Corrosion behaviour of cermet-based coatings with a bond coat in 0.5 M H2SO4

The corrosion behaviour of an HVOF Ni–5Al/WC–17Co coating on Al-7075 is investigated in 0.5 M H₂SO₄. In the temperature range of 25–45 °C, the coating exhibits pseudopassivity that effectively protects from localized corrosion. At 25 °C, pseudopassivity proceeds via three stages: during the first stage, oxidation of W in the binder phase occurs. The second stage is characterized by oxidation of W in both the binder and the carbide particles. The third stage is characterized by intensive hydration of WO₃ and formation of Co₃O₄. During the second and third pseudopassive stages, the formation of a bi-layer surface film is postulated. The inner layer, consisting of anhydrous oxides, has a barrier character. The outer layer, composed of WO₃ · xH₂O, is unstable. In case of surface film disruption, the bond coat successfully hinders corrosion propagation into the Al-alloy. Higher electrolyte temperatures lead to faster corrosion kinetics and higher tendency for pitting.     

Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.     

A comparison between the corrosion resistances of some HVOF-sprayed metal alloy coatings

This study compares the corrosion resistance of one Co-based alloy coating, namely Co-28Mo-17Cr-3Si (similar to Tribaloy-800), four Ni-based alloy coatings, namely Ni-17Cr-4Fe-4Si-3.5B-1C (Diamalloy-2001), Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006), Ni-22Cr-9Mo-4Nb-5Fe (similar to Inconel-625), Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700), and a (WC-12Co)-33Ni-9Cr-3.5Fe-2Si-2B-0.5C cermet-Ni alloy blend coating. They were produced by liquid-fuelled HVOF spraying onto AISI1040 steel plates. Electrolytic hard chrome (EHC) plating was characterised as a reference material, to verify whether some HVOF coatings are suitable as an EHC replacement. The microstructure of the coatings was examined by SEM and XRD. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution; the corrodkote test (ASTM B380-97R02) was also performed, to rank coatings qualitatively.The lowest corrosion current densities (I_corr) were recorded for EHC and Tribaloy-700. The latter coating contained few secondary phases and little porosity; the damage was mainly due to corrosion activation along lamellae boundaries. Diamalloy-2001 exhibited the highest I_corr and was significantly damaged after the polarization test, as its multi-phase microstructure had triggered severe galvanic corrosion. During free corrosion in 0.1 M HCl, Tribaloy-700 and Diamalloy-4006 retained rather stable polarization resistance (R_p), whereas the R_p of EHC decreased significantly. Tribaloy-700 survived 40 h of corrodkote test with no apparent damage and EHC underwent limited pitting corrosion. All other coatings had visible corrosion. The Inconel-625 coating failed to protect the substrate after 20 h of testing, due to inadequate processing conditions.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Corrosion fatigue behavior of a steel with sprayed coatings

This paper describes the corrosion fatigue behavior and fracture mechanisms of a steel with different sprayed coatings. Rotating bending fatigue tests were conducted in 3% NaCl solution using specimens of a medium carbon steel with sprayed coatings of a ceramic (Cr₂O₃), a cermet (WC-12%Co) and two metals (Ni-11 % P and Al-2% Zn). The corrosion fatigue process was basically the same for ceramic, cermet, and Ni-11 % P sprayed specimens. That is, the corrosive media could be supplied from the specimen surface to the substrate through cracks initiated during fatigue cycling and/or pores in the coatings, and thus corrosion pits were generated followed by subsequent crack initiation and growth in the substrate. The corrosion fatigue strength of ceramic sprayed specimens was slightly improved compared to that of the substrate steel because the under-coating (Ni-5%A1) could impede the penetration of the corrosive media although the ceramic coating had a poor resistance to cracking under cyclic loading. Cermet sprayed specimens also exhibited improved corrosion fatigue strength because of the high resistance to cracking and the low volume fraction of pores of the coating. In Ni-11 % Psprayed specimens, cracks were initiated in the coating even at low stress levels; thus the corrosion fatigue strength was the same as that of the substrate. Anodic dissolution took place in Al-2 % Zn coating because the coating was electrochemically poor, and thus the substrate was cathodically protected. Therefore, the corrosion fatigue strength of Al-2 % Zn sprayed specimens was enhanced to as high as the fatigue strength of the substrate in room air. Based on the experimental results, a dual-layer coating consisting of WC-12%Co and Al-2%Zn was fatigue tested. The coating was effective at low stress levels and exhibited long life under conditions where corrosion fatigue strength was critical.     

