Corrosion of typical chemical acids and material selection

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I. Hydrogen chloride and hydrochloric acid: Dry hydrogen chloride does not actually corrode carbon steel at temperatures below 200°C; its corrosion rate is no more than 0.1 mm/a, rising to 0.5 mm/a at 250°C. The corrosion caused by hydrogen chloride-containing gases is essentially the same as hydrochloric acid corrosion. As long as the temperature remains above the maximum dew point of hydrogen chloride (80–200°C, varying depending on the HCl content and pressure conditions; ideally 250–300°C), the corrosion rate of carbon steel stays within acceptable limits ； Hydrochloric acid is a typical non-oxidizing acid; iron forms ferrous chloride in dilute hydrochloric acid and ferric chloride in concentrated acid, with ferric chloride being soluble in water. 31% industrial hydrochloric acid is dark yellow due to the presence of trivalent iron chlorides, and its corrosivity is significantly higher than that of chemically pure fuming hydrochloric acid (with a concentration of 37–38%) ； In hydrochloric acid, the corrosion of cast iron is more severe than that of carbon steel ； Ordinary stainless steel will suffer from pitting even in 1% hydrochloric acid ； In hydrochloric acid media, only molybdenum-containing stainless steels, titanium (at 300 ppm and below 90°C), silver, Hastelloy, zirconium, and tantalum are available as options; in particular, tantalum does not corrode in hydrochloric acid of any concentration and temperature (up to boiling) even in the presence of ferric chloride and chlorine ； In hydrochloric acid production, a large amount of non-metallic materials are used. Rigid PVC has better resistance to hydrochloric acid than PP, and can be used within any concentration range as long as the allowable operating temperature is not exceeded. In the presence of dilute acids, PP can be used for extended periods at temperatures below 110°C. PE exhibits excellent acid resistance below 80°C. The acid resistance of vinyl ester resins is better than that of bisphenol A polyesters and epoxy resins, but their temperature resistance is 20–30°C lower than that of phenolic resins. PTFE has excellent acid resistance, with a usable temperature below 250°C. Natural rubber exhibits excellent acid resistance in hydrochloric acid at temperatures below 80°C. Impervious impregnated graphite possesses excellent corrosion resistance and is widely used in hydrochloric acid production. II. Sulfuric Acid: Sulfuric acid is an oxyacid. Dilute sulfuric acid has very weak oxidizing properties and belongs to the category of non-oxidizing acids; it primarily causes hydrogen depolarization corrosion. Hydrogen depolarization corrosion refers to a corrosion process in which a depolarizing agent is involved, and the result of this reaction is the continuous dissolution and thinning of the metal. Concentrated sulfuric acid has strong oxidizing properties and belongs to the category of oxidizing acids; it enables certain metals to develop self-passivation capabilities, forming a dense passivation film on their surface. This film is insoluble in concentrated sulfuric acid, thereby preventing further corrosion from occurring. In sulfuric acid production, common metal materials include lead, carbon steel, cast iron, stainless steel, Hastelloy, zirconium, and tantalum. The reaction between lead and dilute sulfuric acid forms a stable, poorly soluble protective layer of PbSO4 on the surface of the lead; however, this layer dissolves when exposed to concentrated sulfuric acid. Therefore, lead can be used when the sulfuric acid concentration is below 60% and the temperature is below 100°C. It is not recommended for use when the sulfuric acid concentration is above 75%. Due to its low mechanical strength, it is generally used as a lining material ； Carbon steel is severely corroded in sulfuric acid with a concentration of less than 60%, but when the sulfuric acid concentration rises above 75%, an oxide iron protective film forms on the metal surface, thereby **improving its corrosion resistance ； Cast iron is more prone to passivation in concentrated sulfuric acid than carbon steel, especially high-silicon cast iron (with a silicon content of 14%–17%). However, Si in cast iron reacts with free SO3 that penetrates into the cast iron to form SiO2; the increase in volume resulting from this causes cracks in the casting, so it cannot be used in fuming sulfuric acid ； Stainless steel is not resistant to corrosion by dilute sulfuric acid ； Hastelloy possesses a much higher corrosion resistance than