The surface condition and cleanliness of the parts to be plated are the fundamental guarantee for obtaining a high-quality coating. A rough or oily surface cannot produce a smooth, bright, well-adhered, and corrosion-resistant coating. Practice has shown that defects such as peeling, blistering, mottling, and poor corrosion resistance of the coating are often caused by improper pretreatment of the parts.
The pretreatment process includes polishing, degreasing, pickling, and pre-plating.
(1) Degreasing solution: The degreasing solution removes grease from the surface of the parts, giving the surface good wettability with the plating solution. The degreasing solution is a metal cleaning agent mainly composed of NaOH, Na2CO3, Na2SiO3, and emulsifiers.
(2) Pickling solution: The pickling solution removes oxide scale and passive film from the surface of the parts, activating the surface. Pickling solutions can be acidic or alkaline; acidic pickling solutions are widely used and consist of acid, corrosion inhibitors, and surfactants.
(3) Polishing solution: The polishing solution makes the surface of the parts smooth and glossy. There are two types: chemical polishing solution and electrochemical polishing solution. Chemical polishing solution is composed of various acids, emulsifiers, and corrosion inhibitors, while electrochemical polishing solution consists of acid, corrosion inhibitors, emulsifiers, oxidants, etc.
(4) Pre-plating solution: It prevents the substrate from being dissolved and replaced by a coating with poor adhesion when the parts are immersed in certain plating solutions. The composition of the pre-plating solution varies for different metals. The most commonly used are pre-plating nickel, pre-plating copper, and pre-plating chromium solutions. Aluminum and its alloys, as well as magnesium and its alloys, also require immersion in heavy metal solutions before pre-plating.