Corrosion resistance of HVOF-sprayed coatings for hard chrome replacement

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) and electrolytic hard chrome (EHC) coatings corrosion resistances have been compared through electrochemical polarization tests (0.1 N HCl, 0.1 N HNO₃) and Corrodkote test. EHC coatings passivate in HNO₃, but undergo pitting corrosion in HCl and in Corrodkote test too. HVOF coatings do not passivate, but possess more noble corrosion potentials. Both in HNO₃ and HCl, they undergo more generalized corrosion, with similar i_corr; crevice corrosion along splat boundaries is sometimes detected after the HCl test. Their i_corr in 0.1 N HCl solution is lower than in several of EHC coatings. No visible damage in the HVOF coatings has occurred after the Corrodkote test.     

High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating II: Oxidation and hot corrosion

The high temperature oxidation and hot corrosion behaviour of a NiCoCrAlYSi coating and a gradient coating were investigated. The gradient coating showed better performance of re-healing alumina scale due to its possession of more β phase as Al reservoir. The degradation process of the gradient coating was favorably retarded by the formation of Cr(W)-rich σ precipitates in the interdiffusion zone. The corrosion results also confirmed an improved corrosion resistance of the gradient coating. The improved high temperature performance of the gradient coating owes to the Al enrichment of Al in the outer layer.     

Enhanced tribological properties of PECVD DLC coated thermally sprayed coatings

This research investigates the enhancement of the tribological properties of various thermally-sprayed coatings (APS Ni-50Cr, APS Al₂O₃-13%TiO₂ and HVOF WC-17Co) on steel substrate, achieved through the deposition of a thin DLC-based film. Higher adhesive strength between thin films and thermally-sprayed coatings compared to the simple thin film/carbon steel system was found by scratch testing. Dry sliding ball-on-disk tests performed under lower contact pressure conditions (5 N normal load, 6 mm diameter alumina ball) indicated a significant decrease in wear rates and friction coefficients of thermally-sprayed coatings when the thin DLC-based film is employed; little differences exist between the tribological behaviour of the various thin film/thermal spray coating systems and that of DLC-based film on carbon steel. Under higher contact pressure conditions (10 N normal load, 3 mm diameter alumina ball), the thin film/WC-Co system exhibited the best wear performance. These results indicate the superior tribological performance of DLC/thermal spray coating systems, especially under severe contact conditions.     

Preparation and hot corrosion behaviour of a MCrAlY + AlSiY composite coating

A MCrAlY + AlSiY composite coating was prepared by arc ion plating. Hot corrosion behaviours of the composite coating and the reference NiCoCrAlYSiB coating were investigated. The results indicate that the composite coating consisted of β-(Ni,Co)Al with dispersed σ-NiCoCr and Cr₃Si in outer layer and Cr-rich phases in inner layer after annealing. As being corroded for 200 h, the composite coating proved better corrosion resistance than that of reference NiCoCrAlYSiB coating. The enhanced corrosion resistance of the composite coating was due to the gradient distribution of Al-enriched outer layer and Cr-enriched inner layer.     

Corrosion behaviour of a 2219 aluminium alloy treated with a chromate conversion coating exposed to a 3.5% NaCl solution

This study investigates the formation of a chromate conversion coating at Al–Cu–Fe–Mn intermetallic sites of an Al2219 alloy and the corrosion initiation at these sites in a 3.5% NaCl solution, using SEM, AES and EDX. Changes in the surface chemistry were monitored after progressive exposures to the solution up to 42 h. The coating was found to be thinner and more defective on the intermetallic. Initially, Al is dissolved and Al(OH)₃ deposited on and around the intermetallic. After 42 h of exposure, Al(OH)₃, Fe and Mn oxides and small particles of elemental Cu are deposited as corrosion products.     

Experimental and theoretical research on interfacial reaction of solid Co with liquid Al

Interfacial reaction between solid Co and liquid Al was investigated with immersion tests and theoretical modeling. The microstructure characterization indicated that a Co₂Al₅ intermetallic layer was formed at solid–liquid interface during the immersion test. The modeling results indicated that the corrosion rate of a solid metal in a liquid metal was controlled by both the formation and dissolution of the intermetallic layer. In Co–Al reaction system, the formation and dissolution of the Co₂Al₅ layer reached an equilibrium state in a very short time, and the corrosion of the Co matrix was mainly dominated by the dissolution of the Co₂Al₅ layer.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings

The present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of three High Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co-28Mo-17Cr-3Si (similar to Tribaloy-800) coating, a Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe (Diamalloy-4006) coating and a Ni-32Mo-16Cr-3Si-2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests were performed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, to evaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co-28Mo-17Cr-3Si coating and of the Ni-20Cr-10W-9Mo-4Cu-1C-1B-1Fe coating by enhancing their passivation ability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanic microcells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable using ordinary scanning electron microscopy) prevails. The effect on Ni-32Mo-16Cr-3Si-2Co coatings is more ambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment.     

