*2.1. Materials*

All chemicals were used as-received from the manufacturer without further purification. Acetone (CH3COCH3 ≥ 99.5%) grade ACS reagent and hydrochloric acid (HCl ~37%) grade ACS reagent were purchased from SIGMA-ALDRICH, and sodium hydroxide pellets (NaOH ~98%) grade AR were purchased from LOBA Chemie (Mumbai, India). Stainless steel AISI 316L bearing balls, of grade 100 and 8.5 mm diameter, were purchased from Bearing Warehouse Limited (Sheffield, UK). Thirteen square shaped substrates, divided into: (1) four of 3 cm<sup>2</sup> surface area and 0.6 mm thickness stainless steel 316L (supplied by YC Inox Co., Chang-Hwa, Taiwan), (2) four of 0.5 cm2 surface area and 0.6 mm thickness stainless steel 316L (supplied by YC Inox Co.), and (3) five of 1 cm2 surface area and 0.127 mm thickness copper of 99.9% purity (supplied by Precision Brand Products, Downers Grove, IL, USA), were manufactured using a computer controlled machine. The substrates were then cleaned with acetone, using a bath type Soniclean limited 250TD ultrasonicator (Thebarton, Australia), for 15 min at room temperature then carefully wiped to remove any remaining residuals. Three litres of deionised water (DIW), of pH 6.11, produced by an Elga PR030BPM1-US Purelab Prima 30 water purification system (Buckinghamshire, UK) was used after being divided into 3 sets of 1 L's, then their pH value at 25 ◦C were adjusted, via a HACH HQ11D portable pH meter (Loveland, CO, USA) of 0.002 pH accuracy, to 4, 7, and 9, respectively.
