Stainless steels, nickel alloys and other nickel-containing alloys are used extensively in the chemical, pharmaceutical and petrochemical industries. Their corrosion resistance in aqueous, gaseous and high-temperature environments, their mechanical properties at all temperatures from cryogenic to the very high and occasionally their special physical properties make them invaluable. Nickel plating is favoured for specific properties such as hardness and corrosion resistance. In addition, nickel-containing materials are often used as catalysts.
The safe and cost-efficient production of pharmaceutical compounds, fertilizers, plastics, petrochemicals and a host of other goods rely on stainless steel and nickel alloys.
Chemicals and Petrochemicals
The chemical and petrochemical industries pose a unique set of challenges, including cryogenic applications, elevated temperatures, highly corrosive environments, high-pressure handling and the purity of final products.
Therefore it is unsurprising that stainless steels and nickel alloys have been widely embraced by the chemical and petrochemical industries. Their ability to withstand highly-corrosive substances over a wide range of temperatures, as well as their impressive mechanical characteristics help to drive high levels of demand.
Type 304 (UNS S30400) and Type 316 (UNS S31600) are commonly-used grades. Type 304 is deployed in cryogenic, caustic and oxidizing acid environments. The addition of molybdenum in 316 provides superior resistance to chlorides and reducing acids relative to 304. When chlorides pass the resistance to pitting corrosion for 316 or extreme temperatures promote chloride stress-corrosion cracking, duplex stainless steel such as 2205 (UNS S32205) or super austenitics (UNS S31254 and UNS N08367), or super duplexes such as Type 2507 (UNS S32750) and UNS S32760, can provide useful resistance.
When corrosive conditions exceed the capability of the super austenitics and super duplexes, there are several types of nickel alloys, of which C-276 (UNS N10276) is the best known. There are now many derivatives of this composition, including UNS N06622, N06059, N06200 and N06686, which have been specifically developed to offer increased resistance in specific environments.
Nickel-containing stainless steels are widely used in the pharmaceuticals industry because of their inertness, resistance to corrosion, ease of cleaning and ease of fabrication.
Typically Type 304 (UNS S30400) is used in ‘non-contact applications’, while Type 316 (UNS S31600) is used in ‘product contact applications’, as its added molybdenum provides greater corrosion resistance.
Although Type 316 and its derivatives are the most widely-used stainless steels in pharmaceutical plants, for each application materials are selected on the basis of their corrosion resistance in a specific service environment. Selecting a suitable grade of stainless steel must also consider the cleaning regime and cleansing agents used in the plant.
Type 316 and its derivatives may be used with chloride concentrations up to 500 mg/l. However at higher chloride concentrations, particularly where combined with increased operating temperatures, the duplex stainless steel Type 2205 (UNS S32205) is used for its resistance to pitting corrosion and stress-corrosion cracking. For more aggressive service environments, super austenitic stainless steels, such as UNS S31254 and UNS N08367 and super duplex stainless steels, such as Type 2507 (UNS S32750) and UNS S32760 may be required.