Corrosion in SS 316 Pipe in Copper Sulfate + Sulfuric acid solution

Is the flange welded on? If so, look to the weld for the problem. The solution will be a lower heat weld, non-weld joint, or different material. I have no idea why in only one tank, but sulfuric acid and -Cl ions are tough on 316.

This sounds like your problem to me as well, could it be that someone welded the wrong pipe into the system. It all looks the same in the pipe yard and it would explain why it is only happening in the one tank. I have seen once where we bought 316 pipe and we got 304 that was mistakenly marked 316. it wasn't until we had paid for a chemical analysis test that we found what had happened. the pipe supplier was happy to pay for the test and supply a new pc. of pipe when we had the proof.

Once I replaced a section of a carbon steel tank with a ring of 304 stainless steel and in about 1 year the stainless had corroded badly in the HAZ area from galvanic corrosion. If you have dissimilar metals in the system you may need some type of isolation to keep the 2 metals from contacting each other. On smaller bore pipe we use a Di-electric union that prevents this galvanic corrosion. Just a guess but would be worth checking.

pipewelder

• "I have seen once where we bought 316 pipe and we got 304 that was mistakenly marked 316"

Really, it is a serious problem.

• "the stainless had corroded badly in the HAZ area from galvanic"

That is true, because the HAZ is the weakest zone due to heat input during welding and it becomes sensitive to galvanic.

Sometimes the corrosion in the HAZ is due to changing of microstructural of Cristal's at these area due to excessive heat input, although you weld a similar metals. That happened to 10" stainless steel pipe TP304 at 1987, where and after 5 years of construction we found the corrosion started at all HAZ and weld beads. Also, I think that there is another reason to corrosion for that pipe due to using a contaminated wire brush which used before for cleaning carbon steel weldment.

You can proceed a chemical analysis for the old piece of pipe to prove/insure that suspecting of being TP304.

Also, be sure that the new piece of pipe is TP316 and that grade is stenciled/marked on pipe itself.

Devendra,

I previously worked in a copper electorefinery were we used the same solutions. There were no corrosion problems using 316 Stainless Steel as long as all of the proper precautions were taken.

All welded materials must be 316 L grade (low carbon content). If you weld 316 chromimum carbide is formed which allows corrosion to take place.

The steel must be protected from stray electrical currents as a previous post suggested. All of our SS 316 tanks and pipes were electrically isolated from ground.

The other possibility as previouly posted is likely the one pipe that is corroding is a different grade like SS 304 or even SS 316 rather than SS 316 L.

I have seen stainless steel corroded by sulfates in the past, but its usually at the vapor/liquid interface. If you have sulfates in the gas phase, these tend to scavenge the oxygen in the corrosion layer of the stainless steel (the corrosion layer protects the underlying metal from being oxidized, keeping stainless steel stainless). Once these are exposed to the liquid phase, corrosion occurrs. Probably not what is happening in your application, but it may be something to look at.

sorry but what are the materials involved?

S.