Growth protective scale

The intergranular oxidation can be followed or accompanied by internal oxidation of NiAl.Thc internal oxidation must be initiated by a rapid oxidation causing considerable Al-depletion but without formation of a protective scale [5], Then oxygen can diffuse into the NiAl matrix and reacts under formation of inward growing A1 03 precipitates. In the ampoule experiments with various metal/oxide or oxide/oxide mixtures the attack is mostly localized and inward and outward growth of pocks or cones is observed on the material (Fig. 3), but also general attack can occur, especially in the... 

Reaction of N2 with liquid Th spheres has been investigated. Five-mm diameter spheres of Th supported on a bed of ThN powder grow a dense protective scale of ThN when reacted with an N2 atmosphere [8]. For the kinetics of the diffusion-controlled growth of scale thickness, see p. 24. 

If n = 1/2, a parabolic behavior of nonporous, adherent, and protective scale develops by diffusion mechanism. Thus, 1 < PB < 2 and Ae mechanism of sc growth is related to metal cations diffusing through the oxide scale to react with oxygen at the oxide-gas interface. 

D. M. Lipkin, H. Schaffer, F. Adar, and D. R. Clarke, Lateral growth kinetics of -alumina accompanying the formation of a protective scale on (111) NiAl during oxidation at 1100° C, Appl. Phys. Lett., 70,2550 (1997). 

The process described so far occurs repeatedly and, after longer testing times or at higher temperatures, multilayered scales are formed, as shown in Fig. 2-40, which offer only limited protection. Scale growth kinetics resulting from this situation can often be approximated by quasi-linear behavior (Fig. 

Criterion for the Sustained Exclusive Growth of a Protective Scale. 205... 

If an alloy is to have acceptable resistance against high-temperature corrosion, it must react with the environment to form a continuous and adherent slow-growing scale which has sufficient mechanical properties to withstand the effects of both growth and thermal stresses. As discussed in Chapter 3 of this volume, ideal protective scale growth obeys diffusion-controlled, parabolic kinetics, i.e. ... 

Table I. Equations describing the growth of a protective scale on a pure metal by cation diffusion and surface reaction. (Symbols are listed at the end of the paper).

Promotion of preferential nucleation and growth of stable, low diffusivity Cr20j or AI2O3 protective scales. 

This chapter combined speculation regarding new approaches to mitigating oxidation/corrosion with preliminary results in order to highlight oxidation phenomena of potential interest for future development. Many of these approaches appear to show promise for controlling surface chemistry. However, the key to their ultimate technological utility for applications as protective scales lies in the ability to form these phases as dense, continuous surface layers. This appears to be possible, particularly for the growth of protective nitride layers on Ni and Fe base alloys. Of scientific interest will be to study the similarities and differences that arise from the growth of surface nitride, carbide, etc., layers on multi-component/multi-phase alloys, compared to... 

Based on current understanding of temperature dependence, the mechanisms of scale growth and scale compositions and also of the two-stage, oxidation induced chemical failure process of FeCrAIRE alloys, the development and evaluation of two potential procedures to increase alloy lifetimes have been major objectives of the CEC SMILER project. The majority of testing has been undertaken in laboratory air, as described in this chapter but has been substantiated by concurrent evaluation in other simulated relevant industrial environments [26]. The procedures involved first increasing the alloy A1 reservoir, and second gas annealing prior to service to pre-form a stable a-AI2O3 protective scale and also to remove possible deleterious non-metallic alloy impurities, such as sulphur. 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

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