PROTECTIVE ABILITY OF TIN-NICKEL COATINGS

Abstract

The calculation of the corrosion current of the steel – plating allowed us to estimate the protective properties and the porosity of the Tin-Nickel coatings. Measured in 3% NaCl solution the potential of steel electrode covered by tin alloy-Nickel thickness of 5, 10, 15 µm, respectively, has values of -277 mV, -314 mV, -355 mV, and the potential of steel 0.8 KP was 440 mV relative to the standard hydrogen electrode. At a thickness of 15 µm and at the mentioned potential the current density of corrosion for system steel – coating from tin-Nickel oxalate electrolyte was 2.3 µa/cm2, and for the coating from fluoride-chloride electrolyte was 7.5 µA/cm2. Therefore, the protective ability of tin-Nickel coatings obtained from oxalate-sulfate electrolytes is 3 times higher than similar coatings from fluoride-chloride electrolyte. Corrosion tests in salt spray chamber for Sn-Ni coatings obtained from oxalate-ammonium electrolyte confirmed the high corrosion resistance of tin-Nickel coatings when the ratio of the metals in the electrolyte was Ni/Sn = 5/1. The coatings obtained from oxalate electrolytes, distinguished by small size grains, in contrast to the coatings deposited from fluoride-chloride electrolyte. The increase in the microhardness of the coatings contributes a more microcrystalline surface topography of the alloy tin-Nickel deposited from oxalate-sulfate electrolytes, as it was evidenced by the results of crystallographic calculations. The test of specimens for microhardness showed that the samples obtained from oxalate-sulfate electrolytes are a bit harder (255 MPa) than samples obtained from fluoride-chloride electrolyte (245 MPa). The study of the structure of the coatings was performed using scanning electron microscope. SEM data showed that coatings obtained from the fluoride-chloride electrolyte have the pores, whereas poros are practically absent for caotings obtained from oxalate-ammonium. Probably, for these reasons the tin-Nickel coating obtained from oxalate-ammonium electrolyte better resists to corrosion in comparison with similar coating prepared from fluoride-chloride electrolyte. The technological process of depositing tin-nickel alloy from said weakly acid electrolyte is twice as effective and high-speed as compared to alkaline tinning, since the electrochemical equivalent of tin (II) is twice as high as for tin (IV) deposited from the alkaline electrolyte. It has been established that coatings obtained from oxalate-ammonium electrolytes, due to increased polarization during electrodeposition, are finer-grained, corrosion-resistant in comparison with coatings obtained from fluoride-chloride electrolytes. High dissipation ability of oxalate-ammonium electrolytes makes it possible to deposit tin-nickel coatings on composite products. The new technological processes studied in the result of the work made it possible to improve the physical and chemical properties of the coatings, to reduce material costs and to reduce environmental pollutionForcitation:Shekhanov R.F. Protective ability of tin-nickel coatings. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 10. P. 75-81