Uniform corrosion
Alloys 321 and 347 have the same resistance to general corrosion as the unstable nickel chromium alloy 304. Long time heating in the temperature range of chromium carbide degree may affect the corrosion resistance of alloys 321 and 347 in harsh corrosive medium.
In most environments, the corrosion resistance of the two alloys is similar; However, the corrosion resistance of annealed alloy 321 in strong oxidizing environment is slightly inferior to that of annealed alloy 347. Therefore, alloy 347 is better in water environment and other low temperature environments. When exposed to the temperature range of 800 ° F - 1500 ° F (427 ° C - 816 ° C), the overall corrosion resistance of alloy 321 is much worse than that of alloy 347. Alloy 347 is mainly used for high temperature applications. High temperature applications require strong anti sensitization of materials to prevent intergranular corrosion at lower temperatures.
Intergranular corrosion
Unstable nickel such as alloy 304? Steel is sensitive to intergranular corrosion, and alloy 321 and alloy 347 are developed and applied in this field.
When unstable chromium nickel steel is placed in an environment with a temperature of 800 ° F - 1500 ° F (427 ° C - 816 ° C) or is slowly cooled within this temperature range, chromium carbide precipitates at the grain boundary. When placed in some highly corrosive media, these grain boundaries are eroded first, which may weaken the effectiveness of the metal and may completely collapse.
In organic medium or corrosive water, milk or other dairy products or under atmospheric conditions, even if there is a large amount of carbide precipitation, intergranular corrosion will rarely occur. When welding thin plates, because the time to stay in the temperature range of 800 ° F - 1500 ° F (427 ° C - 816 ° C) is very short, intergranular corrosion is not easy to occur, so unstable grades can be qualified. The extent to which carbide precipitates is harmful depends on the length of time that the alloy is exposed to 800 ° F - 1500 ° F (427 ° C - 816 ° C) and the corrosive medium. Although the heating time for welding thicker plates is longer, due to the unstable level L, the carbon content is 0.03% or lower, and the carbide precipitation is not enough to harm this level.
The strong sensitization resistance and intergranular corrosion resistance of stable 321 and alloy 347 stainless steels are shown in the following table. (Copper copper sulfate - 16% sulfuric acid test (ASTM A262, Practice E)). Before the test, the samples annealed in the steel mill shall be subject to soaking photosensitive heat treatment at 1050 ° F (566 ° C) for 48 hours.
Stress corrosion cracking
Alloy 321 and 347 austenitic stainless steels are sensitive to stress corrosion cracking in halides, similar to alloy 304 stainless steels. This result is due to their similar nickel content. The conditions causing stress corrosion cracking are: (1) exposure to halide ions (usually chloride), (2) residual tensile stress, and (3) ambient temperature exceeding 120 ° F (49 ° C). Cold deformation in forming operations or thermal cycles encountered in welding operations may produce stresses. Stress relief heat treatment after annealing or cold deformation may reduce the stress level. The stable alloys 321 and 347 are suitable for the operation environment that eliminates stress and may cause intergranular corrosion to unstable alloys.
321 and 347 are particularly useful in environments where polythionic acid stress corrosion occurs on unstable austenitic stainless steels, such as alloy 304. Unstable austenitic stainless steel will produce chromium carbide precipitation at grain boundary if exposed to temperature where sensitization will occur. When cooled to room temperature in a sulfur containing environment, sulfide (usually hydrogen sulfide) will react with water vapor and oxygen to form hydropolysulfuric acid that attacks the sensitized grain boundary. Under the condition of stress and intergranular corrosion, the stress corrosion cracking of polythionic acid occurs in the refining environment where sulfide is ubiquitous. Stable alloys 321 and 347 solve the problem of polythionic acid stress corrosion cracking due to their anti sensitization in the heating operation environment. If the operating environment conditions will cause sensitization, these alloys should be used under thermal stable conditions to achieve good anti sensitization.
Pitting corrosion
The pitting corrosion resistance and pitting corrosion resistance of stable alloys 321 and 347 in the environment containing chloride ions are similar to those of alloy 304 or 304L stainless steel, because their chromium content is similar. In general, for unstable and stable alloys, the upper limit of chloride content in the water environment is 100 ppm, especially when crevice corrosion exists. High chloride ion content will lead to crevice corrosion and pitting corrosion. In case of severe conditions such as high chloride content, low PH value and/or high temperature, molybdenum containing alloy, such as alloy 316, shall be considered. Stable alloys 321 and 347 passed the 100 hour 5% salt spray test (ASTM B117), and the tested samples did not produce rust and fade. However, if these alloys are exposed to salt fog from the sea, pitting corrosion, interstitial corrosion and severe discoloration may occur. Exposure of alloys 321 and 347 to the marine environment is not recommended.