Zinc anodizing & coloring by chemical conversion & spark discharges

Zinc is an active element and is often employed as a sacrificial coating on steel articles for cathodic protection. Conventional hot dip galvanized st eel objects, however, exhibit a monotonous silver gray that lacks aesthetic s and limits the field of usage. For this reason, various colored surface l ayers on zinc were developed in aqueous environments basically containing s odium silicate, sodium tetraborate and sodium hydroxide as an electrolyte, The anodic treatment in this study involves a two-step process, the steps a re: (1) ii chemical conversion layer on zinc in the form of a bright film p roduced in 0.25 M Na2Cr2O7 solution at 50-70 degrees C for 1.5 min, and (2) an alternating current of 20 A/dm(2) applied to zinc at 0 degrees C for 10 min. The result of the first step provides an oxide film that is able to r etard the anodic dissolution of zinc and therefore to act as a barrier laye r for the subsequent spark discharge (second step). In the second step, an adequate amount of transition metal salts is mixed into the bath to obtain various colors on the finished anodic coating, Salt fog tests prove that th e treated pure zinc sheets possess much better corrosion resistance than th e untreated ones. According to ASTM B 117, white rust spots appear on zinc after 120 hr of the salt fog test, while the treated zinc can last at least 500 hr before rust spots are observed. The resultant coating provides exce llent adherence to the substrate and a hardness of 20 percent more than for untreated zinc. Moreover, the colored appearance is steady and uniform, in dependent of viewing angles.