Dry gaseous chlorine at mild temperature is not extremely corrosive and can be handled using carbon steel. Dry hydrogen chloride acts in the similar manner. But the powerful hydrochloric acid is deleterious to steel.

Corrosion

Corrosion is a very complicated mechanism. Apparently unnecessary factors like nominal presence of humidity, contaminants or the metal chlorides can manipulate the corrosion level entirely. Nickel and High Nickel alloys are one of the proven metallic materials that offer great corrosion resistance in chlorine, hydrogen chloride and hydrochloric acid. These alloys have been used traditionally as more or less standard choices in caustic, brine and salt handling.

In the chemical treatment industries, the design factor for tubing and internals is 0.075 mm per year or 0.003 inch per year, the maximum permissible corrosion rate, while for pipe and vessels, higher corrosion rate about 0.50 mm per year or 0.020 inch per year is commonly followed with a corrosion allowance of three to six mm or 1/8 inch or ¼ inch. It must offer a secured service for minimum ten years or above.

 Chlorine

Gas chlorine at low temperatures and in the absence of moisture is not extremely attacking and is widely dealt using carbon steel. Often a highly corrosion resistant alloy Monel 400 or alloy C276 is described for crucial components like valve trim, stem, equipments and orifice plates in chlorine pipes. Rather wet chlorine is highly attacking to steel and nickel alloys and needs Hastelloy C276 or titanium. The higher limit of using carbon steel in dry chlorine is about 200oC or 390oF, at this temperature, the secured effects of the materials fade away.

The surface coating of chlorides on alloys offers security at temperatures at which the melting, vaporization or cracking damages the security layers. The corrosion rate becomes equivalent to the vapor pressure of the metal chlorides.

Monel 400 and Inconel 600 are the commonly used alloys in reactor coils, agitators and pipes at temperature limits of 250oC to 500oC or 480oF to 930oF. Carbon steel has still significant application at temperatures lower than 150oC or 300oF. When the operation unit is not in use, complete shutdown processes are followed to dry the equipments to free them from chlorides in order to avoid corrosion by wet residual chlorine on the steel or nickel. Using SS 304 or SS 316 that can be utilized up to 350oC or 660oF, the availability of humidity while downtime, also increases the feasibility of stress corrosion cracking. The limit about 500oC or 930of appears as cautiously higher limit for use of nickel in arid chlorine.

For instance, ethylene is reacted with chlorine using ferric chloride as a catalyst, to develop ethylene dichloride. The unit temperature varies from 60 oC to 100 oC or 140 oF to 210oF. It involves exothermic process; water quenching eradicates the heat from the unit. Carbon steel can be employed as a reactor material and primary apparatus, offering that the chlorine supply is arid and the suitable temperature control is sustained by complete mixing of reactants to avoid warm regions and runway temperatures. Close mixing can be confirmed using ethylene dichloride as a reaction medium. Nickel 200 is possibly chosen for reactor internals and crucial materials, if faces any trouble in controlling temperature lower 150oC or 300oF or it is necessary to perform at temperature above this limit.

Hydrogen Chloride Gas (HCl)

Arid hydrogen chloride acts equivalent to chlorine gas, in this condition. The analyses for 500 hours period in dry hydrogen chloride at 500oC or 930 oF showed the significantly small corrosion rates. Distinguished exception in the conduct of materials in this medium and chlorine are platinum and gold that offer very high resistance to the corrosion by hydrogen chloride. Nickel and high nickel alloys like Inconel 600 and Hastelloy B and Hastelloy C are the commercial materials for application at temperatures up to 547oC or 1000oF.

The family of chromium – nickel stainless steels seems to have a considerable corrosion resistance at temperatures about 425oC or 800oF. Carbon steel gives more suitable service in hydrogen chloride to somehow elevated temperatures rather in chlorine and the attack rate increases with increase in temperature of hydrogen chloride, blazing has not been noticed at temperatures up to 760oC or 1400oF. On the base of allowable corrosion, carbon steel is significant up to 260 to 315oC or 500 to 600oF.

