Corrosion and Material Selection of Typical Chemical Acids

1、 Hydrogen chloride and hydrochloric acid dry hydrogen chloride do not actually corrode carbon steel below 200 ℃, and their corrosion rate is not greater than 0.1mm/a, rising to 0.5mm/a at 250 ℃. The corrosion of water containing hydrogen chloride gas is actually the corrosion of hydrochloric acid. As long as the temperature is above the highest dew point of hydrogen chloride (80-200 ℃, depending on the HCL content and pressure conditions), it is best to be at 250-300 ℃, and the corrosion rate of carbon steel remains within an acceptable range; Hydrochloric acid is a typical non oxidizing acid. Iron forms ferrous chloride in dilute hydrochloric acid and ferric chloride in concentrated acid, which is soluble in water. 31% of industrial hydrochloric acid is dark yellow due to the presence of ferric chloride, and its corrosion is significantly stronger than chemically pure fuming hydrochloric acid (concentration 37-38%); In hydrochloric acid, the corrosion of cast iron is more severe than that of carbon steel; Even in 1% hydrochloric acid, ordinary stainless steel will undergo pitting corrosion; In hydrochloric acid medium, only molybdenum containing stainless steel, titanium (with a content of 300ppM and a temperature below 90 ℃), silver, Hastelloy alloy, zirconium, and tantalum are available for selection, especially tantalum. This metal will not corrode even in the presence of ferric chloride and chlorine in hydrochloric acid at any concentration and temperature (until boiling); In the production of hydrochloric acid, a large amount of non-metallic materials are used, and the resistance of hard PVC to hydrochloric acid is better than that of PP. It can be used in any concentration range as long as it does not exceed its allowable operating temperature. In dilute acid environments, PP can be used for a long time below 110 ℃. PE has excellent acid resistance below 80 ℃. The acid resistance of vinyl ester resin is better than that of bisphenol A polyester and epoxy resin, but its temperature resistance is 20-30 ℃ higher than that of phenolic resin. PTFE has excellent acid resistance and can be used at temperatures below 250 ℃. Natural rubber has excellent acid resistance in hydrochloric acid below 80 ℃. Impermeable impregnated graphite has excellent corrosion resistance and is widely used in hydrochloric acid production.
2、 Sulfuric acid is an oxygen-containing acid, and dilute sulfuric acid has weak oxidizing properties. It belongs to the non oxidizing acid class and mainly produces hydrogen depolarization corrosion. Hydrogen depolarization corrosion refers to the corrosion process used as a depolarizer, resulting in the continuous dissolution and thinning of the metal. Concentrated sulfuric acid has strong oxidizing properties and belongs to the category of oxidizing acids. It can give some metals the ability to self passivate, forming a dense passivation film on the metal surface. This film is insoluble in concentrated sulfuric acid, thus hindering the continued occurrence of corrosion. In sulfuric acid production, commonly used metal materials include lead, carbon steel, cast iron, stainless steel, Hastelloy, zirconium, and tantalum. Lead reacts with dilute sulfuric acid to form a stable and insoluble PbSO4 protective film on the surface of lead, but if it comes into contact with concentrated sulfuric acid, this film will dissolve again. So when the sulfuric acid concentration is below 60% and the temperature is below 100 ℃, lead can be used. When the concentration of sulfuric acid exceeds 75%, it is not recommended to use. Due to the low mechanical strength of lead, it is generally used as a lining material; Carbon steel strongly corrodes in sulfuric acid below 60%, but when the concentration of sulfuric acid increases to over 75%, a protective film of iron oxide will be formed on the surface of the metal, greatly improving its corrosion resistance; Cast iron is more prone to passivation in concentrated sulfuric acid than carbon steel, especially high silicon cast iron (silicon content between 14% and 17%). However, Si in cast iron will react with the free SO3 that infiltrates into the cast iron to form SiO2, and its volume increase will cause casting cracking, so it cannot be used in sulfuric acid production; Stainless steel is not resistant to the corrosion of dilute sulfuric acid; Hastelloy alloy has much higher corrosion resistance than ordinary austenitic stainless steel. In addition to sulfuric acid, it can be applied in various corrosive media, such as nitric acid (concentration less than 50%), hydrochloric acid, phosphoric acid, acetic acid, and various organic acids; Zirconium has good ductility and weldability. Zirconium has a corrosion rate of 0.0036-0.0063 mm/a at boiling temperature in 10% to 40% sulfuric acid, and 0.55 mm/a at 100 ℃ in 75% sulfuric acid. Tantalum is a pure metal with high chemical stability. It has strong corrosion resistance in many media except for hydrofluoric acid, such as inorganic acids, aqua regia, and organic acids. However, tantalum is not resistant to fuming sulfuric acid; In the production of sulfuric acid, a large amount of non-metallic materials are used. Hard PVC is generally used at temperatures below 60 ℃, and PP, PE, PTFE, natural rubber, and impermeable impregnated graphite are also widely used.
