How to choose the correct and reasonable use of sulfuric acid corrosion resistant materials!
Due to the strong corrosiveness of sulfuric acid, the production of sulfuric acid and the equipment using sulfuric acid have caused serious corrosion. In order to prevent or reduce this corrosion and ensure the smooth production, various materials of sulfuric acid equipment must be selected correctly and used reasonably according to the corrosive properties of sulfuric acid and the corrosion resistance of various materials.
First
the corrosion characteristics of sulfuric acid.
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Sulfuric acid is mainly produced by contact method and lead chamber method. Sulfur or sulfide, such as copper sulfide ore after combustion to generate sulfur dioxide, under the action of oxygen and catalyst, sulfur dioxide into sulfur trioxide, sulfur trioxide dissolved in water that is sulfuric acid.
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When the concentration of sulfuric acid is greater than 100%, and contains free sulfur trioxide gas is called fuming sulfuric acid. For example, an acid containing 20% free sulfur trioxide gas is called 20% fuming sulfuric acid or 104.5% sulfuric acid.
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Dilute sulfuric acid is a non-oxidizing acid, and as the concentration increases, sulfuric acid becomes oxidizing, and it can be reduced to sulfur dioxide. Therefore concentrated sulfuric acid can passivate steel and iron so that ordinary steel becomes corrosion resistant in concentrated sulfuric acid. -
However, sulfuric acid absorbs water from the air, and once sulfuric acid is diluted to a concentration of 68% or less, carbon steel and cast iron equipment will suffer severe corrosion. In non-oxidizing dilute sulfuric acid, steel and iron cannot be passivated by generating an oxide film on the surface. The presence of oxygen and other oxidizing agents alters the corrosion characteristics of dilute sulfuric acid.
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But in concentrated sulfuric acid, because the concentrated sulfuric acid itself is an oxidizing agent, so oxygen and oxidizing agents will not change the corrosion characteristics of concentrated sulfuric acid on the metal, as stainless steel in nitric acid as unaffected by the passage of oxygen or not.
Second
the carbon steel corrosion resistance of sulfuric acid characteristics.
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Carbon steel is widely used as sulfuric acid equipment with a concentration greater than 70% at room temperature, such as storage tanks, pipelines, tankers and ship bins. Usually contain 78%, 93%, 98% sulfuric acid and fuming sulfuric acid.
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Higher flow rates can increase the corrosion of carbon steel, such as carbon steel pump life is very short, but a few feet per second flow rate, it does not seem to change its corrosion characteristics. Solid suspensions can cause wear corrosion of carbon steel.
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Air-filled carbon steel in the concentrated sulfuric acid corrosion effect is very small, because the concentrated sulfuric acid itself is oxidizing. However, it is found that “air bubbles” have a destructive effect on the pipeline through the concentrated sulfuric acid. For example, a 93% sulfuric acid through the carbon steel pipeline, after a period of time found along the top of the tube wall grooves, deep and sharp grooves, while on the other internal surfaces of the tube almost no corrosion.
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This was apparently caused by “air bubbles” floating along the top of the inner wall of the pipe. Air is drawn in through the pump bedding, and with the flow of sulfuric acid into the system inside, this “air bubble” the destructive effect can be overcome by expelling air or preventing air from the pump, valve, etc. into the pipeline.
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Carbon steel in the use of concentrated sulfuric acid, due to welding and other reasons after the heat, in local areas will appear “psoriasis” corrosion, which is caused by the spheroidization of pearlite organization after heat, the carbon steel by about 850 ℃ after the normalization process can prevent the occurrence of “psoriasis” corrosion.
Third
low-alloy steel sulfuric acid corrosion resistance characteristics.
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on resistance than ordinary carbon steel in room temperature dilute sulfuric acid, and the corrosion resistance in concentrated sulfuric acid is similar to ordinary carbon steel.
Fourth, cast iron corrosion resistance characteristics of sulfuric acid.
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The corrosion resistance of ordinary cast iron in sulfuric acid is similar to that of ordinary carbon steel. But gray cast iron in concentrated sulfuric acid will occur cracking phenomenon. This is because the acid can penetrate into the cast iron along the continuous flake graphite organization, so that corrosion occurs inside the cast iron
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Due to the increase in the volume of corrosion products, and the local high stress. With the increase of corrosion products, the local internal stress increases and finally leads to the cast iron cracking damage. Therefore, ordinary gray cast iron in sulfuric acid is usually not suitable.
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If the continuous flaky graphite organization is turned into discontinuous spherical organization by heat treatment, so that sulfuric acid will not penetrate into the cast iron, thus preventing the cracking phenomenon of cast iron in sulfuric acid. Therefore, the so-called malleable cast iron, ductile cast iron or ductile cast iron will have better serviceability in sulfuric acid.
