Metal finishing Q&As since 1989

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Will there be an interaction between our aluminium finishes?

Can anyone shed any light on the potential for interaction (galvanic corrosion) between an aluminium surface coated with Alochrome 1200 and an aluminium surface coated with a finish described as "black zinc nickel".

Tony Friedlander
Product Engineer - London, UK
2007

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Chemistry 101 Electrode Potentials: Aluminum on top at negative 1.70, then Zinc in the middle at negative 0.762, then nickel on the bottom pulling the zinc farther away just a tad at negative 0.23.

So, after humidity and temperature cycling a cell will set up with 0.762-0.23 = 0.532 volts. Theoretically that will corrode, but very very slowly agravated by time, temperature, humidity, and so-called "acid rain" in the humidity.

All of the above is the thin skin a lawyer would use. Practically speaking, you have a reasonably safe galvanic couple, just leave as much chromate solution to dry with the gel as you can by not heat drying the chromate and letting it drip dry over night and not wrapping in sulfur [affil links] bearing brown paper. You are so very wise to even think of the galvanic problem as so many in our industry just knee-jerk and call the chromate or plating supplier and say your "product does not work".

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2007