Alloys Resisting Chloride Media
Gaseous chlorine at mild temperatures and without moisture is not extremely attacking and is easily handled by carbon steel. Dry hydrogen chloride (HCl) acts in the same manner. Although, the highly acidic HCl adversely affects steel.
Corrosion is a complicated process, probably unessential factors for example minor magnitude of moisture, contaminants or availability of metal chlorides, can totally alter the corrosion scene.
To specify the corrosion data in a concise form, different methods have been considered. Nickel and high nickel alloys are the significant materials in providing excellent corrosion resistance in Cl, dry HCl and HCl acid. These alloys are constantly providing outstanding performance in chlor-alkali industry, becoming a selective material for use in caustic, brine and salt treatment units. Following table shows the description of alloys with their UNS, and trade names.
|Metal/Alloy||Common name||Composition||ASTM/ ASME B||UNS||Common trade name|
|Nickel||Alloy 200||99.6||–||–||–||–||161 – 163||UNS N02200||Nickel 200|
|Low carbon nickel||Alloy 201||99.6||–||–||–||–||161 – 163||UNS N02201||Nickel 201|
|Nickel-copper alloys||Alloy 400||67||–||–||31||1.5||163 – 165||UNS N04400||Monel 400|
|Ni-Cr-Fe Alloy||Alloy 600||76||15||–||–||8||163 – 168||UNS N06600||Inconel 600|
|Ni-Fe-Cr Alloys||Alloy 800||32||21||–||–||46||163 – 407||UNS N08800||Incoloy 800|
|Ni-Fe-Cr-Mo-Cu alloy||Alloy 825||42||21||3||2.3||30||163 – 423||UNS N08825||Incoloy 825|
|High molybdenum alloys|
|Ni-Cr-Mo-Fe alloy||Alloy 625||61||21.5||9||–||4||443 – 446||UNS N06625||Inconel alloy 625|
|Ni-Cr-Mo-Fe alloy||Alloy C-4||63||16||15||–||2||575 – 622||UNS N06455||Hastelloy C-4|
|Ni-Cr-Mo-Fe alloy||Alloy C-276||58||16||15||–||6||575 – 622||UNS N01276||Hastelloy C-276|
|Ni -Fe-Cr –Mo-Cu alloy||Alloy G||48||23||7||2||20||588 – 622||UNS N6007||Hastelloy G|
|Ni-Mo-Fe alloy||Alloy B-2||68||–||28||–||1.5||333 – 622||UNS N10665||Hastelloy B-2|
Chlorine gas at ambient temperatures and without moisture is not adversely corrosive that carbon steel can be used to deal with it. Often superior corrosion resistant alloys Monel 400 and Hastelloy C-276 are used in valve trim, stem, instrumentation and orifice plates in the chlorine pipes. On the other hand, wet chlorine is widely corrosive to steel and nickel alloys and demands Hastelloy C-276 or titanium. The higher limit of significance of carbon steel in dry chlorine is about 200oC or 390oF, at this limit, security effects of the oxide layers vanish.
Nickel 200 and Inconel 600 are widely used for reactors, coils and pipes for the temperature range of 250oC to 500oC or 480oF to 930oF. Carbon steel is significant for use only at temperatures below 150oC or 300oF. During shutdown periods, adequate closing processes should be used to dry the systems and ensure the absence of chlorine to avoid corrosion by wet residual chlorine on steel or nickel. Using SS 304 or 316 that are suitable for use up to 350oC or 660oF, the availability of moisture in shut down also increases the feasibility of stress corrosion cracking. The temperature limit 500oC or 930oF is a maximum limit for nickel in dry chlorine.
For the production of ethylene, Nickel alloy 200 should be chosen for reactor internal components and crucial parts for temperatures below or above 150oC or 300oF.
Hydrogen Chloride gas
Dry hydrogen chloride acts equivalent to chlorine gas and carbon steel will act sufficiently at temperatures of 250oC or 480oF, for higher limits Nickel 200 is suitable. For higher than dewpoint, the moisture doesn’t accelerate the corrosion process if the temperature is constant. In this condition, hydrochloric acid is produced that is extremely attacking in nature. It may be considered that there are several factors and nominal magnitudes of addition elements to monitor catalyst activity such as may leave an effect on the tenacity and vapor pressure of the security layers. So it is tough to determine the absolute corrosion rates at different temperatures.
Nickel 200 offers excellent service in dry and moisture HCl. In the cyclic operating environments, specifically in air or oxygen, Inconel 600 and Incoloy 825 offer outstanding corrosion resistance.
For the reaction process of ethylene with arid HCl and oxygen (O2) in availability of copper chloride catalyst in a fixed bed reactor to develop ethylene dichloride at temperature 275oC or 525oF and pressure 10 atm, eligible alloys are alloy 200 and alloy 600. Nickel 200 reactor tubes, tubesheets are used and interlinking pipes among the reactors are also made from alloy 200. Alloy 800 and alloy 825 used in pyrolysis furnace tubing and fluid bed oxychlorination reactor internals offer adequate performance and show good resistance to chloride induced stress corrosion cracking.
Hydrochloric acid is an essential material that has several applications such as acid pickling of steel, acid processing of oil units, chemical rinsing and processing.
Alloy 200 and alloy 400 offer similar performance at ambient temperatures for acid concentration 20% and at elevated temperatures below 5% content.
Incoloy 825 and Hastelloy C-276 are recommended for use at ambient temperatures for all acid concentrations. These materials offer excellent resistance to SCC.
The availability of contaminants like fluorides, ferric ions and cupric ions have a significant effect on corrosion of several metals. Fluorides are harmful for glass lined steel and zirconium. Hastelloy C-276 offer good performance and resist the pitting corrosion in the oxidizing ferric chlorides.
During the distillation procedure, entrained water is transported and wet hydrochloric acid is produced through hydrolysis of organic acids when stream is quenched down to 125oC or 255of. The eligible candidate is Alloy 400 that offers good performance in these conditions. Hastelloy B-2 can resist the complete concentration and temperature limits.