To passivate or not to passivate 304 SS?

[metman]

A proposed design has 304 SS in contact with Aluminum alloy. The Al is painted with an epoxy base primer. Let us assume the SS is uncoated for now. I left a voice mail for the engineer, David, who asked me the question whether to passivate or not. My question to him is whether the SS is bare or painted. Also I asked him if both the Al and SS are exposed to the atmosphere or is it one or the other. I will have these answers Monday Dec 14 or sooner as I plan to telephone him later today.

This potential bi metal couple would be on the leading edge of an airplane wing as part of a deicing system. At times the airplane will be flying over the ocean and landing on sea water so we have the condition of salt spray coming into contact with this potential couple.

David told me that in his previous experience in aircraft design, they routinely passivated 304 SS.

If the SS is bare and knowing the Al to be painted, would there actually be a galvanic couple? It seems to me the answer is no -- unless the painted Al has a scratch or bare spot, in which case there not only would be a couple but a condition for high current density. However, the paint is not supposed to be scratched but the paint could become abraded from wind blown sand etc but then normal periodic inspection should reveal this possibility.

With these thoughts in mind, I think the expected or prevailing condition is that there is no galvanic couple and therefore the SS should be passivated to make it more corrosion resistant whether painted or not.

Comments please

 

[israelkk]

You should follow the specs dealing with this type of airplane. If it was for military as far as I know it is mandatory at least to passivate all stainless steels. In some cases it is even required to plate the stainless with a sacrificial type plating such as zinc or cadmium.

 

[metman]

I also intended to ask David, “in what physical condition is the SS, i.e. ¼ hard, solution annealed or…?” However, before posting the above, I did a little boning up in Uhlig’s Corrosion and Corrosion Control 1964 edition page 112 Chapter 7 Effect of Stress. Work hardening does not create increased corrosion rate for the reason we would intuitively expect. According to Uhlig, …the residual energy produced by cold working is less than sufficient to account for an appreciable change in free energy… Then he writes about the effect of cold working on the precipitation of carbides or nitrides, dislocations etc and this being the more probable cause of increased corrosion rate in acid. Whereas in natural waters, cold worked vs. annealed steel corrode at the same rate as noted earlier in Uhlig’s discussion.

So then, even though the intuitive reason for increased corrosion rate from work hardening appears to be a moot point, work hardening produces other effects that do increase rate of corrosion at least in acid. Would this effect apply also to salt water?

 

[metman]

israelkk,

Good point and I will pass this along to David but it could be that the specs are not specific enough (no pun intended)so that it becomes a determination by a DER (FAA Designated Engineering Representative).

The airplane in question falls under Part 23 type FAA regulation in case we have an aero gal/guy on board here.

 

[EdStainless]

Does it matter to you if the SS turns light orange or bleeds streaks of rust?
The passivation has nothing to do with the galvanic corrosion. Two separate, but both important issues.
If you want the SS to perform its best then you must passivate.
If you want to avoid galvanic issues then make sure that the two metals are isolated from each other.


= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[SJones]

Even then, passivation will only buy a bit of extra time before the ravages of marine exposure begin to show. Painting the anode in a bimetallic couple is, generally, not the best of ideas as your reference to current density suggests.

Just paint both the materials and the 'issues' will be addressed.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[metman]

Thank you all for the good input. I will email the URL for this thread to David.

Best regards,

Leonard

 

[metman]

Sent: Monday, December 14, 2009 9:08 AM
To: Leonard
Subject: RE: To passivate or not?

Leonard-
I apologize for not returning your call. I was on a dead run all last week.

I did think to try to call the metallurgist where I used to work. He said that in aerospace applications 300 series should always be passivated and then provided the proper protection to prevent corrosion if dissimilar metals are present (IN PART DUE TO THE REGULATIONS). This can be by primer/paint and or sealant if the environment present isn't too corrosive, or cad plating if it is. A point in our favor is that the noble stainless fitting is DIMENSIONALLY small compared to the much DIMENSIONALLY larger active aluminum airframe. The reason is that while the voltage isn't changed, the current flow (METALURGIST'S USE THE TERM "CURRENT DENSITY" WHICH MEANS AMPERES PER UNIT OF AREA) is much less with this arraignment and thereby less aggressive corrosion. He also said that in commercial applications, PROTECTION isn't so important if the location isn't very corrosive OR IF CORROSION DOESN'T MATTER (NEVER ACCEPTABLE ON AN AIRCRAFT).

A cad plated steel washer will be installed between the stainless nut and skin. This should make the washer be sacrificial rather than the skin.

Thanks for sending the link to the Engineering Tips forum. It pretty much confirms most of this.

David
Engineer

israelkk (Aerospace) Quote: 12 Dec 09 8:06
"You should follow the specs dealing with this type of airplane."

The specs say essentially, "proper corrosion shall be provided" or words to that effect.