Corrosion resistant working condition

For valves that come into contact with corrosive fluid, some special precautions are essential.

If the metallic material of the chemical valves is not selected correctly, the risk of damage to the equipment will increase exponentially, leading in a short time to serious accidents. According to the latest statistics, around 60% of damage to chemical equipment is caused by corrosion. Therefore, during the selection of chemical equipment it is necessary to pay close attention to the material of which they are made. Very often stainless steel is chosen by mistakenly thinking that it is a “universal material”. This idea is totally incorrect and dangerous, regardless of the medium and environmental conditions in which the steel will be used.

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Below are the main chemical means, commonly used in the industrial sector:

Sulfuric acid is an important industrial raw material with a wide range of uses, usable in different concentrations and temperatures that cause large differences in corrosion in materials.

For concentrated sulfuric acid with a concentration above 80% and a temperature below 80 ° C, carbon steel and cast iron have better corrosion resistance, but still remain unsuitable if the sulfuric acid flows to high speed.

Even ordinary stainless steel such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) is not the best choice for sulfuric acid media.Therefore, pumps and valves for conveying sulfuric acid are usually made of high silicon cast iron (difficult to cast and machine) and high alloy stainless steel (20 # alloy). Fluoroplastics have better resistance to sulfuric acid and, in addition, choosing a fluorine-coated pump (F46) is cheaper. However, when the temperature is above 100 ℃ or the pressure is above 16kg, all plastic will be destroyed and, in this case, the only valves currently usable are ceramic ball valves.

Most of the metallic materials are not resistant to the corrosion of hydrochloric acid (including various stainless steel materials), and the molybdenum-containing ferrosilicon can only be used for hydrochloric acid below 50 ° C and 30%. Contrary to metallic materials, most non-metallic materials have good corrosion resistance of hydrochloric acid, so rubber coated pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for supplying acid hydrochloric.

A large variety of metals undergo rapid corrosion in contact with nitric acid with the exception of stainless steel, which remains the corrosion resistant material used, for all concentrations of nitric acid at room temperature. For high temperature nitric acid, titanium materials and titanium alloys are usually used.

This medium is one of the most corrosive substances among organic acids. The best option is 316 stainless steel, which contains molybdenum, can also be used at high temperatures and is capable of diluting the vapor of acetic acid. For severe requirements such as high temperature and high concentration of acetic acid or other corrosive fluids, highly alloyed stainless steel or fluoroplastic pumps can be selected.

Steel is widely used in sodium hydroxide solution below 80 ° C and a concentration of 30% and many petrochemical plants still use ordinary steel at 100 ° C and below 75%. Although corrosion increases, it is inexpensive. good sex. Compared to cast iron, the corrosion resistance of ordinary stainless steel to lye has no obvious advantages. As long as a small amount of iron can be mixed in the medium, stainless steel is not recommended. For the high-temperature lye, mainly titanium and titanium alloys or highly alloyed stainless steel are used.

Most metals and non-metals are only slightly corroded into liquid ammonia and ammonia water (ammonium hydroxide), only copper and copper alloys are not suitable for use.

The corrosion rate of ordinary steel in sodium chloride solution, sea water and salt water is not very high and, generally, it is enough to protect it with paint. On the other hand, all types of stainless steel also have a low corrosion rate in contact with salt water. Alcohols, ketones, esters, ethers

All common media such as:

The selection of materials should be based on the properties of the medium and the requirements relevant to the work to be carried out.