galvanic corrosion: C937 Bronze/Stainless Steel 1

[LanaCarrie]

I have been asked if stainless steel bolts can be used to fasten together a two part C937 Bronze (80%Cu, 10%Pb, 10%Sn) sealing ring without risking galvanic corrosion (water (80 deg C) on one side, hydrocarbon vapour on the other side). I've tried to consult the galvanic serious but find myself confused.

Stainless steels are listed in their active and passive forms. I realize that the passive form indicates a chrome-oxide coating. Can it be assumed that a stainless screw would be passive? Do you consider a material active only when it is being eroded causing removal of the passive layer?

Does it make sense to use stainless steel fasteners for bronze material? Any advice would be helpful as my knowlege of galvanic corrosion is a bit lacking. Thank you.

 

[Metalguy]

Your understanding of active/passive is fine for this case. And no, you shouldn't assume any SS bolts are passive.

Can we assume the bolts get wet? How pure is the water? Can't you find bronze bolts?

 

[LanaCarrie]

The bolts will definitely get wet. The water isn't pure. It can have a chloride content of up to about 720 ppm (pH is also around 9.7 for what that's worth).

I believe the thought was to go for stainless bolts out of convenience but I can look into it and see if we can get some bronze bolts in.

How do I determine if the bolts will be active or passive? My thoughts are that if they are passive then I should be fine - bronze will be the anode and will be a lot bigger than the cathode so it shouldn't cause any problems. But, if the bolts are active (still trying to figure out what kind of stainless they want to use by the way) then I may have a tiny anode and large cathode which in my mind wouldn't be a great idea.

Thanks for the help, I look forward to hearing more.

 

[Metalguy]

You're correct about the anode/cathode areas-the thing is to make REAL sure the bolts do not become the anodes!

Given your temps. and Cl level, I'd be concerned with the usual 304/316 bolts, especially when you consider the crevices. McGuire will probably jump in here, but I'd say use 2205 SS as a minimum if you can't find bronze bolts. Do NOT use common brass bolts-they will be the anodes against your bronze.

 

[TVP]

I agree with Metalguy-- 304 is not suitable, and 316 will be iffy (nice scientific word there) for crevice corrosion. I am not sure which of the more corrosion resistant grades are easiest to obtain for fasteners, but 2205 is a duplex alloy that is used for high chloride environments. 317 and 904L are austenitic grades that are suitable, due to the addition of more Mo.

 

[TVP]

I did some more investigation, and it looks like 2205 and 254 SMO are the most commonly available grades for high pitting resistance. 2205 is also known as UNS S31803, 1.4462 (EN & DIN). 254 SMO is a proprietary grade from AvestaPolarit, that has the EN designation 1.4547. It is listed in ASTM A 193 as B8MLCuN, and is extremely resistant to pitting in chloride environments. One supplier of these fasteners is Bulten:

http://www.bulten-stainless.se/eng/index.htm

 

[LanaCarrie]

Thank you your advice everyone (especially for the details you gave TVP). I love this site! Everyone is always so helpful.

Thanks again.

 

[kenvlach]

Metalguy and TVP: why not silicon bronze bolts? Close in the galvanic series to C937 bronze, readily available, and the price is ~1/4 that of 2205 SS.
Ken

 

[Metalguy]

Yes, silicon bronze would probably be OK, but Al bronze is stronger and has quite a bit more corrosion resistance.

 

[TVP]

Ken,

Copper-based alloys are not really my bag...I thought I would let others comment upon suitable bronze alloys for this application.

 

[unclesyd]

Silicon bronze is a lot cheaper and a readily available.

Look at thread 124-71885 concerning Si-Br fasteners.

 

[LanaCarrie]

Thanks everyone's help so far - I recommended to the mechanical engineer that we try to use bronze bolts. I'm waiting to hear back regarding the strength requirements for the bolts to see if/what bronze fasteners might be acceptable. If we can't go with bronze, I think the stainless types TVP mentioned would be a good fit.

On a side note: Metalguy - in your first reply you mentioned that you should never assume SS bolts are passive. I'm guessing you would consider them active if corrosion or erosion is destroying the passive layer - is this correct? Are there any other times that you would assume a metal is active?

 

[Metalguy]

As you're aware, it's the presence of an ultra thin oxide layer (mostly Cr oxide) that gives SS its corr. resistance. Both the chemical resistance and mech. props. of the oxide are important. When we have something like a bolt, the oxide layer can easily be damaged by abrasion or by cracking from being stretched. So what we're really concerned with is the ability of the alloy to reform a good oxide layer when it gets wet (repassivation). 304 will repassivate nicely in fairly pure water with some O, but ions like F and Cl are very effective in preventing this. 316 does a bit better when Cl ions are present, even tho the Cr is less. For some reason (means I don't know why<g>), Mo is VERY effective at resisting the efforts of Cl.

So, the bottom line is to always assume that something like bolting is active, and make sure it will readily repassivate during use. Crevices are especially bad for 304/316, because they concentrate ions, esp. Cl to far higher levels than the bulk fluid.