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How do I prevent darkening of a nickel deposit over steel when baking?

Baking of hardened steel products is a common step to relieve hydrogen embrittlement (see What is Hydrogen Embrittlement and How is it Prevented? ).  A common product requirement for bright nickel plated products to retain their luster after baking.  It is a common misconception that the nickel deposit darkens during baking at common hydrogen embrittlement temperatures of 375-400F.  Bright nickel deposit can maintain their luster at these temperatures as they generally form a relatively passive oxide with good spectral properties.

However, any nickel deposit that is plated to a common (thin) commercial thickness of 0.0001-0.0003 inches is generally a porous deposit that can be thought of metalugically as a screen on the surface of the steel.  During baking the steel itself is susceptible to oxidizing or slightly scaling with a darker oxide film.  The ferrous scale has a tendency to propagate through the pores in the nickel deposit resulting in a loss of luster to the overall deposit.  In order to mitigate this phenomena it is recommended that the nickel deposit be increased to a minimum thickness of 0.0004 inches in order to reduce the porosity of the deposit.  It has been shown that this higher nickel thickness generally works to better “seal” the substrate steel and thus prevent darkening of the deposit during the baking process.  Slight variations from this rule of thumb may be needed but as a generally design guideline this increase thickness provide acceptable results.