On Galvanic corrosion between stainless steel and aluminium 1

[jorgefherrero]

I have a question regarding corrosion because of galvanic coupling.

I am designing a plant where there is a water tank in Stainless steel 316 with a 6" flanged opening on the top. I am planning to mount an aluminium element on that flange. Thus there will be two flanges put together, one in SS, and the other one in aluminium. Bolts, washers and nuts will be in SS also.

My question is: Is there a risk of galvanic corrosion? I suppose stainless steel would "promote" the corrosion of aluminium, but since this metal is by nature passivated I think there should not be any problem. Anyway, the bolts will not suffer.

Is it right?

 

[CoryPad]

There is a risk of galvanic corrosion. How much corrosion occurs will vary on the solution (contents, concentration), temperature, duration, and many other factors. Will the aluminium part have a coating? What alloy? Any electrolyte exposure on the outside of the parts?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strider6]

As Cory said there is possibility of Galvanic Corrosion. The amount of Corrosion is a function of the all the factors stated above and also of the relative areas of anode and catode.
If you want to avoid galvanic corrosion the main possibilities are:

- install dielectric insulation between the flanges and on the bolts
- install a dielectric spool of pipe (it could be an internally lined pipe)
- paint the cathode of the galvanic couple (in this case the SS)

S

http://www.corrosionist.com

 

[jorgefherrero]

I do not know which aluminium alloy it is. In principle, it should be a rather "normal" one. It is not coated.
There will be no water flowing through those elements, only rain at the outside.
One solution may be to put ceramic washers to avoid direct contact between the two metals. But maybe it is unnecessary, since aluminium is already covered by a aluminium oxide layer in natural state. This is what I wanted to confirm.
Regards

 

[chrysa1975]

Flanged joints of dissimilar metals, where potential galvanic corrosion exists, are usually made with an isolation gasket to prevent galvanic corrosion. Insulation kits, like PIKOTEK or equal, include insulating gasket, insulating sleeves and washers and steel back-up washers. Special care shall be taken for the bolting torque in the aluminium flange.

 

[strider6]

jorge

There are several type of Aluminum Alloys for different application, "in general" the Aluminum is protected by an oxide layer but this is a general rule. The behaviour of your aluminum alloy depends on its composition, there are some series like 5000 if a remember well, which are used in marine atmosphere for example.
In your case i don't know if it's a marine exposure but as you said it's expodes to the atmosphere so you will have an electrolyte and two dissimilar material, and your Al will act as an anode and corrode.
Look at this example from NASA:
"For example, when aluminum alloys or magnesium alloys are in contact with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the corrosion of the aluminum or magnesium. This can be seen on the photo above where the aluminum helicopter blade has corroded near where it was in contact with a steel counterbalance."

http://corrosion.ksc.nasa.gov/galcorr.htm

S.

http://www.corrosionist.com

 

[jorgefherrero]

It is not a marine environment. Anyway, it may be safer to put that component in Stainless Steel too. Thanks everybody!