Oxidation reactive element effect

W. E. King, ed.. The Reactive Element Effect on High Temperature Oxidation After Fifty Years, Materials Science Forum, MRS, 1991. 

Ion implantation is less expensive than VPS or HVOF and can be used in environments that are slightly less corrosive. A concentration of 1016-1018 ions/cm drastically improves corrosion resistance. Rare earth (RE) metals also improve the corrosion resistance. Rare earth metals such as Y improve oxide scale formation on alloys. This improvement is based upon the reactive element effect (REE), which improves resistance in several ways According to Hussey et al. [63], the observed increase in corrosion resistance is due to ... 

In high-temperature corrosion, the protective oxide hlms—usually either chromia, Cr203, or alumina, AI2O3—that form between the metal and the environment are of critical importance. One of the major advances in corrosion science over the past few decades has been the characterization of the reactive element effect (REE), which identifies the role of small additions of reactive elements, such as yttrium, hafnium, lanthanum, zirconium, and cerium, to improve high-temperature oxidation resistance. 

OXIDATION-RESISTANT ALLOYS 11.13.1 Reactive Element Effect (REE)... 

D. P. Moon, "The Reactive Element Effect on the Growth Rate," Oxidation of metals, pp. 47-66, 1989. 

According to the reactive element effect, the reactive element ion, such as beryllium, diffuses in to the native oxide grain boundaries and prevents the outward diffusion of substrate metal cations (Czerwinski and Smeltzer, 1993 Czerwinski and Szpunar, 1998 Czerwinski, 2000, 2004). The inhibitory effect of boron on aluminum alloy oxidation is clearly a surface phenomenon given the effectiveness of very low levels of boron. This could occur through a combination of boron migration into the MgO lattice and/or boron bonding to the defect-rich MgO surface (Choudhary and Pandit, 1991). 

Y ions into an aluminide ((3-NiAl) on a nickel-base alloy and confirm that while initially the implanted reactive element effectively imparts increased scale adhesion, both in air and oxygen at 1000 -1200 C, the beneficial influence is not long lasting. They attributed this loss to the influence of the substrate Ni-base superalloy, since lasting benefits of reduced rates of oxidation and improved scale adherence were maintained when Y was implanted into bulk 3-NiAl (Jedlinski and Mrowec 1987). 

The reader is directed to three comprehensive reviews and a conference publication, dealing with the reactive-element effects on oxidation Whittle and Stringer (1980), Stott and Wood (1987), Moon and Bennett (1989) and Lang (1989), thus only a broad summary of proposed mechanisms, classified as either chemical, physical or mechanical effects is presented here. These may be further, or even alternatively, sub-divided into effects relating to (i) initial oxidation, (ii) growth-rate, (iii) scale adhesion, or (iv) cracking. 

B.A. Pint, Experimental observations in support of the dynamic segregation theory to explain the reactive element effect. Oxidation of Metals, 45, 1/2, 1-31 (1996). 

The reactive element effects on the transition can also be explained by the present model. Application of reactive elements or their oxides onto the alloy surface or into the alloy substrate can change the growth mechanisms of oxide scales [12]. For undoped M-Cr alloys, the growth of Cr203 scales is sustained mainly by the outward diffusion of Cr" ions, resulting in the... 

Pint B A (1996), Experimental Observations in Support of the Dynamic Segregation Theory to Explain the Reactive Element Effect, Oxid Met, 45, 1-37. 

The effects of heteroatoms on autoxidation reactions are reviewed and discussed in terms of six phenomena (1) the effect on reactivity of a-hydrogens in the hydroperoxide chain mechanism in terms of electron supply and withdrawal (2) the effect on a-hydrogen acidity in base-catalyzed oxidation (3) the effect on radical ion stability in base-catalyzed redox chains (4) the possibility of heteroatom hydrogen bond attack and subsequent reactions of the resulting heteroradical (5) the possibility of radical attack on higher row elements via valence expansion (6) the possibility of radical addition to electron-deficient II and III group... 

