Selection of Anti-corrosion Materials for Valves (Part One)
Anti-corrosion is the key point for a valve when it is used for transporting the corrosive medium. If the metal materials of chemical valves are not selected correctly, any carelessness will damage the equipment, and even cause accidents and disasters. According to relevant statistics, about 60% of the damage of chemical equipment is caused by corrosion, so attention should to paid to scientific selection of materials during the selection of chemical equipment. There is usually a misconception that stainless steel is a "universal material", and it is incorrect and dangerous to always select stainless steel valves regardless of media and environmental conditions.
In view of some commonly used media, the main points of material selection for corrosion-resistant valves will be discussed in the following.
1. Ammonia (ammonium hydroxide)
Most metals and nonmetals just have slight corrosion in liquid ammonia and ammonia water (ammonium hydroxide), except copper and copper alloys.
2. HCl (hydrochloric acid)
Most metals are not corrosion resistant to HCl (including various stainless materials), and molybdenum-containing high silicon iron can only be used at 50℃, less than 30% hydrochloric acid. Contrary to metal materials, most nonmetal materials have good corrosion resistance to HCl, such as polypropylene, fluoroplastics.
3. HNO3 (nitric acid)
Generally, most metals are rapidly corroded and destroyed in HNO3, and stainless steel is the most widely used nitric acid resistant material, which has good corrosion resistance to any concentrations of HNO3 at room temperature. It is worth mentioning that the corrosion resistance of stainless steel containing molybdenum (e.g. SS 316 and SS 316L) to HNO3 is inferior to that of ordinary stainless steel, such as SS 304 and SS 321. For HNO3 at high temperature, titanium and titanium alloys are usually selected as the valve material.
4. Acetic acid
It is one of the most corrosive substances in organic acids. Ordinary steel will be seriously corroded in acetic acid of any concentrations and temperatures. Stainless steel is a good acetic acid resistant material. Molybdenum-containing stainless steel 316 is also suitable for high temperature and dilute acetic acid vapour.
In view of some commonly used media, the main points of material selection for corrosion-resistant valves will be discussed in the following.
1. Ammonia (ammonium hydroxide)
Most metals and nonmetals just have slight corrosion in liquid ammonia and ammonia water (ammonium hydroxide), except copper and copper alloys.
2. HCl (hydrochloric acid)
Most metals are not corrosion resistant to HCl (including various stainless materials), and molybdenum-containing high silicon iron can only be used at 50℃, less than 30% hydrochloric acid. Contrary to metal materials, most nonmetal materials have good corrosion resistance to HCl, such as polypropylene, fluoroplastics.
3. HNO3 (nitric acid)
Generally, most metals are rapidly corroded and destroyed in HNO3, and stainless steel is the most widely used nitric acid resistant material, which has good corrosion resistance to any concentrations of HNO3 at room temperature. It is worth mentioning that the corrosion resistance of stainless steel containing molybdenum (e.g. SS 316 and SS 316L) to HNO3 is inferior to that of ordinary stainless steel, such as SS 304 and SS 321. For HNO3 at high temperature, titanium and titanium alloys are usually selected as the valve material.
4. Acetic acid
It is one of the most corrosive substances in organic acids. Ordinary steel will be seriously corroded in acetic acid of any concentrations and temperatures. Stainless steel is a good acetic acid resistant material. Molybdenum-containing stainless steel 316 is also suitable for high temperature and dilute acetic acid vapour.
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