When the chief concern is corrosion resistance at temperatures lower than dew point, but condition against elevated temperature corrosion is also necessary, the specified materials for use are Hastelloy B, Monel 400, Nickel, Inconel 600 and copper. Nickel 201 and Inconel 600 are rated better than Hastelloy alloys for service at temperatures from 450oC to 540oC or 850oF to 1000oF.

In the production of hydrogen chloride using hydrogen and chlorine, the recommended design of combustion equipment uses the metal with temperature controlled in the specific limits through water jackets. In such case, the corrosion risk due to condensed hydrochloric acid at temperature close to dew point is larger than by arid hydrogen chloride at the high temperature.

For choosing the materials for elevated temperature chlorine or hydrogen chloride service, specifically at temperatures above 370oC or 700oF, not just the corrosion rate of alloy should be taken into account, even also the effect of temperature on its mechanical properties must be considered.

Nickel alloy 200 is often utilized. Maximum corrosion limits of 0.075 mm per yr and 0.50 mm per yr are taken as design factors for specific materials. It is trusted that these limits are conservative. In services with higher dewpoint, the availability of moisture doesn’t significantly increase the corrosion rates as long as the temperature and moisture reduces. In this condition, hydrochloric acid if produced is of extremely attacking nature.

It has been described that nickel and high nickel alloys have significant corrosion resistance at temperatures about 540oC or 1000oF. But Nickel 200 suffers from embrittlement by intergranularly precipitated carbon at temperatures from 425oC to 760oC or 800oF to 1400oF for longer periods. Nickel 201 prevents this embrittlement offering carbonaceous materials are not kept in its contact. However both alloys are attacked by sulfur based intergranular embritllement at temperatures exceeding 315oC or 600oF. In services including carbon or sulfur media, when elevated temperature strength is needed, Inconel alloy 600 is the best replacement for Nickel 200/201. Nickel and Inconel 600 both have been commonly employed in chlorination units at temperature limits about 540oC or 1000oF.

It should be figured out that there are several factors and even the nominal magnitudes added agents limit the catalyst action, such as these may put an effect on the tenacity and vapor pressure of the security corrosion layer. Hence the corrosion rates for various temperatures are tough to determine in conditions of chlorine and hydrogen chloride gas equipments. The functionality of Nickel 200 alloy in dry and HCl gas is reliable. In cyclic application conditions, specifically in availability of air or oxygen, Nickel 600 and Nickel 825 are used to provide necessary corrosion resistance. It is cautious to consider that SS 304 and 316 stainless steels are attacked by chloride induced stress corrosion cracking in the downtime besides of many precautions that may be followed.

For instance, ethylene needs to be interacted with arid hydrogen chloride gas and oxygen in availability of copper chloride as a catalyst inside a fixed bed reactor to manufacture ethylene dichloride. The operation temperature is 275 oC or 525 oF and pressure is 10 atm. It is a heat releasing process and reaction heat is discarded by the steam production in the reactor.

Alloy 200 and alloy 600 prevent attack by wet and dry hydrogen chloride. Often Nickel 200 is utilized for reactor tubes, the tubesheets and other components of reactor are cladded with nickel, the linking pipes between the reactors are constructed of Nickel alloy 200. The temperatures are carefully controlled in this heat releasing process because of production of side product and deactivation of the catalyst at temperatures above 325 oC or 615 oF. Nickel 200 has a higher limit of 550 oC or 1020 oF and with localized hot regions at 750oC or 1380 oF, catastrophic attack rates and tube failure will take place.

Stainless steel 304 and SS 316 are attacked by  chloride induced stress corrosion cracking lower the dewpoint and while shutdown, unless the complete caution is followed to confirm a bone dry supply to the system and to sustain shutdown and initialization precautions of gas blanketing and maintaining the dryness of system. Incoloy 800 and Incoloy 825 prevent the chloride stress corrosion cracking and these are specifically employed in the production of ethylene dichloride (EDC) in pyrolysis furnace tubes and reactor internals.

Hydrochloric Acid (HCl)

Hydrochloric acid is an essential mineral acid with several applications such as acid pickling of steel, acid processing of oil wells, chemical cleaning and chemical treatment. It is introduced in 4 content percentages following from 27 to 37 percent.

