Chemical Industry

1.Application in sulfoacid industry and sulfoacid medium.
Corrosion of sulfoacid is connected with its concentration. Sulfoacid with low and medium concentration have comparatively low oxidizability, and belong to reducing acid; while the high concentration sulfoacid has strong oxidizability, and belongs to oxidizing acid. According to such characteristic of sulfoacid, different grades of stainless steel should be used in sulfoacid with different concentration and different temperature.

In the process of producing high concentration sulfoacid, cast iron and bricks are often chosen, while stainless steel is not often used for lack of corrosion resistance. In recent years, due to strict control to environmental pollution and for the purpose of improving production environment, there is the trend for the cast iron rack pipe to be substituted by the plate type heat exchanger in the sulfoacid production. For high concentration sulfoacid （93%-98%）, high silicon austenite stainless steel containing molybdenum is often chosen, like SS-920. When temperature is above 130℃, in the 93%-98%H2SO, corrosion of SS-920 is less than 0.1mm/a, but such steel is not resistant to corrosion by low and medium concentration sulfoacid, and special attention should be paid during the operation. For high concentration sulfoacid under the temperature of 80℃, acid resistant pump is often made of high silicon nickel-base alloy, like IlliumG, when temperature is higher, 1N504 alloy can be used. （80Ni-9.5Si-3Mo-2.5Cu-3Ti）.

In the medium concentration sulfoacid, common stainless steel does not possess satisfying corrosion resistance, under the temperature of 80℃, high quality Austenite stainless steel that contains molybdenum or copper, like 00Cr20Ni29Mo3Cu4Nb and 00Cr20Ni25Mo4.5Cu, etc is often used.

Application range of stainless steel in sulfoacid

When sulfoacid contains enough oxygen or oxidant, application range of stainless steel can be extended, e.g.: in sulfoacid below the concentration of 65%, the 18-8 type Cr-Ni stainless steel commonly cannot be chosen, but when sulfoacid contains ≥5%HNO3, such steel can be used. In producing ammonium sulfate, crystallizing tank is commonly made of  TP316, when chromic salts exist, storage tank and pump can also be made of  TP304.

2.Nitric Acid Industry.
Stainless steel has good corrosion resistance in nitric acid, thus it is widely used in manufacturing equipments for producing nitric acid or that will contact with nitric acid.
In nitric acid producing technique of high pressure process,  TP304L, TP321 and TP347 are used widely. Absorbing towers all use TP304L, ammonia-air mixer uses TP304L, waste gas burner uses 0TP347， TP310S(Nb). High speed moving parts of air compressor can use 1Cr17Ni2 Martensitic stainless steel.

In nitric acid with an azeotropy concentration above 68.4%, the conventional 18-8 Type Cr-Ni Austenite stainless steel does not have satisfying corrosion resistance. When there is concentration ≤85%,  TP310S stainless steel is used, in the nitric acid above the concentration or that contains Cr⁺⁶, then the high silicon austenite stainless steel with different silicon content, like the 1Cr17Ni11Si4AlTi, 00Cr14Ni14Si4(Ti), and Cr-Ni stainless steel with the silicon content of 5%-6% can be used.

3.Phosphoric Acid.
The phosphoric acid with dry process does not contain impurity that enhances corrodibility, and can use 18-8 Type Cr-Ni stainless steel or Cr-Ni stainless steel that contains Mo as material, different grades should be chosen according to the requirements of corrosion resistance.

Phosphoric acid with wet process is basic material for making phosphatic fertilizer, it is a sort of 54% P₂O₂ phosphoric acid that contains impunities, Cl-, F-, SO4- particles in the medium have increased its corrodibility, the main technique equipments are commonly made of t rubber lining, lining bricks, graphite, etc, but some heat exchangers and pumps use high-Cr-content super austenite stainless steel, like 00Cr27Ni31Mo3Cu to make the evaporator, substituting original heat exchanger made of graphite, and 0Cr20Ni29Mo3CuNb is used to make the pumps.

4.Chlorin & Alkali Industry.
The main representative product of caustic alkali is NaOH, in the technique environment of producing NaOH, producing equipments are mainly made of synthetic resin, carbon steel, cast iron, Ti, Ni, etc, stainless steel with Ni<40% may have caustic stress corrosion breach, and is commonly inapplicable. In producing NaOH with diaphragm process, content of impurities in the alkali is comparatively higher, especially NaClO₃, this will cause intensified corrosion to the conventional nickel material, under such condition, the high purity and high chromium ferritic stainless steel （00Cr26Mo1,00Cr30Mo2）has qualities of excellent regular-corrosion resistance and excellent caustic stress corrosion resistance, thus it can substitute nickel in the manufacture of high temperature concentrating technique equipments.

