Process view
Electroplating is an electrodeposition process for producing a dense, uniform, and adherent coating, usually of metal or alloys, upon a surface by the act of electric current.The workpiece to be plated is the cathode (negative terminal). The anode, however, can be one of the two types: sacrificial anode (dissolvable anode) and permanent anode (inert anode).The sacrificial anodes are made of the metal that is to be deposited. The permanent anodes can only complete the electrical circuit, but cannot provide a source of fresh metal to replace what has been removed from the solution by deposition at the cathode. Platinum and carbon are usually used as inert anodes.
SURFACE PREPARATION
Workpieces to be plated may be put through a variety of pretreating processes, including surface cleaning, surface modification, and rinsing. A process flow diagram for hard chromium electroplatinga is depicted in figure 1 , including surface treatment and waste treatment. The purpose of surface pretreatment is to remove contaminants, such as dust and films, from the substrate surface. The surface contamination can be extrinsic, composed of organic debris and mineral dust from the environment or preceding processes. It can also be intrinsic, such as a native oxide layer. Contaminants and films interfere with bonding, which can cause poor adhesion and even prevent deposition. Therefore, surface pretreatment is important to ensure plating quality. Most (metal) surface treatment operations have three basic steps: surface cleaning, surface treatment, and rinsing.
Surface Cleaning
Cleaning methods should be able to minimize substrate damage while removing the contaminants, such as dust and film. Cleaning processes are based on two approaches: chemical approach and mechanical approach. Chemical approaches A chemical approach usually includes solvent degreasing, alkaline cleaning, (soak cleaning), and acid cleaning (acid pickling).
Solvent Degreasing
Contaminants consist of oils and grease of various types, waxes, and miscellaneous organic materials.These contaminants can be removed by appropriate organic solvents, either by dipping the workpieces in the solvent or by vapour decreasing.
Alkaline Cleaning
Workpieces are immersed in tanks of hot alkaline cleaning solutions to remove dirt and solid soil. A special type of alkaline cleaning is electro-cleaning. In electro-cleaning, the workpiece can be either the cathode (namely direct cleaning) or the anode (reverse cleaning). Electro-cleaning adds to the chemical action of the cleaner the mechanical action caused by plentiful gas evolution at the surface of the workpiece.
Acid Cleaning
Acid cleaning can move heavy scale, heat-treat scale, oxide, and the like. The most commonly used acids include sulfuric and hydrochloric. Pickling can also be combined with current to be more effective.
Mechanical approaches
Mechanical preparations include polishing, buffing, and some variations. Polishing is to remove small amounts of metal by means of abrasives. It produces a surface that is free of the larger imperfections left by grinding, and is a preliminary to buffing. Buffing is similar to polishing, but uses finer abrasives to remove very little metal. Buffing can produce an extremely smooth surface.
Rinsing
In wet plating, when workpieces are transferred from one treating solution to another, or when they leave the final treating solution, they carry some of the solution in which it has been immersed. This solution is called drag-out. In most cases, this residue solution should be removed from the workpieces surface by rinsing before the workpieces enter the next step in sequence, or come out of the final processing solution. The dirty rinse water will be sent to the wastewater treatment facilities before being discharged to a public sewage system. The importance of using the proper rinsing procedures between the various baths can't be overemphasized. When a part is transferred from one bath to another, or when it leaves the final bath, it carries before the part enters the next step in the sequence. To accomplish this, the parts must be rinsed free of this adhering solution. With it some of the solution in which it was immersed. This adhering solution must be removed