Pin hole leaks in stainless steel ozone tanks

A. Just as you would get galvanic currents when two dissimilar metals are in a conductive solution and have a metallic path connecting them, you get similar (though smaller) galvanic currents between the stressed 'hot spots' on a surface and the rest of that surface. And you also get galvanic currents between highly oxygenated areas and less oxygenated areas.

I don't know the exact solution to your problem, but I am confident that the pinholes are galvanic corrosion. Perhaps bead blasting after the welding will limit this galvanic differential. Naturally, the degree of ozone aeration at different spots is also a factor that you may possibly be able to address.

Although I can't come up with the term at the moment, there is a name for the phenomenon whereby corrosion occurs underneath bolt heads due to oxygen deprivation compared to the surfaces that are not under the bolt heads.

A. William,

One option you can look at would be to Brush Electroplate the seam areas to fill in the porosity. This process can be done at the facility on an as needed basis using Nickel, Copper or other metals for the repair, of course this would depend on the definition of pin hole. For bigger pin holes the tanks may be able to be welded, then cleaned with a weldment cleaner.

A. You have a major problem developing from your description. Was the welding certified by the mfg. of the tanks? Were the welds passivated? I think you will have to have the welds ground down, the correct filler applied, and all weld areas repassivated in situ.

A. There are two possible failure mechanisms for the leaks in your tanks. One is pitting corrosion. The second is stress corrosion cracking.

Pitting corrosion is a problem since the leaks will gradually get worse until the damage is repaired or the tank is replaced. If the leaks are due to pitting corrosion, you may be able to repair and rehabilitate these tanks if the damage is not too severe. The location of the leaks along the weld is not surprising. Corrosion resistance is diminished by welding, especially if the weld areas are not cleaned properly after welding.

If the leaks are due to stress corrosion cracking (SCC), there is a risk of catastrophic failure of these tanks. The cracks will grow with time, and may cause a dangerous collapse of the tank without having severe leaks. SCC would also occur preferentially at the weld heat affected zone.

I strongly recommend that you have these tanks inspected soon by someone familiar with nondestructive testing and corrosion. This recommendation is made for safety sake in case the damage is SCC and to give you the best chance to make repairs without losing these tanks altogether.

Good luck.

A. A very big problem in welding Austenitic Stainless is sensitization. Stainless is corrosion resistant because of the thin film of chromium oxide formed on the surface. When welded, the chromium in the heat affected zone (800-1400 F) forms chromium carbide. This ties up the chromium allowing corrosion. Heat treat after the weld will prevent this.