High performance materials to meet challenges of corrosion in chemical plants

There are many instances in chemical and allied industries where the material chosen for construction of the plant and equipment is expected to encounter highly aggressive, extremely severe corrosive environments. A large variety of service environments, is encountered in practice where challenges to materials in terms of corrosion attack are major. Metallic materials based on (i) conventional stainless steels - austenitic, martensitic and precipitation hardening (ii) pure nickel, (iii) copper based alloys such as monel may prove inadequate in such situations. This is all the more so because the material of construction is not only to withstand severe corrosion attack, but also has to have optimum combination of mechanical properties - strength, toughness etc. Further, in fabrication of chemical plant and equipment, welding is often involved and it is essential that weld joints have good corrosion resistance, comparable to that of base metal. This imposes a major restriction on choice of construction materials. Special grades/ categories of materials have been developed over the years for use in extremely high duty applications in chemical and related industries. Large data bank has also been generated with reference to performance of these materials in a wide spectrum of service environments. The situation with respect to expertise available in the country on these materials has also improved substantially over the years. Many of these materials are now produced indigenously at Mishra Dhatu Nigam Limited (Midhani), a Public Sector Undertaking based in Hyderabad.. This paper gives an overview of important categories/ grades of special materials available to the designers, with particular reference to the outstanding resistance they offer to aggressive chemical environments encountered in practice. Emphasis is given to coverage of grades that are produced in the country.