Heat Tint On Duplex Stainless Steel 6

Any visible scale from oxidation is indicative of some degree of de-chromization beneath the scale. This means the resistance to pitting is reduced. There are no two ways about it.
Furthermore, a non-annealed weld is intrinsically inferior to the base metal. EdStainless( I'm sure he will have some valuable input) and I might disagree on the mechanism, there is as yet no generally accepted theory for why it is true, but the weld metal is inferior in pitting resistance without a homogenizing thermo-mechanical treatment.
Not having had any problems is a lame technical qualification for omitting weld cleaning and passivation.

How about, 'it all depends.'
If we are tlking about a light straw or yellowish color then it probably doesn't matter.
If the scale is darker (brown blue black) and not shinney (transparent) then it needs to come off.
They can pickle to remove it. Or grind, using fresh media, and then passivate. No polishing allowed. This will just smear the metal around and buff the oxide. It will look nice but not help corrosion resistance.

Take coupons from weld trials and have G-48s run on them. I would expect to loose 5-8 C just due to the welds. If you are using the correct over-alloyed filler you can minimize this.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

Generally I concur with mcguire's post but there some more parameters that need to be considered whether or not to clean/pickle 2205 SS.

Does your process require the ultimate corrosion resistance?

If there is some corrosion to 2205 by the process will it take care of the heat tint? There are degrees of heat tint.

2205 is not the easiest material to pickle, it requires a more agressive pickling medium than used on the 300 series SS. This may require mechanical cleaning with iron free media and then pickling.

Use the cleanest welding process for fabrication. 2205 SS is a material that doesn’t tolerate sloppy welding practices well.

Fabrication of 2205 takes more up front planning if your process requires that the material be pickled. How will you get to the inside?

Here is some information on welding and pickling 2205 SS

In response to the comments/questions identified;

1. I agree that not having problems in the past is not a sound definitive basis for approving a fabrication technique, but I also think that some credit should be given for the past successes of a fabricator. Though the technique is not specifically detailed, they are successful.

2. The heat tint I have observed is typically a black brown color, approximately 3/16" wide in the HAZ of the base metal. One proposal is to sandblast the welds instead of pickle/passivation, is this an effective approach?

3. All weld procedures and qualifications are sound and meet all testing as well as the published requirements of major suppliers (e.g. Sandvik, etc..). If the heat tint sections pass the corrosion test as specified in ASTM A923, would this be an acceptable measure of the fabrication quality?


Thanks,
alloy2205

A923 is only a test for intermetallics, not corrosion resistance. It just happens to be run in the same test solution, but the method is different.
Blasting can be problematic. I have seen two papers recently that indicate that a properly blasted surface isn't too bad. But no where near pickled. It is better than a bad grinding job.

I don't like dark, dull scale for many reasons. The problem with 'it has worked before' is that you have no documentation on the exact condition before. Is this the same? better? worse? There is no way to tell.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

I agree with EdStainless. The location of the oxide is also the locus of the least corrosion resistant area, the melted, but un-mixed base metal.

Please start a new topic with a new posting.

Michael McGuire

Pitting corrosion failures have been seen to initiate in locations of yellow to brown heat tinting on the ID surface weld metal and HAZ in 2205. No pitting or very light pitting was seen in unaffected (bright)base metal or weld metal. Failure occurred in only a few months operation. The aqueous service environment was high in chlorides, sulphates, etc. with max. T of 150 F.