How to choose the right sulfuric acid resistant material
Because of the strong corrosiveness of sulfuric acid, the equipment for producing and using sulfuric acid has caused serious corrosion. In order to prevent or reduce this corrosion and ensure the smooth production, the materials of various sulfuric acid equipment must be selected and used reasonably according to the corrosion characteristics of sulfuric acid and the corrosion resistance of various materials.
Corrosion characteristics of sulfuric acid
Sulfuric acid is mainly produced by contact method and lead chamber method. Sulfur or sulfide, such as copper sulfide, is burned to form sulfur dioxide. Under the action of oxygen and catalyst, sulfur dioxide becomes sulfur trioxides, and sulfuric acid is obtained when sulfur trioxides are dissolved in water. When the concentration of sulfuric acid is more than 100%, and contains free sulfur trioxides, it is called smoke sulfuric acid. For example, acids containing 20% free sulfur trioxygas are called 20% fuming sulfuric acid or 104.5% sulfuric acid.
Dilute sulfuric acid is a kind of non oxidizing acid. With the increase of concentration, sulfuric acid will become oxidized acid, which can be reduced to sulfur dioxide. Therefore, concentrated sulfuric acid can passivate steel and iron, and make ordinary steel corrosion resistant in concentrated sulfuric acid. But sulfuric acid absorbs water in the air, and once sulfuric acid is diluted to a concentration below 68%, the equipment of carbon steel and cast iron will be severely corroded. In non oxidizing dilute sulfuric acid, steel and iron cannot be passivated by the formation of oxide film on the surface. The existence of oxygen and other oxidants will change the corrosion characteristics of dilute sulfuric acid. But in concentrated sulfuric acid, because concentrated sulfuric acid is an oxidant, oxygen and oxidant will not change the corrosion characteristics of concentrated sulfuric acid on metal, just as stainless steel is not affected by oxygen or oxygen.
Sulfuric acid corrosion resistance of carbon steel
Carbon steel is widely used as sulfuric acid equipment with concentration more than 70% at room temperature, such as storage tank, pipeline, tank car and ship bin. Usually, 78%, 93%, 98% sulfuric acid and fuming sulfuric acid are contained. The isocorrosion curve shows that the curve is obviously concave near the concentration of sulfuric acid 101%, indicating that corrosion increases rapidly here. Special attention should be paid to the selection of carbon steel. The curve also slightly concave near the concentration of sulfuric acid 85%, indicating that corrosion increased slightly. The dotted line of the curve indicates that the data is insufficient, and the position of the curve is speculative. Carbon steel cannot be used in sulfuric acid less than 65 percent at any temperature. When the temperature is above 65 ℃, no matter how much sulfuric acid concentration, carbon steel can not be used generally.
Higher flow rate will increase the corrosion of sulfuric acid on carbon steel, such as the short service life of carbon steel pump, but the flow rate of several feet per second does not seem to change its corrosion characteristics. Solid suspension can cause wear and corrosion of carbon steel.
The effect of air filling on Corrosion of carbon steel in concentrated sulfuric acid is very small, because the sulfuric acid itself is oxidable. However, it was found that “air bubble” had destructive effect on the pipeline with concentrated sulfuric acid. For example, a carbon steel pipe line with 93% sulfuric acid was used for a while, and it was found that there was a groove along the top of the inner wall of the pipe, which was deep and sharp, and there was little corrosion on the other inner surface of the pipe. This is obviously caused by “air bubbles” floating above the inner wall of the pipe. Air is inhaled through the cushion of the pump and enters the system with flowing sulfuric acid. The damage of this “air bubble” can be overcome by discharging air or preventing air from entering the pipeline from the pump, valve, etc.
Carbon steel is used in concentrated sulfuric acid. After heating due to welding and other reasons, psoriasis corrosion will appear in some areas. This is caused by spheroidization of pearlite tissue after heating. After normalizing the carbon steel at 850 ℃, it can prevent the occurrence of psoriasis corrosion.
The sulfuric acid corrosion resistance of low alloy steel
The corrosion resistance of low alloy steel in sulfuric acid is similar to that of carbon steel, but the corrosion resistance of some low alloy steels containing copper or molybdenum has been improved in dilute sulfuric acid at room temperature.
