MIX AND MATCH resistant materials or linings?

The first choice for corrosion protection is to pick a material that isn’t going to corrode. Materials choice is important, with the various grades of stainless steel predominating. Rhodia, for example, uses stainless steel actuators for the offloading valves on its fleet of ISO tankers for conveying hazardous chemicals. These include sulphur trioxide, sulphuric acid and oleum; caustic soda, liquid chlorine and hydrochloric acid. The slightest splash of these substances can render equipment useless, and as splashing is a constant danger with tanker offloading, Rhodia found itself replacing tanker valves and actuators with monotonous and expensive regularity. Instead, it opted to replace them completely with Morin pneumatic actuators, made from 316 stainless steel.

When the equipment can’t be made inherently corrosion-resistant, the next option is to line it with a material with the required properties. This has several advantages - the lining can be thin, making it cheaper than making the entire equipment out of a specialist material. And in some cases, the lining material would simply not be a practical choice for large-scale construction. One such material, glass, is proving an increasingly popular choice for linings in the pharmaceutical industry.

Once plants become larger than laboratory ‘miniplant’ scale, glass becomes less suitable for reactors, pipes and other equipment. However, the advantages of using glass are compelling. Not only is it resistant to almost all forms of corrosion, and extremely resistant to heat, it has extremely high surface quality, making it very easy to clean. In fact, according to Werner Kohle of specialist glass manufacturer Thale, the adhesion of microorganisms on glass is less than that on stainless steel which has been electropolished to the same smoothness.

Cleaning is of equal importance to corrosion protection for pharmaceutical applications. Glass linings can be made to have no crevices or dead ends where material can gather. Linings are available in blue (generally for chemical processing) or white for pharmaceuticals, as this shows up residual material.

Specialist linings are also finding uses to protect centrifuges. Equipment supplier Alchem has joined forces with coatings specialist Edlon to introduce a service to refurbish old centrifuges, allowing a ‘cost-effective alternative’ to buying new equipment, Alchem claims.

The refurbishment begins with the centrifuge stripped down to its components, cleaned and inspected. Alchem engineers replace all bearings, seals, PTFE chevrons, hydraulic and pneumatic seals. Then Edlon takes over, shot-blasting the outer casing, cover, basket and pan bottom back to base steel. This is then coated with SC-2001 Halar fluoropolymer, which is tough, impervious and resistant to most chemical compounds and a wide temperature range. It also has good surface-release characteristics, which allows it to be used for pharmaceutical and food applications.

In a much larger-scale application, gas producers are turning to corrosion-resistant steel containing nickel additives to handle the conditions found in deep-sea exploration and production efforts. A wide variety of conditions has lead to several different anti-corrosion techniques being used.

The carbon dioxide found in natural gas deposits corrodes normal steels, as chlorine. A gas production project on Sable Island, off Canada’s Nova Scotia coast, is currently installing nickel-coated carbon steel equipment to separate gases and liquids. The project is a joint venture between Mobil, Shell, and Canadian companies, and is scheduled to start producing gas this month. The hostile operating conditions and distance from the coast - some 10km - has dictated the choice of materials.

The separator units operate at about 121 degrees C and up to 15.3MPa pressure, and handle materials such as water, oil and hydrocarbon condensates such as butane and propane. They are made from carbon steel, which is mechanically strong enough to handle these conditions, but is susceptible to pitting corrosion. To counter this, the steel is coated with a 3mm-thick layer of a steel containing 25 per cent nickel.

The pipelines that bring the gas ashore are also made of carbon steel, but in this case another anti-corrosion strategy is used: the engineers add a corrosion inhibitor to the gas before it enters the pipe. This isn’t practical for the subsea piping sections, however; these will be made entirely from S31803 nickel-containing steel. This can be made thinner than carbon steel without sacrificing strength, as the nickel enhances the toughness of the steel, especially at low temperatures.

An even more exotic blend will be helping researchers uncover the secret of the Big Bang at CERN in Geneva. Although not strictly speaking a process plant, CERN represents one of the biggest materials and engineering challenges ever seen: a 27-km long pipe arranged in a perfect circle, buried 100m underground, though which a beam of protons will run at near the speed of light.

The new alloy was developed for the ‘beam screens’ - a section inside a vacuum chamber which houses the superconducting magnets which accelerate the protons. The screens shield the magnets from the synchrotron radiation generated by the circulating particles, and have to withstand very low temperatures - just 10-20K.

The engineers at CERN and Austrian steel specialist Bohler Edelstahl developed an alloy containing 11 per cent nickel and 12 per cent manganese for the beam screens. Designated P506, the metal will be formed into 1mm-thick, 44mm-diameter perforated tubes, which will be continuously formed and laser-welded. The steel is easy to weld, and has high tensile strength, ductility and toughness even down to 4.2K. Along with its inherent corrosion resistance, this will allow the metal to be used for demanding low-temperature process applications, its developers claim. PE