Stainless/Aluminum Corrosion

[vping]

First post here. I am the "engineer" and an architectural sign company. Here is my problem.

We have 18ga 304 stainless steel hollow fabricated letters mounted to a 6063 .25" Aluminum bar with Stainless fasteners. I'm less concerned about the corrosion at the point of connection. This is mounted to the top of a slight overhang above a door and is up against a brick facade.

The problem is that the letters started to discolor.

I claim that the brick above was washed (as it looks in pictures) with "something" that rained down on the letters and caused them to stain. The staining looks like hard water marks and a bit of rust. The client claims that no cleaning of the bricks has occured and that the chemical reaction was caused the by a reaction of disimilar metals. I'll agree that at the point of connection there is some oxidization but these letters are 12" high and there is corrosion on top of these letters.

We've reinstalled a new set of letters in the same manner and are now playing a waiting game.

Any ideas as to if the letters could have been stained 12" away from the contact point - by a metallic reaction?

 

[CoryPad]

The staining likely is due to iron embedded in the stainless surface from processing with iron tools, dies, etc. This is a common problem. This is not due to contact with the Al - the Al will corrode first in that couple.

 

[Ron]

CoryPad...I agree that aluminum will corrode first, but it will then passivate very rapidly and the corrosion will reverse. Also agree that it probably has nothing to do with the observed staining.

Like something more uniform, such as environmental or the noted washdown.

 

[CoryPad]

If the stainless corrodes it has nothing to do with galvanic corrosion.

 

[SMF1964]

I assume this is outside? If so, then I'm thinking that an alternative to CoryPad's thought (embedded iron) that you're dealing with water coming off of the bricks during a rain storm and carrying with it material from the bricks, mortar, etc., onto the stainless steel.

CoryPad: would powerwashing remove the embedded iron?

Your galvanic corrosion will be occurring at the contact points between the aluminum and the stainless steel bolts, not at some distant point removed from that contact location on the face of the letters.

 

[CoryPad]

Some iron contamination can be on the surface and can be removed by pressurized water. Or, iron contamination can be embedded and would need to be removed by more agressive methods, such as acid pickling.

 

[bithkits]

Is electroplating after manufacture not an option?
Or paint/varnish?

I agree with the "bricks washing contaminants onto the letters" theory...

Some types of bricks can contain all sorts of metallic materials

Adriaan.
I am an Engineer/part time student (Mechatronics) from South Africa.
Advice from lecturer: "Be warned - when you go into industry your boss will give you a thousand things to do and he wants them done yesterday!" So far he is right...

 

[strider6]

I agree that this is not a problem of galvanic corrosion even if the possibility when coupling SS to Al shall be considered.
One possibility i see is that the SS is exposed to a marine atmosphere with high chloride level, in this case SS 304 is not the best material selection and you shloud consider a SS 316 wich as an higher corrosion resistance vs chloride

http://www.stainlessarchitecture.org/index.cfm/ci_id/17527/la_id/1.htm

http://www.imoa.info/moly_uses/moly_grade_stainless_steels_backup/architecture/selection_system.html

http://www.imoa.info/_files/pdf/folder_which_stainless_steel_06.pdf

http://www.corrosionist.com/Galvanic_Corrosion.htm

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Corrosion & Rust Prevention Control