Hot corrosion of alloys and coatings

S PRAKASH, Indian Institute of Technology Roorkee, India 

Hot corrosion is a complex phenomenon involving sulphidation, oxidation or both. Hot corrosion is a form of accelerated oxidation which affects the surfaces exposed to high-temperature gases contaminated with sulphur and alkali metal salts. These contaminants combine in the gas phase to form alkali metal sulphates. If the temperature of the alloy or coating surface is below the dew point of the alkali sulphate vapours and above the sulphate melting points, molten sulphate deposits are formed. Molten sodium sulphate is the principal agent in causing hot corrosion. 

According to Hancock (1987), hot corrosion is an accelerated form of oxidation which occurs when metals are heated in the temperature range 700-900°C in the presence of sulphate deposits formed as a result of the reaction between sodium chloride and sulphur compounds in the gas phase around the metals. 

If the concentration of the sulphate exceeds the saturation vapour pressure at the operating metal temperature for turbine blades and vanes (700-1100°C), 

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The degradation modes of coatings are essentially the same as those described for cyclic oxidation of alloys in Chapter 5, mixed-oxidant corrosion of alloys in Chapter 7, and hot corrosion of alloys in Chapter 8. However, additional factors arise with coatings since they are relatively thin, they contain a finite reservoir of the scale-forming elements (Al, Cr, Si), and interdiffusion with the substrate can both deplete the scale-forming element and introduce other elements into the coating. Additional mechanical affects also arise, which can lead to deformation of the coating. Mechanical effects are also critical to the durability of TBCs. 

H. Singh, D. Puri, S. Prakash, An overview of Na2S04/V20s induced hot corrosion of Fe- and Ni-based super-alloys. Surf. Coat. Techol. 192 (2007) 27—38. 

Figure 8.22 Comparison of the isothermal hot corrosion of Na2S04-coated Ni-8 wt% Cr-6 wt% Al and Ni-8 wt% Cr-6 wt% Al-6 wt% Mo. Both alloys undergo basic fluxing, but Ni-8 wt% Cr-6 wt% Al-6 wt% Mo eventually undergoes alloy-induced acidic fluxing.

J. Schaeffer, G. M. Kim, G. H. Meier, and F. S. Pettit, The effects of precious metals on the oxidation and hot corrosion of coatings . In The Role of Active Elements in the Oxidation Behavior of High Temperature Metals and Alloys, ed. E. Lang, Amsterdam, Elsevier, 1989, p. 231. 

Sidhu TS, Agarwal RD and Prakash S, Hot Corrosion of Some Super Alloys and Role of High-Velocity Oxy-Fuel Spray Coatings. Surf Coat Technol, 2005, 198 441-446. 

Keyvani A. Microstructural stability oxidation and hot corrosion resistance of nano-structured Al203A SZ composite compared to conventional YSZ TBC coatings. Journal of Alloys and Compounds. 2015 623 229-237. 

The sensitivity of hot corrosion to alloying element, operation temperature, and service environment makes the reverse engineering of a part subject to hot corrosion challenging. The compositions of both the coating material and the base alloy need to be carefully identified. 

Fruit juices, meat products, milk and milk products, fish and most vegetables, in which tin is likely to be anodic to steel, can be handled open to the air in tinned steel vessels. Some corrosion of the tin occurs at rates similar to those found for pure tin and in due course retinning may be necessary. The alloy layer in hot-dipped tin coatings is cathodic to both tin and steel and, under aerated conditions may stimulate the corrosion of both metals, but this effect appears to be unimportant in practice. 

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