Corrosion behaviour of Al/Al3Ti and Al/Al3Zr functionally graded materials produced by centrifugal solid-particle method: Influence of the intermetallics volume fraction

Intermetallic particles, Al₃Ti and Al₃Zr were formed in Al–5mass%Ti and Al–5mass%Zr alloys, respectively, by centrifugal casting, in order to create functionally graded materials (FGMs). At present, no information is available on the influence of the amount of intermetallics on the electrochemical properties of these alloys.In this paper, the corrosion resistance of Al/Al₃Ti and Al/Al₃Zr FGMs was investigated by open-circuit measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Results suggests that the corrosion resistance of the FGMs is affected by galvanic effects between the intermetallic particles and the metallic matrix. Lower centrifugal forces resulted in an improvement of the electrochemical properties.     

Internal oxidation and phase transformations of multi-phase Fe–Ni–Al and Fe–Ni–Al–Cr alloys induced by KCl corrosion

The corrosion of two multiphase Fe–Ni–Al and Fe–Ni–Al–Cr alloys is studied at 650 °C in KCl-contaminated air. The oxidation rate of the alloys in air alone is low. When KCl is introduced, the corrosion is accelerated, producing a thick external scale of iron oxides, an intermediate layer of spinel, and a region of internal oxidation of Al. Potassium chromate is detected on Fe–Ni–Al–Cr surface that accounts for the degradation of protective chromia. An Al-depleted single phase region is observed in the front of the internal oxidation, due to the selective consumption of Al via an “active oxidation” process.     

Corrosion Behavior of an Abradable Seal Coating System

A novel NiTi/BN composite abradable coating and two traditional Ni/C and Ni/BN coatings were manufactured with NiAl as the bond layer using thermal spray technology and their corrosion behaviors were investigated. In salt spray corrosion testing of the Ni/BN coating, defective sites of the metal matrix were corroded preferentially. Simulated occlusion experiments and electrochemical tests indicated that migration of ions resulted in pH decrease and Cl^? enrichment in defects, and a more aggressive electrolyte led to a decrease of the corrosion potential of the metal inside defects but an increase of the corrosion current density, representing an autocatalytic corrosion process. Moreover, galvanic corrosion between the top and bond coatings of the abradable system was studied via the electrochemical technique. The results showed that, for the NiTi/BN, Ni/BN, and Ni/graphite coatings with a NiAl bond coating, current flow was generated between the anode and cathode. The NiTi/BN coating acted as the cathode due to its passivation, while the Ni/BN and Ni/graphite coatings acted as the anode because of their lower corrosion potential compared with the NiAl coating. The anode suffered serious corrosion damage due to galvanic corrosion, while the cathode corroded only slightly.     

Effect of particle size and co-deposition technique on hardness and corrosion properties of Ni–Co/SiC composite coatings

Ni–Co/SiC alloy matrix composite coatings were electrodeposited in a modified Watt's bath containing micro and nano sized SiC particles by using conventional electro-co-deposition (CECD) and sediment co-deposition (SCD) techniques. The deposits were characterized using SEM, EDX and XRD analyses, and microhardness and potentiodynamic polarization measurements. The maximum incorporation of the SiC micro- and nano-particles was obtained using the SCD technique at deposition current densities of 2 and 3 A/dm², respectively. It was found that in the composite coatings, incorporation of SiC particles improves the microhardness of unalloyed Ni and Ni–Co alloy matrices. The nanocomposite coatings exhibit higher microhardness values than microcomposite ones. The potentiodynamic polarization measurements in 3.5% NaCl solution revealed that the corrosion resistance of the Ni–Co/SiC nanocomposite coatings is much higher than the Ni–Co alloy and Ni–Co/SiC microcomposite coatings. Moreover, corrosion resistance of Ni–Co/SiC nanocomposite coatings deposited by SCD technique is higher than the ones deposited by CECD technique. Corrosion resistance of the studied Ni–Co/SiC composite coatings was considerably affected by Co content, SiC particle size and content. Hardness enhancement was related to the structural features, and corrosion behavior was discussed based on the formation of corrosion micro cells, diminishing the effective metallic area, and increasing and hindering the corrosion paths.     

Corrosion of nanocrystalline Ni–W alloys in alkaline and acidic 3.5 wt.% NaCl solutions

The corrosion behaviour of reverse-pulse electrodeposited nanocrystalline nickel tungsten alloys (nc Ni–W) in pH 3 and 10 3.5 wt.% NaCl solutions is investigated and analysed as a function of grain size. A potentiodynamic polarisation study reveals that the corrosion rate of nc Ni–W generally increases with the reduction of grain size in alkaline condition, but decreases with the reduction of grain size in acidic environment. Furthermore, for both environments, nc Ni–W alloys exhibit superior localised corrosion resistance than a microcrystalline Ni control specimen. Factors controlling the corrosion behaviour of these materials, including grain size, tungsten content, passivation and crystallographic texture are addressed.     

