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Stainless steel turns black upon sterilization


One of our customers (a hopsital)is processing ours and our competitors stainless steel devices in a washer disinfector (approx 90°C)and doing a final rinse with RO water. Over a period of time the steel is blackening and in extreme cases it appears that there is a black plastic coating covering the steel.
They are using an endozyme detergent and a neutralizer - citric acid at a pH or approximately 1-2. I think it is the neutralizer that is causing this problem.
Could any one provide any other suggestions or do they know what could be causing this?

Stuart March
Processing stainless steel - Durham, England


There are a lot of possibilites here. What grade of stainless steel are you using? What is in the enzyme-detergent? If these are surgical instruments you are probably using 410ss or similar. You cannot use any chlorides in the solution. Depending on the grade of steel you cannot immerse the instruments in straight citric acid at this temperature or it could turn black.
There are excellent citric formulations available that would prevent this. Contact us if you need help.

lee kremer

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

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