Corrosion resistance of Nickel and its alloys in hydrogen chloride applications
Different allows show different behavior in dry HCl gas. Maximum corrosion limits of 0.075 and 0.50 mm/yare decided as design factors for specific components and these are conservative limits. For instance, during studies of 650 hours, mild corrosion rate of 0.25 mm/y at 590oC is noticed. In applications above the dew point, the presence of moisture does not significantly increase the corrosion rates. At lower temperatures where moisture condenses, the hydrochloric acid produced by reaction with water is severely corrosive.
It should be kept in mind that there are several factors and that even nominal magnitudes of additives may influence the tenacity and vapor pressure of the security corrosion scales.
Carbon steel: Carbon steel shows similar performance in dry hydrogen chloride as of in dry chlorine. It is used up to 250oC and for higher temperatures, use of nickel 200 is recommended.
Stainless steel: Grade 304L and 316L are prone to chloride stress corrosion cracking below the dew point and during shutdown even at ambient temperature. It can be prevented by taking extreme care to ensure dry feed to the component and to keep care during beginning and shutdown of gas blanketing and maintaining the dryness of the unit.
Nickel and nickel based alloys: The function of Nickel 200 in dry and wet HCl gas has been found consistently good. In cyclic service conditions, specifically in presence of air or oxygen, use of Inconel 600 wire in dry chloride conditions is recommended. Incoloy 825 also offers suitable resistance. Incoloy 800 prevent chloride based stress corrosion cracking and is used for EDC pyrolysis furnace tubes and fluid bed oxy chlorination reactor internals.
In order to react ethylene with dry HCl gas and oxygen in the presence of copper chloride catalyst in a fixed bed reactor to form ethylene dichloride. The temperature 275oC and pressure 10 atm. The process releases heat and excessive heat is removed by the steam production on the shell side of reactor.
The stainless steel, nickel 200 and Inconel 600 are suitable materials which prevent corrosion in dry and moist HCl conditions. Normally Nickel 200 is used for reactor tubes; tubesheets and heads of reactor are clad with nickel on steel and the interconnecting pipes between the reactors are made of Nickel 200. It is important to control temperatures carefully in this exothermic reaction due to production of side product and deactivation of catalyst above 325oC. Nickel 200 can be used up to 550oC and the temperature goes above like 750oC, catastrophic corrosion and damage occur.
Titanium offers suitable performance in dry hydrogen chloride gas up to 150oC due to the presence of a security oxide layer. Presence of moisture causes corrosion and can result into hydrogen embrittlement.
Nickel 200 and Monel are corroded in hydrochloric acid when dissolved oxygen or stronger oxidants are present. Cupric ions are also produced from the Monel 400 to further accelerate the corrosion. On the other hand when Monel 400 is used in steel pickling operations where hydrogen is released in the reduced form. The alloy is used at ambient temperatures in reducing air-free systems.