The carbides of the early transition metals exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals [1], Typically, early transition metals are very reactive elements that bond adsorbates too strongly to be useful as catalysts. These systems are not stable under a reactive chemical environment and exhibit a tendency to form compounds (oxides, nitrides, sulfides, carbides, phosphides). The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability [2]. Furthermore, in a metal carbide, the carbon atoms moderate the chemical reactivity through ensemble and ligand effects [1-3]. On one hand, the presence of the carbon atoms usually limits the number of metal atoms that can be exposed in a surface of a metal carbide (ensemble effect). On the other hand, the formation of metal-carbon bonds modifies the electronic properties of the metal (decrease in its density of states near the Fermi level metal—>carbon charge transfer) [1-3], making it less chemically active... 

Mercury is a reactive element and its toxicity is probably due to interaction with proteins. Mercury has a particular affinity for sulphydryl groups in proteins and consequently is an inhibitor of various enzymes such as membrane ATPase, which are sulphydryl dependent. It can also react with amino, phosphoryl and carboxyl groups. Brain pyruvate metabolism is known to be inhibited by mercury, as are lactate dehydrogenase and fatty acid synthetase. The accumulation of mercury in lysosomes increases the activity of lysomal acid phosphatase which may be a cause of toxicity as lysosomal damage releases various hydrolytic enzymes into the cell, which can then cause cellular damage. Mercury accumulates in the kidney and is believed to cause uncoupling of oxidative phsophorylation in the mitochondria of the kidney cells. Thus, a number of mitochondrial enzymes are inhibited by Hg2+. These effects on the mitochondria will lead to a reduction of respiratory control in the renal cells and their functions such as solute reabsorption, will be compromised. 

At 1200 °C and 1300 °C, incorporation of a reactive element in Fe3Al as an oxide dispersion is more effective in improving scale adhesion than a comparable elemental addition. It is proposed that this is due to strengthening of the substrate. 

In air, carburising atmospheres and sulphidising/oxidising atmospheres all alloys showed excellent corrosion resistance due to the formation of protective oxide scales. It was found that the concentration of aluminium or chromium does not significantly effect the corrosion behaviour in any of these environments. 10 wt% aluminium, probably even less, are sufficient to enable the formation of protective Al203-scales even at temperatures as low as 650°C. Overdoping with reactive elements (mischmetal), however, causes high oxidation rates in air and should be avoided. 

High reactivity elements (RE e.g., cerium, yttrium, zirconium, hafnium) are sometimes added to the Fe-Cr-Al matrix these help the formation of the alumina protective layer that is, they speed up the transition from the less to the more stable crystallographic lattices [5,6] and increase its adhesion to the substrate. Secondly this action is assisted by the precipitation of "pegs" made up by fhe oxides of fhe reactive elemenfs (RE), partially immersed both in the substrate and in the scale of continuous superficial oxide [6]. However, fhe same authors state that the formation of fhe pegs is nof vital for the resistance to the scaling off of fhe layers of superficial oxide. It is important to note that its crystallographic type is a-Al203, which is much more effective than the 5, y, or 9 types. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

It should also be noted that it is the soluble O and not the combined O (e.g. oxides) which affects surface tension. Certain elements such as Ca, Al, Mg react strongly with O and reduce the soluble O (denoted to very low levels e.g. (a few ppm) and form stable metallic oxides (Figure) [3]. The total O is not a measure of the soluble O (Q%) in such cases. Thus very low concentrations of these reactive elements (e.g. Ca) can have a marked effect on the process because of their effect on the surface active elements present. Thus for the Fe-Al-0 system in Figure 2a, 10 % of Al will lower the soluble O to < 5 ppm but 10 % Ca (10 ppm) (Fe-Ca-0 system) will reduce... 

---