H.L. Du

Air oxidation behaviour of Ti6Al4V alloy between 650 and 850

The air oxidation behaviour of Ti6Al4V alloy was studied in the temperature range 650–850°C. Continuous oxidation kinetics showed that the alloy oxidised according to a parabolic rate law at 650 and 700°C after a transient period, whilst at 750 and 800°C, linear-parabolic kinetics dominated. At 850°C, after 50 h linear-parabolic oxidation, the alloy followed a parabolic rate law. The multilayered oxide scales formed on the oxidised alloy consisted of alternate layers of Al2O3 and TiO2. The number of Al2O3 and TiO2 layers increased with increasing exposure time and temperature. The external gas/oxide interface was always occupied by an Al2O3 layer whilst TiO2 always appeared at the oxide/substrate interface.

The high temperature corrosion behaviour of Hf modified chromo-aluminised coatings produced by a single step process

In this paper the introduction of Cr, Al and Hf into the surface of the RENE 80, INC718 and 2.25Cr-1Mo steel by pack cementation in a single step process has been described. The coated alloys were subject to cyclic hot corrosion and cyclic oxidation test at 870 and 875°C. The surface treated alloys were also sulphidised at 750°C in an environment of H 2 /H 2 S/H 2 O yielding Ps 1 = 10 -6 atm. and P O2 = 10 -23 atm. The coated and exposed specimens were characterised by optical microscope, SEM, EPMA, EDX and XRD. Cyclic hot corrosion and cyclic oxidation tests showed that the presence of the Cr-Al-Hf on the surface of the alloys enhanced their corrosion resistance. The rare earth element modified chromium aluminide coatings showed superior protectivity. However the Cr-Al-Hf coating on INC718 delayed the formation of the refractory metal sulphides in sulphidising atmosphere and thereby reduced its sulphidation resistance whilst the coating on 2.25Cr-1Mo steel only slightly increased its resistance to sulphidation attack.

High-temperature corrosion of Ti and Ti-6Al-4V alloy

Pure titanium and Ti-6Al-4V were exposed at 750°C in an H2/H2O/H2S PO2≈10−18 Pa and PS2≈10−1 Pa), H2/H2O (PO2≈10−18 Pa) and air environments for up to 240 hr. The corrosion kinetics, obtained by the discontinuous gravimetric method, showed that the sulfidation/oxidation kinetics were linear for Ti and linear-parabolic for Ti-6Al-4V in the H2/H2O/H2S environment. Both materials obeyed parabolic rate laws in the H2/H2O atmosphere after a transient period, and linear-parabolic rate laws in air. After exposure to the H2/H2O/H2S atmosphere, the titanium specimen displayed a double scale of TiO2 with an intervening TiS2 film between the double-layered scale of TiO2 and the substrate. Ti-6Al-4V also contained a double layer of TiO2 together with a stratum consisting of Al2S3, TiS2 and vanadium sulfide at the junction of the inner TiO2 layer and substrate. Some Al2O3 precipitated in the external portion of the outer TiO2 layer. Following oxidation in the low-PO2 atmosphere a double-layered oxide of TiO2 scale formed on both Ti and Ti-6Al-4V. The scale on Ti-6Al-4V also contained an α-Al2O3 film situated between the outer and inner (TiO2) layers. For both materials, multilayered-scale formation characterized air oxidation. In detail a multilayered oxide scale of TiO2 formed on the air-oxidized Ti, while a multilayered oxide scale with alternating layers of Al2O3/TiO2 developed on Ti-6Al-4V oxidized in air.

Effect of Nb coating on the sulphidation/oxidation behaviour of Ti and Ti-6Al-4V alloy

The environmental response of Nb-coated Ti and Ti-6Al-4V alloy was studied at 750 °C in an atmosphere of pS2 ∼ 10−1 Pa and pO2 ∼ 10 −18 Pa. By acting as a diffusion barrier and through the formation of a Nb1−xS scale the Nb coating deposited enhanced the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corrosion products generated on uncoated titanium in the same environment and temperature were characterized by a double layered oxide scale of TiO2 beneath which a TiS2 layer was formed. For the Ti-6Al-4V alloy, α-Al2O3 was precipitated in the external portion of the outer-layer of TiO2 whilst a layer containing Al2S3, TiS2 and vanadium sulphide (possibly V2S3) was idenitified underlying the inner TiO2 layer. After prolonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO2/TiO2+NbO2+Nb2O5/Nb1−xS/TiO2/ TiS2/(substrate) was observed on the surface of the Nb-coated Ti, whilst a scale with sequence of TiO2/V2S3/TiO2+NbO2+Nb2O5/Nb1−xS/TiO2/Al2S3+TiS2/(substrate) characterized the corrosion products formed on the Nb-coated Ti-6Al-4V alloy.

Oxidation and Sulfidation Behavior of AlTiN-Coated Ti–46.7Al–1.9W–0.5Si Intermetallic with CrN and NbN Diffusion Barriers at 850°C

Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H2/H2S/H2O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10−11 g2/cm4/s. A multi-layered scale developed: an outer rutile (TiO2) layer, a continuous layer of α-Al2O3 beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl3 and TiAl2) between the scale and substrate.The use of an AlTiN coating greatly increased the oxidation and sulfidation resistance of Ti–46.7Al–1.9W–0.5Si. The use of NbN and CrN diffusion barriers further enhanced its corrosion resistance. The protection of the double-layer coatings persisted even after 240 hr exposure. However the mismatch of thermal expansion coefficients between the coating and substrate led to the development of cracks in some locations within the coatings. A 2.5 μm thick AlTiN coating on the Ti–46.7Al–1.9W–0.5Si substrate with an embedded defect was modeled using the general finite element (FE) program ABAQUS. The modeling results showed rapid mode I failure of the coating at a temperature of 774°C. The through-fracture of the nitride film caused the nitride coating to shrink back leading to delamination around the crack in the nitride coating. The cracks formed acted as diffusion paths, for the ingress of oxygen and sulfur species and the outward diffusion of substrate elements, which resulted in the formation of nodular corrosion products with similar morphologies and microstructures to the uncoated alloy in those locations where cracks developed.

