Effect of substrate pretreatment on the porosity in thin nickel electrodeposits

Substrate pretreatment affects the porosity of thin nickel electrolytic coatings which is a combined function of the composition and the roughness of the substrate surface. Electropolishing the substrate surface is an effective method of reducing coating porosity, whereas coatings plated on either electroetched or mechanically ground substrates are relatively porous owing to the remaining oxide layer on the substrate and the increased roughness of the substrate. For nickel coatings plated on mechanically ground rough substrates, the relationship between nickel porosity, apparent coating thickness and substrate roughness has been examined.     

Electrochemical characterisation of the porosity and corrosion resistance of electrochemically deposited metal coatings

Electrochemical techniques for the assessment of porosity in electrodeposited metal coatings are reviewed. The determination of porosity and corrosion, resistance is illustrated by electrochemical data from three coating/substrate systems namely: electroless nickel on aluminium and steel and immersed gold coatings on an electroless copper-plated ABS polymer. Nickel coatings were up to 24 μm thick while gold deposits had thickness between 75 and 190 nm. Tafel extrapolation and linear polarisation resistance methods were used to determine the corrosion rate of the coated substrates. The aluminium samples were tested in 5% w/v (0.85 mol dm^− 3) NaCl, while coated steel and ABS samples were immersed in 0.125 mol dm^− 3 H₂SO₄ and 0.1 mol dm^− 3 NaBH₄, respectively, at 295 K. Current vs. time curves and anodic polarisation behaviour have also been considered.     

Studies of Porosity in Electroless Nickel Deposits on Ferrous Substrates

The effect of soak and electrolytic alkaline cleaners together with chemical polishing on the porosity of electroless nickel coatings on mild steel substrates was investigated. The porosity of the deposit was evaluated by chemical spot tests using ammonium thiocyanate as indicator, the sulphur dioxide test, Corrodkote, electrochemical corrosion rate measurements and scanning electron microscopy studies. The results showed that the various pretreatments had no significant effect on the porosity of the coating. The overriding factor was observed to be the deposition time hence the thickness of the deposit. All the tests indicated that the percentage porosity decreased exponentially with increase in coating thickness. The relative merits of each of the techniques used in assessing the uniformity of the deposit, are discussed.     

The effect of processing gas on corrosion performance of electroless Ni-W-P coatings treated by laser

An investigation of the microstructural and corrosion characteristics of electroless Ni-5.5 W-6.5P coatings on steel substrates after laser treatment in argon and air is presented. The microstructural characteristics of the coatings, in terms of crystallisation, grain size, microstrain, porosity as well as surface chemistry, were examined using quantitative X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical tests, using potentiodynamic polarisation in 0.5 M H₂SO₄ solution and electrochemical impedances spectroscopy (EIS) in 3.5% NaCl solution, were undertaken to evaluate the corrosion behaviour of the coatings. The results indicated that the laser-treated coatings consisted of nanocrystalline Ni and Ni₃P phases, along with retained amorphous phase; further, the dimensions of the Ni crystallites were larger than those of Ni₃P. The laser-treated coating in argon revealed the presence of submicron scale porosity, while no porosity was evident in the coating surface treated by laser in air. The uniform corrosion revealed in 0.5 M H₂SO₄ solution is mainly determined by the microstructural characteristics of the coating. Pitting corrosion in 3.5% NaCl solution depended on the amount of porosity on the surface. The laser-treated coating in air exhibited better corrosion resistance in both acidic and chloride environments than that laser-treated in argon.     

Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al₂O₃ coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al₂O₃ coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al₂O₃ coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 μm and the substrate temperature of 500 °C. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness.     