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When cast iron and carbon steel are used together in sulfuric acid at about 100%, galvanic corrosion may occur. Because of the brittle cast iron and considering the safety reasons, so in the possible conditions should rather use carbon steel and less cast iron.
Fourth
the corrosion resistance of cast iron in sulfuric acid characteristics.
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Corrosion resistance of ordinary cast iron in sulfuric acid is similar to that of ordinary carbon steel. But gray cast iron in concentrated sulfuric acid will occur cracking phenomenon. This is because the acid can penetrate into the cast iron along the continuous flake graphite organization, so that corrosion occurs inside the cast iron
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Due to the increase in the volume of corrosion products, and the local high stress. With the increase of corrosion products, the local internal stress increases and finally leads to the cast iron cracking damage. Therefore, ordinary gray cast iron in sulfuric acid is usually not suitable.
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If the continuous flaky graphite organization is turned into discontinuous spherical organization by heat treatment, so that sulfuric acid will not penetrate into the cast iron, thus preventing the cracking phenomenon of cast iron in sulfuric acid. Therefore, the so-called malleable cast iron, ductile cast iron or ductile cast iron will have better serviceability in sulfuric acid.
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When cast iron and carbon steel are used together in sulfuric acid at about 100%, galvanic corrosion may occur. Since cast iron is more brittle and for safety reasons, carbon steel should be used rather than cast iron when possible.
fifth
Sulfuric acid corrosion resistance of high-silicon cast iron.
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The typical composition of high-silicon iron is 0.95% carbon, 14.5% silicon, the rest is iron. From the composition, it seems to belong to steel, from the organization and performance, it is similar to cast iron, so it is usually called high-silicon cast iron. It does not contain precious alloy 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 media. However, high-silica cast iron is hard and brittle, difficult to machine, and can only be cast. It is sensitive to temperature changes in use, and can be cracked and damaged by extreme cold and heat, and is also more likely to crack when subjected to mechanical vibration or shock, so 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 solids in suspension or solid impurities.
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In recent years, by adding rare earth elements and other methods, the brittleness of high silicon cast iron has been improved, and has been able to manufacture pumps, valves, sulfuric acid concentration heaters, sprayers, small containers, tank outlets, pipes, electrodes, bubble caps, towers another part and other accessories, etc.
sixth
Sulfuric acid corrosion resistance properties of lead.
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Lead is widely used in dilute sulfuric acid. In sulfuric acid below 70%, lead has good corrosion resistance, but not in concentrated sulfuric acid. This situation is just the opposite of carbon steel
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In low concentrations of sulfuric acid, lead is almost corrosion free, but as the temperature and concentration increase, the corrosion rate increases. In more than 90% concentrated sulfuric acid, lead becomes very uncorrosive, apparently because the surface film of lead sulfate, which has a protective effect in dilute sulfuric acid, is dissolved in concentrated sulfuric acid.
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High purity lead has better corrosion resistance in dilute sulfuric acid, but it is soft and low strength, hoof lead has better corrosion resistance than pure lead in concentrated sulfuric acid, but not as good as pure lead in dilute sulfuric acid. Lead containing 4~5% antimony is usually called hard lead, which has higher mechanical strength than pure lead and is suitable for making castings, etc. However, this “high strength” disappears even when the temperature is as high as 87℃, and even becomes lower than pure lead. In addition, the corrosion resistance of “hard lead” is not as good as that of pure lead, which must be paid attention to when choosing, especially for equipment used at high temperature.
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Because lead is soft and easy to wear, so high speed acid or media containing solid impurities will often destroy the protective film of lead sulfate on its surface and accelerate the corrosion of lead, so lead is not suitable for making pumps, and rarely used as valves. Lead-lined containers, in high temperature or with abrasive conditions, often lined with acid-resistant brick to protect the approach.
seventh
chromium-nickel-molybdenum-copper alloy of sulfuric acid corrosion resistance characteristics.
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Chromium-nickel-molybdenum copper alloy is specially developed for use in sulfuric acid, the famous foreign grades Durimet20, Esco20, Carpenter20 (wrought state), FA-20, etc.. The alloys are austenitic and have similar mechanical properties to 18-8 Cr-Ni stainless steel. In China, the grades of these alloys are OCr23 Ni28Mo3Cu3Ti; OCr20Ni24 Mo3Si3Cu2; OCr12Ni25Mo3 Cu3Si2Nb and so on.
