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How do different alloys perform in dry and moist chlorine media?

Carbon steels can easily deal with gaseous chlorine at low temperatures and in the absence of moisture. But if water is present, chlorine becomes vigorously corrosive to several metals.

In the same manner, dry hydrogen chloride is not corrosive to various metals. As soon as it is dissolved in water HCl is developed and it is corrosive to various metals and alloys. Chlorine, hydrogen chloride and hydrochloric acid is evaluated in different conditions. Materials used to evaluate performance in HCl conditions are stainless steels, nickel base super alloys etc.

Corrosion usually takes a devastating picture in the presence of seemingly minor factors for example moisture, contaminants, presence of metal chlorides etc. In chemical plants, the standard design factor for tubes, valve trim and internals is approximate 0.1 mm/y. For heavier wall vessels and pipe, if the corrosion is anticipated to be uniform, an upper corrosion rate is 0.50mm/y can beĀ  handled. When a corrosion is 3 -6 mm, a safe service life of 6 -10 years can be anticipated.

Performance of alloys in Chloride media

Carbon steel offers suitable service up to 150oC and if maintained properly, the limit can be raised to 200oC. Nominal magnitudes of chlorinated solvent contaminants can cause corrosion and accelerate the ignition.

Standard austenitic stainless steel series 300 are basically more corrosion resistant than carbon steel to dry chlorine. Stainless steels are suitable for service up to 350oC or 660oF however are not usually used because unnecessary ingress of moisture can cause chloride induced stress corrosion cracking or pitting.

Duplex stainless steels like 2205 prevent corrosion in dry chlorine however the ferrite phase is damaged by HCl should ingress of moisture occur.

Nickel and nickel base super alloys

Non-chromium based alloys like Nickel 200, 201, Monel 400 and Hastelloy B2 prevent dry chlorine based corrosion but they are vigorously corroded if moisture is present. Monel 400 is widely used as valve trim however problems can occur in refrigerated systems. Any quantity of water present below the dew point attacks Monel 400 and other nickel alloys.

Chromium based alloys such as Inconel 600 and Hastelloy C276 wire prevent chlorine corrosion superior to other alloys even in the presence of moisture. Hastelloy C276 used as a standard valve stem material in carbon steel lines handling dry chlorine as stems sometimes come in contact of humid air. Other highly alloyed grades are although not needed but they are suitable resistant.

Nickel 200 and Inconel 600 have major applications for reactors, coils, agitators and pipes in 250 to 500oC range. A temperature of 500oC is a prudent upper limit for nickel in dry chlorine. Inconel 600 offers stronger performance than Nickel 201.

Copper and copper based alloys

Copper alloys are not commonly used because they are quickly corroded in acidic conditions due to contamination by moisture. Copper and copper based alloys prevent corrosion in dry chlorine up to 200oC. Versatile, annealed copper tubes are used for gas connections in some applications however should be checked regularly for mechanical reasons.