High-temperature intergranular corrosion-resistant materials

Intergranular corrosion: A type of corrosion specific to austenitic and ferritic stainless steels, characterized by selective corrosion occurring at and near the grain boundaries. The main consequence is the fragmentation of the metal and a loss of its strength. Measures to prevent intergranular corrosion: 1. Austenitic stainless steels: (1) Reduce carbon content and increase the purity of the steel (N, P)； (2) Add stabilizing elements Ti, Nb ； (3) Solution treatment ； (4) Follow the principle of cold working first and then heat treatment ； (5) Control grain size to increase the grain boundary area. 2. Ferritic stainless steels: (1) Reduce carbon content and improve the purity of the steel (N, P) ； (2) Add stabilizing elements Ti, Nb ； (3) Solution treatment at 650~850 ℃ depending on the steel grade ； (4) For nickel-containing steels, the heat treatment specifications should be selected based on their nickel content.

•Chloride stress corrosion cracking (ClSCC) • ClSCC generally occurs when the metal temperature is above (~65°C). • The most sensitive to ClSCC are austenitic stainless steels containing 8% Ni (such as the 300SS series, 304, 316, etc.).

Nickel alloys are a good choice, as they resist high-temperature corrosion

As for the selection of specific grades, it needs to be determined through a comprehensive analysis of the working conditions. Generally, a history of use is required. Materials made of Inconel 600 (UNS N06600) include sheets, pipes, forgings, and round bars. The chemical composition of Inconel 600 is as follows: Alloy elements – Nickel, Chromium, Iron, Carbon, Manganese, Silicon, Copper, Phosphorus, Sulfur. For Inconel 600, the minimum values are 72% nickel, 14% chromium, 6% iron; the maximum values are 17% nickel, 10% chromium, 0.15% carbon, 1% manganese, 0.5% silicon, 0.5% copper, 0.015% phosphorus, and 0.015% sulfur. The physical properties of Inconel 600 include a density of 8.4 g/cm3 and a melting point of 1370–1425 °C. The minimum mechanical properties of this alloy at room temperature are as follows: Tensile strength Rm in N/mm2, yield strength RP0.2 in N/mm2, elongation A5 in %, and Brinell hardness HB. For annealed material: 550 N/mm2, 240 N/mm2, 30%, and ≤195 HB. For solution-treated material: 500 N/mm2, 180 N/mm2, 35%, and ≤185 HB. Inconel 600 possesses the following characteristics: 1. It has excellent resistance to corrosion in reducing, oxidizing, and nitriding environments. 2. It exhibits good resistance to stress corrosion cracking at both room temperature and high temperatures. 3. It has good resistance to corrosion by dry chlorine and hydrogen chloride gases. 4. It maintains good mechanical properties at low temperatures, room temperature, and high temperatures. 5. It has high resistance to creep fracture, making it suitable for use in environments with temperatures above 700 °C. Microstructural structure of Inconel 600: 600 has a face-centered cubic crystal structure. Corrosion resistance of Inconel 600: The 600 alloy exhibits corrosion resistance against various corrosive media. The presence of chromium gives this alloy better corrosion resistance under oxidizing conditions compared to Nickel 99.2 (Alloy 200) and Nickel 99.2 (Alloy 201, low carbon). At the same time, the higher nickel content confers excellent corrosion resistance to the alloy under reducing conditions and in alkaline solutions, and it effectively prevents chloro-iron stress corrosion cracking. 600 alloy exhibits excellent corrosion resistance in organic acids such as acetic acid, formic acid, and stearic acid, and moderate corrosion resistance in inorganic acids. High-purity water used in primary and secondary cycles in nuclear reactors exhibits excellent corrosion resistance. The particularly outstanding feature of 600 is its ability to resist corrosion by dry chlorine and hydrogen chloride, with an operating temperature of up to 650°C. At high temperatures, the annealed and solution-treated alloys exhibit excellent oxidation resistance and high strength in air. This alloy can also resist ammonia gas as well as nitriding and carburizing atmospheres; however, it is susceptible to corrosion by partially oxidizing media (such as green death liquid) when redox conditions change alternately. Applications of Inconel 600 include: 1. Thermocouple sheaths in corrosive atmospheres; 2. Production of vinyl chloride monomer: resistance to chlorine, hydrogen chloride, oxidation, and carbonization corrosion; 3. Oxidation of uranium to hexafluoride: resistance to hydrogen fluoride corrosion; 4. Production and use in environments involving corrosive alkali metals, especially those with sulfides; 5. Production of titanium dioxide using chlorine; 6. Production of organic or inorganic chlorides and fluorides: resistance to chlorine and fluoride corrosion; 7. Nuclear reactors; 8. Retorts and components in heat treatment furnaces, especially in carbonizing and nitriding atmospheres; 9. Catalytic regenerators in petrochemical production. For applications at temperatures above 700°C, alloy 600 is recommended to ensure a longer service life.

Chloride ions can cause pitting, and pitting can induce or accelerate intergranular corrosion and stress corrosion. Low-carbon austenitic stainless steels (such as 304L) have good resistance to intergranular corrosion, but they have low strength, especially their mechanical properties are poor at high temperatures. Duplex stainless steels (such as 00Cr22Ni5Mo3N) possess higher resistance to stress corrosion, pitting corrosion, and intergranular corrosion, as well as greater strength, compared to austenitic stainless steels. For high-temperature, high-pressure, large-diameter equipment, using low-alloy steel with corrosion-resistant linings or surfacing layers can reduce material costs. 450°C is not a very high temperature, so there is no need to use nickel-based alloys (nickel-based alloys are too expensive).

Reply to 6# Pinocchio: Thank you for your answer. As far as I know, duplex steel tends to become brittle at temperatures above 300 degrees, so it certainly cannot be used at high temperatures

It is recommended to use 8-18 austenitic stainless steel. Due to the high temperatures, ultra-low carbon stainless steel is not recommended. Nickel-based alloys should work fine, but their cost is too high. Considering that the original poster mentioned only intergranular corrosion and a small amount of chloride ions, it is necessary to select the appropriate stainless steel material in accordance with the requirements of the five methods specified in GB4334. SCC has no special requirements regarding material selection; instead, it has stricter requirements for the manufacturing process, as SCC generally does not occur directly, but is caused by intergranular corrosion, pitting corrosion, crevice corrosion, etc.

0Cr25Ni20 should work

Reply to 7# hgm: 00Cr22Ni5Mo3N is typically used in urea production equipment, and it is indeed not suitable for high temperatures. 310S (also known as 2520, a duplex stainless steel) has a maximum operating temperature of 1200 °C, and a continuous use temperature of 1150 °C, which is also acceptable.

Reply to 8# sunliu*: The composition of the medium is quite complex, and extensive pitting has occurred