In the environment of Chlorine and hydrogen chloride above the temperature of 200℃, the stainless steel has good corrosion resistance, and can deal with the “sparkling” phenomenal, in the dry chlorine and hydrogen chloride gas, the temperature limits for the use of stainless steel are as the follows:

Operation temperature of stainless steel in dry chlorine and hydrogen chloride gas

When carbonizer tower of soda ash (ammonium chloride) is cooled by sea water, 00Cr18Ni18Mo5 is ammonium chloride corrosion resistant and sea water corrosion resistant, thus carbonizer tower made of this steel has a use life of more than 20 years, besides, as the corrosion can be less, and the impurities are reduced, the level of the product is promoted.

5.Organic Acid Industry.
Commonly, corrodibility of Organic Acid is weak, among the organic acid, acetic acid formic acid have stronger corrodibility, and the corrodibility rises with increase of impurities, especially that increase of methanoic acid content will enhance corrodibility distinctly.

For refined acetic acid, those equipments for producing acetic acid under the temperature of 60℃ and with a concentration less than 90%, like the tanks, pipes, etc, are all made of TP304 steel;. When at a high temperature and containing no impurities, these should be made of TP316 steel. Those parts with heavy metal contaminations, high speed flowing, diluted air, should use TP316.

Acetic acid rectifying tower is feeble to the contamination of formic acid and aldehyde products, and under the severe corrosive environment with high temperature and high concentration, when dealing with the liquid under the concentration of 10%, the material should be TP316,  TP316L and  TP317 steel. When dealing with acetic acid under concentration of 50%, we can use 0Cr26Ni6Mo2 double phase stainless steel, under the condition of 12% (acetic acid)+0.5% (formic acid), re-boiling in the rectifying tower made of this steel will present a better operation state than the one made of  TP316 steel. In the high temperature acetic acid that contains methanoic acid, conventional stainless steel can no longer satisfy the requirements, with the increasing methanoic content in the medium, one should use in accordance the material of 00Cr20Ni25Mo4.5Cu, 00Cr25Ni7Mo3N, 254SMO, 00Cr16Ni60Mo16W4. Under high temperature and high pressure, stainless steel cannot be used in acetic acid.

Formaldehyde is produced by oxidation method and dehydrogenation oxidation methods, the material for the producing equipments are as follows:

Material for Formaldehyde Producing Equipments

If the formaldehyde contains formic acid, for the reason of corrosive pitting and intercrystalline corrosion, material must be TP316L.

6.Ammonia and Chemical Fertilizer Industry.
In the synthesis ammonia production, the pipe lines at the downstream part below dew point in serve condition should use  TP304, and the inner cylinder of the ammonia Synthesis tower should use TP304.

In the technique flow for producing ammonium nitrate, equipments like the nitric acid preheater, low pressure neutralizing tower , concentrator, prilling tower , etc, should all be made of  TP304 austenite stainless steel.

The ammonium sulfate is made from synthesizing sulfoacid and ammonium, by- product from coking production can also be used to produce ammonium sulfate, for crystallizing tank and heater under strictly controlled acidity and temperature, 0 TP316L steel can achieve excellent using effect, e.g. , under PH3-3.5, and the temperature of 63-66℃, and the reduction vaporization, the 0 TP316L steel has achieved satisfying result, in the ammonium sulfate that contains impurities, if the temperature is high, then it is required to use steel types like 00Cr18Ni14Mo2Cu2, TP317，or 00Cr20Ni25Mo4.5Cu, etc.

Urea is an efficient chemical fertilizer with ammonium content up to 46%. The urea is synthesized by CO₂ and ammonium, the technique process is high temperature（180-210℃） and high pressure (140-250 Atmosphere ). The medium reaction product in the producing process: ammonium carbamate, etc. (urea methylammonium liquids), will not cause severe corrosion to the stainless steel under high temperature and high pressure, under the oxygen adding condition, for the cause of using stainless steel passivation, the application of stainless steel in urea producing become possible.

Presently, the four large High Pressure Equipments in the urea production with carbon dioxide vaporizing extract process are synthesizing tower, high pressure condenser, high pressure syringe, and carbon dioxide vaporizing extract tower, among these, apart from carbon dioxide vaporizing extract tower that is made of 00Cr25Ni22Mo2N, all the other three are made of 0 TP316L (Urea Grade) stainless steel. The prilling tower uses TP304 or TP316 . In recent years, it is discovered that high chrome double phase stainless steel has good corrosion resistance in the urea production technique environment with carbon dioxide vaporizing extract process, so 00Cr25Ni7Mo3N has been used in Japan formally to make distribution pipes of vaporizing extract tower. Of methylamine pump, 00Cr18Ni5Mo3Si2 and 00Cr25Ni7Mo3N both have good using effect for the cause of corrosion fatigue. The valves in urea production should use 0Cr25Ni6Mo3CuN( Ferralium 255), which shows good corrosion resistant quality.