Hot corrosion mechanism

Figure 8.40 Schematic diagram to show the relationship of the different hot corrosion mechanisms as a function of temperature and SO3 pressure, (i) Type II, gas-phase induced acidic fluxing, (ii) At high SO3 pressures (PSO3 > 10 atm) pronounced sulphide formation accompanied by oxidation of sulphides and fluxing reactions, (iii) Type I (alloy-induced acidic fluxing basic fluxing, sulphidation).

Usually, there are two types of hot corrosion mechanisms (Figure 5.9) ... 

Hot corrosion mechanism of Ti3SiC2 against various molten salts... 

Hot corrosion mechanism of Ti3AIC2 and Ti2AIC against Na2S04... 

Several hot corrosion mechanisms including sulfidation, acid-basic dissolution and electrochemistry have been proposed. No matter what the mechanisms are, it is important to understand the character of Na2S04 first. Na2S04 is an electrolyte, which follows the following equilibrium ... 

So, any reaction that favors the consumption of Na20 or SO3 will lead to the dissociation of Na2S04 and vice versa. Molten Na2S04 is an ionic conductor the hot corrosion mechanism should generally be electrochemistry [78]. In other words, hot corrosion itself is an electrochemical process that includes anodic oxidation, cathodic reduction and ion diffusioa As for the hot corrosion of Ti3AlC2, the anodic oxidation process mainly consists of the anodic dissolution of Ti and Al ... 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

NaCl, interact with the sulphur and vanadium oxides emitted from the combustion of technical grade hydrocarbons and the salt spray to form Na2S04 and NaVCh. These corrosive agents function in two modes, either the acidic mode in which for example, the sulphate has a high SO3 thermodynamic activity, of in the basic mode when the S03 partial pressure is low in the combustion products. The mechanism of corrosion is similar to the hot corrosion of materials by gases with the added effects due to the penetration of the oxide coating by the molten salt. 

The elemental sulfur may also react with chromium to form sulfides and deplete the surface in chromium. Furthermore, the chromium sulfide releases sulfur by thermal decomposition, which in turn diffuses into the metal and depletes the chromium from the surface, which would otherwise offer protection. This mechanism of depletion of chromium due to sulfidation and increase in the rate of hot corrosion can be overcome by increasing the chromium content of the alloy to a high level. 

E. A. Gulbransen and G. H. Meier, Mechanisms of oxidation and hot corrosion of metals and alloys at temperatures of 1150 to 1450 K under flow. In Proceedings of 10th Materials Research Symposium, National Bureau of Standards Special Publications 561,1979, p. 1639. 

The propagation modes of hot corrosion are determined by the mechanisms by which the molten deposits cause the protectiveness of the reaction-product oxide scales to be destroyed. Two possible propagation modes are basic and acidic fluxing... 

Figure 8.33 Schematic diagrams of the reaction mechanisms of hot corrosion of nickel at (a) 700 °C and (b) 900 °C in gases containing O2, SO2, and SO3 ...

R. H. Barkalow and F. S. Pettit, On Oxidation Mechanisms for Hot Corrosion of CoCrAlY Coatings in Marine Gas Turbines, Proceedings of the 14th Conference on Gas Turbine Materials in a Marine Environment, Naval Sea Systems Command, Annapolis, MD, 1979, p. 493. 

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. 

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