aluminum body, stainless hardware, salt fog

[jtowne]

I have an aluminum housing and am using stainless steel screws. After going through a 30 day salt fog test the aluminum housing was showing signs of corrosion. I had the white powder on the screw tested and the primary element indicated was aluminum. The aluminum alloy is 6061. I do not know the composition of the stainless screw.

In the same test chamber we had another aluminum body also with stainless hardware and there was no evidence of corrosion from the body to the screw. I do not know the composition of either the aluminum body or the screw.

My question is this: Is there a stainless alloy that will not permit the aluminum body to corrode? I suspected that the screw was cadmium plated but the lab that I had it tested at said that it was not plated. They said that its material characteristics were in line with that of a copper bearing austenitic stailless steel such as UNS30430.

One other question; What is the difference between an active stainless and a passive stainless if they presumably are of the same alloy?

Thank you.

 

[EdStainless]

First of all 6061 is not very resistant to saltwater attack. As I recall the 5xxx alloys are used in marine applications.

Second, any stainless alloy will be cathodic to Al and cause accelerated galvanic attack.

Your issue may be more a function of trapped moisture under the fastener. If not moisture is trapped then corrosion will be very slow.

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Rust never sleeps
Neither should your protection

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[swall]

Following up on what EdStainless said--the position of the test items in the salt fog cabinet can have an influence on how much fog condenses onto the part, how much solution is trapped and how much drains off. All of this can affect the corrosion rate.

 

[TVP]

jtowne,

Are you sure that neither of the aluminum housings had a surface coating? This could also have an effect.

 

[jtowne]

All of the units were in the same orientation in the test chamber.

The Al housings are silver plated but there is very little if any plating in the M$ screw holes where the corrosion was found. Interestingly, the corrosion was so severe on one unit as to migrate out from the hole and appear on the outer surfaces of the unit. The other unit was pristine! Confusing.

 

[EdStainless]

Your problem may be the silver plating. Is it really pure silver? If so then you will have serious galvanic corrosion of the Al where ever there is a holiday in the plating, SS screws or not. The screws may just be damaging the plating and forming crevices, the metal involved my not be part of the problem.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

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[IRstuff]

Silver?

Doesn't silver die in salt fog all on its own?

TTFN

Eng-Tips Policies FAQ731-376

 

[kenvlach]

Some vital information seems missing.

Silver + salt = silver chloride (white). Easy to form; precipitation as choride is a standard method of analysis for silver solutions (chloride solutions, too).

Silver is never plated directly onto aluminum (other than evaporation in vacuum & similar PVD processes).

Silver can be plated onto anodized aluminum, but this is rarely done. Metalast has a process for impregnating silver into the anodize porosity for the sake of conductivity.

Silver is typically plated onto either an electroless nickel or cyanide coppper strike (after zincating the aluminum). Nickel is preferred for corrosion resistance and to act as a diffusion barrier if heated. Additional nickel plating if necessary, after the strike plating.
My guess: Any silver formed disappeared as white corrosion (silver plating is very thin, anyway, unless more is paid for), and the corrosion resistance is from an electroless nickel layer.

 

[jtowne]

I had an energy dispersive spectroscopy analysis of the white powder residue on the screw done and it indicated the presence of Al, O, and Cl. Their conclusion was that it was a chlorine/chloride induced corrosion residue.

The aluminum is struck/flashed with copper prior to silver plating. I do not believe that the plating extends to the bottom of the screw holes. So there is the possibility of exposed aluminum and copper in the threaded hole. Since there was no Ag in the analysis of the white powder I presume that there was none in the hole.

If I use a cadmium plated screw will the aluminum not give up its ions as readily? What about using an "active" stainless such as a 304?

 

[kenvlach]

jtowne,
The SS caused galvanic corrosion of the Al is presence of electrolyte (salt solution). Normal behavior.

1) Very difficult to electroplate into blind holes (if large enough diameter, an auxiliary anode can be fitted down the center, but laborious & danger of shorting out).
2) Copper causes severe galvanic corrosion of aluminum.
3) Electroless nickel doesn't need current flow, so can work in blind holes provided that there is circulation of the plating fluid w/o trapping gas bubbles).
4) Electroless nickel is compatible with SS.
5) You cannot simply choose 'active 304;' you don't want active anyway, especially not fasteners which have a small surface area relative to whatever is more cathodic.
6) Although you can passivate SS before using, the passive oxide film is extremely thin (see-through) and is disrupted during tightening of threads.

Possible solutions:
Use 6061-T6 aluminum fasteners -- may require a larger diameter. Don't use 2024. Although stronger, the Cu in 2024 causes galvanic corrosion of both the 2024 & any nearby aluminum.

Get the aluminum chromate conversion coated, both the housing & fasteners if used.

Electroless nickel plate the 6061 casing; communicating the requirements with the plater.

 

[EdStainless]

Other thoughts:
1. Watch the hole/fastener geometry to minimize the risk of water being trapped.
2. Move to a fastener alloy that is closer in galvanic potential to the Al. There are a number of brass on bronze alloys that might be close enough to hurt less.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[jtowne]

Thanks to all who have contributed to my understanding of this phenomenon. Just to qualify my efforts, I did refer to my Mat Sci text book and it did help but not to the level that I needed. I've also used the web to do as much research as possible before posting.

So, my understanding is this; If water is present in an aluminum/stainless connection, there will be corrosion. The only way to prevent this short of sacrificial anodes or impressed voltage, is to keep out the moisture.