Evaluation of electroless nickel-phosphorus (EN) coatings

Abstract

The utilization of Electroless Nickel-Phosphorus (EN) coatings has witnessed a staggering increase during the last two decades. Many outstanding characteristics of the EN coating method have generated a lot of interest in various industries including oil and gas, electronic, chemical, automotive, aerospace, and mining. Some of the highlighted characteristics of EN coatings are superior corrosion and wear resistance especially in environments containing H2S and CO2, superior mechanical properties, uniform coating thickness, excellent surface finish properties, superb adhesion characteristics, and wide range of thickness. The EN coating process is based on a redox reaction in which a reducing agent is oxidized and Ni+2 ions are reduced on the surface of the substrate materials. Once the first layer of Ni is deposited, it acts as a catalyst for the process. Consequently, a linear relationship between coating thickness and time usually occurs. If the reducing agent is sodium hypophosphite, the deposit obtained will be a nickel-phosphorus alloy. The objective of this research was to evaluate various properties of three types of EN coatings, namely, low, medium, and high phosphorus. In the first phase of this work an automated prototype EN bath was designed and engineered. As a result, three types of EN coatings were deposited on various substrates. In the second phase of this research, various qualitative and quantitive methods were implemented to evaluate various properties of EN coatings. Also, the effects of various coating parameters including coating thickness and phosphorus content on properties of EN coatings were comprehensively investigated. Furthermore, the effect of post heat treatment on various properties of EN coatings was studied. Heat treatment on EN deposits in the range of 300-400 °C for one hour caused the hardness to increase due to the formation of various types of nickel phosphide (NixPy). The results of this study showed that various properties of EN coatings are directly related to the phosphorus content of the coatings. EN coatings with lower phosphorus content are crystalline, hard and brittle. As a result, they have superior wear resistance. On the other hand, EN coatings with higher phosphorus content are amorphous with superior corrosion resistance. iii EN coatings in general have excellent adhesion properties. However, the degree of adhesion is affected by several parameters including coating thickness, phosphorus content, post heat treatment, and ductility of the substrate. Moreover, it was shown that due their brittle nature EN coatings in general and heat-treated low phosphorus EN coating in particular have a detrimental effect on fatigue properties of their substrates. It was also shown that EN coatings in general, improve the kinetic coefficient of friction. In other words, EN coatings exhibit a self-lubricating behavior. Also, it was shown that EN coatings completely follow the surface profile of their substrate unlike conventional electroplating. Corrosion and wear studies on EN coatings revealed that EN coatings are excellent candidates for materials subjected to excessive corrosion and wear in a potash brine environment. Finally, the microstructure study of EN coatings using TEM and STEM electron microscopy revealed valuable information regarding the phase transformation during the heat treatment. It was shown that heat treatment at 400°C for one hour caused the precipitation of various nickel phosphide particles. As a result, significant changes in various properties of EN coatings occurred.