Alloys for use in chemical plants

Consistent upgrade in the metallurgical and fabrications gives rise to the development of various nickel alloys and their wide use in the chemical plants. The nickel alloys provide great corrosion resistance, strength, hardness, metallurgical stability, fabrication and welding characteristics. Various nickel alloys offer good heat resistance and maintain high durability at the elevated temperatures for which they are widely used in chemical applications

The analysis of wrought nickel based alloys includes different corrosion resistance alloys that are widely used in the chemical plants.

Features

Nickel alloys are more expensive than stainless steel grades. However considering the prolong economical concept, stainless steel types are found to be more expensive because of their costlier maintenance that is needed after some time.

Nickel-Chromium-Molybdenum alloys are five times costlier than chromium-nickel stainless steel and twice times more expensive than super austenitic steel grades. The nickel alloys have alike thermal expansion properties as of carbon steel but smaller than stainless steel 300 types.

The thermal conductivity of pure nickel is superior than carbon steel however many nickel alloys provide low conductivity values, in some conditions even much lower than austenitic steel types.

Except the pure nickel, the nickel based alloys used in chemical units are more durable than steel 300 series. Nickel alloys provide outstanding ductility and hardness.  

The nickel alloys attain fully austenitic microstructures. Many grades utilized in the chemical plants are the solid solution reinforced alloys. They have superior strength properties as compare by the adequate addition of hardeners such as molybdenum and tungsten. Alike to austenitic steel types, solid solution nickel alloys cannot be hardened by heat treatment however but only by cold treatment.

Another primary class of nickel alloys is strengthened by precipitation hardenable heat treatment. They are commonly chosen for ultrahigh strength based applications such as in deep oil and gas production and widely high pressure processes. Except the selected components in valves and rotating machinery, precipitation hardened nickel alloys have a set of applications in the chemical plants. This group of alloys named as heat resistant super alloys that are utilized in the gas turbines, combustion chambers and aerospace industry.

Corrosion resistance properties

Nickel based alloys show a refined form derived from traditional austenitic steel grades and super austenitic iron based alloys in avoiding corrosion in the wide range of acids, alkalis and salts. The outstanding characteristic of nickel alloys is supreme resistance in aqueous media comprising of halide ions. Therefore nickel alloys are superior to austenitic grades for preventing corrosion in wet chlorides and fluorides.

The supreme corrosion resistance characteristics of nickel based alloys not just affects the material loss in fact it also has the potential to prevent the localized attack, pitting and crevice corrosion, intergranular attack and stress corrosion cracking. These types of localized corrosion that are more severe than general corrosion and result into wide material loss in the chemical industry.

The nickel alloys feature corrosion resistance characteristics to nominal reactivity of nickel as compare to iron as seen from its oxidation potential in the EMS series. As compare to stainless steel grades, chromium based nickel alloys develop security layer more quickly. The addition advantage of nickel over iron is the potential to get blended with different chemistry elements without resulting into the development of brittle phases. The common combining elements are molybdenum, chromium and copper. These factors should be considered as the beginning aspects in the material choice.

Welding

Welding in the different alloys is performed by shielded metal arc welding, gas tungsten arc welding and gas metal arc welding. The Nickel alloy weldments are highly ductile and their nominal heat expansion reduces residual stress and warpage. The post weld heat treatment is usually required for precipitation hardenable grades.

The welding process for nickel based alloys is alike to austenitic stainless steels. However due to their higher sluggishness of Nickel based weld puddles and lower penetration characteristics of nickel alloys, the development of complete cut welds may require enhancement of joint structure and welding methods.

The Nickel alloys cannot withstand the contaminants that cause weld embrittlement. The characteristics such as good ductility, small thermal expansion and ability to handle dilution by various metallic elements have made nickel based welding consumables widely accepted for welding with different materials. It only includes welding of nickel base alloys with iron based materials, even also joining stainless steels to carbon and other steel types. In the same way, nickel alloys can be weld gathered on carbon steel without causing cracks.

