Oxide spalling

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

Figure 8.4 presents schematic diagrams of how an oxide film may respond to a compressive stress. One possibility is for the oxide and metal to separate at the interface hy buckling of the oxide (Figure 8.4(a)), i.e. the oxide spalls. The spallation of a compressively stressed film will occur when the elastic strain energy stored in the intact film exceeds... 

If the oxides spall during service, there is the danger of tube blockage as the spalled oxide accumulates inside the tubes, resulting again in overheating. If oxide particles are carried with the steam into the turbine, erosion of the blades may occur. 

Praseodymium is soft, silvery, malleable, and ductile. It is somewhat more resistant to corrosion in air than europium, lanthanum, cerium, or neodymium, but it does develop a green oxide coating that spalls off when exposed to air. As with other rare-earth metals, it should be kept under a light mineral oil or sealed in plastic. 

The metal has a bright silvery metallic luster. Neodymium is one of the more reactive rare-earth metals and quickly tarnishes in air, forming an oxide that spalls off and exposes metal to oxidation. The metal, therefore, should be kept under light mineral oil or sealed in a plastic material. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at 863oC. 

Industrial materials without sufficient scaling resistance frequendy fail after a short period of time as a result of rapid oxidation or hot corrosion, in conjunction with severe spalling owing to poor adherence of the scale to the metallic component. As a result, the permissible limits of metal loss often are exceeded and expensive, and premature replacement of parts is requited. Extensive efforts are made to develop alloys which are not simply strong at elevated temperatures but which also possess the adequate surface stabiUty. 

In practice, thermal cycling rather than isothermal conditions more frequently occurs, leading to a deviation from steady state thermodynamic conditions and introducing kinetic modifications. Lattice expansion and contraction, the development of stresses and the production of voids at the alloy-oxide interface, as well as temperature-induced compositional changes, can all give rise to further complications. The resulting loss of scale adhesion and spalling may lead to breakaway oxidation " in which linear oxidation replaces parabolic oxidation (see Section 1.10). 

Thus for non-ferrous metals, SO is consumed in the corrosion reactions whereas in the rusting of iron and steel it is believed that ferrous sulphate is hydrolysed to form oxides and that the sulphuric acid is regenerated. Sulphur dioxide thus acts as a catalyst such that one SOj" ion can catalyse the dissolution of more than 100 atoms of iron before it is removed by leaching, spalling of rust or the formation of basic sulphate. These reactions can be summarised as follows ... 

Armitt, J., Holmes, D. R, Manning, M. 1. and Meadowcroft, D. B., The Spalling of Steam Grown Oxides from Superheater and Reheater Tube Steels, EPR1-FP-686,TPS 76-655 Final Report (February 1978). 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

The rate of oxidation increases with temperature. Oxygen diffusion becomes more rapid, the oxide film becomes thicker and eventually cracks and spalls off due to internal stress. Oxidation then becomes continuous and rapid. 

Gaffney JS, Streit GE, Spall WD, et al. 1987. Beyond acid rain - do soluble oxidants and organic toxins interact with S02 andNOXto increase ecosystem effects. Environ Sci Technol 21 519-523. 

In addition, further oxidation and cathodic reactions lead to the production of oxides and oxyhydroxides of Fe (III), which produces a low-permeability, passive film that slows down the corrosion rate considerably. Where corrosion can continue (by depassivation), the expansion of corrosion products at the cement-steel interface and the subsequent spalling of cover concrete can occur. Many examples of this can be seen in concrete structures. 

---