Corrosion behaviour of HVOF sprayed SUS316L stainless steel in seawater

SUS316L stainless steel was coated onto the SUS316L plate by the high velocity oxy-fuel (HVOF) spray technique. Its corrosion behaviour in seawater was investigated by the electrochemical method and the microscopy. The coating had corrosion resistance inferior to the bulk plate. The corrosion of the HVOF sprayed SUS316L coating was related to both its porosity and its oxygen content. Depending on them, the corrosion took place at the small pore and the boundary between the spray particles on the surface.     

The effects of microstructural features on the performance gap in corrosion resistance between bulk and HVOF sprayed Inconel 625

It is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5 M H₂SO₄ to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high-velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction. Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 was compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance. Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20-40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.     

超音速火焰喷涂镍基合金层的腐蚀失效过程

采用超音速火焰喷涂(HVOF)方法在低碳钢表面制备NiCrBSi合金涂层,利用交流阻抗法(EIS)研究镍基合金层在3．5％NaCl溶液中的腐蚀失效过程。由于喷涂层中存在气体,介质与基底接触前喷涂层的电极电位高于同种材料堆焊层的电位。涂层浸泡初期,孔隙等缺陷的存在破坏了涂层金属表面吸附层的完整性。随着腐蚀介质在涂层中进行渗透,腐蚀集中在孔隙等局部区域进行。最后,介质通过涂层渗达涂层／基底界面,碳钢作阳极遭受腐蚀。馀层的耐蚀性在浸泡初期急剧降低,后因腐蚀产物阻塞孔隙等腐蚀通道而有所上升。腐蚀介质接触碳钢基底后,复合电极的耐蚀性继续下降。     

Thermal spray and weld repair alloys for the repair of cavitation damage in turbines and pumps: A technical note

The cavitation and erosion resistance of 21 thermal spray coatings and four weld repair materials were investigated in the laboratory using cavitation jet and slurry erosion testing. Of the thermal spray coatings, Stellite® 6 deposited by the high velocity oxyfuel (HVOF) process had the lowest cavitation rate (11.7 mg/h). This was higher than the corresponding cavitation rate (3.2 mg/h) of 308 stainless steel weld metal currently used as a reference. In the slurry erosion testing, the volume loss of Stellite® 6 applied by the HVOF process was 5.33 cubic mm/h, much lower than the corresponding loss (11.17 cubic mm/h) in the currently used stainless steel 304 reference. Furthermore, the electrochemical potential difference between the carbon steel and HVOF sprayed Stellite 6 coating was 0.25 volts, half the potential difference between the 304 stainless steel carbon steel substrate, and will result in reduced galvanic corrosion of the substrate near the contact areas. Stellite 6 deposited by the HVOF process was recommended for repair of damage resulting from erosion and subsequent cavitation by caused by surface roughening.     

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.     

Duplex coating of electroless nickel and HVOF (high-velocity oxygen fuel) sprayed WC-Co

The porosity of thermal sprayed coatings is usually a problem when coatings are used in wet corrosion application. The porosity allows media to penetrate to the surface of the base material. Corrosion spreads rapidly and the coating loses contact with the substrate and delaminates. This problem can be initiated by impregnating different polymers into the pores. An alternative approach has been tested in the present work to prevent corrosion of cemented carbide coated carbon steel in wet corrosion environments. Carbon steel substrates were coated with a thin film of electroless nickel (electroless nickel plating) and then HVOF (High-Velocity Oxygen Fuel) sprayed with cemented carbide. Reference specimens without electroless nickel were sprayed at the same time. The microhardness of the specimens was measured and the coating structure examined using optical microscopy and X-ray diffractometry (XRD). The bond between the layers and the base material was examined by means of a bend test. A salt chamber test was also performed for the specimens. The structure of the electroless nickel layer was crystalline as a result of the HVOF spraying. There were no cracks in the nickel layer, if the layer was about 20 μm thick. According to the results of the bend test, the adhesion between coatings and substrate was good, and there was no difference between the duplex-coated specimen and the reference specimen. A sample with a thin nickel layer under an HVOF sprayed cemented carbide did not exhibit corrosion after 8 h in the salt chamber test.     

Electrochemical behavior of thermally sprayed stainless steel coatings in 3.4% NaCl solution

Four types of stainless steel coatings prepared by a high velocity oxy-fuel spraying system (HVOF) were studied. Differences among coated steels were related to the spraying parameters, which influenced the behavior of the samples against the corrosion. The electrochemical behavior of the stainless steel coatings was strongly influenced by porosity, the presence of micro- and macro-cracks, and also of un-melted particles. Once the electrolyte reached the steel substrate via these defects, the galvanic pair formed between the coating and substrate-accelerated corrosion, leading to the depletion of the coating.     

