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1. Corrosion failure analysis of stainless steel: 1. Stress corrosion: Stainless steel suffers from stress corrosion in a corrosive environment containing oxygen and chloride ions. Stress corrosion failure accounts for about 45% of cases. Common protective measures: Select materials appropriately; materials resistant to stress corrosion include high-purity austenitic chromium-nickel steel, high-silicon austenitic chromium-nickel steel, high-chromium ferritic steel, and ferritic-austenitic duplex steel. Among them, ferritic-austenitic duplex steels have the best resistance to stress corrosion. Stress control: During assembly, minimize stress concentration and ensure that the parts in contact with the medium have minimal residual stress. Prevent any bumps or scratches, and strictly adhere to the welding process specifications. Strictly adhere to operating procedures: strictly control process parameters such as raw material composition, flow rate, medium temperature, pressure, and pH value. Add a corrosion inhibitor within the limits permitted by the process conditions. When chromium-nickel stainless steel is used in chlorides containing dissolved oxygen, the mass fraction of oxygen should be reduced to below 1.0×10^-6. Practice has shown that in water with a chloride ion mass fraction of 500.0 ×10^-6, 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 yields good results. 2. Pitting corrosion failure and preventive measures: Pitting corrosion generally occurs easily in stagnant media. Erosion holes generally develop along the direction of gravity or horizontally; once formed, pitting erosion accelerates automatically as it goes deeper. The oxide film on the surface of stainless steel dissolves in an aqueous solution containing chloride ions; as a result, small pits with diameters of 20–30 μm form on the base metal. These small pits serve as initiation sites for pitting corrosion. As long as there is a certain amount of chloride ions in the medium, it is possible for an erosion nucleus to develop into an erosion hole. Common preventive measures: Adding elements such as molybdenum, nitrogen, and silicon to stainless steel, or increasing the chromium content while adding these elements. Reduce the content of chloride ions in the medium. Adding a corrosion inhibitor increases the stability of the passivation film or facilitates the repassivation of a damaged passivation film. External cathodic current protection is employed to suppress pitting. 3. Pitting corrosion: Since all metal materials contain non-metallic inclusions to varying degrees, these non-metallic compounds will rapidly lead to pitting corrosion under the corrosive action of Cl ions. Due to the effect of a closed battery, Cl ions outside the pits will migrate into the pits, while the positively charged metal ions inside the pits will migrate outward. In stainless steel materials, those with Mo added exhibit better resistance to pitting corrosion compared to those without Mo; the higher the Mo content, the better the resistance to pitting corrosion. 4. Crevice corrosion: Similar to pitting corrosion, crevice corrosion occurs due to the presence of closed cells in the crevices, which leads to the accumulation of Cl ions and thus corrosion. This type of corrosion generally occurs in the gaps of flange gaskets, lap joints, bolt nuts, as well as in the gaps between heat exchange tubes and tube sheet holes. Gap corrosion is closely related to the concentration of the stagnant solution within these gaps; once such an environment for gap corrosion exists, the likelihood of stress corrosion occurring is quite high. II. Application conditions of several stainless steels in chlorinated aqueous solutions 1. 304 stainless steel: This is the cheapest and most widely used austenitic stainless steel (used in industrial equipment in industries such as food processing, chemicals, and nuclear energy). Suitable for general organic and inorganic media. For example, nitric acid with a concentration of <30%, temperature ≤100°C, or a concentration of ≥30% and temperature <50°C ； Carbonic acid, ammonia water, and alcohols of various concentrations at temperatures ≤100℃. Poor corrosion resistance in sulfuric acid and hydrochloric acid ； It is particularly sensitive to crevice corrosion caused by chlorinated media (such as cooling water). 2 304L stainless steel. Its corrosion resistance and applications are basically the same as those of type 304. Due to its lower carbon content (≤0.03%), it exhibits better corrosion resistance (especially against intergranular corrosion, including in the weld area) and weldability, making it suitable for semi-welded or fully welded PHEs. Type 3316 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 alkali solutions with a concentration of <50% ； Alcohols and **solvents ； Dilute nitric acid at a temperature of ≤100℃ (concentration <20%), dilute phosphoric acid (concentration <30%), etc. However, it is not suitable for sulfuric acid. Due to its approximately 2% Mo content, it has better corrosion resistance in seawater and other chlorinated media than type 304, and can fully replace type 304. The corrosion resistance and applications of 4 316L stainless steel are basically the same as those of 316 stainless steel. Due to its lower carbon content (≤0.03%), it exhibits better weldability and corrosion resistance after welding, making it suitable for semi-welded or fully welded PHEs. Stainless steel type 5317 is suitable for applications that require a longer service life than type 316. Due to slightly higher contents of Cr, Mo, and Ni elements compared to type 316, it exhibits better resistance to crevice corrosion, pitting, and stress corrosion. 