Austenitic aqueous corrosion

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

In practice, by far the most common case of stress corrosion is that occurring when austenitic stainless steels are simultaneously exposed to tensile stresses and hot, aqueous, aerated, chloride-containing environments. In this case the major variable is alloy composition and structure virtually all austenitic stainless steels are more or less susceptible to stress-corrosion cracking in these environments, while ferritic and ferritic/austenitic stainless steels are highly resistant or immune. 

Baeumel, A. and Tramposch, O, Investigation of Intercrystalline Grain Boundary Corrosion of Austenitic Manganese-chromium Steels by Water and Aqueous Salt Solutions , Werkst. Korros., 17, 110 (1966) C.A., 64, 13847d... 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

With the desired microstructure, these alloys are resistant to hydrogen stress cracking and much more resistant to chloride stress corrosion cracking than are the austenitic stainless steels. (The threshold temperature for chloride stress corrosion cracking of duplex alloys in neutral pH aqueous chlorides is about 300°F [150°C].) The chloride stress corrosion cracking resistance of the duplex alloys is similar to that of superaustenitic alloys such as Alloy AL-6XN. Because they contain about 50% ferrite, the duplex stainless steels are more susceptible to hydrogen embrittlement. 

COBALT MURIATE (7646-79-9 7791-13-1, hexahydrate) C0CI2 Noncombustible solid. Incompatible with bases, alkali metals, ammonia vapors oxidizers, acetylene reaction may be violent. Contact with acids or acid fumes can produce highly toxic chloride fumes. Aqueous solution is a weak acid. Incompatible with metals can cause pitting attack and stress corrosion in austenitic stainless steels. Thermal decomposition releases toxic HCl, cobalt fumes, cobalt oxides. Cobalt is a known animal carcinogen. 

CHLOROFORMIC ACID DIMETHYLAMIDE (79-44-7) Combustible liquid (flash point 155°F/68°C). Rapidly hydrolyzed in water to dimethylamine, carbon dioxide, and hydrogen chloride. Violent reaction with strong oxidizers. Acid or acid fumes form toxic chloride fumes. Aqueous solution is incompatible with sulfuric acid, alkalis, ammonia, aliphatic amines, alkanolamines, alkylene oxides, amides, epichlorohydrin, organic anhydrides, isocyanates, vinyl acetate. May increase the explosive sensitivity of nitromethane. Attacks austenitic stainless steels, causing pitting and stress corrosion. 

Schmitt [52] reviewed the effect of elemental sulfur on corrosion of construction materials (carbon steels, ferric steels, austenitic steels, ferritic-austenitic steels (duplex steels), nickel and cobalt-based alloys and titanium. Wet elemental sulfur in contact with iron is aggressive and can result in the formation of iron sulfides or in stress corrosion cracking. Iron sulfides containing elemental sulfur initiate corrosion only when the elemental sulfur is in direct contact with the sulfide-covered metal. Iron sulfides are highly electron conductive and serve to transport electrons from the metal to the elemental sulfur. The coexistence of hydrogen sulfide and elemental sulfur in aqueous systems, that is, sour gases and oils, causes crevice corrosion rates of... 

J.I. Dickson, A.J. Russell, D. Tromans, Stress corrosion crack propagation in annealed and cold worked 310 and 316 austenitic stainless steels in boiling (154 °C) aqueous magnesium chloride solution, Can. Metad. Q. 19 (1980) 161-167. 

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