Corrosion and erosion‐corrosion behaviors of carbon steel in naphthenic acid media

The naphthenic acid corrosion (NAC) and erosion‐corrosion (NAEC) behaviors of carbon steel were investigated detailedly in laboratory. The resistance to NAEC of pack‐aluminized carbon steel and carbon steel coated by high velocity oxygen‐fuel (HVOF) thermal‐sprayed AISI 316L stainless steel, was also investigated in both laboratory and an oil refinery. It was found that the control‐step of NAC was primarily dependent on the temperature. The NAC rate of carbon steel increased markedly with the increase of the total acid number and temperature, which may be attributed to the enhanced absorption and active reaction of naphthenic acid molecules on the metal surface. Increasing the velocity of flow seriously aggravated NAEC, especially in the high temperature range. The reasons were closely associated with the enhanced mass transfer and the accelerated active reaction as well as the rapid spallation of corrosion products from the metal surface. Both the aluminized carbon steel and the carbon steel covered by HVOF coating showed better resistance against NAEC compared to the carbon steel due to higher microhardness and corrosion resistance of their surface‐layers. The HVOF coating is hopeful to be applied for NAEC prevention of the components in oil refineries in view of present experimental results.     

Corrosion mechanism of NiCrBSi coatings deposited by HVOF

NiCrBSi alloy powders were coated on a low carbon steel substrate using high-velocity oxygen fuel (HVOF) thermal spraying, and corrosion tests were carried out by immersing the specimens in 3.5% NaCl with pH adjusted to 3 by addition of acetic acid. Techniques such as scanning electron microscope (SEM), spectral analysis, electron probe microanalysis (EPMA) and X-ray diffraction (XRD) were employed to study the mechanistic process of corrosion of the coating surface. It was found that the corrosion of the NiCrBSi coating first occurred around the particles that had not melted during spraying and the defects such as pores, inclusions and microcracks, then followed by the development along the paths formed by pores, microcracks and lamellar structure, resulting in exfoliation or laminar peeling off. Adjusting the thermal spraying parameters to reduce the electrochemical unevenness or sealing the pores can improve the corrosion resistance of the coating.     

NiCrWMoCuCBFe涂层的制备及其力学性能研究

目的 探究NiCrWMoCuCBFe涂层的微观组织结构、物相组成及其力学性能。方法 通过霍尔流速计表征了喷涂粉末的流动性和松装密度,采用超音速火焰喷涂在316L不锈钢表面制备了NiCrWMoCuCBFe涂层,利用SEM、EDS分别对喷涂粉末的形貌、涂层的组织结构以及粉末和涂层中的元素组成进行了表征,采用XRD、Raman分析了涂层中的物相,使用显微硬度计及万能材料试验机考察涂层的硬度、结合强度和抗弯强度,并分析了涂层的断裂失效机理。结果 NiCrWMoCuCBFe喷涂粉末具有良好的流动性,在喷涂过程中没有发生严重的氧化现象。通过超音速火焰喷涂制备的NiCrWMoCuCBFe涂层具有致密的层状组织结构,其物相主要是Ni基固溶体,但是也出现了少量的NiO和Cr2O3。此外,涂层的截面硬度与表面硬度相当,约为600HV300 g,且涂层与316L不锈钢的结合强度较高,大于70 MPa。三点弯曲试验中,持续加载至载荷为1800 N,应变为(4.81±0.3)%时,涂层达到强度极限,产生明显的塑性变形;而当载荷仍然保持1800 N,应变继续增加达到(11.43±0.03)％时,涂层与316L不锈钢基材的结合界面彻底开裂失效,此时涂层内部同时具有横向裂纹和纵向裂纹,样品的抗弯强度约为(1.87±0.02) GPa。结论 超音速火焰技术制备出的NiCrWMoCuCBFe涂层具有良好的致密性及优异的力学性能。     

Corrosion behavior of HVOF-sprayed and Nd-YAG laser-remelted high-chromium,nickel-chromium coatings

Thermal spray processes are widely used to deposit high-chromium, nickel-chromium coatings to improve high temperature oxidation and corrosion behavior. However, despite the efforts made to improve the present spraying techniques, such as high-velocity oxyfuel (HVOF) and plasma spraying, these coatings may still exhibit certain defects, such as unmelted particles, oxide layers at splat boundaries, porosity, and cracks, which are detrimental to corrosion performance in severe operating conditions. Because of the process temperature, only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium, nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3% Cr, 42.5% Ni, 2.5% Mo, 0.5% B). The coatings were prepared by HVOF spraying onto mild steel substrates. A high-power, fiber-coupled, continuous-wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF-sprayed coating using different levels of scanning speed and beam width (10 or 20 mm). Coating that was remelted with the highest traverse speed suffered from cracking because of the rapid solidification inherent to laser processing. However, after the appropriate laser parameters were chosen, nonporous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of a dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as sprayed and after laser remelting were characterized by optical and electron microscopy (OM, SEM, respectively). Dilution between coating and substrate was studied with energy dispersive spectrometry (EDS). The properties of the laser-remelted coatings were directly compared with properties of as-sprayed HVOF coatings.     