6 AISI 904L or SUS 890L stainless steels: These are cost-effective austenitic stainless steels that offer an excellent balance between price and corrosion resistance. Their corrosion resistance is better than that of the other materials mentioned, making them particularly suitable for use in acids such as sulfuric acid and phosphoric acid, as well as halides (containing Cl— and F—). Due to its high contents of Cr, Ni, and Mo, it exhibits excellent resistance to stress corrosion, pitting, and crevice corrosion. 7 Avesta 254 SMO is a super-low carbon high-grade stainless steel that represents an improvement over type 316 through an increased Mo content; it boasts excellent resistance to chloride pitting and crevice corrosion, making it suitable for use in saline solutions, inorganic acids, and other media where type 316 is not appropriate. 8 Avesta 654 SMO is a super-low carbon high-grade stainless steel with chromium, nickel, molybdenum, and nitrogen contents that are all higher than those of 254 SMO; it exhibits better resistance to chloride corrosion than 254 SMO and can be used in cold seawater. 9 RS-2 (OCr20Ni26Mo3Cu3Si2Nb) stainless steel; this is a domestically produced Cr–Ni–Mo–Cu stainless steel. Its resistance to pitting and crevice corrosion is equivalent to that of type 316, while its resistance to stress corrosion is better. It can be used with concentrated sulfuric acid (concentration 90–98%) at temperatures below 80 °C, with an annual corrosion rate of ≤0.04 mm/a. 10 Incoloy 825(S) is a high-grade stainless steel composed of Ni (40%)–Cr (22%)–Mo (3%). Incoloy is a registered trademark of the International Nickel Co. Suitable for sulfuric acid of various concentrations at low temperatures ； It exhibits good corrosion resistance in caustic alkali solutions (such as NaOH) with concentrations of 50% to 70%, and does not suffer from stress corrosion cracking. However, it is very sensitive to chloride-induced crevice corrosion. Furthermore, its stamping performance is also poor, so it is not a commonly used material for sheets. 11 31 alloy: An improved version of 904L (with increased Mo and N contents); a standard high-grade stainless steel containing 6% Mo (31% Ni-27% Cr-6.5% Mo-32% Fe). Better corrosion resistance in many media than 904L ； In sulfuric acid at concentrations of 20% to 80% and temperatures of 60°C to 100°C, its corrosion resistance even exceeds that of C-276. 12 33 alloy: A fully austenitized chromium-based high-grade stainless steel whose corrosion resistance is comparable to that of certain Ni-Cr-Mo alloys such as Inconel 625. It exhibits good resistance to localized corrosion and stress corrosion cracking in acidic and alkaline media, including nitric acid and mixtures of nitric acid and hydrofluoric acid ； Its corrosion resistance in concentrated nitric acid is much better than that of 304L. For example, it is suitable for sulfuric acid with a concentration of over 96%–99%, a temperature of ≤150°C, and a sulfur oxide content of less than 200 mg/L ； Hot seawater ； Highly corrosive boiling solutions with a concentration of ≤50% ； Phosphoric acid with a concentration of ≤85% and a temperature of ≤150°C, etc. However, it is not suitable for reducing media (such as dilute sulfuric acid). The price is roughly the same as that of C-276. 13 C-2000 alloy: A nickel-based alloy developed in the 1990s; its price is similar to that of C-276, and it is one of the materials with the best corrosion resistance among those listed. In media such as sulfuric acid at concentrations below moderate levels, dilute hydrochloric acid, phosphoric acid at concentrations of ≤50% at boiling temperature, and hot chlorides, its corrosion resistance is better than that of C-276 and C-22, showing a trend to replace C-22 alloys. However, for sulfuric acid with a concentration of ≥70%, its corrosion resistance is inferior to that of C-276. 14 59 alloy: In terms of chemical composition, it is similar to C-2000; the only differences are a slightly higher Ni content (59%), lower Fe levels, and the absence of Cu and W, with everything else being essentially the same. It is currently the nickel-based alloy with the best corrosion resistance, thermal stability, formability, and weldability. Since its commercialization in 1990, it has been widely used in sulfuric acid, hydrochloric acid, hydrofluoric acid, as well as many media containing chlorine, oxygen, and at low pH levels.