In production, cyanide-copper-tin alloy plating may sometimes cause blistering or even shedding of the coating. Why does this happen? There are generally the following reasons.
(1) Degreasing is not clean: Part of the oil film that has not been removed before plating isolates the metal from the plating solution. It affects the deposition of metal ions at the cathode and even when it is severe, it cannot even be coated with metal. With thinner oil film parts, even if the plating with a good surface plating, in fact, is not a strong bond. Under certain conditions (increased stress caused by increased thickness of the plating; impact of high temperature and high current during chrome plating; strong impact of mechanical polishing and high temperature due to friction, etc.), the coating foams. It will fall off when it is serious. (2) The influence of the replacement of copper: Before the parts are plated, they must be electrically de-oiled and acid etched. When the electrolytic solution and the acid etching solution contain a certain amount of copper impurities, it is easy to replace the copper layer on the surface of the steel parts. The combination of this layer of copper and the matrix metal is Not strong. Replating the copper-tin alloy on the replacement copper will cause the coating to have a certain stress as the thickness increases. When the stress is large enough, the coating will break away from the substrate together with the replacement layer to form a blistering or even a large area falls off. .
(3) Effect of hydrogen evolution: During electroplating, hydrogen evolution is a very important factor that causes blistering of the coating. This is because if there is cracks and micropores on the surface of the base metal during the electrolysis and electroplating process, a certain amount of adsorbed hydrogen will accumulate. In the electroplating of copper-tin alloys, if the free sodium cyanide content in the plating bath is too high or the cathode current density is too large, a large amount of hydrogen will precipitate on the cathode surface, and a part of hydrogen will also infiltrate or be adsorbed on the surface of the base metal. And micro holes, etc. After a layer of plating is applied on the surface of the hydrogen- or hydrogen-absorbing article, when the coating is coated and the ambient temperature increases, a pressure is applied to the coating due to the expansion overflow, so that the coating is detached from the substrate and protrudes. Foaming.
In order to improve the quality of electroplating and prevent the occurrence of blistering of the coating, appropriate measures should generally be taken. When the plated parts are electrically de-oiled, the colored matrix metal should be cathodically deoiled to increase the current density and shorten the degreasing time; the steel matrix should use the first cathode to remove the oil, and the rear anode to remove the oil, so that not only can the deoiling be improved. The effect can also reduce the occurrence of hydrogen seepage.
On the other hand, the free sodium cyanide content in the plating bath is too high, and the cathode current density used is too large, and when a large amount of hydrogen is evolved on the surface of the plated part, besides causing foaming of the plating layer, the plating layer may also be caused. Pinholes, pockmarks, airflow stripes and local non-plating defects, so pay attention to control swimming, sodium cyanide content can not be too high, the cathode current density can not be too large, so as not to reduce the current efficiency, to avoid a large number of hydrogen evolution on the coating caused The adverse effects