Selecting an Alloy for Chemical Processing

Start with the acid, not the concentration

The single most common specification error is choosing by concentration alone. What actually determines alloy performance is the combination of acid type, concentration, temperature, oxidising or reducing character, and the presence of contaminants such as chloride, iron or oxidising species. A grade that handles 50% sulphuric acid at ambient temperature may be destroyed by the same acid at 90°C.

Oxidising acids: the chromium and molybdenum story

Oxidising conditions attack the passive film. Chromium builds that film, so 254 SMO — UNS S31254, 6% chromium, 6% molybdenum, 0.2% nitrogen — is the reference point for oxidising acids and for chloride pitting and crevice corrosion. It is specified to ASTM A 240/A 240M for plate, A 276/A 276M for bar, A 790/A 790M for pipe and A 182/A 182M for forgings.

For reducing acids the mechanism reverses. Reducing conditions dissolve the passive film, so the resistance comes from the alloy content itself rather than from the film. Here molybdenum and copper carry the load, and the nickel-chromium-molybdenum family — Hastelloy C-276, C-22, C-2000 — is the answer. C-276 contains 16% molybdenum and is specified to ASTM B 366 for pipe, B 622 for seamless tube and B 564 for forgings.

Chlorides: the pitting and crevice problem

Chloride service has its own failure mode, and it is the one most often missed. Chloride does not corrode the surface evenly; it penetrates and opens crevices, and it concentrates inside them. That means a grade chosen only on general corrosion data can fail badly. The relevant figure is the pitting resistance equivalent number, which combines chromium, molybdenum and nitrogen into a single index. Super duplex 2507 — UNS S32750, 25% chromium, 4% molybdenum, 0.3% nitrogen — exists for exactly this reason, and is specified to ASTM A 276/A 276M, A 240/A 240M and A 790/A 790M.

A practical sequence

• Write down the full service condition, including contaminants and the worst case temperature. Not just the nominal.
• Exclude by mechanism first: pitting and crevice in chlorides, then general corrosion.
• Check mechanical requirements against the process. A pressure boundary needs a specification that covers forgings and fittings, not just plate and bar.
• Confirm the weldability of the chosen grade. Nickel-base alloys with high molybdenum and tungsten content need careful heat input control.
• Ask for the product form in the specification, not just the grade. A grade specified only as a material often ends up supplied in whatever form the mill has in stock.

Where we can help

Send us the service condition and the drawing. We will propose the grade and the specification together, and tell you plainly if the case is outside what any alloy can handle. There are combinations of concentration and temperature where the answer is a lined vessel rather than a different alloy, and it is better to say so.