Agents for steel blackening
Blackening is among most common ways of steel colouring for both decoration and, to a lesser extent, anticorrosion protection (optical appliances, hunting arms, fancy-goods). A very thin coating is formed by a mixture of both ferric and ferrous oxides of dark blue-to-genuine black colour. A perfect blackened finish of high gloss may be merely achieved if the steel substrate is machined as much fine as possible. The steel parts for blackening must be cleaned for the pure metal appearance, free of any rust, mill scales and grease which otherwise might result in colour imperfections and spots in the coating formed. Due to a very low thickness, the blackening coating will copy all defects (mars, scratches, local roughness) on the substrate surface which thus should be avoided. For a glossy finish of the utmost quality, the steel parts to be blackened should be polished (e.g. barrel-polished) at first. On the other hand, a matt finish may be achieved through a slight roughening of the substrate. To enhance both corrosion performance and mechanical durability of the blackening coatings, a subsequent passivation with the agents containing adsorption-based corrosion inhibitors is strongly recommended. For further increase in the performance, an impregnation of the blackening with oil-based preservatives for temporary corrosion protection, after its drying out, is advisable which, moreover, will emphasize the black tone of the coating. If the drying operation of the blackened coating is not possible to be included in the line, the preservation with oil-based water-removers containing adsorption corrosion inhibitors should be preferred. As a principle, only plain carbon steels may be blackened in superior quality and the steels of a higher silicone content should be pickled with 10 % hydrofluoric acid at first.