Nickel oxide staining of stainless steel Electroless nickel tanks(2000)
I work in the disc drive industry. Our plant takes aluminium discs, coats them in electroless nickel polishes them and sends them on to the customer.
Part of the EN (Electroless Nickel) process involves cleaning our 17 electroless nickel tanks. We use Nitric acid at 8 normal concentration (35%) to strip our tanks of nickel residue left behind when the tank is taken down.
Part of the cleaning process is building up a passive layer. THis happens as a result of the stripping process. This passivity is important. We also use anodic protection to boost the passivity as it starts to break down during the plating stage.
However, a brownish coating now covers the inside of our tanks. We have no idea what this is but we suspect it is nickel oxide. We believe that we are forming an oxide layer but not a chrome oxide layer which we want.
Has anyone experienced this type of staining of tanks?
What can we do to get back to our clean stainless steel?
I understand we can electro polish but this is not feasible.
Is this brown staining impairing the electrochemical rest potential of stainless steel? If so in what way and how can I counter it?
Any help would be dearly appreciated.Gerard Michael Harkin
Seagate RMG - Northern Ireland
I would almost bet that the brown film is iron. Most passivation solutions are used far past their prime and become "iron immersion coatings" instead of the desired passivation process. Look at your water quality first. If you have a lot of iron in the tap water, your problems are multiplied. We make an electropolish solution that requires no waste treatment and lasts indefinitely and presents no worker hazards. It would be a simple matter to pump this solution into the tank and electropolish it in a matter of minutes leaving the best passive surface attainable.Dan Weaver
- Toccoa, Georgia
Check sulphur content. Sulphur has been found to deplete CR at the surface of metals.T Neumann
- Hamilton, Illinois, USA
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