3、 Phosphoric acid is a strong ternary acid with stable chemical properties and no strong oxidizing or reducing properties. In the production of phosphoric acid, the reaction slurry contains impurity ions and 35-45% solid particles, among which the corrosiveness of impurity acids has a decisive impact. Solid particles can cause abrasion, damage the passivation film of stainless steel, and even cause pitting corrosion. The commonly used metal materials in production include: lead, K alloy (), Hastelloy alloy, 20 steel, 316L, UB6, duplex stainless steel 22-5, and (), etc; Non metallic materials include graphite, rubber, fiberglass, plastic, etc.
4、 Nitric acid is a strong oxidizing acid, and its corrosion of metals gradually becomes severe with increasing concentration. Corrosion resistant metal materials commonly used in nitric acid corrosion include carbon steel, aluminum, stainless steel, etc. The corrosion characteristics of nitric acid on carbon steel are that when the temperature is around 25 ℃ and the nitric acid concentration is below 30%, the corrosion rate increases with the increase of concentration. When the nitric acid concentration is 30%, the corrosion rate reaches its maximum value of 22.5g/m2 · h. When the concentration of nitric acid exceeds 50%, carbon steel is passivated. When the concentration of nitric acid is 70-80%, the corrosion rate is 0.5-0.1mm/a (corrosion depth). When the concentration exceeds 90%, the corrosion rate increases. When the temperature rises, the passivation film of carbon steel is easily damaged, and there is also intergranular corrosion; When the concentration of nitric acid is above 95%, aluminum is very stable; Nitric acid is a strong oxidizing acid that can oxidize chromium in stainless steel into a dense substance. The higher the chromium content in stainless steel, the stronger its corrosion resistance; Non metallic materials that are resistant to nitric acid corrosion include: enamel glass, polytetrafluoroethylene, ceramics, and glass.
5、 Bromine and bromide have strong oxidizing properties, and bromine and bromide are toxic. Dry bromine gas corrosion belongs to the vapor phase corrosion of bromine, which is a single chemical corrosion and is characterized by the oxidation of materials by bromine. Many metal materials are resistant to the corrosion of dry bromine gas, such as cast iron, high silicon cast iron, aluminum, copper, nickel, Monel alloy, Hastelloy alloy, tantalum, etc. Non metallic materials include glass, enamel glass, ceramics, silicon carbide, polytetrafluoroethylene, and polytetrafluorochloroethylene; Bromine reacts with water to form hypobromic acid and hydrobromic acid, both of which are unstable compounds. In bromine water, in addition to bromine water, there is also corrosion from hypobromic acid and hydrobromic acid. There are very few metals that are resistant to liquid bromine corrosion, including nickel, silver, Hastelloy alloys B, C, and tantalum. Non metallic materials resistant to liquid bromine: glass, enamel glass, ceramics, silicon carbide, polytetrafluoroethylene, and polytetrafluorochloroethylene.
