I had already published an article which relates to the most common ferrous materials used in piping industry. Click here to visit the page.
In this article I will mention about the most frequently used non ferrous piping materials.
In comparison with metallic materials, the use of plastics is limited to relatively moderate temperatures and pressures [230degC (450degF) is considered high for plastics]. Plastics are also less resistant to mechanical abuse and have high expansion rates, low strengths (thermoplastics) and only fair resistance to solvents. However, they are lightweight, are good thermal and electrical insulators, are easy to fabricate and install, and have low friction factors. Since plastics do not corrode in the electrochemical sense, they offer another advantage over metals. The important thermoplastics used commercially are polyethylene, polyvinyl chloride (PVC), fluorocarbons (Teflon, Halar, Kel-F, Kynar) and polypropylene. Important thermosetting plastics are general-purpose polyester glass reinforced, bisphenol-based polyester glass, epoxy glass, vinyl ester glass, furan and phenolic glass, and asbestos reinforced. While using non-metallic piping, viz HDPE, PVC, FRP etc, the designer shall take care of the service, pressure & temperature. Manufacturer’s recommendation shall be taken into account.
The most chemical-resistant plastic commercially available today is tetrafluoroethylene or TFE (Teflon). This thermoplastic is practically unaffected by all alkalies and acids except fluorine and chlorine gas at elevated temperatures and molten metals. It retains its properties up to 260 deg C (500 deg F). Perfluoroalkoxy, or PFA (Teflon), has the general properties and chemical resistance of FEP at a temperature approaching 300 deg C (600 deg F). Polyethylene is the lowest cost plastic commercially available. Mechanical properties are generally poor, particularly above 50 deg C (120 deg F), and pipe must be fully supported. Carbon-filled grades are resistant to sunlight and weathering. Polypropylene has a chemical resistance about the same as that of polyethylene, but it can be used at 120 deg C (250 deg F).
Among the thermosetting materials are phenolic plastics filled with asbestos, carbon or graphite, glass, and silica. Relatively low cost, good mechanical properties, and chemical resistance (except against strong alkalies) make phenolics popular for chemical equipment. Furan plastics filled with asbestos and glass have much better alkali resistance than phenolic resins. Polyester resins reinforced with fibreglass, have good strength and good chemical resistance except to alkalies. Epoxies reinforced with fibreglass have very high strengths and resistance to heat. The chemical resistance of the epoxy resin is excellent in non-oxidizing and weak acids but not good against strong acids. Alkaline resistance is excellent in weak solutions.
Rubber and elastomers:
Rubber and elastomers are widely used as lining materials. The ability to bond natural rubber to itself and to steel makes it ideal for lining tanks. Natural rubber is resistant to dilute mineral acids, alkalies and salts, but oxidizing media, oils and most organic solvents will attack it. Hard rubber is made by adding 25 percent or more of sulphur to natural or synthetic rubber and, as such, is both hard and strong. Chloroprene or neoprene rubber is resistant to attack by ozone, sunlight, oils, gasoline and aromatic or hydrogenated solvents, but is easily permeated by water, thus limiting its use as a tank lining. Nitrile rubber is known for resistance to oils and solvents. Butyl rubber’s resistance to dilute mineral acids and alkalies is exceptional. Hypalon has outstanding resistance to ozone and oxidizing agents except fuming nitric and sulphuric acids. Fluoroelastomers (Viton-A, Kel-F, Kalrez) combine excellent chemical and temperature resistance.
A group of (mostly) proprietry alloys with somewhat better corrosion resistance than stainless steels are called medium alloys. A popular member of this group is 20alloy. Made by a number of companies under various trade names. Durimet 20, Carpenter 20 are a few names. This alloy was originally developed to fill the need for a material with sulphuric resistance superior to stainless steels. Other members of this group are Incoloy 825 and Hastelloy G-3 . These alloys have extensive applications in sulphuric acid systems. Because of their increased nickel and moly contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking and molybdenum improves resistance to crevice corrosion and pitting.
The group of materials called high alloys all contain relatively large percentage ofNickel. Hastelloy B2 contains 61% Nickel & 28% Mo. The alloy has unusually very high resistance to all concentrations of HCL at all temperatures in the absence of oxidizing agents. Other materials of this group are Chlorimet 2 & Hastelloy C-276.
Nickel & Nickel alloys :
The metal is widely used for handling alkalies particularly in handling and storing caustic soda. Neutral alkaline solutions, seawater and mild atmospheric conditions do not affect nickel. A large number of nickel based alloys are commercially available. One of the best known out of these is monel 400 with 67% Ni and 30 % Copper. This Ni-Cu alloy is ductile and tough. It’s corrosion resistance is better than it’s components, being more resistant than nickel in reducing environments and more resistant than copper in oxidizing environments.
Copper and copper alloys:
Copper and it’s alloys are widely used in chemical processing, particularly when heat and thermal conductivity is very important. Main copper alloys are brasses(Cu-Zn), Bronzes( Cu- Sn) and Cupronickels. Some of the bronzes are very popular in process industry , like Aluminium and silicon bronzes because they combine good strength with corrosion resistance. Cupronickels have 10-30% nickel and have become very popular because it has the highest corrosion resistance of all copper alloys.This finds it’s application in heat-exchanger tubing and it’s resistance to seawater is especially outstanding.
Titanium has become increasingly important as a construction material. It is strong and of medium weight. Corrosion resistance is very superior in oxidizing and mild reducing media. Titanium is usually not bothered by impingement attack, crevice corrosion and pitting attack in sea water. Its general resistance to sea water is excellent.
The detailed list of commonly used non ferrous materials in hydrocarbon industries is given in following table.