Reactive elements growth mechanism

Another complication with chromia formers is that surface application of yttria (and ceria) has been shown to produce the same effects as a reactive element incorporated into the alloy, but this precludes the operation of many of the mechanisms proposed for the reactive-element effect. The application of MgO has been shown to be ineffective, at least with regard to altering the growth rate of the chromia. ... 

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. 

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... 

Influence of reactive elements in alumina scale growth mechanism... 

Lees D G, On the reasons for the effects of dispersions of stable oxides and additions of reactive elements on the adhesion and growth-mechanisms of chromia and alumina scales - the sulfur effect , OxidMet, 1987, 27(1-2), 75-81... 

The reactions with chlorine ended polymers suggest that in toluene solution the condensation of these dichlorides with sodium contains no element of a condensation polymerization. Chain growth seems to be restricted to the addition of monomer units to the chain end. From the observation that the 27 ppm peak in the Si spectrum remained unchanged at the end of the reaction of chlorine ended polymers with sodium, it must be concluded that sodium does not react rapidly with a chlorine ended chain to form a sodium ended active chain. This was suggested earlier (11c) as part of the polymerization mechanism to account for the low dependence of the rate on the sodium surface area. This low dependence was first shown for the hexylmethyl monomer (1 la), but has been confirmed with the propylmethyldichlorosilane. Low order dependences of the rate of monomer consumption have been found in ionic polymerizations, and attributed to the aggregation of ionic chain ends in non-polar solvents (17). The reactive species is then a small concentration of monomeric chain ends in equilibrium with the aggregate. These conditions could well exist here in toluene solution. However, reaction with the sodium surface is an essential part of the reaction. 

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