Corrosion resistance and catalytic activity of nickel coatings electrodeposited from a chloride-glycinate electrolyte with the addition of thiourea

Electrolytic nickel deposition is one of the most widely used electroplating processes for obtaining functional Ni-coatings. Depending on the required properties, various surfactants are introduced into the deposition electrolytes, acting as brighteners, levellers, wetting agents (antipitting additives), as well as contributing to the chemical modification of the coating surface by introducing heteroatoms into their composition. Thiourea is characterized by a strong adsorption interaction with the nickel surface, which contributes to the inclusion of sulfur in the composition of coatings and variation of their properties. Nickel alloys containing sulfur are widely used in the electroplating industry and are also promising materials for the electrolytic production of hydrogen. However, the corrosion potential and passivation ability of nickel alloys containing sulfur are relatively low, that is why it is important to study their corrosion resistance in combination with other characteristics. In this paper, the effect of thiourea on the catalytic activity and corrosion properties of nickel coatings electrodeposited from a chlorideglycinate electrolyte is studied. It was found that coatings obtained in the presence of thiourea are characterized by the presence of cracks on the surface regardless of the concentration of the additive, an increase in which contributes to a decrease in the size of the crystallites. It is shown that with an increase in the content of thiourea in the glycinate electrolyte, the sulfur content in the coating increases, which becomes morphologically more developed, and due to the amorphization of the sulfur -containing deposit, more ductile. The rate of hydrogen evolution reaction in sulfuric acid solution increases with increasing coating thickness and with the inclusion of sulfur in its composition, which indicates the electrocatalytic properties of synthesized materials. However, the open -circuit potential of Ni-coatings with various thicknesses is about 0.2 V more negative than compact nickel, and the addition of thiourea further shifts its value to the more cathodic values. The introduction of sulfur into the nickel coating contributes to an increase in the rate of corrosion, while systems containing 9 - 11 wt.% S are optimal in terms of corrosion resistance in a sulfuric acid solution and high catalytic in the evolution reaction.