Effects of thiourea stripping of 14 karat white gold alloys with the addition of SDS surfactant

A stripping solution with thiourea, iron(III) sulfate, and sodium dodecyl sulfate(SDS) was employed to strip Ni-based 14 karat white gold alloys, and the formation of the NiS byproduct and elimination of passivation were investigated in the presence of 0.0-0.2 g/L SDS. White gold alloy samples with a flat shape were cast by gypsum investment and were stripped using the prepared stripping solution. Subsequently, the surface morphology, elimination of the passivation layer, weight loss, microstructure, elemental composition, and electrochemical properties of the samples were analyzed by optical microscopy, Raman spectroscopy, precision scale, scanning electron microscopy, energy dispersive X-ray spectroscopy, and linear sweep voltammetry, respectively. It was found that passivation layers of the as-cast samples were removed by the suggested stripping solution. Upon the addition of SDS, the stripped sample showed a bright silver color without NiS, while the sample showed a dark tarnished appearance due to NiS formation without SDS. The weight loss ratio decreased with increasing SDS content and stabilized at 0.2 % for SDS concentrations exceeding 0.15 g/L, and the sample showed a uniformly etched microstructure. EDS results showed that NiS was formed without SDS addition, while linear sweep voltammetry results indicated that NiS formation was restrained upon SDS addition because SDS suppresses the formation of formamidine disulfide from thiourea. Thus, the suggested thiourea stripping with SDS addition was successfully applied to Ni-based 14 karat white gold alloys.