Oxide coatings

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. 

Titanium dioxide-ferric oxide coated mica... 

Aluminum containers are recommended for many appHcations because of the very hard, corrosion-resistant oxide coating. They are deficient in only one respect once the protective skin has been penetrated, aluminum corrosion accelerates. 

Corrosion. Ammonium bifluoride dissolves in aqueous solutions to yield the acidic bifluoride ion the pH of a 5% solution is 3.5. In most cases, NH4HF2 solutions react readily with surface oxide coatings on metals thus NH4HF2 is used in pickling solutions (see Metal surface treatments). Many plastics, such as polyethylene, polypropylene, unplasticized PVC, and carbon brick, are resistant to attack by ammonium bifluoride. 

Fig. 2. Curves 1, 2, and 3 show the spectral radiance factor for equivalent coatings of separate toluenesulfonamide—melamine—formaldehyde Day-Glo pigments containing 0.5% of a dye, either Alberta Yellow, Rhodamine F5G, or Rhodamine B Extra. Curve 4 is for a bright nonfluorescent red-orange printing ink. The illuminant was Source C. A magnesium oxide-coated block was used as a comparison white.

D. E. Bartak, T. D. Schleisman, and E. R. Woolsey, "Electrodeposition and Characteristics of a SUicon—Oxide Coating for Magnesium AUoys," Paper T-91-041, North American Die Casting Association 16th International Die Casting Congress and Exposition, Detroit, Mich., 1991. 

Many competitive programs to perfect a metallic anode for chlorine arose. In one, Dow Chemical concentrated on a coating based on cobalt oxide rather than precious metal oxides. This technology was patented (9,10) and developed to the semicommercial state, but the operating characteristics of the cobalt oxide coatings proved inferior to those of the platinum-group metal oxide. 

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

Fig. 1. Scanning electron microscope photograph of DSA mthenium oxide coating, showing typical cracked surface.

Iridium Oxide. Iridium dioxide [12030 9-8] coatings, typically used in combination with valve metal oxides, are quite similar in stmcture to those of mthenium dioxide coatings. X-ray diffraction shows the mtile crystal stmcture of the iridium dioxide scanning electron micrographs show the micro-cracked surface typical of these thermally prepared oxide coatings. 

The blue-black Hon oxide formed in this process fills some of the interconnecting porosity and much of the surface. Hence the density is increased, resulting in higher compressive strength. Furthermore, the oxide coating increases hardness (qv) and wear resistance. 

Antimicrobial agents are used where there is a need to inhibit bacterial and fungal growth. The additives can consist of copper, germanium, zinc and zinc compounds, metal oxides or sulfides, metal zeofltes, as well as silver and copper oxide-coated inorganic core particles (154—159) (see Industrial ANTIMICROBIAL AGENTS). 

Magnesium methylate is used as a drying agent for alcohols and other organic solvents and as an intermediate in various manufacturing processes, eg, for organomagnesium compounds (79), orthocarbonic esters (80), and for oxide coatings. 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Aluminide and sUicide cementation coatings such as TaAl on tantalum and MoSi2 on molybdenum oxidize at slow rates and possess some inherent self-repair characteristics. Fine cracks that appear and are common to these coatings can be tolerated because stable, protective oxides form within the cracks and seal them. Thermal cycling, however, accelerates faUure because of thermal expansion mismatch that ultimately dismpts the protective oxide coating. 

Ammonium chloride is used as a flux ia the melting furnace because the large surface of the cathodes favors the formation of dross, ie, oxide-coated globules of ziac. The dross is separated by Hquation or air-swept milling iato metal and oxide fractions. In the latter, the oxide fraction is swept out of the mill and can be returned to roasting for the elimination of chloride. Metallic ziac is recycled. Overall melting efficiency is 96—98%. 

Carbon monoxide and dioxide oxidize zinc vapor below 1100—1300°C although only the carbon dioxide reaction is significant. Rapid condensation of the zinc vapor avoids the formation of ziac-oxide-coated droplets, so-called blue powder. 

Economic Aspects. Table 3 shows that mbber production is the largest market for zinc oxide the downturn in 1980 resulted from a drop in tire production because of the production trend to smaller tires, more importation of tires, and a recession. The drop in paint usage reflects the trend to water-base paints, which originally contained no zinc oxide. However, its growing use in such paints is based upon improved formulations based on zinc oxide. The increased use in agriculture is a result of the realization of the importance of zinc as a trace element. The rise in use of zinc-oxide-coated paper for photocopying is followed by a slackening in use because of a shift to plain-paper copiers. 

A.STM B487, Std. Test Methodfor Measurement of Metaland Oxide Coating Thickness by Microscopical Examination of a Cross Section, American Society for... 

---