Selection of copper-clad steel plating process

The special nature of wire processing requires different copper plating processes than those used for conventional steel parts. Therefore, choosing a suitable copper plating process for wire processing is a crucial issue.

The choice of electroplating process is usually based on the requirements for the coating of the product itself. That is, the product is processed first, and then electroplated. For example, thickness, brightness, hardness, and dispersion ability. Some also have requirements for current efficiency and deposition rate. Then there is the pre-plating layer, intermediate plating layer, and surface plating layer required for the copper plating.

The characteristic of wire electroplating is that the wire processing and electroplating processes are sometimes carried out simultaneously. That is, some are drawn first and then plated, and some are plated first and then drawn. Even the electroplating of finished wires is carried out under the traction of a winding machine. The selected process, in addition to the requirements for the coating according to the different Applications of the wire, must also consider the adaptability of the electroplating process to factors such as the wire running speed, the number of drawing heads, and the length of the processing line.

For CO2 gas-shielded welding wires, due to the strict limitations on the amount of copper attached to the welding wire, for example, the copper content per unit volume should be within 0.52 (mass fraction) of the solder, which is an extremely thin coating, the use of chemical immersion, i.e., displacement copper plating, can meet the requirements. However, due to the traditional displacement copper plating method currently used in China, its bonding strength and coating color cannot meet the product requirements. In practice, a process of immersion plating followed by drawing is used, so that the copper layer is expanded and thinned during drawing to meet the product requirements. This method often results in coating peeling and base metal exposure. Therefore, choosing a copper plating process with good bonding strength and ductility is crucial for gas-shielded welding wires.

Until a new process that can successfully chemically plate finished wires in one step appears, it is still only possible to adopt the process of drawing to an intermediate wire diameter, plating copper, and then drawing into finished products. Cyanide copper plating or matte acid copper plating should be suitable for this processing process. Because cyanide is highly toxic, it is rarely used in the metallurgical wire processing industry. The currently more popular method is still chemical immersion copper plating followed by thickening copper plating and then drawing. The thickening copper plating used is acid copper plating or pyrophosphate copper plating. The chemical method is also mostly used for copper plating in the flat wire processing industry.

Traditional displacement copper plating uses a process of copper sulfate and sulfuric acid. Only a very thin coating can be displaced in a very short time, and the bonding strength is not strong when the copper plating is thickened. If the immersion time is too long in this chemical displacement plating, not only will the coating thickness not increase, but the coating will become loose and porous, and the iron substrate will also be corroded, greatly reducing the strength of the wire. The improved method is to add additives with surface retarding effects to the displacement copper plating solution to make the displacement process proceed in an orderly manner. The ideal additive also has a certain brightening effect.

For wire products that require a certain thickness or even a thicker coating, the use of displacement plating for the base layer must be carried out with great care. At least until a reliable base plating process suitable for thick copper plating is available, for thick copper plating, the reliable process should still be pre-plating using electrochemical methods. Mature pre-plating processes include cyanide copper nickel plating and high P ratio pyrophosphate copper plating. Weighing the various advantages and disadvantages, in the wire drawing electroplating industry, it is better to use nickel plating as the pre-plating base. Thick copper plating can then be carried out using acid sulfate copper plating. This is because, after process adjustment, acid copper plating can meet the requirements of high-speed electroplating. The current density can reach 30～50A/dm2, greatly increasing the deposition rate. In comparison, cyanide copper plating is not suitable due to environmental pollution problems, while pyrophosphate copper plating is not only expensive due to its complex composition but also not suitable for working under high currents. Tests show that in high-speed electroplating in acid copper plating, when the current density increases, the deposition rate of the coating also increases simultaneously. The approximate relationship is shown in Table 1.