Corrosion and Protection in Chlor-alkali Industry Fields such as Brine Purification, Electrolysis of Brine, Cooling and Drying of Chlorine Gas, Hydrogen Chloride, Hypochlorous Acid, Hypochlorites, Bleaching Powder, Vinyl Chloride, Calcium Carbide, and Alkali Concentration (High-Temperature Alkali, Molten Alkali)

The chlor-alkali industry is a large-scale sector and also represents a key end-market for resin-based anti-corrosion solutions, where such applications are particularly concentrated. The Chengdu Eighth Institute has likely developed the largest number of design solutions in this area within China. In this post, I’ll briefly summarize the resin-based anti-corrosion solutions used in the chlor-alkali industry.

What is the chlor-alkali industry, and what are its typical corrosive media?

The electrolysis of brine solutions produces caustic soda, hydrogen gas, chlorine gas, and chlorine-based products—this is known as the chlor-alkali industry. All raw materials, intermediates, semi-finished products, and finished goods involved in this process exhibit strong corrosivity. During electrolysis, due to the large currents being passed through the system, stray-current corrosion becomes particularly severe.

The main corrosive media in the chlor-alkali industry include: brine and the medium inside the electrolytic cell, wet chlorine gas, inorganic acids, oxides, and alkaline solutions.

Currently, common corrosion-resistant structural materials used in the industry include carbon steel, cast steel, austenitic stainless steel, ultra-pure ferritic stainless steel, nickel and high-nickel alloys, copper, titanium, and other metallic materials; as well as non-metallic materials such as graphite, polyvinyl chloride, fiberglass, and rubber-lined materials. Resin-based corrosion protection falls under the category of polyvinyl chloride (thermoplastic lining) and fiberglass.

What are the main corrosion-prone areas in the chlor-alkali industry, and what are the corresponding mitigation strategies?

First is saltwater corrosion.

Steel materials cannot be used directly in equipment for saltwater systems, because the corrosion of metals in saltwater is oxygen-depolarized corrosion, which quickly leads to rusting. In China, equipment for saltwater systems—such as brine preparation units, elevated tanks, water distribution tanks, and pipelines—is mostly protected by rubber lining or glass-fiber reinforced plastic (GRP) coatings, which generally meet production requirements. Settling tanks can also be lined with epoxy GRP or epoxy coal tar. Among these components, the preheater for saltwater pumps experiences the highest degree of corrosion, as the stray currents generated by hot saltwater can be extremely damaging. Therefore, some chlor-alkali plants now opt for a one-time, larger-scale investment—in titanium metal—to avoid these potential problems with preheaters. (Stray currents, explained in more detail: These are leakage currents that deviate from their intended path. When the leaked current comes into contact with areas of poor insulation on the ground, it can cause corrosion of metallic parts such as carbon steel.) Common methods to prevent corrosion caused by stray currents include: improving insulation to a higher standard, installing additional short-circuit devices, and employing sacrificial anode protection for cathodic protection.

The raw materials and products in electrolytic cells are highly corrosive. Whether it’s a diaphragm cell or a mercury cell, the cell walls suffer from severe corrosion—especially at the interface where the liquid level has been in prolonged contact. At the bottom of the cell, where the raw materials and products come into direct contact, the temperature is higher and the concentration is greater, resulting in long-term immersion corrosion by highly alkaline solutions. At the top of the cell, the cover is primarily subjected to high-temperature gaseous-phase corrosion, most commonly from high-temperature, high-concentration wet chlorine gas. Currently, design institutes in China—including the main proposal from the Eighth Institute—typically recommend the following lining options: rubber lining, granite brick lining, heavy-duty anti-corrosion fiberglass reinforced plastic (FRP), and fiberglass-reinforced thermoplastic polyvinyl chloride (PVC/FRP) lining. These are common approaches and, relatively speaking, represent cost-effective solutions currently available from design institutes. To further extend the service life, titanium metal could be used instead; however, this material is currently limited by its high cost, and instances where clients fully adopt titanium are still extremely rare.