ordinary austenitic stainless steels, and can be used in a variety of corrosive media aside from sulfuric acid, such as nitric acid (at concentrations below 50%), hydrochloric acid, phosphoric acid, acetic acid, and various organic acids ； Zirconium has good ductility and weldability. In 10%–40% sulfuric acid, at boiling temperature, the corrosion rate of zirconium is 0.0036–0.0063 mm/a, while at 75% sulfuric acid and 100°C, the corrosion rate is 0.55 mm/a. Tantalum is a pure metal with high chemical stability, exhibiting excellent corrosion resistance in many media aside from hydrofluoric acid, such as inorganic acids, aqua regia, and organic acids; however, it is not resistant to fuming sulfuric acid ； In sulfuric acid production, a large amount of non-metallic materials are used. Rigid PVC is generally used at temperatures below 60°C, while PP, PE, PTFE, natural rubber, and impermeable impregnated graphite are also widely employed. III. Phosphoric acid: Phosphoric acid is a triprotic medium-strong acid with stable chemical properties; it has no strong oxidizing or reducing properties. In phosphate production, the reaction slurry contains impurity ions such as ,, and , as well as 35–45% solid particles. The corrosivity of these impurity acids has a decisive impact, while the solid particles cause abrasion; ,, and other substances can destroy the passivation layer of stainless steel, even leading to pitting. Commonly used metal materials in production include: lead, K alloy, Hastelloy, steel grade 20, 316L, UB6, duplex stainless steel 22-5, and others ； Non-metallic materials include graphite, rubber, fiberglass, plastics, etc. IV. Nitric Acid: Nitric acid is a strong oxidizing acid, and its corrosive effect on metals increases as the concentration rises. Commonly used corrosion-resistant metal materials in nitric acid corrosion include carbon steel, aluminum, stainless steel, etc. The corrosion behavior of carbon steel by nitric acid is such that, at a temperature of around 25°C and a nitric acid concentration below 30%, the corrosion rate increases as the concentration rises; when the nitric acid concentration reaches 30%, the corrosion rate reaches its maximum value of 22.5 g/m2·h. When the nitric acid concentration exceeds 50%, carbon steel passivates. When the nitric acid concentration is 70–80%, the corrosion rate is 0.5–0.1 mm/a (corrosion depth). When the concentration exceeds 90%, the corrosion rate increases. As the temperature rises, the passivation film of carbon steel is easily damaged, along with intergranular corrosion ； Aluminum is very stable when the nitric acid concentration is over 95% ； Nitric acid is a strong oxidizing acid that can oxidize the chromium in stainless steel to form a dense layer; the higher the chromium content in stainless steel, the greater its corrosion resistance ； Non-metallic materials resistant to nitric acid corrosion include glass-lined steel, polytetrafluoroethylene, ceramics, and glass, among others. V. Bromine and bromides Bromine has strong oxidizing properties, and both bromine and its compounds are toxic. Dry bromine gas corrosion is a type of vapor-phase corrosion caused by bromine; it is a simple chemical corrosion process, characterized primarily by bromine’s ability to oxidize materials. Many metal materials are resistant to corrosion by dry bromine gas, such as cast iron, high-silicon cast iron, aluminum, copper, nickel, Monel alloy, Hastelloy, tantalum, etc. Non-metallic materials include glass, glass-lined steel, ceramics, silicon carbide, polytetrafluoroethylene, and polytrifluorochloroethylene, among others ； When bromine reacts with water, hypobromous acid and hydrobromic acid are formed; both hypobromous acid and hydrobromic acid are unstable compounds. In bromine water, in addition to the effects of bromine itself, there is also corrosion caused by hypobromous acid and hydrobromic acid. Few metals are resistant to liquid bromine corrosion, including nickel, silver, Hastelloy B and C, and tantalum. Liquid bromine-resistant non-metallic materials: glass, glass-lined steel, ceramics, silicon carbide, polytetrafluoroethylene, and polytrifluorochloroethylene, etc. VI. Hydrofluoric acid: Hydrofluoric acid is a colorless, clear fuming liquid that is a moderately strong acid with extremely high corrosivity. It is highly volatile and emits white fumes when exposed to air. Reacts with metal salts, oxides, and hydroxides to form fluorides. It releases hydrogen when in contact with metals, and can easily cause explosions or combustion when exposed to sparks. Hydrofluoric acid is highly corrosive and