Influence of microstructure on corrosion properties of multilayer Mg–Al intermetallic compound coating

A multilayer Mg–Al intermetallic coating was fabricated on the Mg alloy in molten salts at 400 °C with treatment time range from 2 to 8 h. The coating consists of a single Al₁₂Mg₁₇ intermetallic layer or Al₁₂Mg₁₇, Al_0.58Mg_0.42 and Al₃Mg₂ intermetallic layers. The corrosion resistance of the coating which is obtained at 400 °C for 2 h is the best. When the treatment time is higher than 2 h, some cracks developed in the layers. The cracks were resulted from the thermal stress due to the different thermal expansion coefficient of the substrate and the intermetallic layer during the rapid cooling process.     

热镀锌沉没辊的耐锌腐蚀失效过程

采用超音速火焰喷涂技术在316L不锈钢基体上制备WC-12Co涂层,并测试其在430℃锌液中的耐腐蚀性,分析沉没辊的腐蚀机理以及失效过程。采用SEM、EDS和XRD分析了腐蚀前、后涂层表面显微结构、化学成分及其相组成的变化。结果表明,WC-12Co粒子之间通过高速碰撞发生强烈的塑性变形而结合在一起,涂层内部致密性很好,喷涂过程中没发生明显氧化现象;涂层的显微硬度平均为1 215HV0.2,结合强度达到85MPa;无涂层试样在腐蚀59h后直径减少20%;经过10天的浸锌试验后,涂层开始产生均匀性腐蚀,涂层中的Co基体受液锌腐蚀生成Co5Zn21化合物,涂层开始遭到破坏;经过15天浸锌试验后,部分锌液穿过涂层与基体中的Fe化合生成FeZn13(即锌渣),涂层局部开始脱落,涂层失效开始。     

钛合金与WC-17Co涂层界面结合性能分析

采用超音速火焰喷涂技术在Ti-6Al-4V合金表面制备WC-17Co涂层,从显微形貌、界面氧化行为、裂纹扩展形式等方面系统分析了WC-17Co涂层与Ti-6Al-4V基体界面断裂失效原因,并利用四点弯曲法测量了WC-17Co与Ti-6Al-4V合金的平均断裂能量释放率.结果表明,热喷涂过程使Ti-6Al-4V的表面氧化相含量显著提高.氧化后Ti-6Al-4V的表面显微硬度达到322.4 HV,熔融粒子撞击到Ti-6Al-4V表面很难对基体造成明显的塑性变形,不能形成有效咬合,使得钛合金与WC-17Co涂层的结合性能偏低.     

镁合金表面冷喷涂纳米WC-17Co涂层及其性能

采用冷喷涂和超音速火焰喷涂(HVOF)在AZ80镁合金表面制备了纳米WC-17Co涂层。利用SEM分析了原始粉末形貌、喷涂粒子沉积行为及涂层显微结构,并采用球盘式摩擦磨损实验机考察了涂层的摩擦磨损性能。结果表明:采用冷喷涂工艺可在AZ80镁合金基体上制备出高质量的WC-17Co涂层,涂层的显微硬度为(1 380±82)HV,磨损率为9.1×10-7 mm3/Nm,其耐磨性较HVOF制备的WC-17Co涂层提高了1倍,较镁合金基材提高了3个数量级。研究表明,冷喷涂WC-17Co涂层在不对镁合金基体产生热影响的情况下,可以显著提高镁合金的表面性能,是一种新型镁合金表面强化工艺。     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.     

The effect of Al content on the galvanic corrosion behaviour of coupled Ni/graphite and Ni–Al coatings

In this study, the corrosion behaviour of 75Ni/25graphite abradable coating and Ni–Al bonding coatings with different Al content were investigated with open circuit potential and polarization tests. The galvanic corrosion of the coupled Ni/graphite and Ni–Al coatings was studied by using a zero-resistance ammeter in 5 wt% NaCl solution. The experimental results showed that the corrosion resistance of the Ni–Al coatings decreased with increasing Al contents. In the galvanic corrosion test, the Ni–Al coatings with high Al contents (80Ni–20Al, 85Ni–15Al, and 90Ni–10Al) were the anodic element of the couples, while, the 95Ni–5Al acted as the cathode element in the couple.     

Corrosion behaviour of Al–29at%Co alloy in aqueous NaCl

Microstructure and corrosion behaviour of a binary Al–29 at%Co alloy have been studied. The alloy was prepared by arc-melting of Al and Co in high purity Ar and rapidly solidified on a water-cooled Cu mould. The alloy chemical composition and microstructure were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, the corrosion behaviour was studied by potentiodynamic polarization in aqueous NaCl (0.6 mol dm⁻³) at room temperature. The alloy was found to consist of three phases: hexagonal Al₅Co₂, Z-phase and AlCo (β). The corrosion resistance of different intermetallic phases is characterized. The results are compared to previously published results of Al–TM (TM = transition metal) alloys.