Electrochemical corrosion behaviour of Mg–Al alloys with thermal spray Al/SiCp composite coatings

The corrosion protection of Mg–Al alloys by flame thermal spraying of Al/SiC particles (SiCp) composite coatings was evaluated by electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The volume fraction of SiCp varied between 5 and 30%. The as-sprayed Al/SiCp composite coatings revealed a high number of microchannels, largely in the vicinity of the SiCp, that facilitated the penetration of the electrolyte and the subsequent galvanic corrosion of the magnesium substrates. The application of a cold-pressing post-treatment reduced the degree of porosity of the coatings and improved the bonding at the coating/substrate and Al/SiC interfaces. This resulted in improved corrosion resistance of the coated specimens. The effectiveness of the coatings slightly decreased with the addition of 5–30 vol.% SiCp compared with the unreinforced thermal spray aluminium coatings.     

Effect of Ni–Al SHS reaction on diamond grit for fabrication of diamond tool material

Diamond grits were added to Ni–Al self-propagating high-temperature synthesis system for fabrication of diamond tool materials. The propagating processes and the microstructure of the reacted composite were investigated. The surface characteristics of diamond grits after Ni–Al SHS reaction were examined by scanning electron microscopy (SEM) and Raman spectroscopic analysis. Owing to the presence of diamond grits, the flame-front propagation velocity of Ni–Al/diamond was reduced by 33% comparing to that of Ni–Al. Intermetallics such as NiAl, Ni₂Al₃, Ni₃Al and NiAl₃ were identified in the reacted Ni–Al/diamond composite. Due to the high temperature of Ni–Al SHS, some of the diamond grits’ surface was graphitised. There were a large number of craters on the surface of graphitised diamond grits, in which the spherical and rod-like graphite particles were found. The mean compressive strength of the SHS-treated diamond grits decreased about 20% comparing to that of the initial diamond grits.     

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

Corrosion behaviour of Mg–Al alloys with Al–11Si thermal spray coatings

The corrosion resistance of AZ31, AZ80 and AZ91D Mg–Al alloys with Al–11Si thermal spray coatings was evaluated by electrochemical and gravimetric measurements in 3.5 wt% NaCl solution. The changes in the morphology and corrosion behaviour of the Al–11Si coatings induced by a cold‐pressing post‐treatment under 32 MPa were also examined. The as‐sprayed Al–11Si coatings revealed high degree of porosity and poor corrosion protection, which resulted in galvanic acceleration of the corrosion of the magnesium substrates. The application of a cold‐pressing post‐treatment produced more compact Al–11Si coatings with better bonding at the substrate/coating interface and slightly higher corrosion resistance. However, interconnected pores remained in the cold‐pressed coatings due to the low plasticity of the Al–11Si powder and galvanic corrosion of the substrate was observed after immersion in 3.5 wt% NaCl for 10 days.     

0Cr13Ni8Mo2Al钢与铝合金和钛台金接触腐蚀与防护研究

通过测定0Cr13Ni8Mo2Al钢与铝合金（LY12）和钛合金（TC4）组成的电偶对的电偶电流的方法,研究了0Cr13Ni8Mo2Al钢在使用中与铝合金和钛合金接触时发生电偶腐蚀的敏感性。研究结果表明:0Cr13Ni8Mo2Al钢与铝合金接触时会产生严重的电偶腐蚀,必须进行防护处理方可使用;与钛合金接触时产生的电偶腐蚀很轻微,可以不进行防护。0Cr13Ni8Mo2Al钢表面进行镀镉钛防护后,与铝合金接触时的电偶电流密度大为减小,相差近10倍;采用环氧锌黄底漆、XM-33-4双组分密封胶防护可以有效地防止0Cr13Ni8Mo2Al钢与铝合金和钛合金接触产生的电偶腐蚀。     

Electrochemical behaviour of cermet coatings with a bond coat on Al7075: Pseudopassivity, localized corrosion and galvanic effect considerations in a saline environment

The behaviour of an HVOF WC-17Co/Ni-5Al coating on Al7075 in aqueous NaCl is investigated. The coating susceptibility to localized corrosion depended on the potential of polarization reversal. A two-stage pseudopassivity was observed for WC-17Co: At low overpotentials, oxidation occurred in the binder leading to surface films composed of anhydrous Co- and W-oxides. At high overpotentials, oxidation extended to the carbide phase leading to the formation of hydrated WO₃ films. Ni-5Al notably reduced the galvanic effect between WC-17Co and Al7075, whereas it hindered corrosion propagation into the substrate. The coating showed a high corrosion resistance during salt spraying for 49 days.     

