Corrosion of stainless steel by chloride ions

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Problem description: Regarding the corrosion of austenitic stainless steel in chloride environments, various authoritative books specify strict requirements – the chloride content must be less than 25 ppm; otherwise, stress corrosion, pitting corrosion, and intergranular corrosion will occur. However, in practical engineering applications, austenitic stainless steel is often used in situations with high concentrations of chloride ions. Therefore, it is necessary to analyze the corrosion effect of chloride ions on stainless steel, take preventive measures to extend its service life, or select appropriate materials. Content: Cr and Ni are the main alloying elements that confer corrosion resistance to stainless steel. Cr and Ni enable stainless steel to form a highly dense oxide film in oxidizing media, thereby passivating the steel and reducing its corrosion rate in such media, which improves the steel’s corrosion resistance. The activating effect of chloride ions plays an important role in both the formation and destruction of the oxide film on stainless steel. Due to their small size and strong penetration ability, chloride ions can easily penetrate the tiny pores in the oxide film to reach the metal surface, where they interact with the metal to form soluble compounds. This leads to changes in the structure of the oxide film, resulting in corrosion of the metal. The fundamental reason why chloride ions destroy the oxide film is their strong ability to be adsorbed by metals; they are preferentially adsorbed by the metals, displacing oxygen from the metal surface. Since oxygen determines the passivation state of metals, chloride ions compete with oxygen for the adsorption sites on the metal surface; they can even replace the passivating ions that are adsorbed there and form chlorides with the metal. The adsorption of chlorides on the metal surface is not stable, resulting in the formation of soluble substances, which accelerates corrosion. Results of electrochemical studies on the passivation state of stainless steel show that the activation effect of chloride ions on the metal surface occurs only within a certain range; there is a specific potential value at which stainless steel begins to activate. This potential is the breakdown potential of the membrane; the higher the breakdown potential, the more stable the passive state of the metal. Therefore, the stability of the passivation state of stainless steel and its corrosion resistance in various media can be measured by the breakdown potential value. Corrosion failure analysis of stainless steel: 1. Stress corrosion failure: Stainless steel suffers from stress corrosion in a corrosive environment containing oxygen and chloride ions. Stress corrosion failure accounts for about 45% of the 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. (1) Type 304 stainless steel: This is the cheapest and most widely used austenitic stainless steel (used in industrial equipment such as those in the food, chemical, and nuclear industries). 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 at 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). The applicable conditions in chlorinated aqueous solutions are shown in Table 1-34. PRE is 19. (2) 304L stainless steel: Its corrosion resistance and applications are essentially the same as those of 304 stainless steel. Due to its lower carbon content (≤0.03%), it exhibits better corrosion resistance (especially intergranular corrosion resistance, including in the weld area) and weldability, making it suitable for semi-welded or fully welded PHEs. (3) Type 316 stainless steel: Suitable for general organic and inorganic media. For example, natural cooling water, cooling tower water, demineralized water ； carbonic acid ； Acetic acid and caustic alkali solutions with a concentration of <50% ； Alcohols and propylene glycol as solvents ； Dilute nitric acid at a temperature ≤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, as shown in Table 1-34. PRE is 25. (4) 316L stainless steel) S9 M: Its corrosion resistance and applications are essentially the same as those of 316 type. 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. PRE is 25. (5) Type 317 stainless steel: Suitable for applications where a longer service life than that of Type 316 is required. Due to slightly higher contents of Cr, Mo, and Ni elements compared to Type 316, it exhibits better resistance to crevice corrosion, pitting corrosion, and stress corrosion. PRE is 30. (6) AISI 904L or SUS 890L stainless steel: This is an austenitic stainless steel that offers a good balance between price and corrosion resistance; it has better corrosion resistance than the materials mentioned earlier, and is 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. The applicable conditions in chlorine-containing media are shown in Table 1-34. PRE is 36. (7) Avesta 254 SMO premium stainless steel: This is an ultra-low-carbon premium stainless steel that represents an improvement over Type 316, achieved by increasing the Mo content. It exhibits excellent resistance to chloride-induced pitting and crevice corrosion, making it suitable for use in saline water, inorganic acids, and other media where Type 316 cannot be utilized. The applicable conditions in chlorine-containing media are shown in Table 5-11. The PRE is 47. (8) Avesta 654 SMO premium stainless steel: This is a ultra-low carbon premium stainless steel whose contents of Cr, Ni, Mo, and N are all higher than those in 254 SMO. Its resistance to chloride corrosion is even better than that of 254 SMO; it can be used in cold seawater. PRE is 64. (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. The PRE is 29. (10) Incoloy 825(S): This is a high-grade stainless steel containing Ni (40%), Cr (22%), and Mo (3%). Incoloy is a registered trademark of the International Nickel Co. Suitable for sulfuric acid at various concentrations under 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. In addition, its stamping performance is not very good; therefore, it is not a commonly used material for sheets. PRE is 32. III. Several new types of foreign corrosion-resistant alloys: (1) 31 alloy: an improved version of 904L (with increased Mo and N contents); it is 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. PRE is 34. (2) Alloy 33: A fully austenitized chromium-based premium stainless steel whose corrosion resistance is comparable to that of certain Ni-Cr-Mo alloys such as Inconel 625. In acidic and alkaline media (including nitric acid, and mixtures of nitric acid and hydrofluoric acid), it exhibits good resistance to localized corrosion and stress corrosion cracking ； 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 greater than 96%–99%, a temperature ≤150°C, and a sulfur oxide content of less than 200 mg/L ； Hot seawater ； Strongly corrosive solution at boiling point with concentration ≤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. PRE is 50. (3) C-2000 alloy: A nickel-based alloy developed in the 1990s. Its price is similar to that of C-276. It is one of the materials with the best corrosion resistance among those mentioned above. 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 the C-22 alloy. However, for sulfuric acid with a concentration ≥70%, its corrosion resistance is inferior to that of C-276. The PRE for the 76 (4) 59 alloy is 59: In terms of chemical composition, it is similar to C-2000, with the exception that its Ni content is slightly higher (59%), it contains less Fe, and it has no Cu or W; otherwise, the compositions are essentially the same. This is currently the material with the best corrosion resistance, thermal stability, formability, and weldability among nickel-based alloys. Since its commercialization in 1990, it has been widely used in sulfuric acid, hydrochloric acid, hydrofluoric acid, as well as many other media containing chlorine, oxygen, and having a low pH value. PRE is 76, the same as C-2000.