Next is chlorine gas.

Chlorine corrosion affects a wide range of media, including high-temperature, wet chlorine gas, dry chlorine gas, concentrated sulfuric acid containing chlorine, high-temperature hydrochloric acid, concentrated hydrochloric acid, and organic chlorides. The corrosion caused by wet chlorine gas is essentially the corrosion induced by hydrochloric acid and hypochlorous acid. For wet chlorine gas at temperatures below 80°C, FRP pipes lined with bisphenol A-type vinyl ester resin can be used. If the temperature drops further, PVC pipes—easier to process and less costly—can be directly employed. However, when the temperature approaches or even exceeds 100°C, and the wet chlorine gas reaches temperatures above 120°C for short periods (typically within 30 minutes), pipes lined with phenolic-type vinyl ester resin become necessary. At extremely high temperatures, pipes made from vinyl ester resins with very high cross-linking densities may even be required. In principle, titanium metals or alloys remain the superior material choice; yet their high cost continues to pose a significant challenge. Nevertheless, in recent years, many manufacturers have increasingly adopted these expensive titanium alloys for critical high-temperature components. One reason is that design institutes, having experienced various accidents over the years when using alternative materials, have lost confidence in those materials. Moreover, without engaging in thorough discussions with R&D personnel from vinyl ester resin manufacturers, they simply opt for the safer and more straightforward approach of specifying special alloys—thus avoiding liability. Of course, thermoplastic fluoroplastics, coatings, and unsaturated resins of the chlorobenzene type can also be used in wet chlorine gas environments. However, due to numerous limitations, these materials currently appear relatively infrequently in actual engineering bid specifications.

Third is concentrated alkali and high-temperature alkali.

Heating causes the water in the alkaline solution to evaporate, resulting in a high-temperature, concentrated alkali. During this process, the raw material—low-concentration chlorinated alkaline solution—as well as the intermediate product—high-concentration chlorinated alkali—and the final high-temperature concentrated alkali are all highly corrosive. In particular, the high-temperature concentrated alkali exhibits extremely severe corrosivity; above 80 degrees Celsius, its corrosiveness becomes remarkably intense. Currently, for low-temperature dilute and concentrated alkalis, the most cost-effective solutions remain rubber-lined materials, glass-reinforced plastics (GRP), and PVC/FRP. However, when it comes to high-temperature concentrated alkalis ranging from 40 to 80 degrees Celsius, the choice of corrosion-resistant materials is currently the subject of much debate. Both PVC/FRP and direct vinyl ester GRP have their own advantages and disadvantages. At present, most equipment manufacturers in this industry adopt one of two approaches: for slightly lower temperatures—generally below 60 degrees Celsius—PVC/FRP is more commonly used; whereas at temperatures around 70 degrees Celsius and higher, whole FRP structures made with vinyl ester resins tend to be preferred. Different manufacturers’ vinyl ester resins vary in their ester bond content. Failure to select the right resin grade or to choose a manufacturer offering better alkali resistance is often the primary cause of subsequent accidents. To date, the most widely adopted material for FRP equipment in the downstream chlor-alkali industry remains glass-reinforced plastic made from bisphenol A-type vinyl ester resins with relatively low ester bond content. Even for concentrated alkalis nearing boiling point at nearly 100 degrees Celsius, some manufacturers still use the aforementioned vinyl ester resins. However, regardless of the resin brand, the service life of such equipment tends to be short—in general, if exposed continuously near boiling point, repairs will certainly be required within two years. Under these conditions, design institutes and project owners tend to favor special alloys such as nickel and chromium, which offer similar corrosion resistance to that of molten, high-temperature alkalis. For instance, equipment like concentrated-effect evaporators and heating tubes more often employ these high-cost alloys, which provide greater assurance in terms of corrosion resistance and service life.

I’ll stop here for now. Later, when I have more time, I’ll share with everyone the FRP equipment manufacturers that excel in the chlor-alkali industry. You guys can definitely spend more time networking and learning from those true experts!