If your corroded component is magnetic (you can easily test it), it is a ferritic stainless steel with fairly low corrosion resistance, e.g. AISI 420 (13% Cr). In this case, you can use a standard austenitic stainless steels, e.g. AISI 304 (18% Cr, 10% Ni) or AISI 316 (17% Cr, 12% Ni, 2% Mo), which are maybe solve the problem. However, if the steels 304 and 316 also corrode under these conditions, then you should use either a titanium or nickel base alloys, e.g. Ti 6-4 or Hastelloy C22. Somewhat cheaper alternative may be bronze, e.g. Al-bronze or Sn-bronze.

Dear Dr. Brendan Howe,

from my point of view the problems encountered can be overcome but paying attention to the material used in your high pressure distribution block but not only... First I would make sure of exactly the type of material that gave problems and I'd use a better one, probably a 316L steel would be enough, or I would turn my attention to an even more resistant to corrosion duplex type 2205 or similar). I would not overlook the fact that crevice corrosion, in a more or less present way, is however a critical point for all steels. As a consequence of this, in particular the flanges / joints are critical areas from the corrosive point of view both to offer a good seal and to avoid a possible galvanic coupling with incompatible materials.

Below I attach the link to a site containing the technical data sheets of the different types of steels, which could be useful to you: http://www.worldstainless.org/Files/issf/non-image-files/PDF/Atlas_Grade_datasheet_-_all_datasheets_rev_Aug_2013.pdf

Best Regards, Pierluigi Traverso

You said internal corrosion, not pitting, so I imagine generalized corrosion on the inside of the device? Rusty stainless indicates the chromium levels on the surface have been depleted.

Just a comment: Make very sure there is no electrical involvement, such as an electrical potential between the parts due to some electrical power connection/grounding. It isn't obvious. However it can be the cause of corrosion in an otherwise perfectly fine assembly. A voltmeter will spot the potential difference. Having a 12VDC power supply hooked up wrong can cause rapid corrosion.

Do you know if the appearance of rust on the internal surface is actually corroded stainless from the device, or rust deposited from another piece of iron in the system? A "dielectric coupling" would prevent galvanic corrosion issues between the dissimilar metals in the system. Once you commit to stainless steel, ALL the wet parts must be stainless steel or you can have galvanic corrosion. Even the weld rod must be enriched in chromium to match the corrosion potential of the stainless. Mixing plumbing parts (steel/brass/copper/stainless) is commonly done, but can be the source of corrosion (it isn't obvious) - check for material incompatibility. You can use "dielectric coupling" plumbing fittings if you need to mix metals (your home water heater has one; copper flex tubing and steel water heater).

A pH of ~8 doesn't sound like the source of the problem, but I can't tell about the chlorides. 180 mg/L is 180 ppm - kind of a lot(?) - is that a typo?

If the device is operated under high pressure, take care for stress corrosion cracking. Given enough time and stress, a level of as little as 1 ppm can cause damage at high temperatures around boiling water levels if pH is less than 10. Then, it is a matter of how fast the crack growth is, not if it will crack or not. I was going to answer that the maximum is 0 ppm but many people would dismiss that as impossible, and it depends on the oxygen availability. A practical maximum for room temperature pressurized austenitic stainless is 10 ppm.

Stainless isn't the best choice for chloride environments. If you have a lot of chlorides, investigate other materials. Electropolishing can help, but you have to have an internal anode in order to polish internal surfaces.

I don't know your device dimensions, but crosslinked epoxy powder surface treatments are resistant to chloride and bromide attack. Not great for rotating seals, but OK for static seals. I only offer it because when you check out the price of resistant metals, trying an organic coating remains a possibility.