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Why is a Heavier Nickel or Tin Deposit Required to Provide Equal Corrosion Protection of a Thinner Zinc Coating on Steel?

Coatings such as electrolytic nickel, electroless nickel or tin can provide excellent corrosion resistance but this only occurs if the deposit thickness is sufficient to develop a pore-free surface on a steel substrate.  This is due to the fact that nickel or tin protect steel by “sealing” the surface from the atmosphere.  This method of corrosion protection is due to the fact that the nickel or tin are less reactive (more noble) than the steel basis material – referred to as a cathodic coating.  This is the opposite compared to zinc on steel.  Zinc is an extremely reactive (less noble) metal when compared to steel – referred to as an anodic coating.  As such, the zinc will corrode in preference to the steel even if the surface is not pore free such as in thin coatings of zinc on steel.  This is why white rust (zinc oxide) is seen prior to red rust (iron oxide) on zinc plated fasteners.  The zinc sacrifices itself to protect the steel until the zinc is consumed near the pore and then red rust begins.

The amount of nickel or tin that is required to achieve a certain level of corrosion protection is very much a function of the surface finish (smoothness, burrs, pits, etc..) of the substrate.  A smooth surface will require less nickel or tin to achieve good corrosion protection as compared to a rough, pitted surface with edge burrs.  However, a good general guideline is that “good” corrosion protection begins around 0.0005 inches (20um) of nickel or tin and “excellent” corrosion protection occurs around 0.001 inches (50um) on reasonable substrates.  This is very different from an anodic coating such as zinc where the surface condition will not play as large of a role in the overall corrosion performance.  In addition, thinner coatings of zinc on steel can afford better corrosion resistance than tin or nickel since zinc coatings do not have to seal the surface to provide scarification corrosion protection.