Black Oxide Stainless Steel Washers

Black oxide stainless steel washers demonstrate quantifiable advantages in galvanic corrosion control. When in direct contact with aluminum alloy components, untreated austenitic stainless steel gaskets will induce a potential difference of up to 220 millivolts, thereby accelerating pitting corrosion of the aluminum matrix. After the black oxide conversion coating treatment, the potential difference is compressed to less than 45 millivolts, which reduces the corrosion rate of aluminum parts by about 80%. At the same time, the black oxide layer can stabilize the contact resistance on the surface of the stainless steel gasket below 0.2 ohms, and after twenty temperature alternating cycles from minus 40 degrees Celsius to 85 degrees Celsius, the resistance value only increases by 12%.

The treatment temperature of the black oxidation process is usually controlled between 130 degrees Celsius and 150 degrees Celsius, which is more than 300 degrees Celsius lower than the process temperature of mechanical galvanizing or hot-dip galvanizing, so it will not change the original hardness distribution of stainless steel. The measurement data shows that the average thickness change of the gasket before and after treatment is only 0.0015 mm, and the inner diameter tolerance can still be maintained within the ISO 2768 medium precision range. Flatness test results on a batch of 500 pieces further showed that 97 percent of the gaskets deformed by no more than 0.02 millimeters under a load of 4.5 Newtons, which means that in stacked assemblies of precision valves or fuel cell bipolar plates, black oxide stainless steel gaskets can provide an anti-glare surface without causing a significant increase in the dispersion of preload force.

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