Decoating is the process of removing the old coating on the workpiece before the coating. The work piece can be repaired after the initial coating is used and the coating can be used again. It can also be repaired and repeated several times. However, the continuous accumulation of multiple coatings will have a negative impact on the service life of the workpiece and the accuracy of the workpiece, so there is a need for repainting.
The decoating process is mainly divided into chemical decoating and electrolytic decoating. Both decoating processes utilize the chemical reaction between the decoating solution and the coating to oxidize and decompose the coating. For example, the chemical reaction of TiN coating by using oxidation H2O2 (hydrogen peroxide) decoating is as follows:
2TiN + H2O2 + 2H2O → 2TiO2 +2NH3 ↑
TiO2 + 2NaOH → Na2TiO3 + H2O
Ti + H2O2 → Ti(OH)2
The electrolytic decoating method is to put the decoating workpiece into the chemical (such as ammonium nitrate) solution. The workpiece is connected to the positive electrode, and the negative electrode is placed near the surface where the workpiece is to be decoating in the solution, and then the coating is ionized and dissolved into the solution. The reaction equation is as follows:
Ti → Ti2+ + 2e-
Ti2+ → Ti4+ +2e-
The electrolytic decoating method can remove all kinds of coatings on almost all substrates, and the plating speed is fast, about 10um/min, but the equipment is more complex. The chemical decoating method generally can only decoating the coating on the steel substrate or some hard alloy substrate.
As a decoating process, regardless of the methods, the decoating solution may also damage the substrate while oxidizing and dissolving the coating. Because some steel contains the same alloy components as the coating, such as Ti, W, Al etc., especially P-type hard alloy, contains about 15% TiC, which is more sensitive to the corrosion solution. In addition, cobalt is used as bonding agent in cemented carbide, and cobalt is easily lost in chemical solution. After cobalt loss, the cemented carbide composition around it loses the binding force, easy to fall off, and the surface of the workpiece is destroyed. Therefore, for the workpiece that needs to be decoated, should make full preparation before decoating and understand the characteristics of the workpiece substrate material. In addition, after the workpiece is decoating, do some necessary post-treatment, such as sandblasting, polishing, etc., to re-process and repair the workpiece surface.