Chloride ion corrosion ring material selection

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Analysis of corrosion failure of stainless steel 1. Stress corrosion: Stainless steel produces stress corrosion in a corrosive medium environment containing oxygen and chloride ions. The proportion of stress corrosion failure is as high as about 45%. Common protective measures: Reasonable material selection and stress corrosion-resistant materials mainly include high-purity austenitic chromium-nickel steel, high-silicon austenitic chromium-nickel steel, high-chromium ferrite steel and ferrite-austenite dual-phase steel. Among them, ferrite-austenite dual-phase steel has the best resistance to stress corrosion. control stress: When assembling, try to reduce stress concentration as much as possible, and make the parts in contact with the medium have minimal residual stress to prevent bumps and scratches, and strictly abide by the welding process specifications. Strictly abide by operating procedures: Strictly control the raw material composition, flow rate, medium temperature, pressure, pH value and other process indicators. Add corrosion inhibitors within the scope allowed by process conditions. When chromium-nickel stainless steel is used in dissolved oxygen chloride, the mass fraction of oxygen should be reduced to below 1. 0×10 - 6. Practice has proved that in water containing chloride ions with a mass fraction of 500. 0 × 10 - 6 , good results can be obtained by simply adding a mixture of nitrate with a mass fraction of 150. 0 × 10 - 6 and sodium sulfite with a mass fraction of 0. 5 × 10 - 6 . 2. Piting corrosion failure and preventive measures Small hole corrosion is generally prone to occur in stationary media. Corrosion pits usually develop along the direction of gravity or transverse direction. Once pit corrosion is formed, it will automatically accelerate to depth. The oxide film on the surface of stainless steel is dissolved in an aqueous solution containing chloride ions, resulting in small corrosion pits with a pore size of 20 μm to 30 μm on the base metal. These small corrosion pits are pitting corrosion nuclei. As long as the medium contains a certain amount of chloride ions, corrosion nuclei may develop into corrosion holes. Common precautions: Add molybdenum, nitrogen, silicon and other elements to stainless steel or increase the chromium content while adding these elements. Reduce the chloride ion content in the medium. Adding corrosion inhibitors can increase the stability of the passivation film or help the damaged passivation film to be re-passivated. Use external cathode current protection to inhibit pitting corrosion. 3. Pointing corrosion: Since non-metallic inclusions exist in any metal material to varying degrees, these non-metallic compounds will quickly form pit corrosion under the corrosive action of Cl ions. Under the action of a blocked battery, the Cl ions outside the pit will migrate into the pit, while the positively charged metal ions in the pit will migrate out of the pit. Among stainless steel materials, Mo-added materials have better pit corrosion resistance than materials without Mo. The more Mo content is added, the better the pit corrosion resistance. 4. Crevice corrosion The mechanism of crevice corrosion is the same as that of pit corrosion. It is a corrosion phenomenon caused by the concentration of Cl ions due to the presence of blocked cells in the crevices. This type of corrosion generally occurs in the gaps between flange gaskets, lap joints, bolts and nuts, and the gaps between heat exchange tubes and tube plate holes. Crevice corrosion is closely related to the concentration of static solution in the gaps. Once there is a crevice corrosion environment, the probability of inducing stress corrosion is very high. Applicable conditions for two types of stainless steel in chlorine-containing aqueous solutions 1 Type 304 stainless steel This is the cheapest and most widely used austenitic stainless steel (such as food, chemical, atomic energy and other industrial equipment). Suitable for general organic and inorganic media. For example, nitric acid with a concentration of <30% and a temperature of ≤100℃ or a concentration of ≥30% and a temperature of <50℃ ； Various concentrations of carbonic acid, ammonia and alcohols at temperatures ≤100°C. Poor corrosion resistance in sulfuric acid and hydrochloric acid ； It is especially sensitive to crevice corrosion caused by chlorine-containing media (such as cooling water). 2 Type 304L stainless steel. Corrosion resistance and uses are basically the same as type 304. Due to the lower carbon content (≤0.03%), it has better corrosion resistance (especially resistance to intergranular corrosion, including the weld area) and weldability, and can be used for semi-welded or fully welded PHE. 3 Type 316 stainless steel is suitable for general organic and inorganic media. For example, natural cooling water, cooling tower water, softened water ； carbonic acid ； Acetic acid and caustic lye with concentration <50% ； Solvents such as alcohol and propylene ； Dilute nitric acid (concentration <20%=) with temperature ≤100°C, dilute phosphoric acid (concentration <30%=, etc.). However, it is not suitable for use with sulfuric acid. Since it contains about 2% Mo, its corrosion resistance in seawater and other chlorine-containing media is better than that of Type 304, and it can completely replace Type 304. 