The corrosion of the chromium layer of the gravure cylinder is related to the external objective conditions on the one hand and the intrinsic quality of the coating on the other hand.
For the corrosion of the chromium layer, we used a magnifying glass and a dot tester to observe the plate cylinder for a long time and found that it can be roughly divided into three types from the shape: the first is that the corrosion point is not obvious, the area is small, and the circle is round Dots or grapes, mostly distributed at both ends of the plate cylinder; the second manifestation of the corrosion shape and area is obvious, can be observed directly with the naked eye, its shape and distribution is not the law; the third manifestation of the local chromium layer peeling The bottom layer is exposed, and its area is larger. It can also be seen with the naked eye. This situation is generally less. Among them, the corrosion of the first and the second case is divided into two types, one is the corrosion from the outside to the inside; the other is the corrosion from the inside to the outside. The third situation is mainly from the inner and outer chromium layer peeling phenomenon.
The following first analysis of the corrosion from the outside to the inside. In any bath, regardless of the pH level, there are certain hydrogen ions due to the electrolysis of water molecules. As the metal precipitates at the cathode, it is often accompanied by the evolution of hydrogen. Chromium plating solution is the most serious hydrogen evolution solution, and hydrogen evolution has a great influence on the plating layer. Hydrogen adsorbed in the pores of the base metal or the intermediate plated layer will expand as the temperature of the surrounding medium increases, and a bulge may be generated in the plated layer. When hydrogen is trapped on the surface of the plate cylinder, deposition of metal ions at these sites will be prevented and pitting or pinholes will appear on the plated layer. These are potential pitfalls causing "point corrosion" of the chromium layer.
To overcome the phenomenon of hydrogen evolution, one must control the current density of the chrome plating cathode does not exceed the upper limit, because the current efficiency of the chrome plating solution decreases with increasing current density. The second is to overcome the hot phenomenon of the conductive system in the electroplating process, because the hydrogen adsorbed in the fine pores of the substrate metal, due to the increase of ambient temperature, hydrogen expansion will produce a smaller bulge in the coating. The third is to keep the bath clean and minimize the mixing of organic impurities and inorganic impurities. Otherwise, the impurities are easily adsorbed on the surface of the cathode, so that the chromium layer has pitting and pinholes.
Another major cause of corrosion from the outside to the inside is external factors. For example, if ink stays on the chromium layer for a long period of time, acidic substances in the ink will corrode the chromium layer. If the platen roller is placed in a humid and ventilated place, copper and green carbon dioxide and water can easily form a patina. This forms a microbattery on the chromium layer and accelerates the corrosion rate of the chromium layer. Therefore, the printing plate is used on the plate cylinder. Afterwards, the ink should be thoroughly scrubbed, and a layer of anti-rust oil should be evenly coated on the surface of the chromium layer and placed in a ventilated and dry place.
Corrosion from the inside to the outside generally has a pronounced snare pack, mainly due to improper handling of the chrome-plated surface and the poor combination of the chromium layer and the copper layer. The third case mentioned above is also due to this reason.
(Beijing Nanfeng Precision Manufacturing Co., Ltd. Huo Jihong)
Reprinted from: Printing Technology