Synthesis and performance of electroless Ni–P–TiCN composite coatings on Al substrate

Abstract

Electroless Ni-P and Ni-P-TiCN composite coatings have been deposited successfully on Al substrates. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques were applied to study the surface morphology and the chemical composition of the deposited films. Moreover, X-ray diffraction (XRD) proved that Ni-P and Ni-P-TiCN deposits have amorphous structures. The properties of Ni-P-TiCN/Al composite films such as hardness, corrosion resistance and electrocatalytic activity were examined and compared with that of Ni-P/Al film. The results of hardness measurements reveal that the presence of TiCN particles with Ni-P matrix improves its hardness. Additionally, the performance against corrosion was examined using Tafel lines and electrochemical impedance spectroscopy techniques in both of 0.6 M NaCl and a mixture of 0.5 M H 2 SO 4 with 2 ppm HF solutions. The results indicate that the incorporation of high dispersed TiCN particles into Ni-P matrix led to a positive shift of the corrosion potential and an increase in the corrosion resistance for all aluminum substrates after their coating with Ni-P-TiCN. In addition, Ni-P-TiCN/Al electrodes showed a higher electrochemical catalytic activity and stability toward methanol oxidation in 0.5 M NaOH solution compared with that of Ni-P/Al.

Effect of Cu in electroless Ni–Cu–P coating on passivation process

“…After passivation treatment, the passivated coatings have a more negative E corr than the as‐plated coating. The i corr of passivated Ni–P coating is 1.370 μA/cm, while only 0.314 μA/cm is determined for passivated Ni–Cu–P coating. To further confirm the anticorrosive properties of two passivated coatings were different, the passivated coatings, which were immersed in 3.5 wt.% NaCl solution for 72 h, were investigated by electrochemical corrosion test again.…”

“…As shown in the data from 0‐h immersion, the E corr of as‐plated Ni–Cu–P coating shifts 46 mV towards positive direction compared with the as‐plated Ni–P coating. The as‐plated Ni–Cu–P coating has a lower i corr with value of 1.751 μA/cm, indicating that the anticorrosive performance of coating can be improved by co‐deposition of Ni with Cu. After passivation treatment, the passivated coatings have a more negative E corr than the as‐plated coating.…”

“…Potentiodynamic polarization and electrochemical impedance spectroscopy measurements were carried out in three electrodes system by electrochemical workstation (IM6ex, Zahner, Germany). After 0‐h or 72‐h immersion in 3.5 wt.% NaCl solution, the sample with exposed area of 1 cm was tested in 3.5 wt.% NaCl aqueous solution at room temperature (25 °C). A sheet of platinum was used as counter electrode.…”

“…Because of its comprehensive properties of uniform deposition, high corrosive resistance and wear resistance, electroless Ni–P coating has been widely used in mechanical industry such as hydraulic support column, gear, thermal slug and so on . However, electroless Ni–P coating is easily oxidized in air and formed a layer of oxide film on the surface, which makes the coating lose metallic luster, even discolor .…”

“…As the content of P increases, Ni–P coating will change from crystalline state to amorphous state, leading Ni and P more difficult to be reacted in passivation solution. Zhou et al investigated chromate passivation treatment for amorphous Ni–P coating and the passivated coating exhibited moderate corrosion resistance with the i corr of 2.032 μA/cm in 3.5 wt.% NaCl solution. Lu et al concluded that the co‐deposition of Ni with W or Co would not improve the passivation effect, attributing to a slower dissolution reaction of Ni when alloyed with W and Co.…”