氧化铝陶瓷表面化学镀镍工艺及镀层性能研究

目的在氧化铝陶瓷表面化学镀金属镍镀层,研究施镀时间对Al_2O_3陶瓷表面化学镀镍层的表面形貌、组织结构、显微硬度、表面粗糙度和镀镍层结合力的影响。方法所用镀液组成及工艺参数为:NiSO_4·6H_2O_25g/L,NaH_2PO_2·H_2O 22g/L,Na_3C_6H_5O_7·2H_2O 64g/L,(NH_4)SO_4 62g/L,pH=5.0~6.0,水浴加热至90℃,施镀时间1~4h。采用NovaNanoSEM50型场发射扫描电子显微镜观察镀层的表面微观形貌,采用TH765型自动显微硬度仪测试镀层硬度,采用OLS4000型三维形貌测量仪测量镀层表面粗糙度,采用压入法和热震试验评价镀层的结合性能。结果施镀时间为1~4h时,1h镀层表面金属光泽性好,呈银白色,4h镀层表面更为细腻,但表面光泽性较差。随着施镀时间的增长,Al_2O_3陶瓷表面化学镀镍层表面越光滑,显微硬度越大。不同施镀时间下的化学镀层均没有出现起泡、片状剥落或者与氧化铝基体分离等现象。结论施镀时间为1~4h时,在温度和pH不变的情况下,随着施镀时间增加,化学镀镍层厚度变化不大,但是镀层颗粒更细小,显微硬度明显提高,表面粗糙度降低,镀层结合力良好。     

Porosity of nickel electrodeposits on mild steel using electrochemical impedance spectroscopy

Abstract

Impedance spectroscopy recorded at the open circuit potential in 1M H₂SO₄ at 293 K is shown to provide a convenient test for detecting the degree of through porosity in electroplated nickel coatings, electrodeposited from a Watts bath, on mild steel. The impedance data can be interpreted in terms of a simple equivalent circuit, and the corrosion resistance provides a reliable guide to the quality of the deposit. The quantitative estimation of porosity from these data is considered.     

A Survey of the Porosity in Gold and other Precious Metal Electrodeposits

The causes of pores in precious metal coatings are briefly reviewed and anomalies in the surface roughness theory mentioned. Close investigation of the relation between porosity and coating thickness reveals three stages: an initial fast coverage of most of the substrate, followed by an intermediate region where the useful reduction in porosity depends to a large extent upon the plating bath composition and operating parameters, and a third region where porosity is insensitive to coating thickness. Mechanisms of pore formation are dealt with.     

Korrosionsverhalten von Hybridschichtsystemen

Corrosion behaviour of hybrid coating systems Potentiodynamic methods were used to investigate the corrosion protection value of various hybrid coating systems. Combinations of galvanic chromium and electroless nickel interlayers with titanium nitride and chromium nitride coatings produced by sputterion-plating method were tested. Anodic polarization curves show that corrosion resistance is strongly connected with the microdefects present in the coatings. The coating system consisting of chromium nitride with electroless nickel as an interlayer exhibits excellent corrosion resistance properties in 1N H₂SO₄ solution. The reason for this behaviour is presented and discussed in detail.     

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.     

TiAlNb intermetallic compound coating prepared by high velocity oxy-fuel spraying

Ti_28.35Al_63.4Nb_8.25 (at.%) intermetallic compound coatings were sprayed onto 316 L stainless steel substrates by HVOF processes using various parameters. By varying the grit blasting pressure between 0.11 and 0.55 MPa, the effects of substrate roughness on the adhesion of TiAlNb thermal sprayed coatings were investigated. The microstructure, porosity and microhardness of the coatings were characterized by SEM, XRD, Image Analysis and Vickers hardness analysis. The tensile adhesion test (TAT) specified by ASTM C 633-79 was used to measure the tensile bonding strength of the coating. The results show that the coatings with substrate roughness of 8.33 μm displayed the best combined strength. TiAlNb coatings had a lamellar microstructure with different spraying parameters. The porosity, bonding strength, microhardness of coatings were assessed in relation to the spraying processes. The thickness of bond coat on the bond strength of coatings was also discussed.     

