Stainless steel with black finish

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There are three methods for blackening stainless steel: molten dichromate method, alkaline low-temperature oxidation method and vulcanization method. The first two methods are introduced below. 1. Using the molten dichromate method, first degrease and purify stainless steel parts, use 8~12% sulfuric acid, heat to 50~60°C for pickling, and activate in a mixture of 1~2% hydrofluoric acid and 10~15% nitric acid for 1~2 minutes. Finally, immerse in the molten salt solution of sodium dichromate and stir thoroughly for 20 to 30 minutes to form a black film. Take it out and cool it, then wash it with water. The temperature of molten salt is 450~550℃. Below this temperature, it cannot be fully oxidized and the black effect cannot be achieved. 2. Alkaline low-temperature oxidation method: The black film generated by this method is strong. Due to the low processing temperature, it can maintain a beautiful surface. The processing temperature is 104~108℃. The required time varies depending on the material, and is generally . The composition of the treatment solution is as follows: 15% sodium hydroxide, 3% sodium phosphate, 2% sodium nitrite, 2% lead peroxide, 0.1% ferric hydroxide, and the remainder of water. It should be noted that the alkaline low-temperature oxidation method mainly utilizes the passivation film of stainless steel itself. Therefore, during pretreatment, after the workpiece is degreased and purified, there is no need to perform pickling activation because pickling will dissolve the passivation film.