General and localised corrosion by acids, with pitting and crevice corrosion where chlorides are present.
Almost every alloy decision comes back to these. Get them right and the grade usually picks itself.
General corrosion, pitting, crevice, stress-corrosion cracking, oxidation, creep. Each one excludes different alloys. Deciding which mechanism governs removes most of the shortlist immediately.
Nominal conditions are not design conditions. Peak temperature, peak concentration, upset conditions and shutdown states are what actually consume the material.
The same grade carries different ASTM specifications for bar, plate, pipe and forgings. Name the form with the grade, or accept whatever the mill has in stock.
The grades that suit this service, with their UNS numbers and the specifications they are supplied to.
254 SMO is a high performance austenitic stainless steel. Its high alloying elements content imparts excellent resistance to uniform, pitting and crevice corrosion, and stress corrosion cracking.
Hastelloy C-276 has excellent resistance to localized corrosion, stress corrosion cracking, and to both oxidizing and reducing media, thus making it suitable for a wide variety of chemical process environments, including ferric and cupric chlorides, hot contaminated media (organic and inorganic),…
Hastelloy C-22 is a versatile Ni-Cr-Mo-W alloy with better comprehensive corrosion resistance performance than other Ni-Cr-Mo alloys, including Hastelloy C-276, Hastelloy C-4 and Inconel 625. Hastelloy C-22 has outstanding resistance to pitting, crevice corrosion and stress corrosion cracking.
Hastelloy C-2000 is a versatile corrosion resistant alloy with excellent resistance to uniform corrosion in oxidizing or reducing environments. It resists all acids, especially hydrochloric, sulfuric and hydrofluoric acids, at wide temperature ranges, and also resists pitting corrosion, crevice…
Incoloy 825 has excellent resistance to chloride-ion-induced stress corrosion cracking, reducing environments containing phosphoric and sulfuric acids, oxidizing environments containing nitric acid and nitrates, and pitting, crevice corrosion and intergranular corrosion.
Incoloy 020 has excellent resistance in chemicals containing sulfuric acid, and has good resistance to environments containing chlorides, phosphoric acid and nitric acid. It contains niobium for stabilization against sensitization and resultant intergranular corrosion.
Monel 400 has excellent resistance to corrosion by many reducing media such as sulfuric and hydrochloric acids. It is generally more resistant to corrosion by oxidizing media than higher copper alloys. Monel 400 resists pitting and stress corrosion cracking in most fresh and industrial waters.
2205 is a duplex stainless steel with nearly equal proportions of austenite and ferrite when properly heat treated. This microstructure makes it much more resistant to stress corrosion cracking than stainless steel 304 and 316L.
2507 is a super duplex stainless steel with higher alloy content, thus making it superior to 2205 in corrosion resistance and strength. It has excellent resistance to uniform, pitting, crevice corrosion and chloride stress corrosion cracking.
The wrong grade in the right service fails quietly for a long time and then all at once. These are the pairings that go wrong most often.
Falls apart in oxidising acids and in ferric or cupric salts. It is the right answer for de-aerated hydrofluoric acid and reducing conditions, and the wrong one almost everywhere else.
Duplex grades have a limited upper temperature and limited acid resistance. They are excellent in chloride-bearing process streams but are not a substitute for a nickel alloy in hot, strongly acidic service.
