Improving the formation and protective properties of La-conversion coatings on brass by use of La2O3 nanoparticle incorporation with electrodeposition

The La₂O₃ nanoparticles incorporation and electrodeposition were used together to prepare the La-conversion coatings on brass surface in a basal solution containing rare earth salt and benzotriazole. The results showed that both of these techniques can improve the coatings formation and their protectiveness. A critical nanoparticulate La₂O₃ content and a critical deposition potential were observed, under which the conversion coatings had the highest protective properties. The composite La-conversion coatings could provide important protection against brass corrosion for considerable immersion periods in 3.5% NaCl solution because it ennobled the corrosion potential and decreased the anodic current.     

Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on Mechanical Properties and Corrosion Resistance of H62 Brass

In this article, the effects of lanthanum oxide (La₂O₃) on the microstructure and mechanical properties of H62 brass were investigated by using the universal testing machine, Brinell hardness tester, optical microscope, and scanning electron microscope (SEM). Immersion corrosion and electrochemical measurements were carried out to identify the influence of La₂O₃ on the corrosion behavior of the H62 brass. The phase constitution, microstructure, and phase composition of the H62 brass were analyzed by x-ray diffraction, SEM, and energy-dispersive spectrometer, respectively. The results show that the microstructure of α phase changes from dendrite grains to equiaxed grains, and the content and distribution of β phase are improved significantly. When the La₂O₃ content reaches 0.8 wt.%, the H62 brass obtains favorable comprehensive mechanical properties and the strength and hardness decrease but elongation increases, which is conducive to plastic processing. In addition, under the optimum amount of 0.8 wt.% La₂O₃ content, the corrosion rate of immersion corrosion attains the minimum values: As 12.6 g m^?2 h^?1, it decreases by 24%; as the corrosion potential changes from ?1.1327 V to ?0.328 V, it increases by 70.9%; and as the corrosion current density decreases from ?2.833 mA mm^?2 to ?3.28 mA mm^?2 corrosion, it decreases by 15.78%, when compared with H62 brass.     

Electrochemical corrosion behavior of plasma sprayed Al2O3‐13%TiO2 coatings in aqueous hydrochloric acid solution

One kind of conventional and two kinds of nanostructured Al₂O₃‐13%TiO₂ coatings were prepared by plasma spray process. The phase composition and microstructure of coatings were examined by means of scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to investigate the corrosion behavior of coatings in aqueous hydrochloric acid solution. The results showed that nanostructured coatings had superior corrosion resistance compared to conventional Metco 130 coating. The corrosion resistance of coatings was mainly related to their microstructure and defects density. The EIS measurement for long time immersion in hydrochloric acid solution revealed that the corrosion resistance of coatings decreased with the increasing of immersion time. During the immersion period, electrochemical corrosion mainly occurred on the carbon steel substrate under NiCrAl coatings. In addition, the Al₂O₃‐13%TiO₂ coatings were also failed during corrosion in aqueous hydrochloric acid solution.     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.     

Effect of La2O3 on corrosion resistance of laser clad ferrite-based alloy coatings

The ferrite-based alloy powders with different contents of La₂O₃ were laser clad on a steel substrate. The microstructural features and phase structure of these coatings were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. The results showed that the microstructure of the coatings with La₂O₃ was refined and purified. The main phase of the coatings was γ (Fe, Ni). Moreover, the electrochemical properties of the coatings were investigated by anodic polarization curves and electrochemical impedance spectroscopy (EIS). Anodic polarization results indicated that both polarizing voltage and polarization current density were reduced with the addition of La₂O₃. EIS results showed that, with the increment of La₂O₃, the inductive arcs shrunk and capacitive arcs expanded. The inductive arc at low frequency was disappeared and changed to capacitive arc thoroughly when the content of La₂O₃ reached 1.2 wt.%. The corrosion weight loss experiment results showed that the corrosion rate was lower and the corrosion attack was lighter in the coatings with La₂O₃ than that without La₂O₃, resulting in a substantial improvement of the corrosion resistance.     

