The initial surface of a metal substrate is often covered with oxide layers, oil films, and general contaminants. These surface deposits hinder full contact between the electroplating solution (or coating material) and the metal substrate, thereby adversely affecting the appearance, crystal grain size, density, adhesion, and uniformity of the resulting coating. Only by removing these deposits—revealing the metal’s natural color and its underlying metallographic structure—can a proper coating be formed. Surface features such as machining marks, burrs, rust, embedded debris, nodules, and solid particles must be thoroughly cleared away; the surface roughness should be minimized to ensure the workpiece is smooth and level. Failure to do so makes it difficult to achieve a dense, low-porosity, and highly smooth coating. In electroplating, the substrate and the coating are bonded via metallic bonds; lower surface roughness generally leads to stronger adhesion. In thermal spray processes (such as ceramic coating), the coating bonds to the substrate through mechanical interlocking; here, lower substrate roughness results in a smoother coating surface, lower porosity, and superior corrosion resistance. Furthermore, a lower surface roughness leads to a higher "true" current density during electroplating, making it easier to reach the metal's deposition potential. Conversely, if the surface is rough, reaching this potential becomes more difficult, often resulting in significant hydrogen evolution during the electroplating process.
Mirror Polishing
Electrochemical mirror polishing typically employs an electrolyte based on phosphoric acid, with the workpiece serving as the anode. During the process, the microscopically rough surface of the metal workpiece undergoes anodic dissolution, which reduces surface roughness and produces a mirror-like, leveled finish. This technique is frequently used as a pretreatment for decorative electroplating. Due to the inherent complexity of electrochemical mirror polishing, various theoretical models have been proposed to explain the mechanism; however, each has its own limitations, and to date, no single, universally accepted theory has been established. Depending on the composition of the electrolyte and the nature of the anode metal—and by selecting appropriate process parameters such as electrolyte concentration, temperature, and current density—the following three types of reactions may occur at the anode surface:
① Evolution of gaseous oxygen: 2H₂O - 4e⁻ → 4H⁺ + O₂↑
② Dissolution of the metal: Me - 2e⁻ → Me²⁺
③ Formation of a passivation film on the anode surface: Me - 2e⁻ + H₂O → MeO + 2H⁺