Sulfuric acid corrosion resistance of cast iron
The corrosion resistance of common cast iron in sulfuric acid is similar to that of common carbon steel. However, gray cast iron will crack in concentrated sulfuric acid. This is because acid can infiltrate into the cast iron along the continuous sheet graphite structure, causing corrosion inside the cast iron (official account: Pump housekeeper). Due to the increase of the volume of corrosion products, the local stress is very high. With the increase of corrosion products, the local internal stress increases continuously, which finally leads to the cracking and damage of cast iron. Therefore, it is not suitable to use ordinary gray cast iron in sulfuric acid. If the continuous flake graphite structure is transformed into discontinuous spherical structure by heat treatment, the sulfuric acid will not penetrate into the cast iron, thus the cracking of cast iron in sulfuric acid can be prevented. Therefore, the so-called malleable cast iron, ductile cast iron or ductile cast iron have better performance in sulfuric acid.
Galvanic corrosion may occur when cast iron and carbon steel are used together in about 100% sulfuric acid. Because of the brittleness of cast iron and the consideration of safety, it is better to use carbon steel than cast iron when possible.
Sulfuric acid corrosion resistance 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 composition, it seems to belong to steel, from the structure and properties, it is similar to cast iron, so it is usually called high silicon cast iron. It does not contain valuable alloy elements such as chromium and nickel, and has good corrosion resistance. In sulfuric acid, it has better corrosion resistance than all other engineering alloys in the concentration of 0 ~ 100%, 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 change in use, and it will crack and damage in extreme cold and heat. It is also easy to crack when it is subjected to mechanical vibration or impact. Therefore, special care must be taken in manufacturing, installation and use. However, because it is hard, so it is very wear-resistant, especially suitable for use in sulfuric acid containing solid suspended solids or solid impurities.
In recent years, with the addition of rare earth elements, the brittleness of high silicon cast iron has been improved, and pumps, valves, sulphuric acid concentrating heaters, sprayers, small containers, tanks, outlets, pipes, electrodes, bubble caps, other parts of the tower and other fittings have been manufactured.
High silicon cast iron has good corrosion resistance in sulfuric acid up to boiling temperature. However, high silicon cast iron is not suitable for use as common gray cast iron in fuming sulfuric acid with concentration more than 100%. In addition, when the sulfuric acid contains sulfur dioxide and fluoride, the corrosion rate of high silicon cast iron in sulfuric acid will increase significantly. High silicon cast iron can be used at 540 ℃, but slow heating and cooling, shock and vibration must be avoided.
The sulfuric acid corrosion resistance of lead
Lead is widely used in dilute sulfuric acid. In sulfuric acid below 70%, lead has good corrosion resistance, but it is not good in concentrated sulfuric acid. This is the opposite of carbon steel.
In low concentration sulfuric acid, lead is almost free from corrosion, but with the increase of temperature and concentration, the corrosion rate increases. Lead in 90% of the concentrated sulfuric acid becomes very non corrosion-resistant. This is obviously due to the dissolution of the lead sulfate surface film in dilute sulfuric acid in concentrated sulfuric acid.
High purity lead has better corrosion resistance in dilute sulfuric acid, but it is soft and low in strength. Hoof lead has better corrosion resistance than pure lead in concentrated sulfuric acid, but it is not as good as pure lead in dilute sulfuric acid. The lead containing 4-5% antimony is usually called hard lead, which has higher mechanical strength than pure lead, and is suitable for casting, etc., but this “high strength” even disappears when the temperature reaches 87 ℃, even becomes lower than pure lead. In addition, the corrosion resistance of “hard lead” is not good as pure lead, which must be noted when selecting, especially for equipment used at high temperature.
Because lead is soft and easy to wear, high-speed acid or medium containing solid impurities will often destroy the protective film of lead sulfate on its surface, which will accelerate the corrosion of lead. Therefore, lead is not suitable for pump and rarely used as valve. Lead lined containers are often protected by lining acid resistant bricks under high temperature or wear conditions.
Sulfuric acid corrosion resistance of chromium nickel molybdenum copper alloy
Chromium nickel molybdenum copper alloy is specially developed for sulfuric acid. The famous foreign brands are durimet20 (darimet), esco20 (ESCO), carpenter20 (kappint) (forged), fa-20, etc. Since they are similar in composition and properties and are all of 20 serial numbers, they are generally referred to as alloy 20. All of these alloys are austenitic structure, and have similar mechanical properties as 18-8 Cr Ni stainless steel. In China, the alloy has the name ocr23ni28mo3cu3ti; OCr20Ni24Mo3Si3Cu2； Ocr12ni25mo3cu3si2nb, etc.
