Select appropriate materials to reduce the costs of corrosion protection projects

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Looking back at this year, the increase in epoxy resin prices has been the highest in a decade. Taking E-44 as an example, its price has risen almost by half, from 15,000 yuan per ton at the beginning of the year to 26,000 yuan per ton now. Epoxy resin is the most commonly used resin material in chemical anti-corrosion projects, and its rising price directly contributes to increased project costs. This article aims to discuss with colleagues in the anti-corrosion field how to reduce corrosion prevention costs through appropriate material selection, given the persistently high prices of epoxy resins. Epoxy resin, commonly known as \"universal glue,\" is also the most widely used in anti-corrosion projects, and can almost be considered \"universal\" as well. Using epoxy resin to make fiberglass reinforced plastic ； Lining brick slabs and granite with epoxy mortar ； Applying epoxy paint and the like has almost become the most common choice. In fact, making a proper choice of corrosion-resistant resin can sometimes save money and may also yield better results. Especially with the high prices of epoxy resins these days, it is even more necessary to consider the possibility of using other resins. Here is a comparison of the prices of several current corrosion-resistant resins and epoxy resin: E-44 epoxy resin (bisphenol A epoxy resin) – 24,500–26,500 yuan per ton; 2130 phenolic resin (thermosetting phenolic resin) – 14,500–16,000 yuan per ton; furan resin (furfuryl alcohol type) – 16,000–18,000 yuan per ton; furan resin (furfuryl alcohol and furfural type) – 14,500–15,500 yuan per ton; xylene-based unsaturated polyester resin – 16,000–17,000 yuan per ton; vinyl ester resin – 22,000–25,000 yuan per ton. It appears that among these corrosion-resistant resins, epoxy resin has the highest price. In fact, every resin product has its reason for existing. There is no single product type that is perfect for everything, and there is no need to favor one over another. In today’s competitive society driven by the economy, many companies have become private enterprises; it is something worth considering that it is possible to get things done with less money. Let’s talk about phenolic resin first. Phenolic resin is a type of resin with a longer history than epoxy resin. Its current price is 60% of that of epoxy resin. It is well known that epoxy resins have good adhesiveness and physical-mechanical properties. In this regard, phenolic resin is slightly inferior, but it is still quite close. In terms of corrosion resistance: phenolic resin has better acid resistance than bisphenol A epoxy resin ； Its resistance to neutral salts and acidic salts is no worse than that of epoxy resins ； Its solvent resistance is also similar to that of epoxy resins. Phenolic resins are not resistant to alkalis and alkaline salts, which is a disadvantage compared to epoxy resins. In terms of heat resistance, phenolic resins are superior to epoxy resins. Typically, epoxy resins can be used at temperatures below 80°C in the liquid phase, whereas phenolic resins can be used at 100°C, or even below 120°C. Phenolic resins have good flame-retardant properties, which is an advantage over bisphenol A epoxy resins. In certain anti-corrosion applications, epoxy resin cannot be used; instead, phenolic resin must be employed for corrosion protection. For example, in the phosphate fertilizer industry, the phosphate reaction tank used for wet phosphoric acid production has a reinforced concrete structure, and it is equipped with a mixer inside. Phosphorus ore powder reacts with concentrated sulfuric acid in a reaction tank to produce phosphoric acid. The corrosive environment in the phosphoric acid reaction tank is a mixed acid consisting of concentrated sulfuric acid, phosphoric acid, and hydrofluoric acid (hydrofluoric acid is generated under the action of sulfuric acid due to the presence of fluorides in phosphate ore) ； Reaction temperature 90℃ ； Due to the medium being a mixture of solid and liquid phases, erosion is also a concern. Phosphoric acid reaction tanks are commonly lined with multiple layers of graphite sheets using phenolic resin mortar for corrosion protection. Phenolic resin is suitable as a substitute for epoxy resin for corrosion protection in the following situations: – Fiberglass or brick-lined acid storage tanks; – Corrosion protection for floors, tanks, and trenches that are in contact with acids, neutral substances, and acidic salts. – In environments where the temperature is around 100°C and there is no alkaline medium. Secondly, among the options for corrosion-resistant resins, xylene-type unsaturated polyester resin deserves special attention. Xylene unsaturated polyester resin is synthesized by polycondensing xylene formaldehyde resin and unsaturated dicarboxylic acids as the main raw materials. Its corrosion resistance and temperature resistance are similar to those of epoxy resin. The corrosion resistance of the two types of resins at room temperature in certain media is compared as follows:

**Medium and concentration:**
- Bisphenol A epoxy resin:
- Sulfuric acid ≤60%: Resistant; Resistant
- Sulfuric acid ≤80%, 70–80%: Not resistant; Resistant
- Hydrochloric acid ≤31%: Resistant; Resistant at any concentration
- Hydrofluoric acid ≤5%: Moderately resistant; Not resistant at 10%
- ≤30%: Resistant
- Nitric acid ≤10%: Moderately resistant; Not resistant at 20%
- ≤40%: Resistant
- Sodium hydroxide 40%: Resistant; Resistant
- Sodium chloride: Resistant; Resistant
- Sulfuric acid anhydride: Resistant; Resistant
- Ethanol: Resistant; Resistant
- Benzene: Resistant; Not resistant

Under alternating exposure to 5% sulfuric acid and 5% sodium hydroxide, both resins are resistant.

