Due to the strong corrosiveness of sulfuric acid, the equipment for producing sulfuric acid and using sulfuric acid has caused serious corrosion. In order to prevent or alleviate this kind of corrosion and ensure the smooth progress of production, the materials of various sulfuric acid equipment must be correctly selected and used reasonably according to the corrosion characteristics of sulfuric acid and the corrosion resistance of various materials.
1. The corrosion characteristics of sulfuric acid:
Sulfuric acid is mainly produced by contact method and lead chamber method. Sulfur or sulfide, such as copper sulfide ore, is burned to generate sulfur dioxide. Under the action of oxygen and catalyst, sulfur dioxide becomes sulfur trioxide, and sulfur trioxide dissolves in water to obtain sulfuric acid. When the sulfuric acid concentration is greater than 100% and contains free sulfur trioxide gas, it is called oleum. For example an acid containing 20% free sulfur trioxide gas is called 20% oleum or 104.5% sulfuric acid.
Dilute sulfuric acid is a non-oxidizing acid. As the concentration increases, sulfuric acid will become an oxidizing acid, which can be reduced to sulfur dioxide. Therefore, concentrated sulfuric acid can passivate steel and iron, making ordinary steel become corrosion-resistant in concentrated sulfuric acid. However, sulfuric acid will absorb moisture in the air. Once the sulfuric acid is diluted to a concentration below 68%, carbon steel and cast iron equipment will be severely corroded. In non-oxidizing dilute sulfuric acid, steel and iron cannot be passivated by forming an oxide film on the surface. The presence of oxygen and other oxidizing agents alters the corrosive properties of dilute sulfuric acid. But in concentrated sulfuric acid, since concentrated sulfuric acid itself is an oxidant, oxygen and oxidant will not change the corrosion characteristics of concentrated sulfuric acid to metals, just as stainless steel is not affected by oxygen or no oxygen in nitric acid.
Second, the sulfuric acid corrosion resistance of carbon steel:
Carbon steel is widely used as sulfuric acid equipment with a concentration greater than 70% at room temperature, such as storage tanks, pipelines, tank cars and ship warehouses. Usually contain 78%, 93%, 98% sulfuric acid and oleum. Figure 1 shows the effect of sulfuric acid concentration and temperature on the corrosion rate of carbon steel in concentrated sulfuric acid. The isocorrosion curve shows that when the sulfuric acid concentration is around 101%, the curve is obviously concave, indicating that the corrosion increases rapidly here, and special attention should be paid when selecting carbon steel. Around 85% sulfuric acid concentration, the curve is also slightly concave, indicating a slight increase in corrosion here. The dashed portion of the curve indicates insufficient data and the position of the curve is somewhat speculative. Figure 1 also shows that carbon steel cannot be used in less than 65% sulfuric acid at any temperature. When the temperature is above 65°C, no matter how high the concentration of sulfuric acid is, carbon steel generally cannot be used.
Higher flow velocities increase the corrosion of carbon steel by sulfuric acid, as pumps made of carbon steel have a short service life, but flow velocities of a few feet per second do not seem to alter its corrosive properties. Suspended solids can cause wear and corrosion of carbon steel.
Air filling has little effect on the corrosion of carbon steel in concentrated sulfuric acid, because concentrated sulfuric acid itself is oxidizing. However, it was found that "air bubbles" have a destructive effect on the pipelines that pass through concentrated sulfuric acid. For example, a carbon steel pipeline passing through 93% sulfuric acid, after a period of use, it was found that grooves appeared along the top of the inner wall of the pipe, the grooves were deep and sharp, and there was almost no corrosion on the other inner surfaces of the pipe. This is apparently caused by "air bubbles" floating above the inner wall of the tube. Air is sucked through the gasket of the pump and enters the system with the flowing sulfuric acid. The destructive effect of this "air bubble" can be overcome by venting air or preventing air from entering the pipeline from pumps, valves, etc.
Carbon steel is used in concentrated sulfuric acid. After being heated due to welding and other reasons, "psoriasis" corrosion will appear in some areas. This is caused by the spheroidization of the pearlite structure after heating. The carbon steel is normalized at about 850 °C Finally, it can prevent the occurrence of "psoriasis" corrosion.
3. The sulfuric acid corrosion resistance characteristics of low alloy steel:
The corrosion resistance of low alloy steel in sulfuric acid is generally similar to that of carbon steel, but the corrosion resistance of some low alloy steel containing copper or molybdenum in dilute sulfuric acid at room temperature is improved. For example, 09CuWSn steel has much better corrosion resistance than ordinary carbon steel in dilute sulfuric acid at room temperature, and its corrosion resistance in concentrated sulfuric acid is similar to that of ordinary carbon steel.
Fourth, the sulfuric acid corrosion resistance of cast iron:
The corrosion resistance of ordinary cast iron in sulfuric acid is similar to that of ordinary carbon steel. However, gray cast iron will crack in concentrated sulfuric acid. This is because the acid can penetrate into the interior of the cast iron along the continuous flake graphite structure, causing corrosion inside the cast iron (public account: pump steward). Due to the increase in the volume of corrosion products, high local stress is generated. With the increase of corrosion products, the local internal stress continues to increase, which finally leads to cracking and damage of cast iron. Therefore, ordinary gray cast iron is usually not suitable for use in sulfuric acid. If the continuous flake graphite structure is changed into discontinuous spherical structure through heat treatment, the sulfuric acid will not penetrate into the cast iron, thus preventing the cast iron from cracking in sulfuric acid. Therefore, the so-called malleable cast iron, nodular cast iron or ductile cast iron has better performance in sulfuric acid.
Galvanic corrosion can occur when cast iron and carbon steel are used together in around 100% sulfuric acid. Due to the brittleness of cast iron and the consideration of safety, etc., carbon steel should be used instead of cast iron when possible.
5. The sulfuric acid corrosion resistance characteristics of high silicon cast iron:
The typical composition of high-silicon iron is 0.95% carbon, 14.5% silicon, and the rest is iron. From the perspective of composition, it seems to belong to steel, and from the perspective of structure and performance, it is similar to cast iron, so it is usually called high-silicon cast iron. It does not contain precious alloying elements such as chromium and nickel, and has good corrosion resistance. In sulfuric acid, from 0 to 100% concentration, it has better corrosion resistance than all other engineering alloys, so it is widely used in sulfuric acid medium. However, high-silicon cast iron is hard and brittle, difficult to process, and can only be cast. It is very sensitive to temperature changes during use, and it will be cracked and damaged by severe cooling and heat. It is also easy to crack when subjected to mechanical vibration or impact, so special care must be taken in manufacturing, installation and use. However, because it is hard, it is very wear-resistant, especially suitable for solid-suspended
It is used in sulfuric acid with substances or solid impurities.