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corrosion allowance for stainless steel?Helpful Member! 

Johnae (Mechanical)
23 Nov 06 11:17
Is there any specification for corrosion allowance for stainless steel? I think we don't need to give stainless steel corrosion allowance. Am I right?
moltenmetal (Chemical)
24 Nov 06 7:38
Corrosion allowance is an application-specific judgment call.  If you're using stainless steel in a situation where carbon steel would probably serve for 30 years with a 1/8" corrosion allowance, you may be able to get away with a zero corrosion allowance on stainless steel in that service- or not- depending on the specific details of the service!  If you're using the stainless steel in a situation where it will be subject to corrosion, you do need to spec a corrosion allowance on it.  
GL431 (Chemical)
24 Nov 06 11:38
We have also had a case in which no CA was specified for the stainless steel, but it turned out in operation that there is pitting attack due to chloride salts. The lifetime of the equipment is limited to 12 years by this corrosion.
Helpful Member!  EdStainless (Materials)
24 Nov 06 14:52
There is no sense in a CA if the attack will be pitting or crevice corrosion.  Added thickness when the failure is localized will not significantly extend service.  But it will raise the cost a lot.
I doubt that there is any environment that took 12 years to pit stainless.  My guess is that it worked fine for 11.75 years and then there was a process upset or chemistry change and the system failed in three months.

If you are in a chemically reducing environment and may see general corrosion, then yes a CA can be used.

In general, you select a stainless or Ni alloy grade with near zero corrosion in you situation and then you do not use a CA.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

GL431 (Chemical)
25 Nov 06 6:14
Ed, so what you are saying is that there is no "slow" pitting of stainless steel, but when there is pitting, it proceeds at a rapid rate?

The equipment I am talking about is the shell of a stainless steel reboiler on a condensate stabiliser on an oil field. Pitting corrosion was found upon an internal inspection after 7 years of operation (this was the first internal inspection). The pits were 2 mm deep. The postulated mechanism is that corrosive species including mineral salts concentrate themselves in the reboiler liquid circuit (the heavy ends in the circuit essentially circulate in a closed loop) and that these species cause the pitting. This is only a postulate still without proof. Based on the 2 mm pitting in 7 years, the expected lifetime was projected. Is this approach wrong?

Do you know of any method to detect active pitting non-intrusively? The reboiler surface is too hot for manual UT (187°C). Have acoustic emmision methods been used for such applications?
TomEun (Materials)
27 Nov 06 8:22
In general, corrosion resistance alloys (i.e. stainless steels, Ni alloys, etc.) do not require the corrosion allowance because they are usually selected under no-expected corrosion rate, less than 1 mpy.  However sometimes they may be exposed to not only pitting due to crevice corrosion or upset condition (i.e. once or twice a year with a short time and/or some corrosive trace elements) but also erosion due to some particles or severe turbulent flow.
In these cases, most users are applying some corrosion allowance for CRA, such as 1/16~1/8” (or 0.05~0.10mm) CA.

Thomas Eun
EdStainless (Materials)
27 Nov 06 12:47
GL, My experience is that pitting is fairly rapid.  I would suggest that your situation may be that you only have the local conditions to drive the pitting during some portion of your operation.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm

Jeffrey888 (Petroleum)
27 Nov 06 20:01
I have been reading your request.In our tests we took sulfuric acid to the boiling point with 316 and 316L.We lost 10.053 grams in 6 hours at 40% sulfuric.We have also surpassed 1000 degree F.I am new here so I am not sure if I am allowed to tell you the product we are using.We are NOT looking to sell anything,but would do a test for you at no charge.We are working in the Alberta oil patch and understand your situation.Please let me know if I can release the info.


Jeffrey C

0707 (Petroleum)
30 Nov 06 13:07
thread338-159732

Corrosion allowances for carbon steels are given by the project based on service conditions and expectable uniform corrosion rates.

For stainless steel designers follow the same criteria.

Some specify 0 mm, others 1mm and others 1.6mm.

See the thread338-159732 with some opinions on this subject

Regards

Luis

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