Xylene-based unsaturated polyester resin can essentially replace epoxy resin in most applications where epoxy resin is used for corrosion protection, and its operating temperature is also below 80°C, just like that of epoxy resin. Its current price is only 60% of that of epoxy resin. Xylenic unsaturated polyester resin has a lower viscosity, making it easier to apply. The resin is cured by adding initiators and accelerators. Xylene-based unsaturated polyester resin can be used for the following purposes: – Manufacturing corrosion-resistant fiberglass-reinforced plastic equipment and pipes; – Using resin mortar for lining acid-resistant bricks and granite; – Applying fiberglass reinforcement to concrete tanks and basins such as wastewater tanks; – Creating corrosion-resistant flooring using resin mortar; – Utilizing it in the form of flake coatings. Examples of applications include Baosteel’s 2030 cold-rolling wastewater treatment system, where the wastewater contains dilute sulfuric acid, hydrochloric acid, sodium hydroxide, chromium ions, oil, etc ； Medium temperature: 50-70°C. Anti-corrosion structure: One coat of epoxy resin primer ； Dibutyl trioleate xylene-based unsaturated polyester resin fiberglass insulation layer ； A 150X150X20 acid-resistant brick surface layer constructed with resin mortar. Construction began in September 2000, and an inspection in March 2004 showed excellent results, with no signs of corrosion detected. Finally, it should be noted that when the temperature of the corrosive medium is high (100–160°C), furan resin should be chosen. Vinyl ester resins have better corrosion resistance than epoxy resins, and they also offer better workability; therefore, they are often the preferred choice in environments with high corrosion levels. The current prices of these two types of resins are also lower than those of epoxy resins. In addition to considering the choice of resin type, using composite resins can also be considered to reduce costs. A commonly used combination is epoxy resin and furan resin ； Epoxy resin combined with phenolic resin. Composite resins are cured using curing agents for epoxy resins, with epoxy resins generally accounting for over 70% of the composition. The common ratio is: epoxy resin/furan resin = 70/30 ； Epoxy resin/phenolic resin = 70/30. Some colleagues have suggested increasing the proportion of furan or phenolic resin; in the author’s opinion, this is not appropriate for curing at room temperature. Furan resins and phenolic resins can undergo cross-linking reactions with epoxy resins when heated; if heating for polymerization is possible (to 110–160°C), it is indeed possible to increase the proportion of furan or phenolic resins to 50%. But our building corrosion prevention projects basically lack the conditions for heat polymerization. To ensure the degree of resin curing, it is not advisable to increase the proportion of furan or phenolic resins. When using epoxy/phenolic composite resins, it is best not to use T31 curing agents; instead, aliphatic amine curing agents with a lower molecular weight and a higher amine value can be used directly. The reason is that the T31 curing agent itself is a phenol-formaldehyde-modified phenolic amine curing agent, and there seems to be duplication in the formulation ； Additionally, mixing epoxy resin, phenolic resin, T31, and propylene can cause the formation of slag. Using coal tar-modified epoxy resin for corrosion protection can significantly reduce costs. Choosing the appropriate one is indeed a good approach. The composite material with an epoxy resin/coal tar ratio of 1/1 can resist corrosion by media such as ≤60% sulfuric acid, ≤31% hydrochloric acid, ≤10% nitric acid, ≤10% acetic acid, ≤10% chromic acid, ≤5% hydrofluoric acid, sodium hydroxide, sodium carbonate, sodium chloride, sodium sulfate, and benzene. Epoxy coal tar with a usage temperature of ≤60°C can be used to manufacture grout, mortar, fiberglass-reinforced plastic, and coatings. If the ingredients are inconvenient to use, the epoxy coal tar adhesive can be processed by a factory. We have specifically formulated materials such as epoxy coal tar cement for certain projects. Examples of epoxy coal tar usage: An electroplating plant used this material as a fiberglass insulation layer for the workshop floor ； Acid-resistant bricks built with clay mortar and granite flooring ； FRP lining for electroplating wastewater tanks. The factory has been using this material for over 8 years with satisfactory results.