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Electrodeposition of Zn–Ni coatings performed in acidic baths are not suitable for high strength steels due to their high susceptibility to hydrogen embrittlement.In this work, Zn–Ni coatings were deposited on a high strength steel (4340) upon stirring conditions from an alkaline bath. A complete characterisation of the coatings (corrosion, morphology and composition) has been accomplished, correlating the electrodeposition conditions with these features. The best protective properties of the grown coatings were achieved for the alloys with a single phase structure of γ-Ni₅Zn₂₁ and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.     

Corrosion behaviour of Mg/Al alloys with composite coatings

The corrosion behaviour of aluminium/silicon carbide (Al/SiC) composite coatings deposited by thermal spray on AZ31, AZ80 and AZ91D magnesium-aluminium alloys was investigated by electrochemical and gravimetric measurements in 3.5 wt.% NaCl solution at 22 °C. Corrosion products were examined by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and low-angle X-ray diffraction (XRD). Al/SiC composite coatings in the as-sprayed state revealed high level of porosity with poor bonding at the Al/SiC and coating/substrate interfaces, which facilitated degradation of the magnesium substrates by a mechanism of galvanic corrosion. Cold-pressing post-treatment produced more compact coatings with improved corrosion performance in 3.5 wt.% NaCl compared with as-sprayed coatings.     

Corrosion behaviour of thermally sprayed Al and Al/SiCp composite coatings on ZE41 magnesium alloy in chloride medium

Thermally sprayed Al and Al/SiCp composite coatings have been deposited on ZE41 magnesium alloy and mechanical compaction at room temperature was applied to the Al and Al/SiCp coatings to reduce their porosity. Corrosion behaviour of coated samples was evaluated and compared to that of uncoated substrate in 3.5 wt.% NaCl solution using electrochemical measurements. Al and Al/SiCp composite coatings reduced the corrosion current density of Mg-Zn alloys by three and two orders of magnitude, respectively, and reductions up to four orders of magnitude were obtained after mechanical compaction.     

Anodic dissolution of Al sacrificial anodes in NaCl solution containing Ce

The corrosion behaviour of Al–Zn–In sacrificial anodes has been investigated in a sodium chloride solution containing CeCl₃. Scanning electron microscopy, energy-dispersive X-ray analysis, and inductively coupled plasma mass spectrometry have been employed to gain knowledge of the micro-morphology and corrosion process of the Al alloy. Cerium, both as the alloy element and as the additive in the NaCl solution, improves the electrochemical properties of the Al–Zn–In alloy. The activation of Ce in the Al–Zn–In alloy in the NaCl solution has been studied.     

Q235上电弧喷涂Zn55Al涂层在3.5wt%溶液中的腐蚀行为研究

本文使用电弧喷涂通过包套挤压+拉拔的方法制备的Zn55Al伪合金丝材成功的在Q235钢上喷涂出了Zn55Al涂层。通过扫描电镜和微区XRD研究了Zn55Al 伪合金丝材的显微结构。通过浸泡腐蚀实验和电化学方法研究了Zn55Al涂层、Zn15Al涂层和 Al涂层的腐蚀行为,并对比了三种涂层之间的差异。结果表明Zn55Al伪合金丝材由纯锌和纯铝组成,在整个成型过程中没有产生合金化。Zn55Al涂层由层片状的富锌相和富铝相组成。经过20天的浸泡实验,Zn55Al涂层形成了一层致密的钝化膜,比其他两种涂层有更好的耐腐蚀性。Zn55Al涂层的自腐蚀电位大约是-1.25v,高于Zn15Al涂层低于纯Al涂层和Q235基体.电偶腐蚀实验表明,Zn55Al涂层比Zn15Al涂层具有更好的点虎穴保护作用。这些结果说明Zn55Al涂层具有更好的耐腐蚀性和可以给Q235基体提供更强的电化学保护.本文也讨论了Zn55Al涂层的的腐蚀机理。     

Pulsed laser deposition of composite Mo–Se–Ni–C coatings using standard and shadow mask configuration