Nickel based alloys

The Nickel based alloys are developed in the various forms for example wire, sheet, plate, strip, tubes, pipe, fitting and flanges. Various nickel alloys are also created as castings. They generally provide several characteristics unlike to their wrought forms. These alloys are usually classified to their main alloying elements. These are commonly utilized in the chemical units such as:

Pure Nickel:

Nickel 200 called as pure nickel provides outstanding corrosion resistance properties to the wide range of reducing media and salts although it is not fit for use with powerful oxidation materials such as nitric acid. The major characteristics of Pure nickel is prevention of attack by caustic alkalis, also in the melt form. Besides of offering great resistance to dry halogen media, Nickel is not perfect for use at temperature below water dewpoint. In application temperature above 600oF, Nickel 201 is recommended for use.

Monel Alloy

The corrosion resistance performance of Monel 400 wire is supreme as of pure nickel, however it cannot perform well in aeration and oxidizing conditions. Alloy 400 attains outstanding security from attack of halogen media particularly hydro-fluoric acid, hot gases containing fluoride or hydrogen fluoride. It is commonly used in dealing with sulfuric acid solutions, sea water and brine solutions. The components that require high strength such as valve and pump fittings, demand Monel K-500 that is a precipitation hardenable form of alloy 400.

Inconel 600: Chromium in addition of nickel improves the service of Inconel alloy 600 in the oxidizing conditions. Although only fit for mineral acids, Inconel 600 provides extreme resistance to organic acids and is commonly used in fatty acid treatment. It is also commonly used in the development and dealing with caustic and alkali media. Alloy 600 is also fit for serve in the high temperature conditions that demand heat and corrosion resistance. The alloy offers outstanding service in the hot halogen media therefore it is fit for use in the organic chlorination processes. In the various elevated temperature degradation processes, Inconel 600 offers good resistance to oxidizing, carburizing and nitriding media.  

Inconel 625: An inclusion of molybdenum to Ni-Cr combination provides security from mineral attack and also from oxidizing and reducing salts corrosion. The presence of molybdenum improves pitting and crevice resistance in wet chlorides. The alloy offers good mechanical strength and fatigue strength. Alike to alloy 600, corrosion resistant Inconel 625 wire is a super alloy material that is a proven candidate for use in chemical and petrochemical processing units for service in the severe and elevated temperature applications.

Incoloy 825: It is a member of austenitic stainless steel group and is commonly used in handling sulfuric and phosphoric acid, they are the major challenges to be met by alloy 825. Although suitably resistant to HCl, alloy 825 prevents attack of chloride pitting and crevice in sluggish and unaerated conditions. However alloy 825 is mildly resistant to alkali and halogen media unlike to high nickel alloys.

Hastelloy G: Alloy G3 offers improved corrosion resistance than Monel 400, alloy 600 and alloy 825 in the various media. It is specifically resistant to sulfuric acid and contaminated H3PO4 and can serve excellent in the vigorous oxidizing and reducing conditions. The advanced Hastelloy G30 provides improved weldability and resistance to different kinds of corrosion and also in the weld heat affected regions.

Hastelloy C: Alloy C276 is the main material that is used in the chemical processing systems for service in the highly corrosive media where the stainless steel grades do not withstand. Hastelloy C276 provides outstanding resistance to different acids, salts and various media that are involved in the chemical industry. It is particularly significant in the extreme conditions involving wet chlorine and hypochlorites. The materials offering superior metallurgical and corrosion resistance properties as compare to Hastelloy C276 has led to the development and popularity of different kinds of alloys for example Hastelloy C22, C2000 and Inconel 622. They have higher molybdenum and chromium content as compare to Hastelloy C276. Few grades also contain tungsten and copper. The effect of nominally alloying elements on the metallurgical properties and corrosion resistance is not focused in this article.

Hastelloy B grades: Hastelloy B2 provides excellent resistance to sulfuric acid, phosphoric acid and hydrochloric acid in the reducing media. It is ideally fit for use in systems that deal with HCl at whole concentration and temperature up to the boiling temperature.

The oxidation media severely decrease the corrosion resistance, particularly the powerful oxidizers like ferric and cupric salts that may be present as contaminants. Hastelloy B3 provides superior service than alloy B2. The main benefit of new grade is limited development of unnecessary matrices during the production that may result into embrittlement.