The chromium in austenitic stainless steel undergoes oxidation reactions with corrosive substances, forming a protective film on the surface of the steel that provides corrosion resistance. In a 40% sodium hydroxide solution at 100°C, the corrosion rate of austenitic stainless steel is less than 0.05 mm/a. Low-carbon stainless steels, such as SUS304 and SUS310S, have low yield strength but high flexibility. When the concentration of sodium hydroxide is around 30% and the temperature is approximately 90°C, these low-carbon stainless steels exhibit good resistance to stress corrosion. As a result, ultra-low carbon austenitic stainless steels are often chosen for non-standard equipment or piping.
Catholyte circulation tanks and caustic outlets are exposed not only to high-temperature, highly alkaline substances but also to electrical currents. For these facilities or surrounding pipes, nickel or carbon steel is often used as lining material. When the sodium hydroxide concentration exceeds 50% and the temperature is above 85°C, nickel materials are typically used. To reduce the alkali embrittlement of carbon steel, strict material control is essential. Additionally, heat treatment is frequently applied to weld joints, and weld joint inspections are enhanced to reduce the impact of external forces.
2.2 Common Corrosion Protection Strategies for Chlorine
The chlorine gas generated from the electrolysis of brine typically has a temperature between 75°C and 85°C and contains a large amount of water vapor, making it moist chlorine. Moist chlorine has strong oxidizing properties and can react with metals like iron and copper, leading to severe corrosion of production equipment. To address this issue, corrosion protection measures must be strengthened, such as using titanium or fiberglass materials for moist chlorine pipelines. Titanium is a highly reactive material, but at room temperature, it forms an oxide protective film on its surface that can resist the corrosion of moist chlorine, hypochlorous acid, and other oxidizing acids. Titanium also has good electrical resistance, can withstand high current densities, and maintains stable geometric properties, which effectively extends the service life of production facilities.
In the current chlor-alkali industry, especially during the process of removing moisture from moist chlorine or during cooling, titanium pipes are used to resist chlorine corrosion. Additionally, many other devices, such as anolyte storage tanks, dechlorination towers, scrubbing towers, and coolers, are also made from titanium. At room temperature, titanium can resist hydrochloric acid corrosion at concentrations below 10%, and at 50°C, it can resist 3% hydrochloric acid corrosion. Therefore, when selecting titanium materials for chlorate decomposition, it is essential to carefully control the concentration of hydrochloric acid
Since titanium reacts strongly with dry chlorine, potentially causing severe fire accidents, titanium equipment cannot be used with dry chlorine. To ensure the proper application of titanium materials, the water content in chlorine should be kept above 1.5%. Carbon steel, on the other hand, exhibits strong stability in dry chlorine environments below 90°C, making it suitable for use in materials for chlorine compressors and some pipelines that transport dry chlorine after cooling. It is important to strictly control the temperature below 90°C.
In facilities where dry chlorine and circulating water are used for heat exchange, protective coatings are typically applied to the surface of the pipes to prevent corrosion caused by chloride ions (Cl−) in the circulating water, thereby enhancing corrosion resistance. Additionally, rubber is a commonly used anti-corrosion material, often used as a lining in metal equipment, due to its excellent impermeability and corrosion-resistant properties.
2.3.Common Corrosion Prevention Measures for Acids
Hydrochloric acid is highly corrosive and reacts chemically with active metals, often causing severe damage to production equipment. Equipment in contact with hydrochloric acid typically uses carbon steel lined with hard rubber to enhance its corrosion resistance. For hydrochloric acid with strict requirements on calcium and magnesium content, rubber with low calcium and magnesium content should be used for the lining. Given hydrochloric acid’s strong permeability, crystallization issues often arise at certain temperatures, leading to cracks in the lining. Therefore, regular inspections of rubber linings are necessary. If significant damage is detected, repairs should be made promptly to prevent pipeline ruptures that could disrupt production. In some processes with low flow rates and pressures, CPVC (Chlorinated Polyvinyl Chloride) material can be used for pipelines to provide convenient maintenance and management while resisting hydrochloric acid corrosion.
Fiberglass-reinforced plastic (FRP) has excellent physical flexibility and good corrosion resistance, making it easy to process and shape. It is often used as a lining for pipelines to protect against hydrochloric acid corrosion. Fluoroplastics, known for their strong corrosion resistance and high-temperature endurance, are also effective as liners, though their high production costs and difficulty in forming make them less commonly used. Impermeable graphite offers excellent resistance to hydrochloric acid corrosion and can withstand temperatures of 2000-3000°C, as well as 400°C in oxidative environments. As a result, graphite is often used in hydrochloric acid synthesis processes.
In chlor-alkali production, other acidic corrosives like sulfuric acid and hypochlorous acid can also cause corrosion to metal equipment. PVC (Polyvinyl Chloride) is highly corrosion-resistant and can withstand corrosion from dilute nitric or sulfuric acids. However, PVC’s thermal resistance is weak, typically only functioning under temperatures of up to 65°C, and its flexibility is poor, making it prone to deformation. From a cost and lifespan perspective, carbon steel is commonly used for concentrated sulfuric acid transport pipelines. Hypochlorous acid pipelines often use PVC or CPVC materials, while low-temperature hydrochloric acid pipelines also frequently use CPVC. For short-distance transport, PE (Polyethylene) pipes supported by pipe racks are suitable, while high-temperature hydrochloric acid pipelines require steel-lined PTFE (Polytetrafluoroethylene) or steel-lined PO (Polyolefin) materials.
Considering the environmental impact of acidic substances, their transportation usually occurs in sealed environments with minimal leakage to prevent environmental damage.
