Corrosion and Protection of Acidic Water Tanks in Sulfur Plants

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Corrosion and Protection of Acidic Water Tanks in Sulfur Plants 1 Introduction The main function of acidic water tanks is to buffer and store the sulfur-containing wastewater discharged from the plants. The composition of the raw water is quite complex, containing various substances such as H2S, NH3, CO2, CN-, phenols, and oils; it is highly corrosive. This corrosion has a severe impact on the anti-corrosion coatings made of epoxy materials used in the tanks, causing bulging, hardening, and damage to these coatings within less than 3 months, rendering them ineffective as anti-corrosion layers. V103 was scrapped after less than two years of use, and the other one suffered from stress corrosion. 2 Analysis of corrosion causes: Due to the relatively harsh operating conditions of the acidic water tank V103 in the sulfur recovery unit, the main operating media are listed in Table 1. When carbon steel operates under such conditions, severe corrosion occurs on its metal surface. Its form of corrosion is electrochemical corrosion, and stress corrosion cracking tends to occur at welds as well as in the base metal where bending forces are applied (such as at the areas where steel plates are compressed and deformed near the bottom of columns in tanks). Furthermore, using ordinary anti-corrosion coatings provides very poor protection; for example, epoxy-based anti-corrosion coatings lose their effectiveness within less than 4 months. Table 1: Major components in wastewater
Serial number | Name | Unit | Maximum concentration range
Serial number | Name | Unit | Maximum concentration range
1 | pH value | 9–10.5 | —
5 | CN－ | mg/L | 3–15
2 | Oil | mg/L | 100–200
6 | 6NH3－N | mmol/L | 2000–15000
3 | S2－ | mg/L | 2000–4000
7 | Temperature | °C | 35–70
4 | Volatile phenols | mg/L | 100–300

2.1 Damage to the coating surface
The corrosion of epoxy, furan, and phenolic coatings that cure at room temperature due to acidic wastewater is caused by small phenolic molecules that can penetrate the coating, leading to swelling and breakdown of the molecular structure of the organic coating. Furthermore, the higher temperature of 65–70°C also causes the anti-corrosion coating to deteriorate over time. Organic coatings have the property of being able to be used at higher temperatures in pure water, as they possess a high glass transition temperature. When used in an aqueous solution containing certain corrosive agents, the glass transition temperature decreases significantly. For example, epoxy enamel can be used for a long time in clean water at around 80°C, but in corrosive aqueous solutions it can only be used below 60°C, and long-term use cannot be guaranteed. 2.2 Metal corrosion beneath the coating: As H2S dissolves in water, it undergoes a corrosive reaction with metals: H2S + Fe → FeS + H2 ↑. FeS reacts with NH3 to form NH4HS, which deposits on the metal surface and leads to corrosion beneath the scale. It can also cause sulfide stress corrosion cracking (SSCC) in areas with stress concentration. NH3 + H2S――NH4HS Ammonia is highly soluble in water (solubility volume ratio 700:1). Ammonia combines with water to form the relatively stable crystalline hydrate NH3 under low-temperature conditions. H2O。 However, it has a low melting point (-78.85°C), and its electrolytic formula is NH3. H2O－NH4＋＋OH－ produce ions and electrolyte, leading to electrochemical corrosion. Furthermore, the combined effect of hydrogen sulfide and ammonia exacerbates the corrosion. In the presence of cyanide (CN－), at a pH greater than 7.5, cracking increases as the concentration of CN－ in the medium rises. When NH4HS reacts with NH3: HN4HS + NH3 → (NH4)2S. Ammonium sulfide, (NH4)2S, increases the solubility of H2S in water, thereby raising the HS– concentration. On the other hand, when ammonia dissolves in water, it raises the pH of the water, creating more favorable conditions for the reaction between CN– and FeS. However, the concentration of NH3 in aqueous solutions is 6000 mg/L, which is far above the acceptable range (the concentration of NH3 should generally be less than 1000 mg/L). Electrochemical heterogeneities always exist on the surface of metal materials. Defective areas or weak points on the metal surface, having a lower potential than other areas, act as active sites that provide crack initiation sites for stress corrosion. If the material already has scratches, holes, or gaps, they are sources of cracks. Therefore, corrosion cracking mostly occurs at the welds on the tank wall and in the heat-affected zones, as well as at the stressed areas of the columns at the bottom of the tank. 3 Criteria for Material Selection 3.1 Selection of Corrosion-Resistant Materials Based on an analysis of metal surface corrosion, determining how to choose a corrosion-resistant coating that can be used under those conditions in order to address the issue of metal corrosion. At the beginning of February 2003, coating coupons were gradually used to select corrosion-resistant coatings suitable for such conditions. Four types of coating coupons were used for the tests, with the coupons immersed in water by suspension. The longest period was 127 days, and the shortest was 34 days. The specific details are shown in Table 2. It can be seen from the table that the titanium nanopolymer coating performs well on test specimens, while the furan-modified coating performs satisfactorily overall, but still has certain issues. Other materials have significant issues and are not suitable for use in such environments. Table 2: Results of the coating hanging test on V103 acidic water tanks. Sequence Number, Surface Material, Method of Placement, First Placement Date (Month.Day), Conditions during Intermediate Inspections, First Removal Date (Month.Day), Surface Changes, Second Removal Date (Month.Day), Immersion Time (days), Surface Changes: 1. Furan-modified coating – upper part of the water; 2.11, 5.7 – Small bubbles appeared on the surface; 6.18, 127 – More small bubbles appeared, with liquid seeping from within them. 1. The coating was not damaged, but it softened and its strength decreased. 