Resin corrosion protection in the chlor-alkali industry

Thread Content

The chlor-alkali industry is a large sector, and it is also a key end-market for heavy-duty corrosion protection where resin-based coatings are widely used. Chengdu Eighth Hospital likely has the most design plans in this field in China. In this post, I will briefly summarize the resin-based anti-corrosion solutions for the chlor-alkali industry. What is the chlor-alkali industry, and what are its typical corrosive agents? Alkali, hydrogen, chlorine, and chlorine-based products are produced by electrolyzing salt solutions; this is the chlor-alkali industry. All the raw materials, intermediates, semi-finished products, and finished products involved in this process are highly corrosive. During electrolysis, due to the large amount of electricity applied, stray current corrosion is particularly severe. The main corrosive agents in the chlor-alkali industry include: brine and the fluids inside electrolyzers, wet chlorine gas, inorganic acids, oxides, and alkaline solutions. Common corrosion-resistant structural materials used in the industry today include carbon steel, cast steel, austenitic stainless steel, ultra-pure ferritic stainless steel, nickel and high-nickel alloys, copper, titanium, and other metal materials ； Non-metallic materials such as graphite, polyvinyl chloride, fiberglass reinforced plastic, and rubber-lined materials. Resin-based anti-corrosion methods fall under polyvinyl chloride (thermoplastic lining) and fiberglass-reinforced plastic. Main corrosion sites in the chlor-alkali industry and corresponding strategies? First is saltwater corrosion. Steel materials cannot be used in equipment with saline systems, as the corrosion of metal in saline water is oxygen-induced depolarization corrosion, which causes rapid rusting. Domestic saltwater system equipment, such as salt dissolving devices, surge tanks, water distribution tanks, and pipelines, is mostly protected with rubber lining or fiberglass, which basically meets the requirements of production. Settlers can also be lined with epoxy fiberglass or epoxy coal tar pitch. Among them, the preheater of the salt water pump suffers the most severe corrosion, as the stray currents generated by hot salt water are extremely harmful. As a result, some chlor-alkali plants now make a one-time larger investment in titanium to avoid such problems with the preheaters. (Stray current, detailed explanation: Leakage current that deviates from the normal path.) When the leaked part comes into contact with areas where there is poor insulation from the ground, it will corrode metal parts such as carbon steel. Common methods to prevent corrosion caused by stray currents include improving insulation, using additional short-circuiting devices, and employing sacrificial anode protection. Both the raw materials and products in electrolyzers are highly corrosive. Whether it is a diaphragm electrolyzer or a mercury electrolyzer, corrosion of the cell walls is severe, especially at the areas where there is prolonged contact with the liquid surface ； The temperature of the raw materials and products in contact with the bottom of the groove is higher, their concentration is greater, and they are all highly alkaline solutions that cause corrosion through prolonged immersion ； The cover at the top of the groove is primarily subject to high-temperature gas-phase corrosion, with high-temperature, high-concentration wet chlorine gas corrosion being a common form of this corrosion. The design proposals currently developed by domestic design institutes (the main proposal from the Eighth Research Institute is also of this kind) include: rubber lining, granite tile lining, highly corrosion-resistant fiberglass, and PVC/FRP lining made by winding thermoplastic polyvinyl chloride around fiberglass. The methods commonly used for these purposes represent, relatively speaking, the most cost-effective solutions available among design firms at present. To achieve a longer service life, titanium metal can be used; however, due to its high cost, it is rare for clients to opt for titanium metal in all aspects of the construction. Next is chlorine gas. Chlorine corrosion covers a wide range of substances: hot and humid chlorine gas, dry chlorine gas, chlorinated concentrated sulfuric acid, hot hydrochloric acid, concentrated hydrochloric acid, and organic chlorines among other products. The corrosion caused by wet chlorine is actually the corrosion caused by hydrochloric acid and hypochlorous acid. Wet chlorine gas with a temperature below 80 degrees can be handled using FRP pipes lined with bisphenol A-type vinyl ester resin ； If the temperature is even lower, it is sufficient to use PVC pipes, which are easier to process and less costly ； When the temperature is close to 100 degrees, or even above 100 degrees, and the wet chlorine gas reaches temperatures over 120 degrees within a short period of time (usually within 30 minutes), pipes lined with phenolic vinyl ester resin are required; in cases of extremely high temperatures, vinyl ester resins with a high cross-linking density are even necessary. It’s the same principle: titanium or its alloys are better materials, but there is also the issue of high costs. But in recent years, many manufacturers have begun to use this expensive titanium alloy in more and more critical high-temperature applications. On the one hand, design institutes have encountered various accidents over the years due to the use of other materials; lacking confidence, and without conducting in-depth discussions with the R&D personnel from vinyl ester resin manufacturers, it is simpler and less troublesome to opt for special alloys, thereby avoiding any responsibility. Of course, thermoplastic fluoroplastics as well as coatings and chlorinated unsaturated resins can also be used in wet chlorine environments, but due to various limitations, they are not commonly seen in actual engineering project proposals at present. Third is the use of concentrated alkali or alkali at high temperatures; heating is used to evaporate the water from the alkaline solution, thereby creating a concentrated alkali at high temperature. During this process, the chlorine-containing dilute alkaline solution as a raw material, the chlorine-containing concentrated alkaline solution as an intermediate product, and the concentrated alkali at high temperature are all corrosive. In particular, concentrated alkali at high temperatures is extremely corrosive – its corrosiveness is very high when the temperature is above 80 degrees Celsius. At present, for low-temperature dilute alkali solutions and low-temperature concentrated alkali solutions, the most cost-effective options remain rubber-lined materials, fiberglass-reinforced plastics, and PVC/FRP. The anti-corrosion approach using high-temperature concentrated alkalis at 40-80 degrees is currently a subject of much debate; whether to use PVC/FRP or direct vinyl-based fiberglass, each option has its advantages and disadvantages. Currently, equipment manufacturers in this mainstream industry generally adopt two approaches: at lower temperatures, usually below 60 degrees, PVC/FRP is more commonly used, while vinyl-based monolithic FRP is more frequently used at temperatures around 70 degrees. Vinyl resins from different manufacturers have varying levels of ester bonds; failing to select the appropriate grade or a manufacturer that offers better alkali resistance are among the main reasons for accidents. To date, bisphenol A-type vinyl ester resin fiberglass, with a low content of ester bonds, is the preferred material for most FRP equipment manufacturers in the downstream chlor-alkali industry. For concentrated alkalis that are nearly 100 degrees Celsius hot, vinyl resins as mentioned earlier are also used, but no matter which type of resin is employed, its service life is ultimately short (under conditions of constant near-boiling temperatures, it will definitely need to be repaired after two years). In such cases, design institutes and clients prefer special alloys such as nickel and chromium, which can resist corrosion similar to that caused by molten, high-temperature alkalis. Equipment such as concentrated evaporation units and heating tubes often uses these alloys, despite their higher cost, because they offer greater reliability in terms of corrosion resistance and longer service life.