The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Why is cadmium more "compatible with aluminum" than zinc is?

2003

In the version of the electrochemical series in my possession Zinc, Cadmium and Aluminum are listed as follows:

Aluminum -1.66 Volts
Zinc -0.76 Volts
Cadmium -0.40 Volts

If so, the potential difference between Aluminum and Zinc is 0.90 Volts whilst the potential difference between Aluminum and Cadmium is 1.26 Volts. In other words there is a greater potential and a stronger galvanic cell between Aluminum and Cadmium as opposed to Aluminum and Zinc.

If my facts are correct then I cannot understand the statement "Cadmium displays low galvanic corrosion in contact with aluminum, accounting for its widespread use in the plating industry"

I have seen this statement in many publications on the subject. Could someone explain what I am missing or which facts I have incorrect?

Thank you.

Tony van der Spuy
electroplating consultant/trainer - Cape Town, Western Province, South Africa

---

The problem is that while your numbers are correct, the chart they are from isn't fully applicable to the situation. The EMF series you are quoting lists the tendency of a metal to go into or come out of a 1-Normal solution of its own ions. While this gives some useful info, some of the rankings don't hold when the solution is seawater instead of a 1-Normal solution of the metal in question.

In seawater and real world corrosion situations, the galvanic potential of cadmium almost exactly matches the grades of aluminum which cadmium plated fasteners typically join. Look up the "seawater series" instead of the EMF series.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003