2. For the olefin coating, in the upper part in water: 2.115.7 – bubbling on the surface; 6.18127 – an increase in the number of bubbles, swelling and delamination, resulting in loss of usability. 3. For the titanium nanocoating, in the upper part in water: 2.175.7 – no changes on the surface; 6.18123 – no changes on the surface, the coating remained shiny; scratch testing using a scratch tester showed that the coating retained its original strength and adhesion. 4. For the WHJ coating, in the upper part in water: 3.205.7 – partial bubbling and cracking; 6.1890 – extensive swelling of the coating with local cracking; it was no longer usable. 3.2 Reason for using titanium nanocoating: Based on the test results, titanium nanopolymer coating was chosen. Titanium nanopolymers are created by ultra-fining titanium to the nanoscale, thereby **increasing its surface activity**. At the same time, the double bonds of the organic compounds are broken to form free bonds, and these combine with each other through chemical adsorption and chemical bonding to produce titanium nanopolymers. Its coating is a two-component coating composed of titanium nanopolymers, resins, curing agents, additives, and a small amount of solvent. It has the following characteristics: 3.2.1 Strong resistance to permeation – The Ti nanopolymer and resin form chemically bonded and chemically adsorbed structures, thereby blocking the pathways for permeation between the filler and the resin. Tiny titanium nanopolymer particles are filled into the molecular pores; since water, oxygen, and other ions cannot pass through the titanium nanopolymer particles themselves, they must find alternative paths to penetrate, which lengthens the penetration route and creates a maze-like effect. C titanium nanopolymers have an organic coating layer, which gives them wetting resistance and reduces capillary action, thereby preventing polar media and ions from passing through the coating. 3.2.2 High corrosion resistance: Type A has good impermeability, which prevents the passage of water, oxygen, and ions, thereby endowing the coating with shielding properties. When curing, B titanium nanopolymer coatings exhibit a low volume shrinkage rate. Moreover, the bonding state between the free titanium and other components renders the molecular chains flexible and capable of rotating, which helps to eliminate internal stresses. As a result, these coatings have very low stress levels; there are no microcracks within them, and they possess strong resistance to cracking and peeling. The above two improve the ability to prevent physical damage. The C titanium filler has good corrosion resistance on its own. D. Chemical bonding and chemical adsorption create stable structures that prevent water, oxygen, and other corrosive agents from taking their place, thereby reducing the likelihood of corrosion reactions and enhancing the resistance to chemical corrosion damage. Therefore, these four factors determine that titanium nanopolymer coatings possess excellent corrosion resistance. 3.2.3 Good temperature resistance: In titanium nano-coated materials, when the temperature reaches the glass transition temperature of the resin, the movement of the resin bonds is restricted by the chemical bonding and filling effect of the titanium nanoparticles; as a result, the free volume voids cannot increase, and the material retains good impermeability, making it difficult for corrosive agents to penetrate. Its temperature resistance is more than 50°C higher than that of coatings based on the same resin. For example, the temperature resistance of epoxy coatings is generally 80°C, while that of epoxy titanium nanopolymer coatings reaches 150°C. 4 Performance and Economic Analysis 4.1 Performance After one year of use, an inspection of the container revealed that the anti-corrosion coating remained intact; its surface was glossy, with no signs of peeling, bubbling, cracking, or flaking. There are no rust products on the surface of the anti-corrosion coating. 4.2 Economic analysis: High solid content: The solid content of titanium nano-polymer coatings is over 75%, whereas that of ordinary coatings is around 35%. Therefore, when applying the same dry film thickness, it is about 1/2 of that of conventional coatings. Light density: The density of titanium nanopolymer coating layers is 1.17–1.20 g/cm3, while the density of other heavy-duty anti-corrosion coating layers is above 1.50 g/cm3. Therefore, when the dry film thickness is the same, this coating saves 20–30% more compared to other coatings in terms of the amount required to coat the same area. Dry film thickness of the coating: With the same dry film thickness, the service life of this coating is more than twice that of conventional coatings. In other words, when the required service life is the same, the dry film thickness of this material can be 2/3 of that of conventional coatings, yet its corrosion resistance remains no less than that of conventional coatings. 5 Conclusion: Strong impermeability: It has greater impermeability than ordinary special anti-corrosion coatings. High corrosion resistance: When used under these conditions, it is more resistant to corrosion than ordinary anti-corrosion coatings. Good heat resistance: Its heat resistance is more than 50°C higher than that of coatings based on the same resin. Good water resistance: The anti-corrosion coating does not stick back or become brittle with long-term use. Therefore, it relates to the ability of this anti-corrosion coating to be used in various media containing H2S, NH3, CO2, CN－, phenols, etc., as well as in water at temperatures around 60°C. It solves the problem of conventional special anti-corrosion coatings, which are resistant to corrosion but not to temperature changes. A new method has been found for the corrosion and stress corrosion prevention of the acidic water tank in the desulfurization unit.