## 1 Introduction  
Stainless steel has often been used as a material for heat exchangers because of its excellent corrosion resistance and heat resistance. Nickel-based filler alloys are commonly used for brazing stainless steel, owing to their good corrosion resistance and oxidation resistance. To lower the melting point of the nickel-based filler metal, it generally contains a large amount of a melting point depressant, such as P, B, or Si. However, nickel-based alloys containing a melting point depressant easily form a eutectic structure that causes the brazed joint to be brittle [1; 2]. When brazing the components with a small thickness, owing to the rapid diffusion of boron into the substrate and its low solubility in austenite, large amounts of brittle borides precipitate in the bonding-affected zone, increasing the brittleness of the bonding. Furthermore, the borides can decrease the corrosion resistance of the brazed joint, shortening the life span of the components. Compared with boron, it is difficult for phosphorus to diffuse into the substrate and generate metal compounds in the bonding-affected zone owing to its low diffusion and large atomic radius. In addition, it was found that nickel-based alloys containing phosphorus exhibit very good wettability and may form a ductile solid solution in a joint in the case of a very narrow gap [3]. Therefore, the Ni-P filler alloy is more suitable for the brazing of stainless steel with a small thickness. For a Ni-P alloy, some melting will occur at the lowest melting point (875 \({}^{\circ}\)C) of the Ni-P system, regardless of the phosphorus content, provided it is greater than 0.2%. Therefore, it is easy to set the brazing temperature using a eutectic Ni-P alloy in industrial applications. Currently, a eutectic Ni-(10-12)P (wt.%) alloy, code name BNi-6 (JIS Z 3265), is commercially used as the filler metal owing to its lowest melting temperature in the Ni-P system, extremely good fluidity and excellent brazability [4]. However, with the reduction in the weight of heat exchangers, especially multilayer plate fin heat exchangers, the stainless steel components of heat exchangers have become thinner and smaller. Therefore, a filler metal with a thickness of around 20 \(\upmu\)m is required to join the stainless steel. Usually, BNi-6 filler metal is formed as a foil or paste because of its inherent hardness and brittleness, making it difficult to supply the correct quantity of the filler metal to the brazed area. An inappropriate quantity of the filler metal causes degradation and decreases the reliability of the brazed joint. In addition, the thickness of the filler metal is usually below 0.1 mm, and it is difficult to form a foil or paste with such a thickness [5]. It is widely known that Ni-P alloy layers can be produced by rapid quenching, ion implantation, vapor deposition, and chemical deposition [6; 7; 8]. However, for various reasons, these techniques do not lend themselves to industrial-scale applications. The chemical deposition method is more economically favorable than these techniques, and alloy layers of uniform thickness can be obtained by controlling the plating conditions. However, the plating solutions used in chemical deposition are laborious to control and produce large amounts of waste owing to their short life span. Furthermore, it is difficult to accurately control the phosphorus content of the plated layers because the properties of the plating solution change with time. Compared with the case of electrodeposition, the deposition rate of chemical deposition is low and a high operating temperature (about 80-90 \({}^{\circ}\)C) is required [8]. On the other hand, electrodeposition has been found useful at the industrial scale in preparing thin alloy layers, owing to its higher deposition rate and lower operating temperature than chemical deposition. Furthermore, variations in solution composition and operating parameters have resulted in a number of stable solutions capable of producing high-quality Ni-P plating layers [9; 10; 11; 12; 13].