During the electroplating process, the presence of an oxide layer on the substrate surface can severely hinder the adhesion between the plated layer and the substrate. This is because the physical bond between the oxide layer and the metal substrate is far weaker than the chemical bond between the electroplated layer and the fresh metal surface. Therefore, completely removing the oxide layer and exposing the active metal lattice is a key goal of the electroplating pretreatment process, directly impacting the adhesion, corrosion resistance, and service life of the subsequent coating.
Chemical pickling is a common method for removing oxide layers during electroplating. It dissolves metal oxides using an acidic solution. Steel substrates are often pickled with hydrochloric acid or sulfuric acid. Hydrochloric acid, due to its high volatility, is suitable for rapid rust removal at room temperature. Sulfuric acid requires heating to improve the reaction efficiency and is particularly suitable for thick oxide scales. For aluminum alloys, a mixture of nitric acid and hydrofluoric acid effectively breaks down dense oxide films while neutralizing residual alkaline residues and preventing secondary oxidation. For copper and copper alloys, a sulfuric acid-hydrogen peroxide system is often used. Hydrogen peroxide acts as an oxidant, accelerating the dissolution of the oxide film and avoiding the use of toxic chromic acid. During the pickling process, corrosion inhibitors, such as hexamethylenetetramine, are added. The nitrogen atoms in its molecules can adsorb onto the metal surface, forming a protective film that inhibits excessive corrosion. The addition of mist suppressants can reduce acid mist volatilization and improve the working environment. Immediately rinse with running water after pickling to prevent acid residue from causing rust recurrence and ensure contamination during subsequent electroplating steps.
Electrolytic pickling enhances rust removal during the electroplating process through electrochemical action and is particularly suitable for high-precision workpieces. When the substrate acts as the anode, metal ions in the surface oxide film dissolve under the action of an electric current, and the generated oxygen bubbles mechanically exfoliate the oxide layer. Cathodic electrolysis utilizes the reducing properties of hydrogen to remove the oxide layer, but the risk of hydrogen embrittlement must be considered, making it more suitable for low-strength steels. The current density, temperature, and time parameters of electrolytic pickling must be closely matched to the substrate characteristics. For example, when electroplating aluminum alloys, the voltage must be controlled to prevent excessive corrosion, ensuring the stability and consistency of the electroplating process.
Specific electroplating processes require tailored treatment plans for specific substrates. The naturally formed oxide film on aluminum alloy surfaces is dense and stable, requiring alkaline etching (sodium hydroxide solution) to disrupt its structure. Subsequently, nitric acid polishing is used to remove the remaining black dust. Acid pickling of copper and copper alloys requires a sulfuric acid + hydrogen peroxide system, avoiding the use of toxic substances such as cyanide. For easily passivated metals such as stainless steel, activation pre-plating with Wood's nickel solution (containing nickel chloride and hydrochloric acid) is required. The highly acidic environment corrodes the passive film and deposits a nickel layer on the surface, providing catalytically active sites for subsequent electroplating and ensuring compatibility with the electroplating process.
Mechanical treatment can complement chemical methods in the electroplating process, particularly for localized oxidation or stubborn stains. Sandblasting, through high-velocity blasting of quartz sand or steel shot, not only removes oxide scale but also creates a uniform roughness on the substrate surface, enhancing the mechanical anchoring of the electroplated layer. Grinding and polishing are suitable for precision parts, but the force must be controlled to avoid dimensional deviations. The tumbling process involves rolling the workpiece against an abrasive in a drum. It is suitable for batch processing of small parts, effectively removing burrs and improving surface finish, providing excellent foundational conditions for the electroplating process.
Activation is a key step in the electroplating process after removing the oxide layer. Its purpose is to form active crystal nuclei on the substrate surface, improving wettability with the electroplating solution. For steel, immersion in dilute hydrochloric acid or sulfuric acid can remove residual oxides. For aluminum alloys, immersion in a zincate solution is required. The zinc layer acts as an intermediate transition layer, significantly improving electroplating adhesion. Pre-treatment for electroplating on non-metallic substrates, such as plastics, is more complex, requiring a two-step process of sensitization (adsorption of Sn²⁺ by stannous chloride solution) and activation (deposition of catalytic centers by palladium salt solution) to provide reactive sites for electroless plating and ensure the feasibility of the electroplating process on non-metallic substrates.
Process consistency and bath maintenance are crucial for ensuring the effectiveness of the electroplating process. All pre-treatment steps must be performed continuously to prevent re-oxidation after the workpiece dries. For example, aluminum parts must be polished immediately after alkaline etching. Prolonged intervals will cause dust buildup and hardening, making cleaning more difficult. Bath composition requires regular testing and adjustment. For example, excessive iron ion concentration in the pickling solution can reduce rust removal efficiency and require filtration or bath replacement. Furthermore, wastewater treatment must comply with environmental standards. Acidic and alkaline wastewater must be neutralized and discharged to meet standards, and acid mist must be collected and treated using equipment such as spray towers to ensure the environmental friendliness of the electroplating process.
Through chemical pickling, electrolytic strengthening, specialized substrate-specific treatment, mechanical assistance, activation process optimization, and rigorous process management, the electroplating process systematically addresses the problem of surface oxide layers on substrates. These methods not only enhance the adhesion of the electroplated layer to the substrate but also lay the foundation for the corrosion resistance, wear resistance, and functionality of the subsequent coating. In actual production, a flexible combination of pretreatment processes is required based on substrate type, oxidation level, and electroplating requirements to achieve a balance between quality and efficiency in the electroplating process.
