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Up Topic Welding Industry / Technical Discussions / Corrosion after pickling
- - By timczx6 Date 10-30-2007 10:56
We recently did a stainless steel pipe spool job made of 316L sched 10 pipe ranging in size from 8" to 12".  The spool flanges were 304L with backing rings. We decided to get the spools bath pickled by a professional company.  The company was aware that the pipes were 316L and the "loose" flanges were 304L so I am assuerd that the pickling conditions were suitable.  After pickling, the pipes came out very clean but the flanges were severely corroded with loss of thickness of up to about 2mm.  The pickling company said the flanges must be something other than 304L but a test with a spectrograph confirms it is 304L.  I understand that 316L is more corrosion resistant than 304L but if the pickling conditions were set for 304L why has this happened. I am wondering if anybody else has come across this problem?
Parent - - By G.S.Crisi (****) Date 10-30-2007 13:09 Edited 10-30-2007 19:47
2 mm of corrosion? Good heavens! That's a lot of stuff!
1st possibility among many. The flanges were not 304L. I suggest you to run a chemical analysis in addition to the spectrography you've already done.
2nd possiblity. The pickling company forgot that the flanges were 304L and prepared the solution for 316L.
Stress corrosion perhaps? I don't believe so.
Anyone has another explanation?
Giovanni S. Crisi
Sao Paulo - Brazil
Parent - By RANDER (***) Date 10-30-2007 13:35
Mr. Chuck Meadows leaves quite a hole to be filled don't you think?
Parent - - By timczx6 Date 10-30-2007 19:52
A sample flange has been sent for chemical analysis and we are waiting for a result.
The 2nd possiibility is possible but the pickling company assures us it is not the case... Still possible I think.
Lots of theorys have been floating around our work place. One is that this similar to the sacrificing blocks on the hull of a ship.
Parent - - By G.S.Crisi (****) Date 10-30-2007 19:59
Sacrificing blocks (or anodes) belong to a cathodic protection system. I can't see how your case compares with cathodic protection. I'd be glad if someone explains it to me.
Giovanni S. Crisi
Parent - - By DaveBoyer (*****) Date 10-31-2007 03:06
Giovanni: The 304 is less noble than the 316. The less noble material always gets disolved the fastest. In a situation where they were immersed in a week electrolite without being hooked to a current what would happen is the 304 would be sacrificed as the anode and therby protect the 316 as the cathode. In the case of the pickling, they were immersed in acid and possibly an electric current applied [electropolishing] either of these would accelerate the material removal. In this case a posibility is that the materials were left in the process way too long. Another posibility is that if an electrical process was used the flanges were close to the cathode and took the brunt of the action, while nobody realised what was happening.
Parent - - By G.S.Crisi (****) Date 10-31-2007 22:50
Thank you very much for the clear explanation, Dave.
Now, if things happened as you said, then the fault is on the so called "professional company" that was appointed to carry out the pickling.
Giovanni S. Crisi
Parent - By DaveBoyer (*****) Date 11-01-2007 03:33
Yes, it is My asumption that there was a process problem of some sort that someone should have picked up on before wrecking the part.
Parent - - By timczx6 Date 11-01-2007 11:07
Chemical analysis of sample flange is as follows:

Fe     C        Mn     Si       S        P      Ni     Cr     Mo     Cu     Nb     Ti
Bal.   0.07    1.15   0.73   0.01   0.03   8.1   18.1   0.14   0.23   0.02   0.02

I'm told this is consistant with 304.

Thanks for the comments.
Parent - - By js55 (*****) Date 11-01-2007 17:10
Well, its not an L Grade thats for sure.
I think Dave is on to something with perhaps too much time in the bath, or some other control issue.
Giovanni is right 2mm for a pickling bath is extreme.
Parent - By timczx6 Date 11-01-2007 22:41
'L' was a typo.    Bad habbit.
Parent - - By G.S.Crisi (****) Date 11-01-2007 19:52
Yes, it is 304 stainless steel, although, as js55 pointed out, the carbon content is somewhat high for an L grade.
In any case, this is not the reason for a corrosion of 2 mm.
I'd say that the reasons explained by Dave Boyer should be further investigated.
By the way, and this has nothing to do with corrosion. When I was a child, many and many years ago, there was a famous movie star called Charles Boyer. Anything in common?
Giovanni S. Crisi
Parent - - By js55 (*****) Date 11-01-2007 21:22
Every little breeze, Heh Giovanni?  :>)
Parent - - By G.S.Crisi (****) Date 11-01-2007 21:50
js55,
as English is not my mother language, sometimes I don't understand some popular (or slang) expressions.
What does "little breeze" mean? A 10 miles per hour breeze?
Giovanni
Parent - By TRC (***) Date 11-01-2007 22:03
Humor does'nt have a language barrier, your very funny Giovanni!@#$%$
Parent - By js55 (*****) Date 11-02-2007 13:21
Giovanni,
Just in case.
A reference to what I believe is Boyer's most famous song.
Parent - By DaveBoyer (*****) Date 11-02-2007 03:44
The actor may be a distant [really distant] relative. My Sister is interested in the family geneology and never mentioned a connection. We are Pensylvania Dutch, We pronounce the "r" on the end.
Up Topic Welding Industry / Technical Discussions / Corrosion after pickling

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