Investigating Corrosion Behaviour of Nickel-Plated Steel for Circulation Coinage
Authors:   , Waldram Rebecca K. 
Published: November 2021

Nickel plating has been used as a barrier coating to protect more reactive metals against corrosion, to improve their wear resistance or as a decorative feature since the early 1800’s. Since then there have been many changes to the bath composition to develop different deposit characteristics. The most widely used bath for nickel electrodeposition was developed by Professor O.P. Watts in 1916, which, with a few minor alterations is still commonly used today. Circulation coinage is made from a diverse range of metals and alloys, which varies widely across the world. Until recently, these tokens were produced solely from solid metal sections; however rising metal prices for those commonly used such as nickel, copper and zinc meant alternative, cheaper production methods required investigation. Nickel plating was selected to reduce raw metal costs, by only using a thin layer of this more expensive metal over a cheaper substrate, such as steel. Nickel has excellent corrosion resistance in a wide variety of environments, good wear resistance and an attractive lustre. Part of this thesis examines the use of nickel-plated coin sections in conjunction with other metallic sections to discover their compatibility for circulation, in terms of the risk of a galvanic corrosion cell forming. Comparisons between such sections were made using in-situ SVET and SKP analysis to obtain current density distribution maps to show the locations of anodic and cathodic activity, as well as classic electrochemical techniques including open circuit potential to form a galvanic series and zero resistance ammetry to measure the current generated when sections were immersed in the same electrolyte. The results from these experiments indicated that where bi-metallic construction is used there is a risk of galvanic corrosion to varying extents under both immersion and atmospheric conditions. Different countries use coins of varying compositions, and results from four bimetallic coins showed some compositions are more prone to galvanic corrosion than others. Nickel-plated coatings have previously been found to suffer from porosity, and therefore can be susceptible to pitting corrosion when immersed in aggressive chloride-based environments. To monitor this in atmospheric conditions, defects were made in the nickel-plated deposits and electrolyte droplets containing chlorides were introduced. A variety of hole sizes and chloride concentrations were used, and the spreading of these droplets was monitored using time-lapse photography and a Scanning Kelvin Probe. Overall, increased chloride concentration up to 2 M led to an increase in the observed spreading, although above this a decrease was seen. Defect size was shown to have little effect on the spreading distance. As a possible alternative to standard electrolytic nickel plating, electroless nickel plating solutions were investigated. This technology plates via a chemical process and requires no applied current, which could potentially reduce manufacturing costs. Three commercially available plating solutions were compared to a basic in-house bath, where it was found changing the bath pH and temperature could significantly alter the properties of the final deposit. Increasing plating temperature led to an increase in the phosphorous content, plating rate, corrosion resistance and Vickers hardness of the deposit, whilst increasing bath pH led to a decrease in the phosphorous content and corrosion resistance, and an increase in the plating rate and Vickers hardness of the deposit. Additions of organic compounds taurine and thiamine hydrochloride were made to the in-house bath in varying quantities to effect changes in the deposits and improve the properties. These compounds were selected as they are both amino acids, similar in structure to some additives previously used in electroless nickel baths, are non-toxic, low cost and contain sulphur, previously shown to have a brightening effect on nickel deposits. Taurine additions led to similar phosphorous contents and plating rates and lowered the corrosion resistance of the deposits. Thiamine hydrochloride lowered the phosphorous content of the alloy, increased the plating rate and lowered the corrosion resistance of the samples produced.