Improvements on the wear resistance of high thermal conductivity Cu alloys using an electroless Ni-P coating prior to PVD deposition

An attempt to improve the load support for hard PVD coatings on soft Cu alloys has been made by using a medium phosphorous content electroless Ni-P coating prior to PVD deposition, envisaging the application of these coating/substrate systems in plastic injection moulding. Several PVD coatings, including TiN, CrN, CrAlN, multilayered CrAlN, WC-C, multilayered CrAlN/WC-C were deposited onto an Ampcoloy 940 Cu alloy in two conditions: ‘standard’ and electroless Ni-P plated. The effect of the electroless Ni-P coating on the coating/substrate performance was evaluated by pin-on-disc wear and impact tests. Ultra-microhardness and surface roughness measurements were also used to characterise the resulting coating/substrate systems. The electroless Ni-P coating reduced the wear rates of the PVD-coated Cu alloys and increased the impact wear resistance. Among the PVD coatings trialled, CrN, CrAlN and multilayered CrAlN coatings on electroless Ni-P provided the lowest wear rates.     

Abrasive wear resistance of electroless Ni-P coated aluminium after post treatment

Electroless nickel (EN) coatings are recognised for their hardness and wear resistance in automotive and aerospace industries. In this work, electroless Ni-P coatings were deposited on aluminium alloy substrate LM24 (Al-9 wt.% Si alloy) and the effect of post treatment on the wear resistance was studied. The post treatments included heat treatment and lapping with two different surface textures. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and micro-abrasion tester were used to analyse morphology, structure and abrasive wear resistance of the coatings. Post heat treatment significantly improved the coating density and structure, giving rise to enhanced hardness and wear resistance. Microhardness of electroless Ni-P coatings with thickness of about 15 μm increased due to the formation of Ni₃P after heat treatment.     

A study of corrosion resistance and formation characteristics of electroless Ni-P alloy coatings on Al18B4O33w/6061 Al composite with a simple surface pre-treatment

Ni-P alloy coatings were successfully deposited on Al₁₈B₄O₃₃w/6061 Al composite by electroless deposition. A simple surface pre-treatment method prior to the coating process was used for the first time. The coatings were characterized by SEM/EDX, TEM and electrochemical methods. It was found that the pre-treatment prior to the coatings had an important effect on the electroless plating, which offered an active surface for the electroless deposition, resulting in the Ni-P coatings being uniformly formed on the substrate and providing the protection for the composite substrate.     

Corrosion resistance of electroless nickel coatings in mining environments

Abstract

Recent experience with electroless (autocatalytic) nickel coatings for the corrosion protection of mining components has been disappointing, particularly in view of their potential for certain applications. This paper presents some laboratory results concerning the corrosion resistance of various commercially available electroless nickel coatings when subjected to chloride containing waters. The laboratory experiments consisted of porosity and thickness measurements, of corrosion tests using a humidity and temperature cycle similar to that found on some coal faces, and of examination by a scanning electron microscope to observe the mechanism of breakdown. It was found that the porosity of the coatings increased with exposure to the corrosive environment, thereby allowing galvanically enhanced attack of the steel substrates. The porosity increase was a result of progressive pitting of the nickel coatings with time and the results supported previous field experience. The various coatings tested exhibited different degrees of corrosion resistance, but all of them eventually showed some pitting of the nickel. It appeared, with one exception, that the coatings with the best corrosion resistance had been plated using a strong complexing agent and an organic thiocompound stabilizer.     

Effect on Corrosion Resistance of Ce-Rich-Sealed Ni-P Coatings on Cf/Al Composite Surface

In this study, Ni-P coatings and sealing of the coatings by Ce-rich solution on Cf/Al composite surface for enhanced corrosion resistance are investigated. The corrosion resistance of uncoating sample in 3.5 wt.% NaCl solution was investigated and a comparison with Ni-P and Ce-sealed Ni-P coatings is given. Effect of Ce-sealing on Ni-P coating is discussed. The results of electrochemical measurements of corrosion performance of Cf/Al composites show that sealing of Ni-P coatings with Ce-rich solution can improve the corrosion resistance. The Ce-rich-sealed Ni-P coating has higher corrosion resistance than the coating without Ce, and the electroless plated Ni-P coating on composite surface has higher corrosion resistance than the bare sample, as evidenced by EIS and potentiodynamic polarization measurements. The microstructure of the Cf/Al composites and the two kinds of coatings (i.e., Ni-P coating and Ce-sealed Ni-P coating) were examined by scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy. The Ce-sealed Ni-P coatings on Cf/Al composite surface have a total thickness of ~11 μm of which 10 μm is the thickness of the Ni-P coating and ~1 μm is the thickness of the Ce-rich sealing. It shows that the selected area electron diffraction ring pattern of Ce-rich sealing on Ni-P plated composite is consistent with Ce₆O₁₁ or CeO₂. X-ray photoelectron spectroscopy results show that Ce⁴⁺ was the dominant oxidation state for Ce-rich sealing on Ni-P plated composite. The Ce-sealing treatment on Ni-P coating has improved the corrosion resistance over and above the corrosion resistance offered by the Ni-P mono-coating to the bare substrate.     

