Galvanic Corrosion of dissimilar metals 1

[Vesselsntanks]

This will probably come off as a very generic and broad question so I can expect broad answers. I am quite new at this and learning via mentoring and trying to get as much field experience as possible. If I wanted to use a spool piece of stainless steel, approximately 5 feet, in a carbon steel line, approximately 100 ft., can I expect a lot of galvanic corrosion at the interfaces of the carbon steel and stainless steel? The setup will be like this: 100 feet of carbon steel piping - flanged connection to stainless steel - 5 ft of stainless steel - connected to the side of a large carbon steel column. Piping size is 3". Caustic soda(30%) is being added into the process stream(3/4" line injected into aforementioned 4" line). There have been corrosion/leakage issues downstream of the injection point hence the need to replace this portion of piping. Any issues you guys can think of? I figured with the large ratio of anodic-to-cathodic area, it should not be a big issue.

 

[Doove117]

If you're using flanged joints then ensure the metals are electrically isolated; this is usual way. Nonconductive gaskets, nonconductive 'isolating' washers under the nuts or metal washers.
No electrical contact no galvanic corrosion.
Area effect can be considered (often hear the example that you can use stainless bolts in large galv pieces but not galv bolts in large stainless pieces) but the reality is you would (as I think you suspect) probably not get a text book perfect spread of the electrolytic action.
Welded austenitic (304, 316 etc) stainless pipe will more than likely have chromium depleted or ferritic zones in the heat affected zones around the welds which may make them likely sites. Make sure your pipe fabricator knows what he's doing.

 

[SJones]

API RP 571, 4.3.1

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[Vesselsntanks]

Thanks for the responses. I will consult 571 as well.