Stainless Steel Bolts & Mild Steel Potential Corrosion

[fetier]

Hi,
We are working in a water treatment plant and will be installing stainless steel bolts in a piping system. The piping system will consist of mild steel flanges connecting with ductile iron body butterfly valves. Piping is located indoors in northern Texas so while subject to humidity it will not be subject to any salt spray. Can anyone see a reason Stainless Steel bolts would need to be isolated from the mild steel/Ductile Iron?

 

[DAVIDSTECKER]

If are using SSTL nuts and bolts I'd HIGHLY recommend using anti-seize to keep the threads from galling up.
And as long as you have the anti-seize out it may not be a bad idea to put some between the fasteners and the steel and iron to at least reduce the SSTL being contaminated with iron which could cause the appearance of the SSTL rusting.

 

[Metalero]

I see no reason for the use of stainless steel. I even see the possibility of forming a galvanic couple that can activate corrosion of carbon steel and iron in contact or vicinity of the bolts and nuts.

 

[metengr]

I agree there is no reason for stainless steel bolting in this application. I would use carbon steel bolts.

 

[Tmoose]

Hi Fetier,

Why are you changing to SS hardware ?

Difficulties with removal for service, perhaps ?
Or Spiffy-ing up the station for public tours?

regards,

Dan T

 

[fetier]

Water District specifically wants Stainless Steel.

 

[Metalero]

If you can't reverse that situation, thus take care to minimize the risk of galvanic corrosion, if you can try to avoid direct contact between both metals, without compromise de stiffness and secuurity of the joint of course.

Here some guidelines that i found on the web.

http://www.anzor.com.au/blog/the-dos-and-donts-of-using-stainless-steel-fasteners/

 

[fetier]

Thanks for that link. One thing that keeps coming up in my research is the presence of salt water to trigger the galvanic corrosion. In an environment with no salt water(500+ miles from coast) is this corrosion still a threat. If so are we talking about a reduced service life of more than 50%?

 

[EdStainless]

We always used insulation pads under the washers and bolt heads.
If this gets wet it will bleed rust all over and you will have accelerated galvanic attack there which can result in loss of tightness.
You need bolts with rolled threads and you need a good anti-seize compound designed for Stainless Steel.
I am partial to the ones with Cu in them, but there are many that work.
And pray that it is rare that these will be taken apart.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

What are the markings on the bolts in place now?

I'd be checking bolt strength requirement for the flange class of piping. Some SS bolts are pretty cheesy, with yield strengths a lower percentage of UTS than "real" bolts.

 

[WKTaylor]

Are Your Mild-Steel flanges/bolt-hole 'coated' with good quality primer and sealing washers? IS SO, probably no problem...

IF NOT then CRES [nickel alloy] will be highly dissimilar to steel and initiate corrosion on a mass scale

Regards, Wil Taylor

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