Improving corrosion resistance of aluminium metal matrix composites using cerium sealed electroless Ni–P coatings

Abstract

A conversion coating treatment using cerium salts was developed for the surface sealing of electroless nickel–phosphorus (Ni–P) coatings on carbon fibre reinforced aluminium (Cf/Al) composites. The corrosion resistances of uncoated and coated materials (i.e. the Ni–P coating, the Ce conversion coating and Ce sealed Ni–P coatings) were evaluated in 3·5 wt-%NaCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy. Ce sealed Ni–P coating showed the highest corrosion resistance and clearly improved the overall corrosion resistance of Cf/Al composites. Thus, the Ce sealed Ni–P coating had no obvious microcracks that were generally evident in the more conventional Ce conversion coatings. This is presumed to occur because the electroless nickel surface is relatively homogeneous, compared with the Cf/Al composite surface on which different local coating thicknesses would encourage increased microcrack formation. X-ray photoelectron spectroscopy analysis showed that the Ce conversion coating mainly contained both Ce³⁺ and Ce⁴⁺ species; however, Ce⁴⁺ species were the dominant oxidation state on Ce sealed Ni–P coatings.     

化学镀镍内应力的研究

用于非金属电镀、电铸、化学镀、化学铸的现行许多化学镀镍工艺,总是在零件尖端部位发生崩裂,在镜面光洁的地方易起泡,这种难题至今未得到很好解决,这大大限制了化学镀镍的应用.针对这种情况,采用正交实验方法开发出一个低温、低内应力化学镀镍工艺,初步探讨了沉积层内应力与各工艺参数之间的关系,以及作为添加剂的糖精在化学沉积层中的作用机理,并对内应力和结合力的关系作了简介.实践证明了该工艺在短时间内能于多种复杂形状或镜面光亮的非金属表面上获得完整的化学镍沉积层.     

Corrosion and metal release rates of Ni substrates under a porous gold coating

Noble metal coatings intended to protect devices from corrosion may have small porosity that allows corrosive fluid to attack the substrate alloy. A technique combining electrochemical tests and chemical analysis is described that measures low levels of porosity and low corrosion product release rates from the substrate. The technique has been applied to an electrodeposited gold coating on a nickel substrate. The results yielded a porosity of 1.8 × 10⁻³, defined as the area of substrate exposed to corrosion/area of the coating. Galvanic coupling between gold and nickel was found to increase the rate of nickel corrosion by about a factor of 10. A simplified model showed that if the substrate alloy does not passivate, the corrosion product release rate is expected to increase with exposure time.     

预化学镀镍时间对铝基化学镀镍层性能的影响

目的揭示试样的显微形貌随预化学镀镍时间的变化规律,并探讨试样的显微形貌、镀层的结合强度及耐蚀性能的相关性。方法以预化学镀镍时间为变量,通过化学沉积方法制得化学镀镍层。采用扫描电镜观察预镀层及化学镀镍层的表面形貌,采用热震试验、弯曲试验和划格试验测试镀层的结合力,并对化学镀镍层与铝基体之间的结合力进行评价。采用电化学方法对镀层在模拟燃料电池腐蚀介质中的耐蚀性进行评价。结果随着预化学镀镍时间的延长,颗粒尺寸不断增大,预化学镀镍层形貌先逐渐变得均匀、致密,之后又变得粗糙不均匀。化学镀镍层的耐蚀性以及与基体的结合力呈现出先增加后降低的趋势。结论预化学镀镍时间在5 min时,所得化学镀镍层的表面形貌最平整,结合力最好,耐蚀性最佳。