Stainless steel welding (tig)

By Lawrence (*****)

Date 07-29-2004 20:35

Welcome to the forum!

Tell us how your corrosion is manifesting itself. Pitting? Cracking? etc.

Caustic Soda effects on stainless steels depend greatly on the nickle content of the steel (higher is better as a rule) an on the temperature and concentration of the caustic in the pipes.

Here is a couple of links with good data.

Corrosion in caustic solutions
http://www.hghouston.com/naoh.html

Alloys for Caustic Soda Service by Corrosion Resistance Category
http://www.hghouston.com/naoh_tbl.html

By michaelab Date 07-29-2004 21:10

I welded the 304L with 2%thoriated tungsten after a few days there is corrosion on the pipe. The corrosion is only in the area that I welded . there was cleaning of the joints with caustic acid and all the areas that have heat or grinding to them are now showing signs of rusting .

By GRoberts (***)

Date 07-29-2004 22:01

Sounds like there are two possibilities. The most likely sounding is that you may have used a contaminated tool on the pipe. Were all the wire brushes you used stainless steel and never used on carbon steel before? Same for grinding wheels. The other possibility is that your stainless is 304, (not 304L?) could have a case of sensitization, or chrmium depletion at the grain boundaries. This normally happens in the HAZ just outside the weld.

By jamesrodr (*)

Date 07-30-2004 14:45

hi michaelab,

with regard to your question regarding effect of caustic soda on SS304 . I quote from the ISSF( Belgium ) notes : hydroxides like caustic soda and hydrogen sulphide can cause SCC ( Stress corrosion cracking )under conditions of high temperature and concentrations.
You can use a nitric acid - hydrofluoric acid pickling paste pack for cleaning the heat tint and other contaminants instead of caustic acid.

By michaelab Date 07-30-2004 20:11

We used two brushes one we were not sure if it was stainless steel. We went back later and cleaned all of the areas with stainless steel wire brush using grinder. What do you recommend to correct this?

By GRoberts (***)

Date 07-31-2004 00:13

If it did get iron contamination from a mild steel brush, or even a stainless brush that had been used on mild steel before, the only way I know about that will get rid of the problem is pickling the contaminated area. Your local welding shop might have a pickling paste that you can use. Brusing over it with stainless steel brushes only contaminates the stainless brush and spreads around the contamination.

By ajoy

Date 07-31-2004 09:51

I fully agree with GRoberts that it is a case of sensitization in the HAZ of an unstabilised stainless steel. Even if you do not contaminate the weld area with carbon steel tools, the weld joint is liable to show traces of rusting by picking up moisture from the environment under such conditions.

Here the focus should also be on the welding procedure to avoid recurrance of the same. Control the interpass temperature to 150 deg. C max and ensure relatively fast cooling of the weld in between passes. this will inhibit the precipitation of carbides to some extent.

Hope you agree with me.