Because of the potentially powerful reactions between chloride ions and base metal, the magnitude and temperatures of hydrochloric acid are required to be known to find if the metal will stand and has a suitable corrosion resistance rate. The corrosion rate of hydrochloric acid similar to several other acids is widely based on the temperature. Chloride based acids in several conditions describe an attacking nature identical to hydrochloric acid at analogous acid and / or chloride contents.

Hydrochloric acid (HCl) is a standard reducing acid for its whole concentration. It possesses powerful acidic nature that is deleterious to stainless steel. Alloy 200 and Monel 400 act equivalently and offer service at low temperatures about 20 percent content. For higher limits, Hastelloy B2 is utilized.

At low temperatures, Incoloy 825 and Hastelloy C276 can be utilized for the entire acid content range. These materials are chosen for resisting chloride stress corrosion cracking. Moreover, Hastelloy G and Inconel 625 act well in these media.

The availability of contaminants like fluorides, ferric ions and cupric ions has a specified influence on the corrosion of several metals. The fluoride corrodes glass lined steel and the refractory metals like zirconium and tantalum. The ferric chlorides corrode the nickel alloys such as Hastelloy B2. The cupric salts have an increased effect equivalent to the ferric ions, causing pitting and stress corrosion attack.

When the acid content may be small in iron, acid can be easily contaminated while transportation and handling, and from attack on steel in the service vessels. In these conditions, in availability of oxidizing ferric chlorides, titanium and Inconel C276 offer suitable functionality and prevent pitting corrosion.

For instance, in a distillation operation, entrained water is handled, and dilute hydrochloric acid is produced by hydrolysis of organic acids when the stream is quenched lower than 125oC or 255oF. Extensive attack happens on the carbon steel condenser tubes and piping in the edge of accumulator.

On the base of temperature and hydrochloric acid content, corrosion rates on carbon steel quickly reach to 0.25 to 4 mm per year. It is very tough to confirm a bone dry equipment without inadvertent moisture spontaneous at flanges and seals, or to avoid entrainment of water. It should be observed that acid contents in these cases are extremely lower than 0.5 percent. Monel 400 can withstand in these environments.

Nickel-Copper alloy 400 has been utilized at ambient temperature in reducing unaerated systems about 20 percent hydrochloric acid content. Hastelloy B2 can prevent corrosion by complete range of acid magnitude and temperature.

Most of nickel alloys are resistant to dry chlorine and hydrogen chloride, even at the very high temperatures. Monel 400 is a typical alloy for trim on chlorine cylinder and container valves for orifice plates in chlorine pipe lines and is employed for components of chlorine dispensing systems.

Diluted hydrogen chloride at temperatures lower than dew point will often act equivalent to the concentrated hydrochloric acid. Hastelloy B is the best nickel resistant alloy. Diluted chlorine at temperatures lower than dew level or aqueous solutions comprising of significant magnitudes of free chlorine are extremely attacking to standard alloys except Hastelloy C, Alloy 400, Nickel and Alloy 600 that are commonly utilized in the solutions concentration 3 gm per liter or less existing chlorine in irregular operations like cyclic textile bleaching with hypochlorite solutions where bleaching process is performed by washing and acid souring process in the same container. In larger concentrations corrosion is extremely intense and caused by pitting.

Short time analyses of Monel 400 in sodium hypochlorite solutions of 3 g/l chlorine have described average corrosion rates of 0.001 ipy in contact with solutions for eight hours every day for 365 days in cyclic textile bleaching. Smaller to larger rates have been discovered to apply for short or long time spans of every day contact with solutions. Nickel offers equivalent performance to Monel 400 in weak hypochlorite solution media, however is lower than alloy 400 in contents of chromium above 3 g/l. Analyses with alloy 600 have shown that it is partially more resistant than Monel 400 or nickel to attack caused by hypochlorites particularly where available chlorine magnitude is above 3g/l.

