Hydrogen evolution reaction on electrodeposited Zn-Cr alloy coatings

The hydrogen evolution reaction (HER) on electrodeposited Zn-Cr alloy coatings with different thickness and Cr content was investigated.in a 0.5 M Na2SO4 solution by electrochemical methods. The hydrogen evolution on the alloys starts at less negative potentials in comparison to that on Zn. Subsequently, with the potential shift in negative direction the rate of the HER on the alloy increases significantly faster. At low polarizing currents the hydrogen overvoltage on the alloys is closer to that on Zn, whereas at high polarizing currents it is closer to that on Cr. In the latter case the hydrogen overvoltage on the alloys is more than 200 mV lower than that on Zn. The lowering of the hydrogen overvoltage is related to the Cr content in the alloy. Almost similar values of the exchange current density for the HER on the alloys (Cr content in the range of 3 to 10 mass %) are determined. Based on the derived Tafel slopes Volmer reaction mechanism for the hydrogen evolution reaction on the alloys within the 0.25-10 mA cm (-2) current density range could be proposed. At higher polarizing currents the effect of Cr becomes more pronounced and presumably the electrochemical desorption appears to be the likely rate determining step. (C) 2016 Elsevier B.V. All rights reserved.