Darimer 20 alloy is different from lead and carbon steel, and it has good corrosion resistance in the whole sulfuric acid concentration range. In about 78% sulfuric acid, the curve is concave downward, indicating that the corrosion rate of the alloy is relatively large. Iron sulfate and copper sulfate play an important role in corrosion inhibition of alloy 20 in sulfuric acid, which reduces the corrosion rate. However, when the concentration of iron chloride and cuprous chloride is high, the corrosion of alloy 20 will occur.
Alloy 20 is mainly used as pump and valve in sulfuric acid. Many industrial sectors, including factories that make sulfuric acid by contact, use such alloys and are listed as standards. Because the service life of the pumps and valves of carbon steel and cast iron is not long in concentrated sulfuric acid, the lead pump and valve are easy to produce abrasion in dilute sulfuric acid.
Sulfuric acid corrosion resistance of Ni Mo alloy and Ni Cr Mo alloy
The nickel aluminum alloy has the cast chlorimet2 (keloly) and hastelloyb (Hastelloy) alloy which can be cast or forged. They are made up of roughly two-thirds of nickel and one-third of molybdenum. The best corrosion resistance can be obtained after solution heat treatment. Fig. 6 shows the isocorrosion of rimei 2 and hastroyib alloy in sulfuric acid.
It shows that the alloy has good corrosion resistance in sulfuric acid. The corrosion resistance of the sulfuric acid under 8% is slightly poor. In the use of such alloys, attention should be paid to the influence of impurities, especially the influence of oxidizing impurities, such as nitric acid, chlorine gas, hypochlorite, cuprous chloride, iron chloride, iron sulfate and air, etc., which will accelerate the corrosion of such alloys and shorten their life. These alloys can be aged and hardened, but aging often makes the alloy brittle and reduces corrosion resistance.
Nickel chromium molybdenum alloy has cast alloy chlorimet3 and hastelloyc alloy which can be cast and forged. They are a kind of nickel base alloy with about 18% chromium and 18% molybdenum, and have good corrosion resistance in sulfuric acid in the whole concentration range.
Sulfuric acid corrosion resistance of stainless steel
Ordinary stainless steel is not usually used in equipment contacting sulfuric acid. 18-12m o stainless steel is occasionally used in medium containing very dilute sulfuric acid at room temperature or in dilute sulfuric acid solution containing oxidizing components. Because stainless steel has unstable corrosion resistance in sulfuric acid, it is difficult to obtain reliable data and make corrosion diagram.
In room temperature sulfuric acid, stainless steel has good corrosion resistance, but in general, it is better to use carbon steel than stainless steel. Stainless steel can only be used if the medium requires to prevent iron ion pollution or when about 101% sulfuric acid medium carbon steel cannot be used. Chromium stainless steel, chromium nickel stainless steel and chromium nickel molybdenum stainless steel are similar, which have good corrosion resistance in room temperature concentrated sulfuric acid, and the corrosion rate also increases with the increase of temperature. In concentrated sulfuric acid above 60 ℃, the service life is shorter and should not be used.
The corrosion resistance of 1cr18ni11si4a1ti containing silicon is much better than that of ordinary stainless steel in concentrated sulfuric acid, even at higher temperature. 1cr18ni11si4a1ti steel has better corrosion resistance than other stainless steel in 97% sulfuric acid. Therefore, 1cr18ni11si4a1ti can be used in the equipment used in concentrated sulfuric acid at 60-100 ℃ without expensive nickel molybdenum alloy. This is the case with the pumps and valves used in the synthetic alcohol equipment in China, and the effect is remarkable.
Stainless steel containing copper and molybdenum, such as ocr18ni18mo2cu2ti and 1cr17mn9ni3mo3cu2n steel, is mainly used in dilute sulfuric acid. This kind of steel can be used in sulfuric acid with temperature less than 40 ℃ and concentration less than 70%. In sulfuric acid with concentration less than 5%, it can also be used when the temperature is up to 80 ℃. The steel is suitable for the manufacture of castings or forgings such as pumps, valves, etc.