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Alloy 20 alloy differs from lead and carbon steel in that it has good corrosion resistance over the entire sulfuric acid concentration range. In sulfuric acid at about 78%, the curve is concave downward, indicating that the corrosion rate of this alloy is larger around here. Iron sulfate and copper sulfate act as corrosion inhibitors in sulfuric acid for alloy 20, resulting in a lower corrosion rate. While ferric chloride and cuprous chloride in sulfuric acid, when the concentration is higher will cause pitting corrosion of alloy No. 20.
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Alloy No. 20 is mainly used as pumps and valves in sulfuric acid. Many industrial sectors, including the contact method of manufacturing sulfuric acid plants, use this type of alloy and has been included as a standard. Because the carbon steel and cast iron pumps, valves in concentrated sulfuric acid service life is not long, and lead pumps, valves in dilute sulfuric acid is easy to produce abrasion.
Eighth
nickel-molybdenum alloy and nickel-chromium-molybdenum alloy of sulfuric acid corrosion resistance characteristics:
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Nickel-molybdenum alloys are cast Chlorimet 2 and can be cast or forged Hastelloy B alloy, etc.. They are roughly composed of two-thirds of nickel and one-third of molybdenum. The best corrosion resistance can be obtained after solution heat treatment. -
These alloys have good corrosion resistance in sulfuric acid. They are slightly less resistant to corrosion in high temperature dilute sulfuric acid below 8%. These alloys in use should pay attention to the influence of impurities, especially oxidizing impurities, such as nitric acid, chlorine, hypochlorite, cuprous chloride, ferric chloride, ferric sulfate and the influence of ventilation air, etc. They tend to accelerate the corrosion of these alloys, so that the life is shortened. These alloys can be age-hardened, but aging often makes the alloy brittle and makes it less resistant to corrosion.
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Nickel-chromium-molybdenum alloys include the cast Chlorimet 3 alloy and the Hastelloy C alloy that can be cast and forged. They are a class of nickel-based alloys with about 18% chromium and 18% molybdenum, and have good corrosion resistance in the entire concentration range of sulfuric acid.
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Due to the presence of chromium, this class of alloys also has some passivation ability under oxidizing conditions, so the NiCrMo alloys are not as sensitive to the effects of oxidizing impurities as NiMo alloys. They are also more resistant to corrosion in high-temperature dilute sulfuric acid under reducing conditions, such as in the condensation bath of rayon. In the medium concentration of high-temperature sulfuric acid corrosion resistance than the nickel-molybdenum alloy.
Ninth
stainless steel corrosion resistance characteristics of sulfuric acid:
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Ordinary stainless steel is not usually used in contact with sulfuric acid equipment, 18-12Mo type stainless steel is occasionally used in media containing very dilute sulfuric acid at room temperature, or dilute sulfuric acid solutions containing oxidizing ingredients. Because stainless steel corrosion resistance in sulfuric acid is unstable, it is difficult to obtain reliable data and make a corrosion map.
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In the concentrated sulfuric acid at room temperature, stainless steel has still good corrosion resistance, but in general, prefer to use carbon steel rather than stainless steel. Unless the medium requirements to prevent iron ion contamination, or in about 101% sulfuric acid carbon steel can not be used, to choose stainless steel. Chromium stainless steel, chromium-nickel stainless steel and chromium-nickel-molybdenum stainless steel similar to the room temperature in concentrated sulfuric acid has a good corrosion resistance, with the temperature increases the corrosion rate also increased significantly. In the concentrated sulfuric acid above 60 ℃, the service life is shorter and should not be used.
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Silicon containing stainless steel 1Cr18Ni11Si4A1Ti, in concentrated sulfuric acid has much better corrosion resistance than ordinary stainless steel, even at higher temperatures also have better corrosion resistance. In about 97% of concentrated sulfuric acid 1Cr18Ni11Si4A1Ti steel has better corrosion resistance than other stainless steels. Therefore, the equipment used in 60~100℃ concentrated sulfuric acid, 1Cr18Ni11Si4A1Ti can be used instead of using expensive nickel-molybdenum alloy. China’s synthetic alcohol equipment used on the pump, valve is this, the effect is remarkable.
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Containing copper, molybdenum stainless steel, such as OCr18Ni18Mo2Cu2Ti and 1Cr17Mn9Ni3Mo3Cu2N steel, etc., mainly used in dilute sulfuric acid. This type of steel can be used in sulfuric acid at temperatures below 40°C and concentrations less than 70%. It can also be used in sulfuric acid with a concentration of less than 5% at temperatures up to 80°C. Table 5 lists the composition and properties of 1Cr17Mn9Ni3Mo3Cu2N steel. The steel is suitable for the manufacture of pumps, valves and other castings or forgings.
