Abstract
This study was carried out to improve the properties and as-plated conditions of 44wt%W Ni-W alloy, since that composition is considered optimum for the electroplated alloy. The experiments were conducted using citric acid-ammonia baths of pH 6, 70°C, containing 40g/L of NiSO4·6H2O, 70g/L of Na2WO4·2H2O plus citric acid and NH4OH, taking the citric acid concentration in the bath as the parameter. When citric acid monohydrate concentration was varied from 80g/L to 140g/L and the current density was kept constant at 10A/dm2, the composition of the plated alloys retained the value of 43±0.6wt%W, which closely corresponds to Ni4W stoichiometry. The alloys assumed the highest ductility at a citric acid concentration of 100g/L, and at citric acid concentrations exceeding 120g/L, the whole surface of the plates was mirror bright. As-plated alloy from 100g/L citric acid bath had a hardness of Hv 620, but showed a room temperature hardness of Hv 1310 after heating at 700°C for 1h, and a hot hardness of Hv 950∼980 at temperatures of 550∼670°C. It was also found that the quantity of the codeposited trace elements, carbon, oxygen, nitrogen and sulfur, was less for alloy plated from 100g/L citric acid bath than for all other alloys. From the results of measurements of electrode surface pH, crystallite size, physical density and trace elements in the alloys, it was inferred that the main species adsorbing on the electrode surface will change from hydroxide to citric acid, when citric acid concentration in the bath exceeds 100g/L.
