An Investigation into a Copper/Tin Alloy as an Underlying Coating to Chromium

Abstract

The electroplating industry has been under intense pressure over the last few years due to numerous factors which include the increase in metal prices, economic climate and the new regulations. These factors have led the electroplating industry to find more economical and less harmful chemicals. The characteristics and properties of a copper-tin alloy as an alternative to the nickel deposit under chromium were investigated due to the increasing concern over health risks associated with nickel metal and its soluble salts. Spectrophotometry was used to identify the colour of the deposits by using the L*a*b* colour space. The results showed that the colour of the chromium did not change regardless of the deposit below. However, there was a colour difference between the Cu-Sn and nickel deposit, the Cu-Sn deposit was lighter, less green and more blue than the nickel deposit with the numeric values L* = 86.26, a* = 0.40 and b* = 3.25, nickel proved to be the most red and most yellow in colour with the numeric values L* = 83.12, a* = 0.81, b* = 6.68. Atomic force microscopy (AFM) characterised the surface roughness of the deposits by reporting the RMS and Rmax values. The results identified the Cu-Sn deposit to have an increased roughness to its structure with values of 8.30 nm and 73.0 nm, respectively compared to the nickel deposit with values of 3.08 nm and 18.6 nm respectively. Corrosion performance was measured by two methods, (1) electrochemical test using linear polarisation and (2) accelerated corrosion test using both neutral salt spray (NSS) and copper accelerated acetic acid salt spray (CASS). The accelerated corrosion tests identified the nickel deposit to provide better corrosion protection compared to the Cu-Sn deposit, however the electrochemical test indicated the Cu-Sn deposit with a calculated corrosion rate of 0.232 mm per year would provide better corrosion protection than the nickel deposit with a corrosion rate calculated as 0.285 mm per year. To conclude, the Cu-Sn deposit could not replace nickel in all applications but it could be used as an alternative to the nickel in applications where appearance was primary, but it would be unable to replace nickel where corrosion protection was foremost.