Spalling 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. 

Porcelain enameling requires the use of frits and melting temperatures of 550 °C or below. Enamels are appHed over chemical conversion coatings that are compatible with the frit. AHoy selection is important to obtain good spall resistance. Alloys 1100, 3003, and 6061 are employed most extensively among wrought products and alloy 356 for castings. 

When alloy steels do not give adequate corrosion protection— particularly from sulfidic attack—steel with an aluminized surface coating can be used. A spray coating of aluminum on a steel is not likely to spall or flake, but the coating is usually not continuous and... 

Monolithic refractory coatings have been applied to metallic components in furnaces for fuel ash corrosion control. Results have been less than satisfactory because of the large thermal expansion mismatch between the metal and refractory. Failure usually occurs upon thermal cycling which causes cracking, eventual spalling of the refractory, and direct exposure of the metal to the effects of the fuel ash. 

It is also doubtful whether the surface condition of the bare metal at the time of burial has much significance. Some authorities consider that the casting skin on cast iron is protective, but the evidence on this point is conflicting. On the other hand, it is desirable to remove the millscale from steel when, as is usual, a protective coating is applied. If the millscale was left on, it might cause the coating to spall. 

Exemplary water-repellent treatments for masonry surfaces include metal stearates, oils, waxes, acrylates (both polymers and monomers), silicones (solvent-based and emulsion), siliconates, silanes and, fluorochemicals. In contrast, to waterproofing coatings, water-repellent coatings, because they are permeable to water vapor, do not trap moisture and, therefore they can reduce spalling. In addition most water-repellent coatings do not alter the appearance of a porous masonry. 

Repair of spalled concrete by use as an adhesive and protective coating... 

It has been shown that welds provide unique environments for the colonization of SRB with the subsequent production of sulfides that affect the weld seam surface of the heat-affected zone. Exposure of sulfide-derived surfaces to fresh, aerated seawater resulted in rapid spalling on the downstream side of weld seams. The bared surfaces became anodic to the sulfide-coated weld root, initiating and accelerating localized corrosion. (Dexter)5... 

Polymeric overlays are used on concrete pavements to protect them from dust, excessive wear, cracking, crazing, spalling and to provide an attractive appearance. These materials are applied on concrete pavements in the form of screeds, self-levelling material or floor coatings. Epoxies, polyurethanes and acrylics are commonly used, epoxies being the most commonly chosen. Table 1.4 gives the suitability of these materials for different environments. 

Epoxy seal coating was first applied as test patches in industrial plants in 1953. In 1957 epoxy polymer concrete was first used as a wearing course in the repair of popouts and spalled areas on the surfaces of various bridge decks in California (ACI, 1993). 

If conditions are severe, for example if there is widespread spalling and corrosion, patch repairs are carried out, followed by application of acrylic coating as the final part of the repair. Surface treatment with a silane... 

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

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