Why do the coatings sometimes blister or even fall off?

In the production, the coating of cyanide copper-tin alloy sometimes causes the coating to blister or even fall off. Why is this happening? There are generally the following reasons.
 (1) Degreasing is not clean: the oil film that has not been removed before electroplating isolates the metal from the plating solution. It affects the deposition of metal ions at the cathode and, in severe cases, does not even plate metal. For thinner oil film parts, even if a good surface coating is applied, the bonding is actually not strong. Under certain conditions (increased stress caused by increased thickness of the plating layer; impact of high temperature and high current during chrome plating; strong impact force of mechanical polishing and high temperature caused by friction, etc.), the plating layer will foam. It will fall off when it is serious.
(2) The effect of replacing copper: Before the electroplating, the parts are generally subjected to electric de-oiling and acid etching. When the electric oil 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 workpiece, and the combination of the copper and the base metal is Not strong. The copper-tin alloy is electroplated on the replacement copper. As the thickness increases, the coating will have a certain stress. When the stress is large enough, the coating will form a foam with the replacement layer together with the matrix, and even a large area will fall off. .
(3) Effect of hydrogen evolution: During the electroplating process, hydrogen evolution is a very important factor in the foaming of the coating. This is because in the process of electric oil removal and electroplating, if cracks and micropores are present on the surface of the base metal, a certain amount of adsorbed hydrogen is accumulated. When electroplating a copper-tin alloy, if the content of free sodium cyanide in the plating solution is too high or the cathode current density is too large, a large amount of hydrogen gas is deposited on the surface of the cathode, and a part of hydrogen is also infiltrated or adsorbed on the surface of the base metal. And micropores, etc. After plating a layer on the surface of such a hydrogen absorbing or hydrogen absorbing material, when the temperature of the coating and the surrounding temperature rises, a pressure is generated on the plating layer due to the expansion overflow, and the plating layer is detached from the substrate to form a protrusion. Foaming.
In order to improve the quality of plating 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 base metal should be degassed by the cathode to increase the current density and shorten the degreasing time; the steel substrate should be first degassed by the cathode, and then removed from the oil by the anode j, which can not only improve the degreasing The effect can also reduce the occurrence of hydrogen permeation. On the other hand, the content of free sodium cyanide in the plating solution is too high, and the cathode current density used is too large, and when a large amount of hydrogen is deposited on the surface of the plated member, in addition to causing blistering of the plating layer, plating is also caused. Pinholes, pitting, airflow streaks and partial uncoated rickets, so pay attention to control travel, the content of sodium cyanide should not be too high, the cathode current density should not be too large, so as not to reduce the current efficiency, avoiding a large amount of hydrogen evolution on the coating Bad effects.