Monel 400, Nickel and Inconel 600 are resistant to the dilute hypochlorite solutions, often comprising of less than 500 ppm, introducing chlorine utilized for sterilizing purposes. Preventers like sodium silicate or trisodium phosphate have remarkable influence in decreasing corrosion of nickel, alloy 400 and Inconel 600 in the hypochlorite solutions. The prevention effect is valid for solutions of 6.5 g/l chlorine. It is essential to observe that in the media containing a preventer, corrosion is more homogeneous and less limited to the local regions. The samples in contact with such media of 6.5 g/l chlorine describe sensitivity to local corrosion, in fact also in the presence of preventer, however the susceptibility towards the corrosion has been decreased by the preventer. Though a preventer content about 3.3 g/l chlorine, the attack of alloy 400 and nickel is even without local corrosion.

In the lab tests, smallest 0.0025 cubic cm of sodium silicate solution per liter of bleaching solution offers a significant effect in decreasing the corrosion.

Hastelloy C offers a great performance in preventing attack of strong chlorine media and diluted chlorine gas, however the temperature is limited to 40oC or 105 oF in presence of wet chlorine gas. The highest corrosion rate of Inconel C in wet chlorine gas at room temperature is 0.001 ipy. It is attacked by 0.0003 in water vapors at temperature of 170oC or 340oF comprising of 1000 ppm chlorine. 

Chlorine and hydrogen chloride at elevated temperatures

Growth in the chemical processing sectors in the recent years have been in the way of utilizing extremely high temperatures to accelerate the reactions or to complete reactions at lower temperatures. This approach has been applied to procedures including chlorine and hydrogen chloride in the formation of new organic or inorganic chlorides, and the processing of titanium and zirconium ores. In several conditions, the complete development of process has based on discovering the suitable materials for production to be used in the elevated temperature halogen gasses.

The corrosion rates of several metals and alloys in arid chlorine and arid hydrogen chloride increase slightly with the increase in temperature up to critical level that depends on the specific material. Exceeding this level by additionally increasing the temperature quickly accelerates the corrosion rate. To some extent the corrosion rate is hardly in proportion to the vapor pressure of the specific metallic chloride included similar to nickel. Although the corrosion resistance cannot be determined alone by this procedure. Few metal chlorides melt or decompose at temperatures at which the vapor pressure is small, and the reaction can occur as long as the security effect of coating vanishes. Other alloys may be resistant to the production of surface chloride layers. However others may catch fire in presence of chlorine above the specific temperature with release of heat hence increasing the metal temperature as well as reaction rate.

The lab corrosion tests were performed for several metals and alloys in dry chlorine and hydrogen chloride for two to six hours and ten to twenty hours, with a gas supply rate of 1.3 foot per minute. Inconel alloy 600 in arid chlorine, in the longer tests showed smaller corrosion rates, because of the influence of time on the production of security layers. The corrosion rates in longer services are expected to be low because of the production of security chloride layers.

In several industrial procedures, the chlorine or hydrogen chloride may not be pure neither dry. Air, dilution or chemicals may be available to obscure the corrosion process.

Chlorine

In the operation temperatures about 540oC or 1000oF, Nickel, Inconel 600 and Hastelloy B are the extremely resistant alloys. Hastelloy C is suitable for service up to 480oC or 900oF. Alloy 400 seems to be useful up to 430oC or 800oF and chromium-nickel stainless steels can provide desired service up to 345oC or 650oF. Pure metals like Gold and Platinum produce unstable chlorides in warm chlorine limiting their significance to the comparatively small temperatures.

Copper, cast iron and carbon steel can be used at temperatures below 205oC or 400oF in arid chlorine, as they catch fire at the temperatures above these limits. Copper catches fire at 345oC or 650oF at a small gas flow speed and at 260oC to 300oC or 500oF to 575oF at the larger speeds.

At small speeds about 40 ml per min, aluminum was not attacked in dry chlorine however at larger velocity of 250 ml per min, the reaction occurred vigorously so to melt the sample. In moisture of 0.4% of chlorine, attack on aluminum was decreased to very nominal rate in fact at the larger speed, because of the production of security oxide layer.

Mild corrosion of carbon steel in dry chlorine at small speed, blazing noticed within fifteen minutes at the larger speed. Further moisture inclusion widely decreased the corrosion of steel because of the production of oxide layer. Copper is attacked at the noticeable rate at the small speed in the presence of moisture and blazed in the larger speeds.