Compatibility of stainless steel with aluminium alloy 3

[Pras]

In one of the applications, we need to fit stainless steel (SS304 grade)threaded caps on a transmitter housing made up of aluminium alloy (GD AlZn10Si8Mg). The transmitter housing is meant for housing electronic modules and expected to withstand temp upto 75 degC. The housing is not expected to be exposed to severe corrosion atmosphere viz. offshore conditions. However we would like to check on the aspect of "Two Metal Corrosion". Any comments on this will be useful.

Regards,

pras

 

[bilge]

The free corrosion potential of aluminum alloys in sea water runs from - 0.75 to -1.0 volts (sce). That of 304, from -0.05 to -0.10 volts (sce). That is a very large differance. You might be asking for trouble by coupling them --- particularly if there is an opportunity for crevice corrosion to occur.

 

[TVP]

Even if these two metals are not exposed to a marine environment, normal humidity can produce enough electrolyte at the interface of the parts to produce a relatively severe galvanic corrosion effect. I would recommend a thorough evaluation of the potential effects of corrosion before approving the use of these two metals. Galvanic corrosion can be minimized by separating the two metals by a non-conductor, if necessary. Coating the stainless steel (instead of the aluminum part) may be an option-- any disruption in the coating would present a more favorable anode to cathode ratio. Overall, this is a suspect couple, and should be used only with an appropriate understanding of the operating conditions.

 

[rustbuster1]

In atmospheric service - occasional rain, etc. - this will pose no problem, although over time the aluminum adjacent to the SS will corrode slightly more. Both materials form self-protective oxide films on their surface and the galvanic currents will be minimal. Immersed or with high times of wetness, there will be a whole different story - the Al will try to protect the SS and the resulting corrosion effects on the Al housing will depend on the area ratio between the two. Small SS and big Al will mean slower Al corrosion, and so on.

Now for the main point of my reply - WHY DO YOU USE A SS CAP? If the housing can be Aluminum why can not the cap also be Aluminum?

 

[CoryPad]

I would like to simultaneously agree with TVP and rustbuster1. This is a suspect pair if the conditions are severe, especially if there is high mechanical loading. However, stainless steel can be coupled with aluminium in non-severe conditions. An example is stainless steel fasteners for aluminium poles for traffic and lighting applications: large anode/cathode ratio, non-severe environment.