The corrosion resistance of copper alloy by sulfuric acid
Copper and copper alloy are very stable in reducing dilute sulfuric acid, because they have much higher electrode potential than hydrogen, and hydrogen cracking corrosion reaction is not easy to occur in dilute sulfuric acid. This makes copper and copper alloy have much better corrosion resistance than other engineering alloys in dilute sulfuric acid. However, copper and copper alloy are sensitive to oxidation impurities. When dilute sulfuric acid contains oxygen, nitric acid, chromic acid, chlorine gas, hypochlorite, cuprous chloride, iron chloride, iron sulfide and ammonia and ammonia salt, the corrosion of copper and copper alloy will be accelerated. Copper and copper alloy are also not corrosion resistant in oxidized sulfuric acid.
In the open dilute sulfuric acid equipment, copper and copper alloy will be seriously corroded at the water line. Tin bronze can be used in sulfuric acid with a concentration of less than 60% and temperature below 80 ℃. The everdur has a slightly better corrosion resistance than tin bronze. Aluminum bronze is used in dilute sulfuric acid, which is not only because it is more wear-resistant and corrosive, but also less sensitive to oxidation conditions than copper alloy mentioned above. In 40% sulfuric acid, aluminum bronze has good corrosion resistance even when it reaches boiling point.
Nickel white copper has good corrosion resistance in dilute sulfuric acid, especially in boiling sulfuric acid with medium concentration. As long as the medium does not contain oxide impurities, it can be used in sulfuric acid below 55%.
Cu88ni9si2 alloy is suitable for casting pumps and valves, and is suitable for the neutral energy of high temperature dilute sulfuric acid with the temperature less than 50%. 110 × 10 ℃, 40 ℃ by synthetic alcohol ± The service life of 10% sulfuric acid for more than six years is far more than that of other engineering alloys including nickel molybdenum alloy.
Corrosion resistance of other metals and alloys in sulfuric acid
Gold and platinum have excellent corrosion resistance in sulfuric acid. Because of their rarity and high cost, they can not be used as engineering alloys in industry. Only in some special occasions, such as the few artificial silk nozzles requiring minimal corrosion and absorption tower of reagent pure sulfuric acid which cannot be polluted can be used in a small amount.
Tantalum is very corrosion resistant except for the very strong and hot sulfuric acid. Zirconium is very corrosion-resistant in sulfuric acid under 60% concentration and boiling sulfuric acid, and also in concentrated sulfuric acid below 92 ℃. Zirconium hafnium alloy also has good corrosion resistance in sulfuric acid.
Titanium is not corrosion resistant in sulfuric acid except for its very thin concentration, but titanium is sometimes used as corrosion inhibitor, for example, in equipment for smelting nickel ore, titanium is used in tons. The addition of some alloy elements in titanium can improve its sulfuric acid resistance. The Ti 68mo32 has good corrosion resistance in sulfuric acid.
Molybdenum has excellent corrosion resistance in sulfuric acid. The corrosion rate of molybdenum in sulfuric acid under 10-95% 70 ℃ is less than 0.025mm / year, and that in sulfuric acid below 50% to boiling is less than 0.125 mm / year, in 204 ℃, 10% sulfuric acid is less than 0.025mm / year, in 204 ℃, 20% sulfuric acid, less than 0.1mm / year.
Corrosion resistance of nonmetallic materials in sulfuric acid
The mechanical properties of nonmetal materials are not as good as those of metal materials. For example, plastics are not only easy to aging, but also easy to creep damage at a higher temperature. Glass and ceramics are brittle.
It should also be noted that the corrosion resistance of some nonmetallic materials, such as plastics and synthetic rubber, is also affected by their composition, plasticizer and filler, as well as metal materials. Therefore, the corrosion resistance of nonmetallic materials of different grades should be determined by test under given corrosion conditions.
Selection of sulfuric acid resistant materials
Because different materials show different corrosion resistance in sulfuric acid of different concentration or temperature, there is no same rule between them, so the selection of sulfuric acid resistant materials must be based on experiments and experience. George.a.nelson first drew the selection diagram of sulfuric acid resistant materials based on a large number of experimental data and factory experience. He divided the concentration temperature state diagram of sulfuric acid into 10 areas, and the materials with corrosion rate ≤ 0.5mm / year in each area are listed in the table.
However, it should be noted that the corrosion rate only refers to the application of the material in the simple sulfuric acid with static or low flow rate. If the acid contains other impurities which can significantly change the corrosion characteristics of the material or the high velocity of acid will cause the material to wear and corrosion, it cannot be directly used.
Source: China Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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