Corrosion behavior of NiCrBSi coatings deposited by HVOF spraying

The corrosion resistance of NiCrBSi coating deposited on steel substrate by HVOF was examined using electrochemical tests and immersion tests so as to offer an experimental basis to expand a promising applied field of HVOF in aqueous medium, comparing with those of coatings deposited by oxyacetylene flame spraying and flame cladding. The results show that the general corrosion rate of HVOF sprayed coatings is quite bigger than that of clad coatings, bat it is less sensitive to local corrosion. There is less and smaller porosity in the coatings deposited by HVOF than that in flame sprayed coatinlgs. The effects of porosity on the corrosion current density was indistinctive, bat the existence of large amount of defects in the coatings damaged the cohesion of the coatings, causing the metallic particles drop off from the coatings under the influence of corrosive medium. Improving the quality and reducing the porosity of coatings is the key to get the coatings with high corrosion resistance.     

Improvement of corrosion resistance of high-velocity oxyfuel-sprayed stainless steel coatings by addition of molybdenum

To improve the marine corrosion resistance of stainless steel coatings fabricated by high-velocity oxyfuel (HVOF) spraying with a gas shroud attachment, the molybdenum (Mo) content of stainless steel was increased to form coatings with a chemical composition of Fe balance-18mass%Cr-22mass%Ni-2∼8mass%Mo. These coatings were highly dense, with <0.1 vol.% in porosity, and less oxidized, with 0.5 mass% in oxygen content at most. The corrosion mechanism and resistance of the coatings were investigated by electrochemical measurement, chemical analysis, and statistical processing. The general corrosion resistance of the coatings in 0.5 mol/dm³ sulfuric acid was improved with increases in Mo content, and the corrosion rate could be decreased to 8.8 × 10⁻² mg/cm² per hour (∼1 mm/year) at 8 mass% Mo. The pitting corrosion resistance of the coatings in artificial seawater was improved with increases in Mo content and was superior to that of the 316L stainless steel coating. The crevice corrosion resistance of the coatings in artificial seawater was improved and the number of rust spots at 4 mass% Mo was decreased to 38% of that for the 316L coating. Accordingly, Mo is highly effective in improving the corrosion resistance of the stainless steel coatings by HVOF spraying.     

Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution

Amorphous/nanocrystalline Ni-Ti powders produced by low energy mechanical alloying were used as feedstock to deposit NiTi intermetallic coatings on 316L stainless steel substrate using high velocity oxy-fuel (HVOF) and air plasma spraying (APS) processes. Electrochemical impedance spectroscopy (EIS) and polarization tests indicated that the corrosion performance and passive behaviour of HVOF coating were far better than those of APS coating. The study also showed that the solution had penetrated through the coating microcracks and caused interior corrosion of APS coating, while the HVOF coating was immune from interior corrosion attack and consequently exhibited a good passive behaviour during long-term immersion.     

Damage tolerant functionally graded WC-Co/Stainless Steel HVOF coatings

In this paper, effective damage tolerance of a functionally graded coating (FGC) deposited by high velocity oxygen fuel (HVOF) spraying is observed. The thick FGC (≈ 1.2 mm) consists of 6 layers with a stepwise change in composition from 100 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WC-12Co (top layer) deposited onto an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an increase in compliance with depth, and 2) an increase in fracture resistance by containment, arrest and deflection of cracks. A smooth gradation in the composition and hardness through the coating thickness is found by scanning electron microscopy and depth-sensing microindentation, respectively. The in-situ curvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist in the lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to high tensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are low because of the gradation. All of this is beneficial for the deposition of a thick coating.The FGC structure shows the ability to reduce cracking with increased compliance in the top layer during static and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disk tests). Results are discussed in comparison to the behavior and properties of coatings of similar individual compositions and thicknesses, as well as a thick monolithic WC-12Co sprayed coating. Further improvements in the processing are proposed to enhance the adhesion strength and avoid coating delamination under high load contact-fatigue conditions.     

