Reactive elements coating

R.J. Hussey, M.J. Graham, The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys, Oxid. Met. 45 (1996) 349—374. 

Fluorine, the most reactive element known, is a dangerous material but may be handled safely using proper precautions. In any situation where an operator may come into contact with low pressure fluorine, safety glasses, a neoprene coat, boots, and clean neoprene gloves should be worn to afford overall body protection. This protection is effective against both fluorine and the hydrofluoric acid which may form from reaction of moisture in the air. 

Bioavailability from Environmental Media. The bioavailability of elemental phosphorus following inhalation, oral, and dermal contact is poorly understood (see Section 2.3). The estimated log Koc for elemental phosphorus is 3.05 (See Table 3-2). Therefore, elemental phosphorus is moderately sorbed to aerosol particles in air, to sediment in water, and to soil. However, due to its high reactivity, elemental phosphorus may not be found in aerobic zones of soil and water, unless the element is protected from oxidation by unreactive oxide coating (Berkowitz et al. 1981). Its bioavailability in the sorbed state from inhaled air, ingested soil, and dermal contact with soil and water may be lower than the free form of the element under identical conditions. 

The objective of surface modification by CVD is to develop additional corrosion protection for high-temperature alloys (e.g., Fe-25Cr-0.3Y) beyond that achieved by reactive-element additions which will be effective for a long period. Because CVD is a high temperature process, the following topics must be discussed before we can develop successful high-temperature corrosion-proteetion coatings. ... 

The colloids adsorb heavy metal ions and waterborne pollutants hence the fates of reactive elements and of many pollutants in the environment depend to a large extent on the movement of colloids in aqueous systems. The colloids in natural waters are characterized by an extreme complexity and diversity organisms, biological debris, organic macromolecules, various minerals, clays, and oxides, partially coated with organic matter. 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

A small addition of Y to the coating resulted in flat A1203 scales [48] as has often been discussed in terms of the so-called reactive element effect. However, the protectiveness of the coating was not improved rather it was slightly decreased. A variation in the nature of the coating by the Y addition may be a reason for this. 

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... 

The addition of small amounts of reactive elements such as cerium, yttrium, hafnium, thorium, lanthanum, or their oxide dispersions greatly increases the high-temperature oxidation resistance of Fe-Cr alloys under isothermal or cyclic conditions. [11], Beneficial effects also result from ion implantation of the active element or from surface-applied coatings [11]. The ion implantation work of Bennett et al. [12] concerns the oxidation behavior of a 20Cr-25Ni-Nb stainless steel in CO2 at temperatures in the range of 900 to 1050°C. SIMS can be used to locate the position of the reactive element after oxidation. 

Additions of reactive elements (RE), such as Y, Hf and Zr, to MCrAlY bond coats enhance the oxide adhesion. Y and Hf promote the formation of... 

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