SS304 Corrosion at Welds 16

[IFRs]

We have some "rust" at or near welds on SS304 tubing. The rust has formed right next to the weld and adjacent to the weld, both on the tube seam weld and the end plates we weld on. The material is all SS304 and we use SS308 weld wire. What is the easiest way of removing the "rust" in the field (China) and of keeping it from happening in the shop (USA)? Will wire brushing with SS wire brush do it? Is there some chemical tretment that will help? Many thanks!!

 

[moltenmetal]

jebrown: you have an in-depth knowledge of tubing manufacture, that's for sure. But I just re-read IFR's posts again and didn't see the grade of tubing mentioned anywhere, aside from the fact that it is welded seam versu seamless. In fact I didn't remark the size of the tubing either, though I presume this is relatively large bore stuff due to its use.

A few questions for you: is the HF induction method you've mentioned the same as the electrical resistance welding (ERW) method? Are these usually fillerless welds, or is filler metal sometimes used? Is scrafing (scarfing?) always used, or is the weld sometimes forged into shape? If the tubing is annealed after welding, does the HAZ and associated problems disappear?

 

[IFRs]

Our tubing is SS304, 12" OD, 18 gauge wall.
Future orders will be for SS304L.
Does that make a difference?

 

[mcguire]

High frequency welding is almost never used for fluid containing 304 pipe or tubing. This process is used for structural stainless pipe or exhaust pipe where perfect weld integrity is not very important. Most good quality stainless pipe is made by TIG or laser, where wall thickness permits. The weld is still scarfed to make it flush with the ID.

TIG welds must be annealed in 304 to minimize ( but not eliminate) the detrimental effect which as-solidified metal, as opposed to the wrought parent metal, has on corrosion resistance. In 316 this effect can be 20C on the critical pitting temperature depending largely on the metal's sulfur level.

Laser welded pipe is the best because the molten zone is miniscule and the resulting quench is so rapid that there is no diminution of corrosion resistance.

High frequency welded carbon steel pipe is common as is the lower quality butt welded.

 

[IFRs]

Our tubing is seal welded on both ends with formed caps, pressure tested and used for flotation.

 

[arto]

For passivating the piping, try:

http://www.derustit.com/products.htm

or

http://www.stellarsolutions.net/products/citacid.htm

 

[stanweld]

IFRs,
If you intend to use these flotation devices in marine service, 304 and 304L SS are inappropriate choices. A very short service life must be expected. You can easily determine if weld HAZ sensitization has occurred, as I and many others in the forum suspect, through metallographic examination, which I urge you to do.

When welding thin stainless steels, heat is transferred away from the joint through conduction unidirectionally, which promotes slow cooling and subsequent chromium depletion through formation of chromium carbides adjacent to the grain boundaries in the HAZ. Convection cooling due to the gas shielding and purging during welding do not appear to be adequate to compensate. To aid in more rapid cooling, you can try copper or aluminum heat sinks adjacent to the weld as well as back purging with helium (an excellent heat transfer medium).

Very likely, the only way to salvage the produced items is to paint/coat the welds if sensitization has indeed ocurred.

 

[rolledalloy]

For superficial rust/iron contamination removal on stainless steels another economical alternative I have been told is to use deck cleaner from a place like Home Depot. Most deck cleaners contain oxalic acid, which will remove the iron oxides on the surface, but not attack the stainless steel.