Corrosion Resistance of 321 321H Stainless Steel

General Corrosion
Alloys 321 and 347 offer similar resistance to general, overall corrosion as the unstabilized chromium nickel Alloy 304. Heating for long periods of time in the chromium carbide precipitation range may affect the general resistance of Alloys 321 and 347 in severe corrosive media.

In most environments, both alloys will show similar corrosion resistance; however, Alloy 321 in the annealed condition is somewhat less resistant to general corrosion in strongly oxidizing environments than annealed Alloy 347. For this reason, Alloy 347 is preferable for aqueous and other low temperature environments. Exposure in the 800°F to 1500°F (427°C to 816°C) temperature range lowers the overall corrosion resistance of Alloy 321 to a much greater extent than Alloy 347. Alloy 347 is used primarily in high temperature applications where high resistance to sensitization is essential, thereby preventing intergranular corrosion at lower temperatures.

Stress Corrosion Cracking
The Alloys 321 and 347 austenitic stainless steel tubing are susceptible to stress corrosion cracking (SCC) in halides similar to Alloy 304 stainless steel. This results because of their similarity in nickel content. Conditions which cause SCC are: (1) presence of halide ion (generally chloride), (2) residual tensile stresses, and (3) environmental temperatures in excess of about 120°F (49°C). Stresses may result from cold deformation during forming operations or from thermal cycles encountered during welding operations. Stress levels may be reduced by annealing or stress-relieving heat treatments following cold deformation. The stabilized Alloys 321 and 347 are good choices for service in the stress relieved condition in environments which might otherwise cause intergranular corrosion for unstabilized alloys.

The Alloys 321 and 347 are particularly useful under conditions which cause polythionic acid stress corrosion of non-stabilized austenitic stainless steels, such as Alloy 304. Exposure of non-stabilized austenitic stainless steel to temperatures in the sensitizing range will cause the precipitation of chromium carbides at grain boundaries. On cooling to room temperature in a sulfide-containing environment, the sulfide (often hydrogen sulfide) reacts with moisture and oxygen to form polythionic acids which attack the sensitized grain boundaries. Under conditions of stress, intergranular cracks form. Polythionic acid SCC has occurred in oil refinery environments where sulfides are common. The stabilized Alloys 321 and 347 offer a solution to polythionic acid SCC by resisting sensitization during elevated temperature service. For optimum resistance, these alloys should be used in the thermally stabilized condition if service-related conditions may result in sensitization.

Intergranular Corrosion
Alloys 321 and 347 have been developed for applications where the unstabilized chromium-nickel steels, such as Alloy 304, would be susceptible to intergranular corrosion.

When the unstabilized chromium-nickel steels are held in or slowly cooled through the range of 800°F to 1500°F (427°C to 816°C), chromium carbide is precipitated at the grain boundaries. In the presence of certain strongly corrosive media, these grain boundaries are preferentially attacked, a general weakening of the metal results, and a complete disintegration may occur.

Organic media or weakly corrosive aqueous agents, milk or other dairy products, or atmospheric conditions rarely produce intergranular corrosion even when large amounts of precipitated carbides are present. When thin gauge material is welded, the time in the temperature range of 800°F to 1500°F (427°C to 816°C) is so short that with most corroding media, the unstabilized types are generally satisfactory. The extent to which carbide precipitation may be harmful depends upon the length of time the alloy was exposed to 800°F to 1500°F (427°C to 816°C) and upon the corrosive environment. Even the longer heating times involved in welding heavy gauge are not harmful to the unstabilized "L" grade alloys where the carbon content is kept to low amounts of 0.03% or less.

The high resistance of the stabilized Alloy 321 and Alloy 347 stainless steel to sensitization and intergranular corrosion is illustrated by data for the 321 alloy in the copper-copper sulfate –16% Sulfuric Acid Test (ASTM A262, Practice E) below. Mill annealed samples were given a sensitizing heat treatment consisting of soaking at 1050°F (566°C) for 48 hours prior to the test.

*Annealed 1100°F, 240 hours

The absence of intergranular attack (IGA) in the Alloy 347 specimens shows that they did not sensitize during this thermal exposure. The low corrosion rate exhibited by the Alloy 321 specimens shows that even though it suffered some IGA, it was more resistant than Alloy 304L under these conditions. All of these alloys are far superior to regular Alloy 304 stainless steel pipe under the conditions of this test.

In general, Alloys 321 and 347 are used for heavy welded equipment which cannot be annealed and for equipment which is operated between 800°F to 1500°F (427°C to 816°C) or slowly cooled through this range. Experience gained in a wide range of service conditions has provided sufficient data to generally predict the possibility of intergranular attack in most applications.

Please also review our comments under the Heat Treatment section.

Pitting/Crevice Corrosion
The resistance of the stabilized Alloys 321 and 347 to pitting and crevice corrosion in the presence of chloride ion is similar to that of Alloy 304 or 304L stainless steel tube because of similar chromium content. Generally, 100 ppm chloride in aqueous environments is considered to be the limit for both the unstabilized and the stabilized alloys, particularly if crevices are present. Higher levels of chloride ion might cause crevice corrosion and pitting. For more severe conditions of higher chloride level, lower pH and/or higher temperatures, alloys with molybdenum, such as Alloy 316, should be considered. The stabilized Alloys 321 and 347 pass the 100 hour, 5 percent neutral salt spray test (ASTM B117) with no rusting or staining of samples. However, exposure of these alloys to salt mists from the ocean would be expected to cause pitting and crevice corrosion accompanied by severe discoloration. The Alloys 321 and 347 are not recommended for exposure to marine environmentsz

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