The authoritative public forum
  for Metal Finishing since 1989
-----

Cadmium Coated Protection

2005

Hello,

This is the second time that I am trying to get some technical opinion. I would request you to reply.

We usually use the stainless steel bolts for connection of Aluminum members on overhead line towers in an aggressive atmosphere with Chlorines, Chlorides and Fluorides (Aluminum Smelter).

Now, we have a proposal to use the Cadmium plated-coated bolts for the same purpose.

You are requested to clarify us the following:

1. Is Cadmium coating possible / recommendable to be used in such atmosphere.
2. Comparing these two types (stainless steel or Cadmium coating) of bolts, nuts, washers, etc., which is better?
3. What are the mechanical characteristics of Cadmium coat (we have to tighten such bolts, some coat damages are expectable?)?
4. What are the criterions for decision?

Nijaz Sijercic
Hatch - Montreal, Canada

---

2005

Sorry that you had to ask twice, Nijaz, but this site never received previous correspondence from you.

4. Cadmium is a cumulative poison, and the world is working hard to get it out of the environment. So the criteria for selection is: don't use it unless it is indispensable.
3. Cadmium is, however, an ideal coating for bolts because it offers cathodic protection (scratches are not a big issue), it is soft and lubricious, and its corrosion products are not bulky and sticky.
2. Stainless is a poor material because it galls (poor stick-slip) and because most types are very vulnerable to pitting from chlorides.
1. All corrosion prediction charts are full of disclaimers but I think cadmium plating would be reasonably resistant.

But how about galvanizing, which is commonly used on line towers? Or, if the aluminum members have proven resistant, how about hot dip aluminizing, or Ivadizing (aluminum ion vapor deposition)? Good luck.

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