preferential weld corrosion

Date 02-27-2003 17:08

I can't answer your question but if you don't get a response here, you might try going on the NACE website (nace.org). They run a forum similar to this one but the focus is on corrosion issues and coatings.
CHGuilford

By Niekie3 (***)

Date 02-27-2003 18:41

Hi LTRN

The weld and HAZ is an area of high energy due to the residual stresses and meta-stable phases present. As such, one would typically expect to see excellerated corrosion in this area. Sometimes it is in the weld metal while at other times it is in the HAZ. Sometimes, for no particular reason, the weld and HAZ appears to be more resistant to the corrosion than the surrounding parent metal. This is however rare.

Generally a Post Weld Heat Treatment (PWHT) such as a stress relieve would reduce this tendancy to prefferential corrosion.

I can not say anything about your particular application, because you give very little detail, but if you are using C/Steel base metal it probably means that this is an adequate material in this application.

I once read a paper that suggested that the accelerated corrosion in the weld area was due to the dissolution of sulphor into the material in this area due to the heat cycle of the welding. I was not entirely convinced by this research, but if you are interested, I can try and get hold of a copy of this and e-mail it to you.

Regards
Niekie Jooste

By M.L.

Date 02-27-2003 18:52

I agree with Niekie that carbon would be a suitable material for this application. This occurance generally would happen in stainless steels more so than carbon I believe.

By LTRN Date 02-27-2003 21:32

ML,
Thanks for your reply.

By LTRN Date 02-27-2003 21:31

Hello Niekie
Thank you for your time and information. The situation is an accellerated metal loss at the welds and HAZ. The service may some times get excessive chlorination that can lead to the presence of HCL if H2O is introduced. I am basically wondering if stress relief (PWHT) would be practical are if anybody had experienced this problem?

By G.S.Crisi (****)

Date 02-28-2003 13:26

Niekie is right in telling that pwht will decrease corrosion rate for the reasons he's explained. However, there are two problems.
The first problem is that ASTM A-106 is carbon steel and no carbon steel withstands corrosion from acids. So, it's imperative that you change your material. Stress relief will DECREASE corrosion, but will not PREVENT or AVOID it.
The second problem is that the corrosive agent, as you say, is hydrochloric acid (HCl), which stainless steel doesn't withstand either. In the presence of chloride ion stainless steel is subjected to a type of corrosion called "stress corrosion".
So you have the following choices:
a) Plastic pipe: fiberglass reinforced polyester or polypropilene would be a good choice, up to a temperature of 190 ºF.
b) If pressure precludes the use of plastic, then rubber lined carbon steel is the other option, also up to a temperature of 190 ºF.
If the temperature involved is higher, then other more sophisticated materials, such as 25-20 metal alloys, should be considered.
Giovanni S. Crisi
Sao Paulo - Brazil

By LTRN Date 02-28-2003 14:23

Giovanni,
Thanks for the material replacement suggestions.