Wednesday, January 30, 2008

Postplating Treatment

Postplating treatments are not commonly used on chromium plate, as the great passivity and tarnish resistance of the metal usually make them unnecessary. The normal “air passivity” of the metal can be considerably increased by treatment with anodizing agent such as nitric acid, where this is practical. Treated thick deposits with hot nitric acid, chromic acid, or permanganate solutions for many hours, and stated that the passive chromium would no longer dissolve in hydrochloric acid, and was more noble than platinum. Except for nitric acid, such treatments tend to discolor the chromium surface.

Electrodeposited chromium foils 250 micro meter or more thick, produced by stripping the brass basis metal from them in concentrated nitric acid and then baking overnight at 165 OC. resisted corrosion for 2 hours in concentrated hydrochloric acid. It should be noted, however, that this is an unstable condition, and any slight disturbance or handling may result in the sudden violent attack of the chromium by the hydrochloric acid with evolution of hydrogen.

Although treatment in a sodium hydroxide-sodium nitrite solution has been recommended for improving the corrosion resistance of thick chromium deposits on steel, other workers have disputed the efficacy of this procedure. Electrolytic polishing of the steel before chromium plating was effective to give hard chromium deposits of greater corrosion resistance.

To improve the corrosion coating of chromium-plated parts with supplementary surface films applied by cathodic treatment for about 1 min in a solution of 50 g/l sodium dichromate and 1 g/l chromic sulfate at 85 to 95 OC and pH 2.0 to 2.5 using 0.32 to 0.64 A/dm2. The extent and permanence of the improved corrosion resistance are not clear. There are indications that the effect tends to be lost after a year or two of outdoor exposure.