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mshimko (Materials) (OP)
8 Feb 06 12:28
I'm investigating leaks caused by pitting on a fuel system constructed from 304L stainless steel using 308L electrodes.  

The system occasionally sees dirty “river” water, occasionally treated fresh water, and may even have fuel contaminated by salt water.   The result has been pitting and leaks (also - I’m sure MIC has a role in this).

I’m considering recommending a higher Mo content wire (such as 316L wire) for repairs and new systems.  Higher Mo should provide a slight increase in pitting resistance.

My question is:  will this increase in Mo also provide greater general corrosion resistance, resulting in the welds being slightly more noble than the base metal, and thereby providing some level of cathodic protection to the welds?
metengr (Materials)
8 Feb 06 16:43
mshimko;
You need to confirm if you have MIC, and where the leaks are occurring (HAZ, weld deposit, etc) in the fuel line. If MIC and general pitting attack is confirmed, your suggested filler metal of 316L will provide no added benefit. Also, having the remainder of the fuel line as 304L is still the weak link in terms of lower pitting resistance should you have similar excursions with contaminated fuel. The Mo will aid in improved pitting resistance but will have no benefit in terms of cathodic protection or increase in general corrosion resistance against the 304L fuel line.

mshimko (Materials) (OP)
8 Feb 06 17:35
Thus far I've a clear "lack of real data".  

From the limited info I've obtained, and ONE instance of examination (one section was cut out sent to a lab for examination), the pitting occurs in the weld deposit of butt welds and belled end fitting welds.  The one sample sent to the lab revealed little (nothing abnormal noted, and the lab didn't check and made no determination of MIC).
 
The "weak link" is clearly the weld; and if the attack is simple pitting in the weld, then I would expect 316L wire to provide an improvement.

The cathodic protection I was refering to was the bulk of the piping being slightly less noble than the weld;  hence a slight galvanic cell and CP of the weld (at least in theory, that is).
metengr (Materials)
8 Feb 06 17:51
Based on your additional information, you certainly can try using the 316L filler metal. A word of caution is that you can get microsegration and pitting problems with the austenitic stainless filler metals like 308L and even 316L. I know that some have even used an over-alloyed filler metal like 309MoL.
kenvlach (Materials)
8 Feb 06 17:57
mshimko,
Anneal the welds to minimize residual tensile stress, then passivate.

An easy test for MIC is to put a few drops of concentrated HCl on the corrosion.  An H2S smell is a strong indication of MIC.
EdStainless (Materials)
8 Feb 06 20:02
If the corrosion is HAZ or oxide realted then I lean toward the surface oxidation and MIC mechanism.
I assume that annealing and pickling is out since these are field welds.
If the pitting is actually in the welds that is different.
I'll go out on a limb here and suggest that you don't try welding 304 with 316, or even 316 with 317.  The segrigation in these alloys is severe.  Instead look at using a duplex.  even the lean alloys have better pitting resistance than 304 and the welds don't have as much segragation.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

mshimko (Materials) (OP)
9 Feb 06 6:22
Ed - you're correct that post weld heat treatment is not an option as at least a significant number of the welds are field welds.

When you suggested looking at a duplex, were you refering to a duplex weld wire, or duplex piping as well?   

"Upgrading" the weld wire only is obviously a much more cost efficient solution - IF it provides any benefit.  A change to duplex is possible, but it would obviously be costly, and I'm not sure how much experience the shipyard has with welding duplex piping.

EdStainless (Materials)
9 Feb 06 15:35
Welding duplex is no more difficult than welding 316 correctly.
I was thinking of using a lean duplex alloy for everything.  They aren't that pricey, and if you reduce the wall thickness you can easily get to a breakeven.
I watch people use semi-automatic orbital welding on sch5 pipe and 0.065" wall tube all of the time.

The problem with using overalloyed filler on piping is that it is really the root of hte weld that you care about, not the filled portion.  You could use weld rings or inserts, but they are expensive and you need to inspect every weld to make sure that there are not crevices from under consumed inserts.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

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