Scale growth process at 1473 K on unmodified and yttrium‐ or chromium‐implanted β‐NiAl

Early oxidation of unmodified and yttrium‐implanted or chromium‐implanted β‐NiAl intermetallic compound at 1473 K was studied using a combination of two‐stage‐oxidation exposure with ¹⁸O₂ as a tracer, SIMS elemental distribution analysis (depth profiling and imaging modes) and photoluminescence spectroscopy analysis of the scale phase composition. It was found that phase transformation of transient aluminium oxides, represented by θ‐ Al₂O₃ into stable and protective α‐Al₂O₃ occurs locally and is affected by implanted additions: Yttrium retarded while chromium appeared to accelerate it. Typical patch‐ and/or web‐like scale morphology of the growing scales was observed.     

Performance of thermal barrier coatings on γ‐TiAl

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at temperatures in excess of 850 °C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long‐term use in automotive and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel‐based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre‐oxidized or coated with PVD‐Al₂O₃, TiAlCrYN, or diffusion aluminides. Yttria‐stabilized zirconia TBCs were deposited applying electron‐beam physical vapour deposition. Cyclic and quasi‐isothermal oxidation tests were carried out at 900 °C in air. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ‐TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl₃ aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1‐h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.     

Effects of cathodic electrodeposition parameters of cerium oxide film on the corrosion resistance of the 2024 Al alloy

Cerium oxide thin films obtained by cathodic electrodeposition on 2024 aluminium alloy have been studied. The coatings, obtained with electrochemical deposition, offer an effective corrosion protection and require a lower deposition time when compared to chemical conversion coatings. The coatings were obtained at room temperature by deposition from CeCl₃/H₂O₂ aqueous solutions and the influence of several parameters (CeCl₃ concentration, H₂O₂ concentration, deposition time, current intensity) on the corrosion resistance was studied. The composition, morphology and microstructure of the films have been characterized by SEM, XPS and AFM. The corrosion resistance was investigated through potentiodynamic tests in 3% NaCl solution.     

Nano‐enamel: a new way to produce glass‐like protective coatings for metals

Nanoparticles and nanostructured films have gained an increasing interest for industrial application in the last years. Based on chemical nanotechnology glass‐like protective coatings thermally processed at comparatively lower temperatures around 500°C were developed for metal substrates by two different ways. Starting from polymeric sols (suspension consisting of branched macromolecules) containing multicomponent oxide of the SiO₂‐B₂O₃‐P₂O₅‐Na₂O system produced by hydrolysis and polycondensation of an alkoxide mixture under acidic condition, thin, hard, transparent, crack‐free and corrosion resistant coatings could be applied to aluminium and steel. Coatings from particulate sols (suspension consisting of solid particles) containing multicomponent oxide synthesized under basic condition by electrophoretic deposition (EPD) proved to be a promising method when very thick coatings are required. Both routes offer the potential of a new type of purely inorganic coatings for corrosion and abrasion protection.     

La2O3包埋渗硼TB2钛合金的腐蚀与磨损性能

为了改善TB2合金的表面性能,采用4％La2O3(质量分数)包埋渗硼法对TB2合金进行1100℃,20 h渗硼处理,研究TB2钛合金的渗硼层组成与厚度以及腐蚀与磨损性能.结果表明,La2O3在渗硼过程中促进硼化物层的生长,提高其连续性和致密性,TiB晶须长度从16.80增至21.84μm.这是因为La2O3能与B反应生...     

A study of conversion coatings on vanadium/galvanic zinc

The protective ability and corrosion resistance of conversion coatings on vanadium/galvanic zinc were assessed with chemical and electrochemical approaches. The conversion coatings were shown to possess high protective properties and corrosion resistance in a 3% NaCl solution. FT-IR and X-ray analysis data indicate that the conversion coatings on vanadium/galvanic zinc are complex systems consisting of ZnO and V₂O₅ oxides, as well as phosphorus-containing zinc and vanadium compounds, V(HPO₄)₂, Zn₃(PO₄)₂, and Zn(VO₃)₂, which are resistant throughout 13–19 h in a salt-spray chamber until the appearance of the first symptoms of white zinc corrosion.     