矿用碳钢罐道表面超音速喷涂不锈钢涂层研究

本文研究了在碳钢罐道表面超音速喷涂不锈钢涂层的腐蚀磨损性能和腐蚀行为。对超音速喷涂不锈钢涂层在3种极端煤矿地下水环境下的腐蚀规律、界面腐蚀特征和腐蚀产物进行了实验,对不同载荷下磨损规律、摩擦系数变化规律和磨损表面形貌进行了观察。实验结果表明,超音速喷涂不锈钢涂层的摩擦系数随着加载载荷的增加而减小,其磨损速率比碳钢罐道降低10倍以上,不锈钢涂层表面磨损失效主要为轻微的粘着磨损;不锈钢涂层的腐蚀电位较碳钢基体上升300mv;不锈钢涂层在5％NaCl和pH2．97矿井水的强腐蚀性环境下的界面腐蚀主要为浓差腐蚀,在pH6．97和pH9．98矿井水的弱腐蚀环境下的界面腐蚀主要是电荷传质腐蚀;XRD结果显示超音速喷涂不锈钢涂层和不锈钢喷涂材料的物相主要为γ-（Fe,Ni）、γ-（Fe,Cr）,合金元素烧损较少。     

NiCrBSi超音速火焰喷涂层在水溶液中的腐蚀行为

采用高速氧燃料火焰喷涂(HVOF)方法在普通碳钢表面制备了镍基合金涂层，对其组织结构进行了观察，并利用电化学方法对其在水溶液中的腐蚀行为进行研究，探讨HVOF涂层应用于水介质环境中的可能性．试验结果表明：NiCrBSi喷涂层在1mol／L NaOH溶液中表面能够形成致密钝化膜，耐碱腐蚀的性能最好．涂层在酸性溶液中的腐蚀速度大于在中性3．5％NaCl溶液中的腐蚀速度。利用冰醋酸将3．5％NaCl溶液的pH值调整到3，可以提高实验结果的重现性。酸性溶液中，只要被测试表面处于活性溶解状态，腐蚀试验重现性都能满足要求。另外，缺陷越少涂层的耐蚀性越好，减少涂层中的孔隙等缺陷是提高涂层耐蚀性的关键。     

Microstructural Development and Deposition Behavior of Titanium Powder Particles in Warm Spraying Process: From Single Splat to Coating

Warm spraying has been developed by NIMS, in which powder particles are accelerated and simultaneously heated, and deposited onto a suitable substrate in thermally softened solid state. In this study, commercially available titanium powder was sprayed onto steel substrate by the spraying process. Microstructural developments and deposition behaviors from a deposited single particle to a thick coating layer were observed by high resolution electron microscopes. A single titanium particle sprayed onto the substrate was severely deformed and grain-refined mainly along the interfacial boundary of particle/substrate by the impact of the sprayed particle. A successive impact by another particle further deformed the previously deposited particle and induced additional grain refinement of the remaining part. In a thick coating layer, the severe deformation and grain refinement were also observed. The results have demonstrated the complex deposition behavior of sprayed particles in the warm spraying using thermally softened metallic powder particles.     

Influence of Plasma Spray Parameters on the Microstructure and Property of ZrO2-7 wt%Y2O3 Coating

The plasma sprayed ZrO2-7 wt%Y2O3(7YSZ)coating has been widely applied in aircraft engine as a thermal barrier coating.In this study,the duplex TBC system(MCrAlY+7YSZ)were deposited onto 304 stainless steel substrate by APS.Four principal spraying parameters during the deposition of 7YSZ coating were optimized by using orthogonal experimental design,including the stand-off distance,plasma power,preheating temperature and powder feeding rate.Their influences on the coating’s microstructure,adhesion and its mechanical properties were studied and the optimized parameters were finally obtained.The results showed that the dependence of the spraying parameters on the properties of 7YSZ coating is highly correlated to the melting condition of the in-flight particles.     

Residual stresses in high-velocity oxy-fuel thermally sprayed coatings – Modelling the effect of particle velocity and temperature during the spraying process

The application of thick thermally sprayed coatings on metallic parts has been widely accepted as a solution to improve their corrosion and wear resistance. Key attributes of these coatings, such as adherence to the substrate, are strongly influenced by the residual stresses generated during the coating deposition process. In high-velocity oxy-fuel (HVOF) thermal spraying, due to the relatively low temperature of the particle, significant peening stresses are generated during the impact of molten and semi-molten particles on the substrate. Whilst models exist for residual stress generation in plasma-based thermal spray processes, finite element (FE) prediction of residual stress generation for the HVOF process has not been possible due to the increased complexities associated with modelling the particle impact. A hybrid non-linear explicit–implicit FE methodology is developed here to study the thermomechanical processes associated with particle impingement and layer deposition. Attention is focused on the prediction of residual stresses for an SS 316 HVOF sprayed coating on an SS 316 substrate.