Tuesday, March 18, 2008

Semi-bright Nickel Plating

In 1945 have introduced the use of comarin to obtain a high leveling, ductile, semibright, sulfur-free nickel plate from Watts baths which became prominent in the plating of automobile bumpers. This plate was easily buffed bright and then chromium plated. The chromium plated sulfur-free nickel gave excellent corrosion protection results. Later, in order to eliminate the buffing, sulfur-containing bright nickel was applied on top of the semibright sulfur-free nickel in a thickness less than that of the semibright plate.

While it was not apparent from static roof exposure tests in industrial atmospheres, the corrosion protection with this chromium-plated double layered nickel coating was superior to even the good performance of the same total thickness of chromium-plated sulfur-free buffed semibright nickel. This superior performance was manifested in the cities that used salt to de-ice the streets in winter. This double layer nickel coating was an important step in obtaining greatly improved corrosion protection results with nickel chromium coatings. The corrosion pits starting in the bright nickel underneath the pores in the top thin chromium plate are diverted laterally on reaching the more noble sulfur-free nickel layer, thus greatly delaying the corrosion pitting penetration to the underlying steel or zinc die cast substrate. Static roof exposure tests did not markedly show this sacrificial protection because the acidity of the industrial atmosphere was due mainly to moist sulfur dioxide which attacks sulfur-free nickel about as readily as nickel containing 0.08% sulfur. On the other hand, in the presence of acidic saline solution, the sulfur containing nickel is attack anodically more rapidly than sulfur-free nickel.

Further improvements in diverting the corrosion pitting laterally were obtained by using a thin layer of nickel (1.2-2.5 ┬Ám), containing 0.15% sulfur, between the sulfur-free nickel and the top bright nickel plate uses a 50:50 ratio of semibright to bright nickel instead of the 70:30 preferred ratio for the double nickel.

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