compounds might be due to an effect on the catalyst balance of the solution.

Other Special Types of Chromium Plate

A "frosty" or satin-finish plate in between cold chromium and bright plate was found desirable for press plates. Such smooth-bubbly or natural rounded nodular plate has been found useful for handling textile materials. Some people used a special cold chromium plate produced in refrigerated electrolytes for printing plates. Black chromium can be obtained from immersing chromium deposits in molten cyanide.