Field welding 316 and passivation

[ATSE]

Service condition:
1. Low pressure air pipe (10 psi to 30 psi), normal humidity
2. 8" to 16" diameter pipe
3. 316 and 316L
4. Some field welded joints and some shop welded joints (bends and bolt-up flanges)
5. In the close vicinity of municipal wastewater, but not submerged and not in the splash zone.

Our specification requires pickling and passivation for shop-welded components.
Questions:
1. For the field welded joints, is cleaning and degreasing followed by citric acid wash sufficient for the given environment?
2. Does it make sense to require pickling of shop fabricated components (half of the contract) when field-welded components do not require pickling? Is pickling even necessary, when passivation alone may work just fine?
3. How much does pickling add to the total fabricated cost of SS 316 air pipe?

 

[SJones]

Assuming that you mean internal pickling and passivation:

1. If your back purge has been up to scratch, it should be
2. Not really if system cleanliness is the target. It's always good to avoid pickling if possible because of the HSE concerns.
3. Can't help with that one.

Steve Jones
Materials & Corrosion Engineer

http://www.oilandgaspeople.com/cv/11499664

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

 

[EdStainless]

Passivation only is fine if the welds are colorless. If they are heat tinted then pickling is the only way to really restore corrosion resistance.
You could mechanically polish (no wire brushes, no SiC abrasives) and then passivate. But most people don't remove enough metal to make it work.

The cost of pickling depends on how you are set up to handle it. In most jobs that I have seen it is an insignificant cost.

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Plymouth Tube

 

[ATSE]

Gentlemen, thank you for the replies.
Dumb question #1: Does the use of a flapper wheel (say, aluminum oxide) qualify under mechanically polish?
Dumb question #2: How is pickling addressed when field welding is required (in California)? Seems like chemical containment associated with pickling is not practical in an open environment (or in a small job-shop).

 

[EdStainless]

Dumb answers.
1. I have never seen anyone actually remove metal with a flap wheel. They just polish the heat tint.
2. You pickle with paste and the wash down with a neutralizing solution. It is easier to get your argon purge correct and not have any heat tint in the first place.

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Plymouth Tube

 

[stanier]

In this engineering world you have time, cost and quality. You cannot have all three. If you are avoiding pickling and passivating field welds because of "cost" your quality will suffer.

Suggest you look at the failures on the Gold Coast Desalination Plant ad the problems they had for not doing what was required

See

http://www.nidi.org.