I'd use galvanized steel fasteners.  Zinc will still protect scratched areas very well.  Coat them with something that will last a long time and forget 'em.

What type exposure are the bolts going to see, total immersion, splash zone, or external?  What are your raw water specifications?

If you have already decided on galvanized make sure you specify HDG (Hot dipped galvanized). They are two other galvanizing processes, electrolytic and mechanical, that will not be satisfactory in your service.

Thanks for your responses.
In relation to unclesyd the  bolts are going to be inside (along the wall) of the tank so at times will be fully immersed, and at other times not.There should be no water splashing upon them (I think I know what you mean by splash zone). The water is rainwater, which has relatively low-moderate pollution.

I would reconsider the stainless steel fasteners.  The company I'm working for uses SST fasteners to join painted carbon steel parts and we do not have any problems with corrosion of disimilar metals.  I should mention that the parts are in an enclosed environment with softened water sprayed on them intermittently.

If you do decide to use stainless steel fasteners make sure an anti-seize lubricant is used on the threads.

kris44,
Is this by any chance going to be cistern/tank for potable water?

This is a rainwater tank for potable water for domestic purposes, so the spraying of softened water is out of the question.

The reason I changed direction is that I checked a local fastener supply company for the presence of lead in their HDG bolts. It could be a problem depending on the origin of the fasteners.
You have to be carful of the materials that you put in the tanks.  Everything installed should be approved for potable water.  I will have to agree with “cooperjer” that one should reevaluate S/S fasteners unless you can verify the Hot Dipped Galvanize Fasteners are free of lead. I would check with the suppliers of the other galvanizes components also. Though it was used for years the limits of exposure to lead and lead containing products have been drastically lowered.  Lead is a tramp metal that comes with the zinc. To make lead free HDG fasteners is very expensive.  

Sorry for the timing
If you have any connections in the trades and warehouse in your area you can get S/S fasteners at reasonable prices.  
Watch your system and if anything starts to corrode change to out.

Another problem with galvanized parts is that they routinely are chromated, which adds hexavalent chromium.  This has been added to the list of bad metals (lead, mercury, cadmium).

Regards,

Cory

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