4 The corrosion resistance and uses of type 316L stainless steel are basically the same as those of type 316. Due to the lower carbon content (≤0.03%), the weldability and corrosion resistance after welding are also better, and it can be used for semi-welded or fully welded PHE. 5 Type 317 stainless steel is suitable for conditions requiring longer service life than Type 316. Since the content of Cr, Mo and Ni elements is slightly higher than that of type 316, it has better resistance to crevice corrosion, pitting corrosion and stress corrosion. 6 AISI 904L or SUS 890L stainless steel. This is a cost-effective austenitic stainless steel that takes into account both price and corrosion resistance. Its corrosion resistance is better than the above materials. It is especially suitable for general sulfuric acid, phosphoric acid and other acids and halides (including Cl-, F-). Due to the high content of Cr, Ni and Mo, it has good resistance to stress corrosion, pitting corrosion and crevice corrosion. 7 Avesta 254 SMO high-grade stainless steel This is an ultra-low carbon high-grade stainless steel that has been improved on type 316 by increasing the Mo content. It has excellent resistance to chloride pitting corrosion and crevice corrosion, and is suitable for media containing salt water, inorganic acids and other media where type 316 cannot be used. 8 Avesta 654 SMO high-grade stainless steel This is an ultra-low-carbon high-grade stainless steel with higher Cr, Ni, Mo, and N contents than 254 SMO. It has better chloride corrosion resistance than 254 SMO and can be used in cold seawater. 9 RS-2 (OCr20Ni26Mo3Cu3Si2Nb) stainless steel; this is a domestic Cr–Ni–Mo-Cu stainless steel. The resistance to pitting corrosion and crevice corrosion is equivalent to type 316, while the resistance to stress corrosion is better. It can be used in concentrated sulfuric acid (concentration 90~98%) below 80 ℃, with annual corrosion rate ≤0.04mm/a. 10 Incoloy 825(S), which is a Ni (40%) – Cr (22%) – Mo (3%) high-grade stainless steel. Incoloy is a registered trademark of the international Nickel Co. Suitable for various concentrations of sulfuric acid at low temperatures ； In a caustic alkali (such as NaOH) solution with a concentration of 50% to 70%, it has good corrosion resistance and does not cause stress corrosion cracking. However, it is very sensitive to crevice corrosion caused by chloride. In addition, the stamping performance is not very good, so it is not a commonly used material for plates. 11 31 alloy: Improved from 904L (increased Mo and N content), standard 6% Mo high-grade stainless steel (31% Ni-27% Cr-6.5% Mo-32% Fe). Better corrosion resistance than 904L in many media ； In sulfuric acid with a concentration of 20% to 80% and a temperature of 60°C to 100°C, the corrosion resistance even exceeds that of C-276. 12 33 alloy: A fully austenitized chromium-based high-grade stainless steel with corrosion resistance comparable to some Ni-Cr-Mo alloys such as Inconel 625. It has good resistance to local corrosion and stress corrosion cracking in acidic and alkaline media (including nitric acid, mixtures of nitric acid and hydrofluoric acid) ； The corrosion resistance in concentrated nitric acid is much better than 304L. For example, it is suitable for sulfuric acid with a concentration greater than 96%~99%, a temperature ≤150℃, and a sulfur oxide content less than 200 mg/L. ； hot sea water ； Concentration ≤50%, boiling highly corrosive solution ； Phosphoric acid with concentration ≤85% and temperature ≤150℃, etc. However, it is not suitable for reducing media (such as dilute sulfuric acid, etc.). The price is about the same as C-276. 13 C-2000 Alloy: A nickel-based alloy developed in the 1990s, the price is similar to C-276, and it has one of the best corrosion resistance among the above materials. Its corrosion resistance is better than C-276 and C-22 in media such as sulfuric acid, dilute hydrochloric acid, boiling temperature, concentration ≤50%, and hot chloride, and has a tendency to replace C-22 alloy. However, for sulfuric acid with a concentration of ≥70%, the corrosion resistance is not as good as C-276. 14 59 alloy: Compared with C-2000, the chemical composition is basically the same except that the Ni content is slightly higher (59%), low in Fe, and free of Cu and W. This is currently the material with the best corrosion resistance, thermal stability, stampability and weldability among nickel-based alloys. Since its commercialization in 1990, it has been widely used in sulfuric acid, hydrochloric acid, hydrofluoric acid and many media containing chlorine, oxygen and low pH. Three material selection tables based on temperature and chloride ion content