Methods for Preventing Blackening of Stainless Steel Quick Connectors

During the passivation of stainless steel quick connectors, it is often observed that some stainless steel parts, when placed in the solution, quickly produce reaction bubbles, and the stainless steel surface gradually turns black. After removal, multiple sandblasting or mechanical polishing steps are required until passivation can proceed successfully. This can sometimes lead to parts exceeding dimensional tolerances or being scrapped. However, some parts still fail to produce a qualified passivation film even with this method.

If salt bath quenching is used, the impurities (SO₂₋₄) in the salt bath must be controlled. The SO₂-₄ content should be measured regularly, and operations should strictly follow the process specifications. This can prevent the formation of sulfides on the stainless steel surface. During tempering, the heat treatment specifications for stainless steel should be changed to vacuum furnace quenching and tempering. The treated parts have a bright surface, although there may still be tempering oxidation colors. However, the surface condition is better than that of previously treated parts, and the film is thinner. This eliminates the need for mechanical polishing or sandblasting to remove the oxidation color, preventing parts from exceeding tolerances or being scrapped, thus improving the passivation quality. At the same time, the correct process specifications should be selected during quenching. During tempering, if the operation is performed within the temperature range of 500-600°C, and if hardness is not the main indicator for the part, multiple tempering should be avoided as much as possible. Due to insufficient understanding of stainless steel passivation in the past, the requirements for surface roughness of parts were not strict. Surfaces with a roughness of 6.3 μm, and even non-machined surfaces, could affect the passivation quality. Now, it is stipulated that the surface roughness of stainless steel parts undergoing passivation should be less than 1.6 μm. This regulation eliminates one factor affecting passivation blackening.