Tenth
copper alloys of sulfuric acid corrosion resistance characteristics:
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Copper and copper alloys in the reduction of dilute sulfuric acid is very stable, because they have a much higher electrode potential than hydrogen, in dilute sulfuric acid is not easy to fold hydrogen corrosion reaction, which makes copper and copper alloys in dilute sulfuric acid has much better corrosion resistance than other engineering alloys. However, copper and copper alloys are sensitive to oxidizing impurities, when dilute sulfuric acid contains oxygen, nitric acid, chromic acid, chlorine, hypochlorite, cuprous chloride, ferric chloride, iron sulfide and ammonia and ammonia salt will accelerate the corrosion of copper and copper alloys. Copper and copper alloys are also not resistant to corrosion in concentrated sulfuric acid with oxidizing properties.
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In open dilute sulfuric acid equipment, copper and copper alloys at the water line will occur serious corrosion. Tin bronze can be used in sulfuric acid at concentrations below 60% and temperatures below 80°C. Silicon bronze (Everdur) has slightly better corrosion resistance than tin bronze. Aluminum bronze works better in dilute sulfuric acid, not only because it is more resistant to abrasive corrosion, but it is also less sensitive to oxidizing conditions than the copper alloys described earlier. In 40% sulfuric acid, aluminum bronze has good corrosion resistance even to the boiling point.
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Nickel white copper in dilute sulfuric acid corrosion resistance is good, especially in the medium concentration of boiling sulfuric acid corrosion resistance is excellent. As long as the medium does not contain oxidizing impurities, it can be used in sulfuric acid below 55%.
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Cu88Ni9Si2 alloy is suitable for casting pumps, valves, etc., and has good performance in high temperature dilute sulfuric acid below 50%. By synthetic alcohol production 110 Shi 10 ℃, 40 ± 10% sulfuric acid in the use of more than six years of life far more than other engineering alloys, including nickel-molybdenum alloy.
Eleventh
other metals and alloys in sulfuric acid corrosion resistance characteristics:
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Gold and platinum in sulfuric acid has excellent corrosion resistance, due to the scarcity and expensive, can not be used in large quantities as engineering alloys in industry, only in some special occasions, such as the requirements of minimal corrosion of artificial silk nozzle and do not allow the pollution of reagent pure sulfuric acid absorption tower and other individual equipment or another piece can be used in small quantities.
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Tantalum is very resistant to corrosion except for very concentrated and hot sulfuric acid. Zirconium is very resistant to corrosion in sulfuric acid below 60% concentration up to boiling, and also very resistant to corrosion in concentrated sulfuric acid below 92°C. Zirconium-hafnium alloys are also very resistant to corrosion in sulfuric acid.
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Titanium is not corrosion resistant in sulfuric acid except at very dilute concentrations, but titanium is sometimes used as a corrosion inhibitor, for example in equipment for smelting nickel ore, where titanium is used in tons. The addition of certain alloying elements to titanium can improve its resistance to sulfuric acid. Chin aluminum alloy (Ti68Mo32) has good corrosion resistance in sulfuric acid.
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Molybdenum has excellent corrosion resistance in sulfuric acid. The corrosion rate is less than 0.025 mm/year in 10~95% 70℃ below sulfuric acid, less than 0.125 mm/year in 50% below until boiling, less than 0.025 mm/year in 204℃, 10% sulfuric acid, and less than 0.1 mm/year in 204℃, 20% sulfuric acid.
Twelfth
non-metallic materials in sulfuric acid corrosion resistance characteristics:
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It should be noted that the mechanical properties of non-metallic materials are not as good as metal materials, such as plastic is not only easy to age, and at a slightly higher temperature that is easy to produce creep damage. Glass, ceramic is very brittle.
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It should also be noted that some non-metallic materials, such as plastics and synthetic rubber corrosion resistance is also affected by their composition, plasticizers and fillers and other changes. Therefore, the corrosion resistance of different grades of non-metallic materials should be tested under the given corrosion conditions to determine.
thirteenth
the choice of sulfuric acid-resistant materials:
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As a variety of different materials in different concentrations or temperatures of sulfuric acid show different corrosion resistance, and there is no the same law between them, so the choice of sulfuric acid-resistant materials must be based on experiment and experience. First of all, according to a large number of experimental data and factory experience in the use of sulfuric acid-resistant materials selection chart was drawn. The concentration of sulfuric acid – temperature state map (below the boiling point of atmospheric pressure) is divided into 10 regions, the corrosion rate of each region ≤ 0.5 mm / year materials listed in the table.
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It should be noted that this corrosion rate refers only to the material at rest or flow rate is not large in the simple sulfuric acid applicable, if the acid contains other impurities that will significantly change the corrosion characteristics of the material or acid flow rate is very high will cause the material to wear corrosion, the selection chart can not be directly transferred.
