Synthesis and properties of high corrosion resistant Ni–cerium oxide nano‐composite coating

The effect of cerium ion on the formation, morphology, composition, and corrosion behavior of Ni–cerium oxide coatings was investigated by SEM, FESEM, XRD, EDS, XPS, EIS, and potentiodynamic polarization. The extremely highest corrosion resistant coating was obtained when the cerium ion concentration in the plating bath was 16 mM. It has been observed that the presence of cerium ion in the plating bath led to changes in the morphology of the coating from pyramid nodular structure to coaxial structure. By adding cerium ion to the plating bath, a considerable grain refinement in the nanometer region was observed.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

The structure and properties of electroless Ni–Mo–Cr–P coatings on copper alloy

In the present study, the quaternary Ni–Mo–Cr–P alloy coatings were deposited on copper alloy by an electroless deposition process. Crystallization behavior and the effect of heat‐treatment on hardness and corrosion resistance of Ni–Mo–Cr–P deposits were detailedly investigated. X‐ray diffraction (XRD) analysis shows that as‐deposited Ni–Mo–Cr–P coatings are Ni–Mo–Cr–P solid solution and mixed crystal structure; the trend of microcrystallinity increases with the introduction of additional types of metal element; Ni–Mo–Cr–P alloy coatings start to occur in the crystallization with the heat‐treatment temperature increasing. With an increase in the annealing temperature, the hardness improves and reaches the maximum value at 500 °C. Further, it is found that Ni–Mo–Cr–P coatings have superior corrosion resistance than Ni–P and Ni–Mo–P deposits after the analysis of electrochemical measurements. Moreover, corrosion resistance increases before annealed at 400 °C, but heat‐treatment at higher temperatures has a negative effect on the corrosion resistance of Ni–Mo–Cr–P alloy coatings.     

Performance of cold sprayed Ni‐20Cr and Ni‐50Cr coatings on SA 516 steel in actual industrial environment of a coal fired boiler

Power plants are one of the major industries suffering from severe erosion–corrosion (E‐C) problems resulting in substantial losses. One way of tackling this problem is by the use of thermal spray coatings. In the current investigation a new emerging technique i.e. cold spray coating process was used to deposit Ni‐20Cr and Ni‐50Cr powder on SA 516 (grade 70) boiler steel. The bare as well as the coated steels were subjected to cyclic experimental studies, in the superheater zone of a coal fired boiler. Weight change, thickness loss, XRD, FE‐SEM/EDS and X‐ray mapping techniques were used to analyse the eroded‐corroded specimens. The uncoated steel showed weight gain after exposure in the actual boiler environment, whereas, for the coated steels there was initial weight loss followed by negligible weight change. Based upon thickness loss data the cold‐sprayed Ni‐50Cr coating was found to provide better E‐C resistance than the Ni‐20Cr coating.     

SKPFM investigations of intermetallic compounds of innovative Er‐ and Zr‐containing Al–Si–Mg alloys and their influence on corrosion localization in saline solution

The paper aims at characterizing the influence of intermetallic compounds on the corrosion localization of innovative Al–Si–Mg Er‐ and Zr‐containing casting alloys. Samples of the investigated materials were studied by means of optical and scanning electron microscope micrographs, immersion tests, and scanning Kelvin probe force microscope (SKPFM) analyses in the T6 temper. Combination of immersion tests and SKPFM analyses allowed to identify those classes of intermetallic compounds promoting localization of the corrosion process. It was found that intermetallic compounds richer in Fe were the most critical for corrosion localization; furthermore, additions of Er caused a marked decrease of the potential difference of intermetallic compounds with respect to the Al matrix and a consequent less intense microgalvanic coupling, which translates into slower corrosion kinetics. Further, Zr additions slightly increased the potential difference of intermetallic compounds with the Al matrix, promoting a faster corrosion process.     

Effect of different aging processes on the corrosion behavior of new Al–Cu–Li–Zr–Sc alloys

The intergranular corrosion and exfoliation corrosion behaviors of Al–Cu–Li–Zr–Sc alloys under different aging effects, such as single‐stage aging, strain aging, and double‐stage aging, were studied. Among the three aging treatments, single‐stage aging resulted in the best resistance to corrosion, followed by double‐stage aging; strain aging resulted in the worst corrosion resistance. A 3.5% precooling strain could increase the dislocation density, which promoted the precipitation of corrosion‐prone T₁ phase and increased the corrosion driving force of the alloy. Double‐stage aging made the precipitated T₁ phases finer and more uniform and reduced the number of equilibrium phases at grain boundaries, thus improving the corrosion properties of the alloy. The corrosion susceptibility of the alloy was attributed to the T₁ phase and precipitate‐free zone (PFZ), and the underlying corrosion mechanism was revealed as preferential dissolution of the equilibrium phase at grain boundaries and its surrounding distortion zone, followed by expansion of the PFZ along the grain boundaries, resulting in the development of corrosion from the grain boundaries to the intragranular regions.     

Short‐term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating

A CrN/Cr_1?xAl_xN coating comprised of an inner layer of CrN and an outer layer of Cr_1?xAl_xN with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot‐corrosion tests of the gradient CrN/Cr_1?xAl_xN coating were performed at different temperatures. The phase compositions and morphologies of the as‐deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr_1?xAl_xN coating exhibited good oxidation resistance at temperatures above 1000 °C owing to the formation of an α‐Al₂O₃‐rich oxide scale. The coating possessed good hot‐corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr₂O₃ after the Al₂O₃‐enriched scale failed due to its dissolution in the molten sulfate.     

Field exposure of FeCrAl model alloys in a waste‐fired boiler at 600°C: The influence of Cr and Si on the corrosion behaviour

The aim of this study was to examine the performance of FeCrAl model alloys in a waste‐fired boiler and investigate the influence of chromium and silicon content on the corrosion behaviour. The investigation was executed by utilising an air‐cooled probe, giving a material temperature of 600°C throughout a 672 hr exposure. The material loss measurements were performed by utilizing an ultrasonic thickness gauge in combination with scanning electron microscopy analysis. It was found that increasing the chromium content significantly reduced the overall material loss of the FeCrAl model alloys but further accelerated the corrosion attack on the windward side. Simultaneously, the increased chromium content caused embrittlement of the material. Minor additions of silicon drastically reduced the material loss of the FeCrAl model alloys, whereas the sample ring with no silicon present was completely deteriorated. The trends observed in this field study correlated well with what has been observed in previous laboratory studies. A state‐of‐the‐art alloy in the present environment, Inconel 625, was simultaneously exposed and showed similar performance to the silicon‐containing FeCrAl model alloys with ≥10 wt% Cr.     

Ball-milling in vacuum of α and σ phases of near-equiatomic FeCr alloys

The structural changes induced by ball-milling of near-equiatomic σ-FeCr and α-FeCr in vacuum were followed. Besides the α-phase, an amorphous phase appears when milling the σ-phase for times longer than 20 h. An amorphous phase forms too, but at a slower rate than the latter, when milling the α-phase. The partial amorphisation of σ-FeCr and α-FeCr ball-milled in vacuum is concluded to be a phenomenon of intrinsic origin. The amorphous phase crystallizes into a bcc Cr-rich phase and a bcc Fe-rich phase during short annealing steps.