bolting aluminum to mild steel

[quiry13]

We are required to bolt a 1/2" aluminum flange to mild steel angle in an outside situtation in a mildly corrosive envirnment.

What would you recommend for gasketing:
- between the flanges
- under the bolt washers
- around the bolt shanks
The bolts are nominally loaded.
Thankyou

 

[0theblacksmith0]

Hi quiry13,
My suggestion would be to gasket the 2 flanges and use
stainless steel fasteners.Would it be practical to coat
the joint with marine grease or a suitable product?
Best of luck.
TWWeld.

 

[Raulz1]

In the old days we used to coat the areas with primer followed by laquer paint.Once fastened we painted it again and it worked well. Now we have epoxy. Used the same way should be terrific. Hope this works for you.
Raul

 

[JLAU]

Hi,

SS316 and aluminium work very well together without galvanic corrosion.
I would use e.g. a NBR rubber gasket to part both flanges and use SS316 fasteners without any coating.

Best of luck
JLAU

 

[butelja]

I have to STRONGLY DISAGREE with JLAU's comments. I have seen severe galvanic corrosion of aluminum piping stub ends that were bolted to 316 mating flanges. In piping work, a nonconductive gasket combined with nonconducting sleeves/washers for the bolts is required. This is known as a dielectric kit.

 

[quattro]

To Butelja:
You and JLAU are talking about two different applications.
In his case, I presume, relatively large aluminum parts are bolted with SS fasteners. Yours is an opposite case, SS parts are larger than aluminum parts. Aluminum is anodic and SS is cathodic. The larger the cathode, the more active galvanic reaction will become and, consequently, the corrosion process of the anode will go faster. In JLAU case, rusting of aluminum will be insignificant if any, because the S/steel fasteners (cathode) are much smaller.
Makes sense?

 

[spooky]

I have an application where two thick aluminum (6061) plates are being used to level a rail system inside a large vacuum chamber. I plan to use stainless steel fasteners (300 series). Will I need to plate the fasteners to prevent galling? I cannot use a thread lubricant inside the chamber.

 

[bogues]

I STRONGLY AGREE with Butelja Yes you may have a large anode however Aluminum quickly forms an oxide coating reducing the effective cathode area. The danger is that oxide removed in the bolt area (eg. by exposing scratching inside the bolt holes)will set up a very active corrosion cell (small anode). This is commenly seen in bottoms of aluminum boats where people have used S/steel bolts and the bolts float around an oversised corroded bolthole.

 

[sbc]

I strongly agree with butelja. I work on various
marine systems and know that aluminum will cause
electrolysis with ANY dissimilar metal.