Repairing corroded Stainless steel

[BCRD]

I was wondering if anybody has any experience repairing corroded SS?

I have been doing some mods to new SS parts to go inside an acid tank for a trucking company. In this particular application, they get about 1 year service before the two 1/4" rods break off. He showed me an example - they look like crap. Hard to imagine they were ever SS they are so rusted. Apparently the vapors near the top of the tank are worse than the submersed portion. So anyways, I was wondering if it was worth offering to fix them to add service life (they are very expensive parts). But if it won't last a day, I don't want my name attached to that kind of repair. I will probably ask for a sample to experiment with, but maybe somebody has some input?

Thanks guys...

Jason

[Oldiron2]

This is another case where a great many more details are needed to understand the situation, IMHO.

[MoonRise]

Stainless steel is just that, stain LESS.

It is not steel that will never corrode no matter what.

And there are lots of different kinds of stainless steel, all with varying degrees of chemical resistance and differeing physical properties.

And there are all sorts of acids, that have varying effects on those different stainless steels.

Some combinations of a specific type of acid and a specific type of stainless steel are just fine.

Other combinations are not.

As to the material failure, I'd say to get back to the tank manufacturer and have them play Sherlock and figure out the material incompatibility.

IMHO.

[BCRD]

Sorry, forgot to mention it is hydrochloric acid. Thanks for the points of consideration Moonrise. Tonight I should be able to grab some scraps to experiment with.

Jason

[forhire]

Originally Posted by BCRD

Sorry, forgot to mention it is hydrochloric acid. Thanks for the points of consideration Moonrise. Tonight I should be able to grab some scraps to experiment with.

Hydrochloric acid when diluted will attack even 316 as in short order due to the ion exchange. Can the parts be made of another material or coated?

You might look at something like this for bolts and such:
http://www.tantaline.com/Fasteners-9.aspx

[76GMC1500]

HCL and 316 stainless steel are relatively friendly... until you put the steel under strain. Try googling stress corrosion cracking. I had some strange cases with this at work last year. I work in the maritime industry. I had a few pnuematic cylinders with external stainless steel rods tying everything together. They were 1/2 inch diameter. During overhauls several rods failed while losening the nuts. It took maybe 10-15 ftlbs of torque to twist the rod apart. The rod looked pristine with no pitting or corrosion on its surface but the fracture was heavily corrode. On other rods, the nuts, which also appeared pristine and new, came loose just fine but I could not fit a wrench over them in order to reinstall them. The nuts had all fractured and as a result spread slightly. Because of the spread I couldn't fit a wrench over the nut. This was the only indication I had that there was a problem. The fractures were corroded. I attributed these failures to stress corrosion in a salt water (sodium chloride vs. hydro chloric).

You've got two choices here. Increase the diameter of the rod to reduce the stress or pick a different alloy. Consider Monel or Hastelloy. A286 or Nitronic 60 may be options? I don't know, I don't deal with acids very often.

The other side of the story, a 1 year lifecycle out of a 1/4 inch stainless rod equals spend $20 and replace the 1/4 inch rods every year before they fail. That's pretty good life by the standards of most platers and acid dippers. You could spend $1000 on rods and have them only last 2 years.

[BCRD]

Thanks guys. I think 76GMC1500 has it nailed. These rods are about 7' long, suspended (a rigid mount) from the top of the tank with a 2lbs weight at the bottom. And this is inside a truck mounted tank. So lots of braking / acceleration / sloshing etc. So there will be stress at the top. These 2 "rods" cost $1800 (part of a level sensor assembly), and I don't believe I will be repairing them.

Jason