Welding two different SS in SW Piping

[nickpa]

Hi,

I am replacing service water (lake water) piping at a power plant and I had a question in regards to welding two different medals. I am told that welding AL-6XN high alloy SS (20.5% Cr, 24% Ni, 6.3% Mo)to a 316 SS flange is still resistant to Microbiological corrosion (MIC). I don't see how this is possible since AL-6XN has higher concentrations of certain elements; however, I am told that since it is welded to a 316 SS flange and not a 316 SS pipe the water will not be able to developing pitting on the pipe. Let me know your opinions on this issue.

Nick

 

[BigInch]

There is no 316 pipe at all. Only a flange. It doesn't sound so good at the weld though.

Independent events are seldomly independent.

 

[SJones]

Interesting concept. It would appear that somebody may be under the impression that the 316L will be a sacrificial anode for the 6Mo and protect it. The 316L would have to be in the active state to do that and the flange would be toast in no time. The pipe is more resistant to pitting by virtue of its much higher molybdenum content. Bimetallic corrosion would not be expected unless a nickel alloy filler material is used in the weld. Introducing MIC is a bit of a red herring, as it will occur on both materials; neither of them have biocidal properties.

Steve Jones
Materials & Corrosion Engineer

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

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

 

[SJones]

Oh! Not to forget intergranular corrosion on the flange as it is 316, rather than 316L.

Steve Jones
Materials & Corrosion Engineer

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

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

 

[MJCronin]

Why does the joint have to be welded ?

Is it possible to make this a flanged joint at the material change and use two different flange types ?

An insulating flange kit could also be used for those management prisses afraid of stray currents or galvanic corrosion...

 

[BigInch]

Isn't he welding the neck of the flange.

Independent events are seldomly independent.

 

[racookpe1978]

We are all thinking of welded joints above.

What if he calls for a lap joint flange at each joint: Then the water will "see": inside of pipe material | gasket material | inside of pipe material and never the 316 (or any other metal for that matter) flange material.

Also, the joints will be easier to remove for mussel cleaning and replacement.