Use of PVD deposited TiN coating in retarding high temperature sulphidation

Abstract TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmosphere comprising a high sulphur potential (pS 2 ∼ 10 −1 Pa) and a low oxygen potential (pO 2 ∼ 10 −18 Pa) at 750 °C for periods up to 72 h. The sulphidation kinetics, determined by a discontinuous gravimetric method, demonstrated that the TiN coating greatly enhanced the sulphidation resistance of the substrates, particularly during the early stages of exposure. For TiN coated Inconel 600 the post-exposure analysis by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and glancing angle X-ray diffraction, showed the formation of an outer layer containing Ni 3 S 2 on the surface of the TiN coating while an inner layer consisting of Cr 2 S 3 developed at the coating/substrate interface. The double layered scale formed on uncoated Inconel 600 consisted of Ni 3 S 2 and Cr 2 S 3 . For the TiN coated Nimonic PE11, sulphide nodules consisting of three layers — (Fe,Ni) 9 S 8 (outmost) Cr 2 S 3 MoS 2 (innermost) were observed to develop and the TiN coating was sandwiched between (Fe,Ni) 9 S 8 and Cr 2 S 3 . The portion of the TiN coating which was enveloped by those sulphide nodules became unstable after long-term exposure and subsequently dissociated thereby causing the loss of environmental protection. Moreover, the coating was damaged mechanically by the growth of the sulphide nodules. The scale formed on the uncoated Nimonic PE11 showed the formation of a similar structure in a similar sequence.

Influence of plasma-sprayed Mo coating on sulphidation behaviour of Inconel 600 and Nimonic PE11 alloys

Abstract Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen ( p O2 ~10 −18 Pa) and high sulphur ( p S2 ~ 10 −1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10 −12 g 2 cm −4 s −1 in an atmosphere of p S2 ~ 10 −1 Pa at 750°C. In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising p S2 ~ 10 −1 Pa and p O2 ~ 10 −18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni 3 S 2 layer and an inner Cr 2 S 3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni) 9 S 8 layer, a mid Cr 2 S 3 layer and an inner MoS 2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni) 9 S 8 and Ni 3 S 2 on the surface of the uncoated Nimonic PE11. The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni 3 S 2 for Inconel 600 and (Fe, Ni) 9 S 8 for Nimonic PE11. A MoS 2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr 2 S 3 layer gradually developed between the outer layer and the inner MoS 2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr 2 S 3 layer.

Enhancement of oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by HfN coating

Abstract An attempt has been made to enhance the oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by the application of an HfN coating produced using Physical Vapour Deposition (PVD). The coated specimens were exposed to an atmosphere comprising high sulphur and low oxygen potentials ( p S 2 ∼ 10 −1 Pa and p O 2 ∼ 10 −18 Pa) at 750 °C for periods up to 240 h. The corrosion kinetics were obtained by a discontinuous gravimetric method. Linear and linear-parabolic kinetics were recorded for Ti and for Ti6Al4V alloy respectively. The HfN-coated Ti and Ti6Al4V specimens showed enhanced corrosion resistance. The scale formed on the uncoated Ti specimen consisted of a double layer of TiO 2 with an underlying TiS 2 film on the substrate. The exposed Ti6Al4V alloy also contained a double layer of TiO 2 but with Al 2 O 3 precipitated in the external portion of the outer layer of the TiO 2 , whilst a layer of Al 2 S 3 and TiS 2 with vanadium sulphide developed beneath the inner layer of TiO 2 . Application of the HfN coating suppressed the formation of the duplex oxide scales. However, extensive cracking developed either parallel or perpendicular to the sample surface, and severely compromised the protectiveness of the coating.

Sulphidation behaviour of FeCoCrAlY alloys containing refractory metals

Abstract The corrosion behaviour of three FeCoCrAlYX (where X = V and Nb)-type alloys has been studied in a sulphur-bearing atmosphere ( p S 2 ∼ 10 −1 Pa and p O 2 ∼ 10 −18 Pa) at 750°C. Weight change data revealed protective kinetics and followed a parabolic rate law. The alloys containing higher levels of V and Nb showed better corrosion resistance. Replacement of Co by Fe improved the sulphidation behaviour. Analysis of the exposed specimens by means of SEM, EDAX and XRD demonstrated that the scales formed were duplex in nature. The outer layer of the scale mainly consisted of sulphides of Fe and Cr (Fe 3 S 4 , Co 9 S 8 and Cr 3 S 4 ). The inner layer principally containing sulphides of V, Nb and Cr was compact, adherent and protective, and substantially resisted the outward diffusion of the base alloying elements.

1.15 – High Temperature Tribocorrosion

A review of high temperature tribocorrosion is presented focusing attention on those contributions that illustrate the general principles, modeling, and scientific theories of the processes of high temperature wear. A significant aspect of this chapter is the inclusion of new information on glaze formation, generated at the sub-microscopic and nano-scale level.