Enhanced corrosioon resistance by sol‐gel‐based ZrO2‐CeO2 coatings on magnesium alloys

The physical, chemical and mechanical properties of magnesium alloys make them attractive materials for automotive and aerospace applications. However, these materials are susceptible to corrosion and wear. This work discusses the potential of using sol‐gel based coatings consisting of ZrO₂ and 15 wt.% of CeO₂. The CeO₂ component provides enhanced corrosion protection, while ZrO₂ impart corrosion as well as wear resistance. Coating deposition was performed by the dip coating technique on two magnesium alloy substrates with different surface finishes: AZ91D (as‐casted, sand‐blasted, and machined) and AZ31 (rolled and machined). All as‐deposited coatings (xerogel coatings) were then subjected to 10 h annealilng: a temperature of 180°C was applied to the AZ91D alloy and 140°C to the AZ31 alloy. Morphological and structural properties of the annealed coatings were investigated by scanning electron microscopy, atomic force microscopy and transmission electron microscopy. Coating composition was examined using energy dispersive X‐ray analysis. Adhesion of the annealed ZrO₂‐CeO₂ coatings on the substrates, assessed by scratch tests, showed critical loads indicative of coating perforation of up to 32 N. Hardness and elasticity, measured using depth‐sensing nanoindentation tests, gave a hardness and elastic modulus of 4.5 GPa and 98 GPa, respectively. Salt spray corrosion tests performed on these coatings showed superior corrosion resistance for AZ91D (as‐casted and machined) and AZ31 (machined), while severe corrosion was observed for the AZ31 (rolled) and AZ91D (sand‐blasted) magnesium alloy substrates.     

Influence of electrodeposition potential,TiO2 nanoparticles and chromium (III) inhibitor addition on the corrosion protection performance of organosilane coating on aluminium

In this work, the anticorrosion properties of phenyl trimethoxysilane (PTMS) films coated on aluminium 5000 series alloys were studied. PTMS films were deposited at various cathodic potentials. The optimum electrodeposition potential was found to be ?0.8?V vs. SCE. The coatings were also modified by different amounts of nano-TiO₂. In order to introduce corrosion inhibition and a self-repair property of the PTMS film, the addition of chromium (III) corrosion inhibitor in the presence of nano-TiO₂ was studied. The anticorrosion performance of coatings was investigated in a 3.5 wt.% NaCl solution. At optimum deposition potential, the ‘critical’ nano-TiO₂ and Cr(III) contents were both observed, under which the obtained PTMS coatings show the highest anticorrosion performance. The surface morphologies of PTMS coatings were examined by scanning electron microscopy. The results showed that the coatings deposited at ?0.8?V vs. SCE, from 20?ppm of nano-TiO₂ and 0.003 M Cr(III) inhibitor present uniform and compact morphologies.     

Korrosionsbeständigkeit von plasmagespritzten Aluminiumoxid- und Chromoxidüberzügen. Wirkung von NiCr und NiAl als Haftgrund

Corrosion resistance of plasma sprayed aluminia and chromia coatings. Effect of coating sublayers NiCr and NiAl The corrosion resistance of plasma sprayed Al₂O₃ and Cr₂O₃ coatings has been studied in 3.5% NaCl and 10% NaOH solutions. In this context the effect of intermediate coatings (NiCr and NiAl) on the protective efficiency of the ceramic coatings has been evaluated, too. The corrosion rates were determined by gravimetry, corrosion potential and polarisation resistance measurements. The two ceramic coatings afford efficient protection. In the alkaline solutions NiCr is superior, while in the chloride solution NiAl offers better protection.     

