The Surface Coating of Car Bodies NZIC（车身NZIC的表面涂层）.pdf

THE SURFACE COATING OF CAR BODIES Cars need a surface coating to prevent them from rusting excessively. Currently, this surface coating is an electrocoat paint, i.e. a paint that is applied using an electrochemical reaction, rather than simply by dipping the car body into a vat of paint. The whole electrocoat process consists of five steps. Step 1 - Precleaning The car body is assembled from imported metal panels and cleaned with kerosene to remove dirt and grease. Step 2 - Phosphating The metal is treated with phosphoric acid, leading to the following two reactions occuring in the zinc-rich surface of the steel: 2H PO + 3Zn → 3H + Zn (PO ) 3 4 2 3 4 2 2H PO + Fe + 2Zn → 3H + FeZn (PO ) 3 4 2 2 4 2 This gives a mixture of zinc phosphates. The electrocoat paint binds better to these zinc phosphates than to the steel alone, and the zinc phosphates corrode much slower than the pure steel. Step 3 - Electrocoating A mixture of resin and binder and a paste containing the pigments are fed into a tank. The vehicle is then lowered into the tank from an overhead conveyor and an electric current applied. This car body becomes the cathode and the tank the anode in an electrocoating reaction that results in a resin polymer being very tightly and evenly bound to the metal surface of the car body. Step 4 - Rinsing The vehicle is rinsed several times, both with water and with a weak paint solution, to remove a thin coat of paint that clings to the surface but hasnt bonded to the metal. If this wasnt removed then the car would have an uneven, messy coat of paint. Step 5