Fuhui Wang

Transient-Alumina Transformations during the Oxidation of Magnetron-Sputtered CoCrAl Nanocrystalline Coatings

The thermally-grown alumina formed at 1000, 1100, and 1200°C on magnetron-sputtered, nanocrystalline CoCrAl coatings, with and without yttrium, has been characterized using photostimulated-luminescence spectroscopy. The measurements enable the evolution of initially-formed transient alumina to its stable, α phase to be followed, and in particular, the effect of yttrium on the transformation. Yttrium retards the transformation from gamma to theta alumina and also its subsequent transformation to α alumina. The retardation of the transformation decreases with increasing oxidation temperature until at ∼1200°C the transformation is complete within minutes. The presence of yttrium in the coatings also affects the residual stress in the thermally-grown oxide. For samples oxidized at 1100 and 1200°C the residual stress is ∼0.3u2009GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.

Corrosion behavior of pure Cr with a solid NaCl deposit in O-2 plus water vapor

The corrosion behavior of pure chromium with a solid NaCl deposit on itssurface has been investigated in air or oxygen containing water vapor at500–700°C. Results indicated that, when pure Cr was corroded inair or in oxygen plus water vapor, the corrosion was restrained even at700°C because of the formation of a compact and uniform Cr2O3protective scale. However, a solid NaCl deposit on its surface acceleratedthe corrosion of pure Cr in air or in oxygen plus water vaporsignificantly. A model of synergistic action to explain the effect of solidNaCl with oxygen and/or water vapor and on the corrosion of pure Cr isproposed.

Influence of Cr content on the corrosion of Fe-Cr alloys: The synergistic effect of NaCl and water vapor

The corrosion behavior of pure Fe and Fe–Cr alloys with different Cr content in the presence of a solid NaCl deposit and water vapor at 600°C was studied. Results indicated that the corrosion of pure Fe was severe even in air at 600°C and the scale formed on the surface was compact and uniform. However, with a solid NaCl deposit on its surface, the corrosion of pure iron in air was suppressed to some extent, but the presence of water vapor in the atmosphere causes accelerated corrosion. Under the synergistic effect of NaCl and water vapor, the corrosion of pure iron is accelerated more significantly. In contrast with the known effect of Cr content on the oxidation of Fe–Cr alloys, an increasing Cr content in Fe–Cr alloys increases the corrosion rate of the alloys under the synergistic effect of solid NaCl and water vapor. A mechanism to explain the effect of water vapor and NaCl on the corrosion of pure iron and Fe–Cr alloy is proposed.

Synergistic effect of NaCl and water vapor on the corrosion of 1Cr-11Ni-2W-2Mo-V steel at 500-700°C

The corrosion behavior of 1Cr-11Ni-2W-2Mo-Vsteel was studied in the presence of a NaCl deposit andwater vapor at 500-700°C. Results indicated thatcorrosion was not obvious without an NaCl deposit, while corrosion was accelerated remarkably by thesimultaneous presence of NaCl and water vapor at anytemperature. Optical microscopy (OM), X-ray diffraction(XRD), scanning electron microscopy (SEM), andelectron-probe microanalysis (EPMA) were used to analyze thecorrosion products. A mechanism of the synergisticeffect of water vapor and NaCl on corrosion of the steelis proposed.

Hot-Corrosion Resistance of a Sputtered K38G Nanocrystalline Coating in Molten Sulfate at 900°C

The hot-corrosion behavior of a nanocrystalline coating of K38G alloy, prepared by magnetron sputtering and cast K38G in molten 75 wt.% Na2SO4+25 wt.% K2SO4 at 900°C was studied. The coating eliminated internal sulfidation during the early stage of corrosion as a result of the formation of a continuous and compact Al2O3 scale. The nanocrystallization of K38G alloy prolonged the incubation of breakaway corrosion and improved the corrosion resistance of K38G. The relevant corrosion mechanism is discussed.

Corrosion Behavior of a Sputtered K38G Nanocrystalline Coating with a Solid NaCl Deposit in Wet Oxygen at 600 to 700°C

A nanocrystalline coating of K38G alloy was obtained by means of magnetron sputtering. The corrosion behavior of cast K38G alloy and its sputtered nanocrystalline coating with a solid NaCl deposit in dry air or wet oxygen at 600 and 700°C have been studied and compared. The results indicated that the mass gain of the coating is higher than that of the cast alloy at 600 and 700°C. Therefore, nanocrystallization reduced the corrosion resistance at these temperatures. Furthermore, the mass gains of the coating and the cast alloy with a NaCl deposit in wet oxygen were less than that with a NaCl deposit in dry air. The relevant corrosion mechanisms are discussed.

Effect of an enamel coating on the oxidation and hot corrosion behavior of an HVOF-sprayed - Co-Ni-Cr-Al-Y coating

The oxidation and hot-corrosion behavior of a Co–Ni–Cr–Al–Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating were investigated in air at 900°C and in molten 75 wt.% NaCl+25 wt.% Na2SO4 at 850°C. The results show that the enamel coating possesses excellent hot corrosion resistance in the molten salt, in comparison with the HVOF-sprayed Co–Ni–Cr–Al–Y coating alone. In the hot-corrosion test, breakaway corrosion did not occur on the samples with the enamel coating and the composition of the enamel did not significantly change. The oxidation resistance of the Co–Ni–Cr–Al–Y coating, which had good adhesion, was also improved by the enamel coating.

Oxidation behavior of TiAl coated with a fine-grain Co-30Cr-4Al film

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-μm thickness has been studied at 1100–1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K. Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded. This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.

Corrosion Behavior of Ti60 Alloy Coated with a Solid NaCl Deposit in O2 Plus Water Vapor at 500-700°C

The corrosion behavior of Ti60 has been studiedin oxygen containing water vapor at500-700°C, with and without a solid NaCldeposit on the surface. No measurable corrosion of theTi60 alloy was observed in atmospheres without NaCl.Corrosion of Ti60 was accelerated in the presence ofNaCl because of the formation of a nonprotectiveTiO2 oxide scale; the corrosion of Ti60 issignificantly aggravated due to the synergistic effect ofNaCl and water vapor. When the temperature was increasedup to the melting point of NaCl, the corrosion of Ti60coated with NaCl both in air-NaCl and inO2-NaCl plus water vapor became more severe. Amechanism to explain the effect of water vapor and NaClon the corrosion of Ti60 is proposed.

Composite coatings of titanium-aluminum nitride for steel against corrosion induced by solid NaCl deposit and water vapor at 600 °C

Composite coatings (Ti,Al)N with different Al content were deposited on a wrought martensite steel 1Cr11Ni2W2MoV by reactive multi-arc ion plating. With the addition of Al to the coatings, the crystallographic structure of them changed from B1 NaCl to B4 ZnS, the relevant hardness and adhesive strength firstly increased then decreased and their oxidation-resistance was also dramatically improved. It was indicated that the introduction of Al was beneficial to (Ti,Al)N coatings against corrosion induced by NaCl(s) in wet oxygen at 600 °C as well as wet corrosion in NaCl solution at ambient temperature.