Corrosion of environmentally friendly lanthanum conversion coating on AA2024-T3 aluminum alloy

In this work, lanthanum conversion coating applied on AA2024-T3 aluminum alloy. The coatings were analyzed by optical and scanning electron microscope. The composition and phase structure of the coating were evaluated by EDS and XRD, respectively. Corrosion properties of the coating were investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. To explain the corrosion behavior of the coating, equivalent circuit was developed from the measurements. Results showed that the coating has a nanocrystaline structure with crystal size less than 50 nm which is composed of La₂O₃. Additionally, EIS measurements showed that the lanthanum conversion coating was monolayer. Measurements taken by polarization tests showed that the coating increased corrosion potential and decreased corrosion current density approximately one order of magnitude.     

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.     

A study on preparation and corrosion behavior of nano rare earth oxide-modified chromized coatings

The corrosion resistance of carbon steel in a harsh environment was improved by preparing a chromized coating on the surface through the method of pack cementation. Nano rare earth-oxides (NREOs), Y₂O₃, CeO₂, and La₂O₃ were added to the pack and to further enhance the performance of the chromized coatings. Morphological results showed that NREO refined the microstructure of chromized coatings by retarding the growth of grains. The Mott–Schottky plots demonstrated that coating Cr–La₂O₃ displayed fewer defects than the other two composite coatings, which was beneficial to reduce the corrosion tendency. In addition, the potentiodynamic polarization and impedance spectroscopy analysis showed that the order of the corrosion rates of the achieved coatings was pure Cr > Cr–Y₂O₃ > Cr–CeO₂ > Cr–La₂O₃, which revealed that the corrosion resistance increased accordingly.     

KCl‐induced corrosion of Ni‐based alloys containing 35–45 wt% Cr

Ni–(35–45)Cr–4Nb alloys containing different fractions of α‐Cr were exposed to potassium chloride (KCl)‐induced corrosion. The corrosion exposures were carried out for 168 hr at 600°C in a 15% (vol/vol) H₂O (g) + 5% (vol/vol) O₂ (g) + N₂ (g; balance) atmosphere using KCl‐free (reference) and predeposited KCl samples. To mimic the KCl deposition in real boilers, 24 hr exposures where KCl vapor condensed continuously onto samples were also performed. The corrosion attack of the studied materials increased significantly when KCl was present compared to the KCl‐free samples. For the KCl exposures, the corrosion attack drastically increased when a significant α‐Cr fraction was present. α‐Cr was either selectively attacked or dissolved through solid‐state diffusion and a layered build‐up of the outer external scale of K₂CrO₄ and chromia could be observed. For the in situ condensed KCl exposure, severe corrosion was observed already within the 24 hr exposure, indicating a higher corrosion rate compared with when KCl was predeposited.     

Atomic Layer-Deposited Al2O3 Coatings on NiTi Alloy

Atomic layer deposition is introduced as a method suitable for preparation of Al₂O₃ layers on the surface of NiTi medical devices such as stents because of the excellent thickness control and conformal protective coating on complex structures. The corrosion properties of NiTi plates with Al₂O₃ coatings of various thicknesses in an environment similar to that occurring in the human body were studied using open circuit potential, potentiostatic electrochemical impedance spectroscopy, and cyclic polarization tests. It shows that the layer thickness plays a key role in the inhibition of corrosion. The thinner layers are more diffuse and make it easier for anodic reaction of passive NiTi with protective TiO₂ underneath of Al₂O₃, while the thicker layers have the barrier effect with local pores initiating pitting corrosion. The results of our electrochemical experiments consistently show that corrosion properties of thick Al₂O₃ coatings on NiTi plate are inferior compared to the thin layers.     

组合超声条件下电沉积 Ni-Nd2O3纳米复合镀层的耐腐蚀性能

分别在单一超声和组合超声条件下制备了Ni-Nd2O3纳米复合镀层,分析了镀层的微观形貌,测定了镀层中纳米Nd2O3的含量,考察了纳米复合镀层的耐腐蚀性能。结果表明:组合超声可以提高Ni-Nd2O3纳米复合镀层中Nd2O3的含量,在组合超声空化效应和协同效应的作用下,复合镀层的晶粒细化,组织致密,腐蚀速率降低,表现出优良的耐腐蚀性能。