6、 Hydrofluoric acid is a colorless and clear smoky liquid, a medium strength acid with strong corrosiveness and high volatility. It emits white smoke when exposed to air. React with metal salts, oxides, and hydroxides to form fluorides. Encountering metals can release hydrogen gas, while encountering sparks can easily cause explosions or combustion. Hydrofluoric acid has strong corrosiveness and toxicity. According to the corrosion characteristics of hydrofluoric acid, it can be divided into three categories: anhydrous (i.e. liquid hydrogen fluoride), high concentration, and low concentration hydrofluoric acid. Due to the fact that hydrofluoric acid is an electrolyte solution, hydrofluoric acid corrosion is a typical electrochemical corrosion. In the presence of hydrofluoric acid corrosion, carbon steel and low alloy steel have good corrosion resistance when the concentration is above 75% to 80% and the temperature is below 65 ℃. Magnesium is an ideal corrosion-resistant material for hydrofluoric acid and is generally only used as a container. Titanium is suitable for concentrations ranging from 60% to 100% (at room temperature). Silver can be used for boiling hydrofluoric acid. Monel alloy is an important material that is resistant to hydrofluoric acid corrosion; The non-metallic materials that are resistant to hydrofluoric acid corrosion mainly include PTFE plastic and impermeable impregnated graphite materials.
7、 The corrosion of chlorine gas. Chlorine gas is slightly soluble in water, generating hydrochloric acid and hypochlorous acid, which have strong oxidizing properties. The corrosion of chlorine water is similar to that of wet chlorine, but decreases with increasing water content. When the water content in chlorine gas is less than 150 ppM, the corrosion rate of carbon steel is less than 0.04 mm/a, indicating that dry chlorine gas does not corrode carbon steel. When the moisture content in chlorine is high, metals are quickly corroded in wet chlorine. When the content of stainless steel exceeds 150ppM, it will damage the passivation film on the surface of stainless steel and cause pitting corrosion or stress corrosion cracking. Titanium has excellent moisture resistance to chlorine (with a moisture content of over 1.5%). The only metal materials that are resistant to dry and wet chlorine gas are tantalum, silver, platinum, and some alloys; Chlorine gas will penetrate and react with the vast majority of polymer materials, forming a yellow oily paste like product on the surface - chlorocream. In a chlorine atmosphere, the corrosion rate of PP is greater than that of PE, while PE is inferior to hard PVC. The chlorine penetration resistance of polytetrafluoroethylene is not as good as that of polyvinylidene fluoride. Although fluoroplastics have excellent corrosion resistance and high temperature resistance, they are not recommended for use in chlorine media.
8、 Hypochlorite corrodes hypochlorite salts, such as sodium hypochlorite and calcium hypochlorite, which are unstable in neutral or weakly acidic conditions. They are highly corrosive and even more severe at high temperatures. Sodium hypochlorite solution has good stability under slightly alkaline conditions. When approaching neutral, it is easy to decompose, but under acidic conditions, it accelerates decomposition and releases chlorine gas. Sodium hypochlorite will gradually decompose and release oxygen during storage, and atomic oxygen has a strong oxidizing effect. Therefore, in sodium hypochlorite, corrosive media include HCL, CL2, O, NaOH, and NaCL, among which O and NaCL are strong permeable media. Therefore, in addition to considering the material's acid resistance, alkali resistance, and strong oxidative corrosion, the material's impermeability should also be considered. The corrosion of calcium hypochlorite is similar to that of sodium hypochlorite, but the degree of corrosion is slightly lower. Titanium metal has excellent corrosion resistance in sodium hypochlorite solution, and equipment made of titanium will not have problems such as oxygen penetration or crevice corrosion; Natural rubber, due to the penetration of oxygen, quickly causes swelling and foaming of the rubber, with a general service life of only 2 years. Although soft PVC plastic has good antioxidant properties, the plasticizers inside the soft board will gradually be extracted by sodium hydroxide and age and crack, and its service life is only 2-3 years. The application effects of butyl rubber lining, PE, and fluoroplastics are relatively ideal.
9、 The anti-corrosion process of acylation generally refers to the process in which hydrogen in organic compound molecules is replaced by acyl groups. The commonly used acylating agents in industry include formic acid (HCOOH), acetic acid, oxalic acid, benzoic acid, phosgene (COCL2), acetic anhydride, etc. The acids and anhydrides of these acylating agents are oxidizing, and stainless steel is corrosion-resistant in oxidizing organic acids.