It’s no surprise that stainless steel has been welcomed across the chemical and petrochemical industries. Its ability to withstand attack from highly corrosive substances, as well as its impressive mechanical characteristics, are key factors that continue to result in high levels of demand. As industrial processes evolve and change and new challenges present themselves, exciting new applications for stainless steel are being explored in order to make the most of the key benefits offered by this material.
The main difference between stainless steel and carbon steels is the chromium content. The chromium forms a thin layer of chromium oxide on the surface – thereby stopping corrosion from penetrating the metal. Even if the oxide layer is damaged, it repairs itself quickly as fresh chromium becomes exposed and oxidised.
Another advantage is that stainless steel is recyclable. Unlike carbon steel, which does corrode, or can be contaminated by paint or plate finishes for example, stainless steel is 100% recyclable. In fact, most stainless steel objects are said to contain up to 60% recycled material.
The conditions in which stainless steel is used within petrochemical and chemical plants are critical. Treated fluids and extremely high temperatures can be particularly aggressive during operation; and environments where chloride is present, or where induced cracking occurs are key challenges.
The chemical and petrochemical industries present a unique set of challenges including cryogenic applications, elevated temperatures, highly corrosive environments, high pressure handling and the purity of final products.
Petroleum refining utilises ever more complex processes, prompting the need for chemical and mechanical engineering advances designed to increase yield and improve operational reliability. The reduction or elimination of potential pollutants from processes and products is a high priority. The industry is also seeing more emphasis on materials engineering, and increased interest in high-alloy, corrosion resistant steels, especially stainless steels, which are able to cope with a variety of raw crudes.
Large markets have been created for stainless tanks, pipes, pumps and valves. Special grades have also been developed with greater corrosion resistance across a wide range of temperatures to maximise performance. This type of product is commonly used in desalination plants, sewage plants and offshore oil rigs.
There are five types of stainless steel:
Ferritic: These steels are based on chromium and have small amounts of carbon (usually less than 0.10%). They are used in thin sections and have a range of applications where welding is not required. They cannot be hardened by heat treatment. Ferritic steels are chosen for their resistance to stress corrosion cracking.
Austenitic: These are the most common and have a microstructure which has nickel, manganese and nitrogen added to improve weldability and formability. Corrosion resistance is enhanced by adding chromium and molybdenum.
Martensitic: These are similar to ferritic steels but have higher levels of carbon which means they can be hardened and tempered. They are used if there is a requirement for high strength and moderate corrosion resistance and are commonly produced as long products rather than in sheets or plates.
Duplex: This has a microstructure of 50% ferritic and 50% austenitic, offering improved strength capabilities and resistance to stress corrosion cracking. Super-duplex steels have enhanced strength and corrosion resistance, weldability and moderate formability. They also offer potential for weight savings.
Precipitation hardening (PH): These super-strong steels have added elements such as copper, niobium and aluminium as well as a strong particle matrix which forms during a heat treatment process. These steels can be machined into intricate shapes with minimal distortion.
Questions and answers about stainless steel
Q: What is stainless steel typically used for?
A: In the chemical and pharmaceutical industries, it can be used effectively for pressure vessels and process piping for example. In the oil and gas sector, it is commonly found in platform accommodation, cable trays and subsea pipelines.
Q: What causes a corrosive environment?
A: This can be atmospheric, water, concentration of particular chemicals, chloride content or the presence of acid.
Q: What is the effect of temperature on stainless steel?
A: Higher temperatures accelerate rates of corrosion whereas lower temperatures require increased strength.
Q: Which type of stainless steel is strongest?
A: Higher strength is provided by austenitic, duplex, martensitic and PH steels. However it is important to consider the processes involved as these can impact on which type is most suitable for a particular application.
Q: Which types of stainless steel can be welded?
A: In general, austenitic steels are the most weldable, however duplex steels can also be welded but require more care. Ferritic steels can be welded in thin sections. Martensitic and PH grades are the least suited to welding.
One of the important developments in the last decade is the emergence of highly-alloyed grades of stainless steel. Known as ‘super-austenitic’, ‘super-ferritic’ and ‘super-duplex’ grades, these stainless steel products have been developed to compete head-on with the nickel and titanium-based alloys being used in the most demanding conditions.
With so many options, it is vital to choose the correct grade to ensure the best match of properties, a suitable design and manufacturing process – and at a reasonable cost of course.