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Cadmium plated CRES vs passivated CRES

What is the difference between Cadmium plated CRES and Passivated CRES?

Thanks,

Amir H Hazrati
Engineer - Seattle, Washington
March 26, 2008

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CReS means "corrosion resisting steel", Amir. As far as I know, that always means stainless steel. Passivated means the CReS has been subjected to a nitric acid or citric acid process to remove surface iron and deter rusting and pitting. Cadmium plated means electroplated with the element cadmium, which exhibits properties very different from CReS. Could you please put this in context, so we understand your situation?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 27, 2008

So what are the major differences and applications of a passivated CRES versus a cadmium plated CRES? It seems to me that both passivated and cad coatings provide an extra degree of corrosion resistance on top of the CRES/stainless steel base.

James Yin
- Seattle, Washington
September 24, 2008

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September 26, 2008

Hi, James. Unfortunately you haven't told us your situation either, and it would take a dozen pages just to cover the important issues of cadmium plating, and another dozen to cover passivation :-)

Although cadmium plating is a sacrificial anodic coating for steel, it is never applied solely for that purpose because it is a toxic bioaccumulative poison that is avoided whenever possible, and there are other sacrificial anodic coatings you could use.

Cadmium plating is only applied when there is a call for a unique combination of properties which can be delivered only by cadmium and nothing else. Cadmium is anodic to steel, it is lubricious and prevents stick-slip, it is galvanically compatible with aluminum, it is malleable, its corrosion products are neither voluminous nor gummy, it has biological properties that may deter biologically aided corrosion processes, it functions differently than other materials in radiation situations. Sometimes cadmium is specified in old documents whereas newer documents specify replacement coatings.

Passivation should almost always be done on stainless components because iron particles left from manufacturing processes will cause rusting and pitting of stainless components if not removed via passivation. Plus, there is a chromium enhancement effect from passivation which improves corrosion resistance.

Cadmium plating is not "optional"; rather, it is either absolutely required or it is avoided with a 10-foot pole :-)

Please tell us your situation. Thanks.

Regards,

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

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Hi Ted.

I just found this thread and thought that you could potentially solve our dilemma. The question I'm faced with is whether to use cad plating INSTEAD of passivation for 15-5PH SS.

We are replacing fittings on a main spar splice of a general aviation aircraft. Currently, all parts of the joint and spar are aluminum but we've selected 15-5PH for its stiffness properties, ease of machining and availability among other reasons.

One designer has called for passivation and mag particle inspection while another (with access to data from some of this area's premier aerospace OEMs) calls for cad plating and no mag particle inspection. Having little experience with SS, I can't say which is correct.

Charles Berguein
- Seattle, Washington
November 2, 2009

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November 3, 2009

Hi, Charles. Although I have some familiarity with metal finishing, and can explain certain things about cadmium plating and passivation of stainless steel, I have no experience at all in aircraft design and would not know "what to look out for", so please don't interpret my comments as a recommendation for a finish on an aircraft component :-(

One of the strong points in favor of cadmium plating, however, is galvanic compatibility with aluminum. To my limited knowledge in this area, fasteners used in aluminum aircraft are always either cadmium plated or coated with aluminum by Ivadizing. Without aircraft experience, I would be very concerned about connecting a passivated stainless steel component to an aluminum one.

There are modern zinc alloy platings that reportedly can be used in lieu of cadmium plating in some instances (https://www.pfonline.com/articles/pfd0019.html) but I don't know if they are appropriate for this spar. Good luck.

Regards,

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