toxic. Based on the corrosion properties of hydrofluoric acid, it can be divided into three categories: anhydrous (i.e., liquid hydrogen fluoride), high-concentration hydrofluoric acid, and low-concentration hydrofluoric acid. Since hydrofluoric acid is an electrolyte solution, hydrofluoric acid corrosion is a typical form of electrochemical corrosion. In environments subject to hydrofluoric acid corrosion, carbon steel and low-alloy steel exhibit good corrosion resistance at concentrations above 75%–80% and temperatures below 65°C. Magnesium is an ideal corrosion-resistant material for hydrofluoric acid, and is generally used only for containers. Titanium is suitable for concentrations of 60–100% (at room temperature). Silver can be used in boiling hydrofluoric acid. Monel alloy is an important material resistant to hydrofluoric acid corrosion ； The non-metallic materials resistant to hydrofluoric acid corrosion mainly include PTFE plastics and impermeable impregnated graphite materials. VII. Corrosion by chlorine Gas Chlorine gas is slightly soluble in water, forming hydrochloric acid and hypochlorous acid; hypochlorous acid possesses strong oxidizing properties. The corrosion of chlorine water is similar to that of wet chlorine, but it decreases as the water content increases. When the water content in chlorine is less than 150 ppm, the corrosion rate of carbon steel is less than 0.04 mm/a; in other words, dry chlorine has little corrosive effect on carbon steel. When the moisture content in chlorine is high, metals are quickly corroded in wet chlorine. When its concentration exceeds 150 ppm, stainless steel loses its passivation film on the surface, leading to pitting or stress corrosion cracking. Titanium has excellent resistance to wet chlorine (with over 1.5% moisture content). At the same time, the metal materials resistant to both dry and wet chlorine are only tantalum, silver, platinum, and certain alloys ； Chlorine penetrates and reacts with the vast majority of polymer materials, resulting in the formation of a yellow, oily paste-like substance on the surface – chlorocream. In a chlorine atmosphere, the corrosion rate of PP is higher than that of PE, while PE is slower than rigid PVC. Polytetrafluoroethylene has a lower resistance to chlorine permeation compared to polyvinylidene fluoride; although fluoroplastics possess excellent corrosion resistance and high temperature tolerance, they are not recommended for use in environments containing chlorine. VIII. Hypochlorite corrosion: Hypochlorites such as sodium hypochlorite and calcium hypochlorite are unstable in neutral or weakly acidic conditions; they possess strong corrosive properties, which increase further at high temperatures. Sodium hypochlorite solution exhibits good stability under slightly alkaline conditions; it decomposes easily at near-neutral pH, and its decomposition accelerates further in acidic conditions, with chlorine gas being released. Sodium hypochlorite gradually decomposes during storage, releasing oxygen; atomic oxygen has a strong oxidizing effect. Therefore, in sodium hypochlorite, corrosive agents include HCl, CL2, O, NaOH, and NaCl, among which O and NaCl are highly permeable agents. Thus, in addition to considering the material’s resistance to acid, alkali, and strong oxidative corrosion, its resistance to permeation must also be taken into account. The corrosion caused by calcium hypochlorite is similar to that of sodium hypochlorite, with only a slightly lower degree of corrosion. Titanium exhibits excellent corrosion resistance in sodium hypochlorite solutions; equipment made of titanium does not suffer from issues such as oxygen penetration or crevice corrosion ； Due to oxygen permeation, natural rubber quickly undergoes swelling and blistering; its typical service life is only 2 years. Although soft PVC plastic has good anti-oxidation properties, the plasticizer within the soft sheets is gradually extracted by sodium hydroxide, leading to aging and cracking; its service life is only 2 to 3 years. Butyl rubber sheet linings, PE, and fluoroplastics offer relatively satisfactory performance. IX. Corrosion prevention in acylation processes. Broadly speaking, acylation refers to the process in which hydrogen atoms in the molecules of organic compounds are replaced by acyl groups. Commonly used acylating agents in industry include formic acid (HCOOH), acetic acid, oxalic acid, benzoic acid, phosgene (COCl2), and acetic anhydride. Both the acids and anhydrides of these acylating agents are oxidizing, and stainless steel is resistant to corrosion in oxidizing organic acids.