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Bridging the gap between superalloys & carbon steel

Losses due to corrosion have been anticipated to be of the order of 2 – 5% of the GNP of any country. In India, the losses due to the impact of corrosion have been estimated roughly of 25,000 crores rupees per year (USD$25 billion), according to NACE, India. Many of these losses are due to the corrosion of iron and carbon steel, although many other metals may corrode as well. Pitting is a common form of corrosion which, if it occurs throughout the surface of metal, is also termed as uniform corrosion and which eventually causes structural weakness and disintegration of the metal. 

The influence of corrosiveness, temperature and pressure on material selection

Steel is considered to be carbon steel when no minimum content is specified or needed for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium, zirconium, or any other element to be added to obtain a desired alloying effect. Stainless steels and super-alloys, on the other hand, owe their corrosion resistance to a thin, passive surface-oxide film, rich in chromium, which spontaneously forms when exposed to air. This film-forming property is common to all alloys containing more than approximately 12% chromium. The composition of the oxide film obviously varies with the composition of the material.

Alternative to super alloys

Superalloys (Hastelloy, Inconel) exhibits several key characteristics: 

  • excellent mechanical strength,
  • resistance to thermal creep deformation,
  • good surface stability,
  • resistance to corrosion or oxidation in comparison to carbon steel.

Stainless steels show several similarities to superalloys and therefore, in some critical applications where severe corrosion and high temperatures are of major concern with the carbon steel, act as a cheaper solution in substituting superalloys. Special grades of stainless steel (UNS S30815, UNS S32750, UNS S32760, 254SMO) have improved corrosion resistance and high-temperature oxidation resistance and strength compared to standard grades like 304 and 316. 

Among the many case studies demonstrating the value of these special grades is Flue Gas Desulphurization (FGD) systems. The new emissions norms for coal-fired power plants regarding Sox and Nox, are stringent enough to mandate the power plants to be equipped with an FGD system. While FGD is essentially a simple process operating under relatively mild conditions in comparison with other chemical processes in industry, numerous materials problems have been encountered. As the system works in a high chloride environment, plastic and rubber linings on carbon steel are frequently specified; however, the failure of such linings due to fires has lead to the shutdown of power stations. 

The development of a ‘wallpaper’ sheet lining of carbon steel with thin (1.6 mm-2.0mm) nickel-containing super-alloys is significant in this respect. An important component is the scrubber, which is mainly subjected to a severe chloride environment. However, with a two-loop system, the 50,000-60,000 ppm chloride environment can be restricted to the quench area of the scrubber.

The remaining scrubber areas with chloride contents in the few thousand ppm ranges will encounter less corrosive conditions and may not require highly expensive superalloys and super duplex stainless steel: instead UNS S32750 could be a cheaper solution. Cost-wise, stainless steel is five times cheaper than the basic grades of superalloys on a per-kilogram basis.

It can, therefore, be concluded that depending upon severity level for the intermediate operating conditions where carbon steel is inferior and super-alloys could be much more superior, stainless steel could be the most appropriate and economical material.