## 1 Introduction  
Wear, friction, and corrosion are surface phenomena that have been early characterized as degradation mechanisms that may lead to catastrophic results reducing the performance and the life cycle of components and various industrial equipment, under extreme conditions. The development of coatings has been recognized early as one of the most promising and effective process towards this direction. The excellent wear and corrosion resistance of the Ni-P (nickel-phosphorous) coatings makes them very good candidates for application where surface is under load and aggressive environment [1; 2; 3; 4]. The two major deposition techniques for Ni-P coating systems are the electro- and electroless deposition, with the first to provide high deposition rates and the second to provide coatings of exceptional thickness uniformity, even at the complicated geometrical features [1; 2; 3; 4; 5]. The development of the electroplating technology is established many decades ago and parameters, such as bath pH values, bath temperature, and current density, are of the most crucial factors affecting both the process and the final coating characteristics. The continuous development of the electroplating technology has recently led to a new electrodeposition approach that is known as pulse current electrodeposition, where some additional process parameters play a key role in deposition control. Among these, \(\mathrm{t_{ON}}\), which is the time that the current is on, \(\mathrm{t_{OFF}}\) the time the current is off, the duty cycle as the ratio \(\mathrm{t_{ON}}\)/\(\mathrm{t_{ON}}\)+\(\mathrm{t_{OFF}}\), and the frequency of the cycle (f), which is defined as the ratio f = 1/\(\mathrm{t_{ON}}\)+\(\mathrm{t_{OFF}}\) control the nucleation and growth of the coatings [6]. Pulse current provides some important advantages, such as: a) a negatively charged region is formed, upon plating, close to the cathode that obstructs metal ions from reaching its surface. In pulse current, during the \(\mathrm{t_{OFF}}\) time, this layer of negative charge is partially discharged and the migration of ions is improved in such way, b) upon the \(\mathrm{t_{OFF}}\) period, areas within the bath that have been depleted of metal ions can recover the bulk concentration, so to increase mass transport and increase the limiting current density value, and c) careful control of both the typical plating parameters along with the pulse current parameters can improve the coating final properties with more compact structure and controlled microstructure and thickness distribution [6]. The outstanding characteristics in the performance of Ni-P coatings are mostly related to their microstructure, which is affected by the P content. Low P content (<4% wt) lead to crystalline coatings, medium additions of P to coatings lead to partially crystalline, and partially amorphous nature and high P level (> 10% wt) leads to fully amorphous coatings [7; 8; 9; 10; 11; 12]. Often, these coating are heat treated to increase their hardness by precipitation hardening. This heat treatment process lead to the crystallization of the amorphous phases and the precipitation of the supersaturated P lead the final microstructure to a Ni Face-Centered Cubic (FCC) matrix and Ni3P intermetallics [13; 14; 15; 16]. It has been reported that post heat treatment close to 400 \({}^{\circ}\)C drastically increases the coating hardness due to the strengthening effect that was caused by the precipitation of Ni3P intermetallic particles [17; 18]. The strengthening effect fades at higher treatment temperatures due to the coarsening of the precipitated particles, coarsening of matrix grains, and due to the reduction of the crystal lattice defects. The Ni-P coatings were further evolved and enhanced towards the development of Ni-P composite coatings due to the continuous increase in the demand for improved wear and corrosion resistance, where parallel deposition of Ni-P matrix and embedment of hard particles occurs. Various systems have been developed so far of exceptional in some cases properties.