Corrosion question : Is it ok to use Stainless bolts in Aluminium plate?

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I am having a tank inspection cover made up from 10mm aluminium.. The cover consists of an inner plate and an outer plate.. The inner plate will be drilled and tapped for M8 bolts at 50mm centers.. I will bolt through the outer plate and plastic tank into the inner plate with stainless bolts..

Question is will using stainless bolts into the ali plate cause a galvanic corrosion problem even though the tank is plastic and there is no conductive connection to anything else?

Thanks..
 
Yes.

Why don't you an inner stainless plate with the stainless set screws from inside the tank then use a plastic cover with either a stainless outer ring to evenly compress the plastic.

That's how I have done it on my water, waste and shower drain tanks.
 
Hmmm.. Bit late, the plates are already being made..

What about aluminium bolts?
They would solve the corrosion problem but would they be strong enough?

I suppose they would be the same strength as the plate so should be strong enough..
 
I think that I'd stay away from the idea of aluminium screws into aluminium tapped holes.

It doesn't take much for ali threads to bind and then seize permanently. You can use various non seizing compounds but just a small bite soon ends up as a solid lump. Also, ali screws are not as strong as you are used to, and are very easily damaged or stretched by over tightening.

I would stick with SS screws, thread lube and probably nylon washers.

However If you can use threaded studs loctited into the inner plate, you could use nylon washers under nyloc nuts on the outside.
 
I did almost exactly the same 3 years ago for a polyethylene diesel tank: S/S screws through alloy plate and no sign of corrosion so far with no particular attention to corrosion prevention, just attention to sealing threads and plate with hylomar/hylotyte. This was though in a dry yacht interior. Could be different if a water tank or if exposed to water.
 
I agree with Rogershaw using the plates you have had made, screw the bolts fully through the inner flanges tapped holes and fit inside the tank with the bolt threads facing out. Fit the outer flange using nuts. The ally thread will only be used to stop the bolt from turning. I would use stainless steel nuts and bolts.
 
The tank is a petrol tank and from what I have briefly read petrol in not very conductive so in theory anyway that should limit the galvanic corrosion right?

Hylotyte seems to be a good fit with petrol resistance and it's a galvanic corrosion inhibitor.. Might go that way..

The nylon washer suggestion is also interesting.. How well do nylon washers stand up to petrol?
 
The only corrosion you risk is from atmospheric moisture. There are thousands (millions?) of aluminium masts and fittings held together with stainless bolts that do not suffer corrosion. Provided they are not immersed in water there can be no corrosion. In your case for a petrol tank just ensure that a decent sealant is used to exclude water and all will be well.
 
Not a fan of aluminium in any application! Experience says it just goes nasty quite quickly. Reinforced my forestay once with an aluminium plate, and yes it lasted couple of years but wasn't in a great condition when replaced. The stainless bolts were fine as was the surrounding of the Alu plate - no worse than the centre.

Masts seem fine with it in the open air.

Mind you 10mm sounds substantial.

And a Lofrans windlass suffered on the Alu parts - no expert, just went off Aluminium generally.
 
There are lots of different Aluminium alloys. If there is a risk of salt water exposure you should be using 5083 alloy. In plate form it's used for ship superstructures. I would use 316 s/s bolts with zinc chromate paste on the threads.
Sailorbaz
 
JSYmartini said:
It'll be fine, as someone already said, buy a tube of Tef-gel and apply to the threads.
Click to expand...

sailorbaz said:
There are lots of different Aluminium alloys. If there is a risk of salt water exposure you should be using 5083 alloy. In plate form it's used for ship superstructures. I would use 316 s/s bolts with zinc chromate paste on the threads.
Sailorbaz
Click to expand...

Tef-Gel is PTFE based and works by sealing the joint between the components against water ingress. It is probably a the better choice where the joint is a neat fit with no large gaps

Not sure if zinc chromate past is available. The well known Duralac paste is barium chromate.

Duralac is probably better than Tel-gel where the joint is not a neat, tight fit.

Duralac is very messy to use.
 
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