Stainless steel with black molten dichromate method, three kinds of alkaline low-temperature oxidation method and curing method, here are the first two processes.
A molten dichromate method
stainless steel parts first defatted with 8-12% sulfuric acid, heated to 50-60 ° C, pickling, activated in the hydrofluoric acid 1-2% and 10-15% nitric acid mixture, 1-2min. Finally, dipping in the molten salt solution of sodium dichromate, full mixing 20-30min the formation of a black film. Washed to remove the cooling. The temperature of the molten salt in the 450-550 ° C, below this temperature can not be fully oxidized, not black effect.
Two, the black film of alkaline low-temperature oxidation
the method to generate strong, able to maintain the appearance of the surface due to the low processing temperature, processing temperature of 104-108 ℃ the time required varies according to material, typically 10-30min world pie network information . Solution composed as follows:
sodium hydroxide 15% Sodium 3%, 2% of sodium nitrite, lead peroxide 2%, 0.1% of ferric hydroxide, water margin.
To note that the alkaline low-temperature oxidationWorld Steel Pipe , the main use of the passive film of the stainless steel itself, pretreatment, the workpiece after defatted, without pickling and activation, because the pickling will dissolve the passive film.