Corrosion resistance in different environments of Ni matrix micro-and nano-composite electrodeposits

摘要

The galvanic codeposition of ceramic particles for the production of metal matrix composite coatings is a research domain of wide interest as these deposits could be used for a large field of applications, especially in cases where high wear and corrosion resistance are required. The aim of this work is the production and characterization of nickel matrix micro-and nano-composite deposits. Three types of deposits have been produced: pure Ni deposits, Ni containing SiC microparticles and Ni containing SiC nanoparticles. A Ni sulfamate electroplating bath containing the particles in suspension has been used and the deposition was carried out using both direct and pulse current at different frequencies. ASTM 387 gr.22 steel plates were used as substrate. The microstructure of the obtained deposits has been observed by Scanning Electron Microscope at both top surface and cross section after metallographic etching. The SiC content along the whole thickness of the deposits has been evaluated by Glow Discharge Optical Emission Spectroscopy. The corrosion resistance of the coatings has been evaluated by potentiodynamic polarization curves in three different corrosive environments: a solution containing 3.5% NaCI, a solution containing S~(2-), SO_4~(2-), CI- ions and a solution containing H_2S and NaCI at 50oC. The degradation mechanisms have been evaluated by SEM observation of the corroded samples on both top surface and cross section. The codeposition on SiC micro-particles decreased the protective properties of the coatings while the codeposition of nano-particles did not penalize the high corrosion resistance of the nickel coatings in all the above mentioned corrosive environments. The microstructural modifications induced by the codeposition of the particles and the use of pulse current seems to have an influence on the degradation mechanisms.