6Highlights 7 Effects of various additives on the corrosion resistance of Ni-P films is studied.
8 Thinner, more uniform, and smoother Ni-P thin films are formed with additives.
9 Excellent improvement of the corrosion resistance is observed.
11
Abstract
12Ni-P amorphous coatings are considered as important engineering alloys. In this paper, the effects 13 of various additives on the properties of Ni-P thin films were investigated, including saccharine, 14 glycine, pyridinium propyl sulfonate, coumarin, sodium citrate, and cerium sulfate.
15Potentiodynamic polarization and electrochemical impedance spectroscopy tests, scanning 16 electron microscopy, X-ray fluorescence and diffraction, and atomic force microscopy were 17 employed to study the properties of the samples. It was found that a proper concentration of these 18 additives, except saccharine, noticeably improved the corrosion resistance, especially at high 19 potentials (about 25% increase in instantaneous corrosion efficiency and about 300% in corrosion 20 efficiency at high potentials) and decreased the surface roughness (by about 10 to 55 %) of Ni-P 21 thin films. Thinner, more uniform, and less porous coatings were formed in the presence of 22 additives. Except saccharine, all the additives enhanced the P content of Ni-P films, which thus 23 kept their amorphous structure; saccharine highly suppressed the incorporation of P inside the Ni 24 lattice and a mixed amorphous-crystalline structure was stabilized. The preparation of thin film coatings using electrochemical methods is attractive due 3 to their simplicity, the opportunity to work generally at ambient temperature, the 4 uniform and controllable deposition rate, the possibility to form multilayers, the 5 ability to coat large surfaces in almost any shape and geometry, and the low cost thus impede crystalite nucleiation. Therefore, the amount of P in the alloy affects its 35 crystallographic structure; increasing the P content changes the microstructure from 36 3 crystalline to nano-crystalline and finally to an amorphous state. According to the 1 literature, there is not a specific content but a range of P contents for the crystalline 2 to amorphous transition of Ni-P coatings [15]. However, coatings with P contents 3 higher than 8% present usually an amorphous structure [14]. Given that the P content 4 largely determines the properties of Ni-P coatings shows the importance of the 5 chemical composition of the electrodeposition bath. it is well known that 6 introducing even a small amount of certain compounds as additives to the electrolyte 7 significantly affects the properties and appearance of deposits [24]. The refinement 8 of the microstructure, with a decrease of the internal stress and the surface roughness, 9 an increase of the brightness , and the improvement of the corrosion resistance and 10 mechanical properties of the coatings are some of the reported benefits of additives 11 [11,16,17,23,[25][26][27][28][29]. Organic compounds are the most commonly used additives, 12 and they...