Thin film Mo–Se–Ni–C coatings for tribologycal applications were prepared by pulsed laser co-deposition from two targets — MoSe₂(Ni) and graphite. Two methods of deposition from the MoSe₂(Ni) target were used: deposition with unhindered expansion of the laser plume (standard PLD) and deposition of a plume scattered in collisions with Ar gas (pressure 2 Pa) in the shadow of a mask (shadow-masked PLD — SMPLD). Doping with carbon by PLD was used in all cases, and carbon content in the composite Mo–Se–Ni–C coatings was varied in the range 35–85.5 at.%. Pure MoSe_x(Ni) coatings were also prepared by PLD and SMPLD. Rutherford backscattering spectroscopy of helium ions, scanning electron and atomic force microscopy, electron probe microanalysis, transmission electron microscopy and micro-diffraction, micro-Raman spectroscopy, X-ray photoelectron spectroscopy and hardness evaluation were all used for comparative studies of the coatings obtained by PLD and SMPLD/PLD. In the PLD coatings, micron-sized particles were found, consisting of pure Ni or MoSe₂, as well as nanometre-sized particles of monocrystalline Mo. The nanoparticles were distributed on the surface and in an amorphous matrix in all PLD coatings. Within the amorphous matrix of the Mo–Se–Ni–C coatings, local ordering of atoms was detected, causing the formation of a mixture of amorphous carbon, Mo–C, and Mo–Se phases. Increasing the carbon content caused an increase in the content of sp³ bonds in the carbon phase, and an increase in the hardness of the coatings. SMPLD/PLD coatings had no micro- and nanoparticles, but these coatings were characterized by high selenium content and reduced density. Doping with carbon in the SMPLD/PLD configuration caused the formation of composite coatings containing Mo–Se and amorphous carbon phases as in the PLD coatings, but the hardness of the composite SMPLD/PLD coatings was significantly lower than even the hardness of the pure PLD MoSe_x(Ni) coatings.     

Long-term atmospheric corrosion properties of thermally sprayed Zn,Al and Zn–Al coatings exposed in a coastal area

The long-term atmospheric corrosion properties of thermally sprayed Zn, Al and Zn–Al coatings have been evaluated using an electrochemical impedance measurement and several analytical techniques. All the thermal-sprayed specimens with 100 μm coating thickness have protected the steel substrates. In case of the Zn–Al coating, the red rust was not observed regardless of the coating thickness. The corrosion products were identified by the XRD analysis. Preferential dissolution of zinc was observed on the Zn–Al coating by EPMA analysis. The electrochemical impedance results provided an insight about the surface structures of each thermally sprayed coating.     

Synthesis and properties of open-cell Ni–Cr–Fe–Al alloy foams by pack co-deposition process

A pack cementation process for the co-deposition of Cr, Fe and Al onto open-cell nickel foam was developed. The reticulated open-cell Ni–Cr–Fe–Al foams were annealed to homogenize the material with 18.8 wt.% Cr, 11.3 wt.% Fe and 7.7 wt.% Al. The microstructure and phase composition of the Ni–Cr–Fe–Al foams were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive analysis (EDS). The results show that the coating is uniform and dense along the perimeter of the Ni strut, and consists of three layers: a Cr–Fe outer layer, an inner layer containing mostly Al and a transition zone. After homogenization annealing, the alloyed foams retain the hollow struts structure of the original pure nickel foams and the low relative densities. The Ni–Cr–Fe–Al alloy foams exhibit enhancement in absolute strength as compared to the pure nickel and Ni–35.2Cr foams. Furthermore, the Ni–Cr–Fe–Al alloy foams show excellent oxidation resistance and outperform the chromia-forming Ni–35.2Cr alloy foam after oxidation at 900 and 1000 °C, which is mainly due to its high aluminum and chromium content leading to the formation of a continuous and adherent duplex oxide layer.     

Degradation behaviour of ZrO2-Ni/Al gradient coatings in molten Zn

In order to improve the corrosion resistance of metallic materials in molten zinc, ZrO₂-Ni/Al gradient coatings were sprayed on the surface of the Fe-0.35-0.44 wt.% C steel. The corrosion behaviour and corrosion mechanism of the ZrO₂-Ni/Al gradient coatings in molten zinc were studied. The ZrO₂-Ni/Al gradient coatings on the surface of steels prolonged the lifetime of samples and changed the corrosion behaviour of the samples in molten zinc. The lifetime of the ZrO₂-Ni/Al gradient coatings immersed in molten zinc at 620 °C is 28 days, which is 4 times as long as that of the general ZrO₂ coatings. The ZrO₂-Ni/Al gradient coatings were corroded in molten zinc at 620 °C, which was caused by zinc atom diffusing along the crystal boundary and pores of the ZrO₂-Ni/Al gradient coatings, and reacting with Ni/Al particle in the ZrO₂-Ni/Al gradient coatings. The corrosion mechanism of the coatings in molten zinc at 620 °C was crystal boundary corrosion, pitting corrosion and reaction corrosion.