Electrochemical analysis of Zinc for biodegradable applications

Abstract: To avoid costly and risky secondary surgery after implantation, biodegradableimplants are of importance to the medical device industry. Theseshould be designed to degrade in a controlled manner, and to haverest products taken care of by the body’s internal mechanisms withoutcausing harm. Finding materials that corrodes predictably, are biocompatiblein itself and when decomposed is a great challenge. This study focused on the corrosion behaviour of zinc, which anearlier study pointed out as a promising subject for such an analysis.The properties of zinc samples were observed by electrochemical methodswhile immersed in phosphate buffered saline (PBS), Ringer solutionand human blood. Scanning electron microscopy (SEM) and energy dispersivex-ray spectroscopy (EDX) were used to characterise the structureand composition of the corrosion products. It was found that thecorrosion mechanisms are different in the three electrolytes studied. Samples immersed in Ringer solution showed a uniform corrosion,with an initial corrosion rate of 0.1 mm/yr - 0.5 mm/yr. Resultsfrom electrochemical impedance spectroscopy (EIS) suggest the corrosionproducts after time (12-24 h) correspond to a two-layer interface,interpreted as an outer diffusion controlled layer appearing first, witha homogenous layer being formed later between the metal and the initiallayer. EDX and SEM indicates that the corrosion products mainlyconsist of zinc carbonates. These findings were confirmed by FourierTransform Infrared (FTIR) spectroscopy. The zinc samples exposed to PBS solution exhibit a localized corrosion,with dendrite like structures growing out from the surface astime passes. The initial corrosion rate was determined to be somewherebetween 6 μm/yr- 22 μm/yr. The EIS model suggest that the corrosionproducts developed on the PBS exposed surface corresponds to a singleoxide layer covering the metal. The main corrosion products weredetermined to be zinc phosphates (FTIR and EDX). Human blood was used for a final set of experiments. Here the zincsamples corroded by an initial rate of 0.08 - 0.38 mm/yr. The corrosionwas uniform like the samples exposed to Ringer solution. Meanwhile,the electrochemical model (from EIS) appeared similar to the one encounteredin the PBS experiments, suggesting a single layer of corrosionproducts over the metal surface.