Three manifestations of corrosion of chromium layer on the surface of printing plate
For the corrosion of the chromium layer, a long-term observation of the plate cylinder with a magnifying glass and a dot tester was found to be roughly divided into three types from the shape:
(1) The corrosion point is not obvious, the area is small, and it is dot-shaped or grape-shaped, which is mostly distributed at both ends of the plate cylinder;
(2) The corrosion shape and area are obvious, which can be directly observed with the naked eye, and its shape and distribution are irregular;
(3) Part of the chromium layer peels off, exposing the bottom layer, which has a large area and can be seen clearly with the naked eye. This situation generally occurs less frequently.
Among them, the corrosion in the first and second cases is divided into two types, one is corrosion from the outside to the inside; the other is corrosion from the inside to the outside. The third case is mainly the skinning phenomenon of the chromium layer from the inside to the outside.
Analysis of the causes of different corrosion phenomena
1. Causes of corrosion phenomena from outside to inside
In any plating solution, regardless of the pH level, there are certain hydrogen ions due to the electrolysis of water molecules. When the metal precipitates at the cathode, it is often accompanied by hydrogen evolution. Chromium plating solution is the most serious solution for hydrogen evolution. Hydrogen evolution has a great influence on the coating. The hydrogen adsorbed in the base metal or the pores of the middle plating layer will expand as the temperature of the surrounding medium rises, causing the plating layer to bulge. When hydrogen gas stays on the surface of the plate cylinder, it will prevent the deposition of metal ions in these parts, causing pitting or pinholes in the plating layer. These are potential hidden dangers that cause "pitting corrosion" of the chromium layer.
To overcome the phenomenon of hydrogen evolution, one is to control the cathode current density of chrome plating not to exceed the upper limit, because the current efficiency of the chromium plating solution decreases as the current density increases. The second is to overcome the hot phenomenon of the conductive system during the electroplating process. Because the hydrogen adsorbed in the fine pores of the base metal, due to the increase in the temperature of the surrounding medium, the expansion of the hydrogen will cause the plating layer to have a smaller bulge. The third is to keep the plating solution clean and minimize the mixing of organic impurities and inorganic impurities, otherwise the impurities are easily adsorbed on the surface of the cathode, thereby causing pits and pinholes in the chromium layer.
Another major cause of corrosion from outside to inside is external factors. For example, if the ink stays on the chromium layer for a long time, the acidic substances in the ink will corrode the chromium layer. If the plate roll is placed in a damp and non-ventilated place, it is also easy to form a patina through copper and carbon dioxide and water in the air, which forms a micro-battery effect on the chromium layer and accelerates the corrosion rate of the chromium layer. After that, the ink should be thoroughly scrubbed clean, and a layer of anti-rust oil should be evenly coated on the surface of the chrome layer and placed in a ventilated and dry place.
2. Causes of corrosion from the inside out
Generally there are obvious bulges, mainly due to improper surface treatment before chromium plating and poor combination of chromium layer and copper layer. In addition, some components in certain inks are corrosive to the chromium plating layer on the surface of the printing plate. They etch away the copper under the chromium layer through the trachoma on the chromium plating surface, causing the copper and chromium layer to peel off.
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