Rotogravure Chrome plating process and quality control principle

Chrome plating process principle:

The chrome plating process is an electrochemical process, a redox reaction process. The basic process is to immerse the parts in a metal salt solution as the cathode, the metal as the anode, and after connecting to direct current, a metal coating will be deposited on the parts. Schematic diagram of the gravure electroplating process: the plate roller is the cathode and the titanium mesh is the anode.

Main components of chromium solution Main salt:

Chromic anhydride Content: 200-260 g/L Catalyst: Sulfuric acid Content: 2.2-2.5 g/L Additives: Leveling and improving efficiency Performance of chromium plating layer: Chromium is a silvery white metal with a slight blue tint, with a relative atomic mass of 51.99, a density of 6.98-7.21 g/cm3, and a melting point of 1875-1920°C. Metallic chromium is easily passivated in the air, forming a very thin passivation film on the surface.

1. The rotogravure chrome-plated layer has a very high hardness. Depending on the composition of the plating solution and the process conditions, its hardness can vary from 400 to 1200 HV.
2. The chrome layer has good heat resistance. When heated below 500°C, its glossiness and hardness do not change significantly.
3. The friction coefficient of the chrome-plated layer, especially the dry friction coefficient, is the lowest among all metals. Therefore, the chrome-plated layer has good wear resistance.
4. The chrome-plated layer has good chemical stability and has high chemical stability in alkali, nitric acid, sulfide, carbonate and most gases and organic acids.
5. The chrome-plated layer is easily soluble in hydrohalic acids (such as hydrochloric acid) and hot concentrated sulfuric acid.

Chrome plating features:

Chromic anhydride aqueous solution is chromic acid, which is the only source of chromium plating. Although the performance of the plating solution is related to the chromic anhydride content, it mainly depends on the acid ratio, that is, the ratio of chromic anhydride to sulfuric acid.

1. The main component of the chromium plating solution is not metal chromium salt, but chromic acid, an oxygen-containing acid of chromium, which is a strong acidic plating solution. During the electroplating process, the cathode process is complex, and most of the cathode current is consumed in two side reactions: hydrogen evolution reaction 2 and hexavalent chromium reduction to trivalent chromium reaction 1. Therefore, the cathode current efficiency of chromium plating is very low (10% to 18%). There are also three abnormal phenomena: 1. The current efficiency decreases with the increase of chromic anhydride concentration; 2. It decreases with the increase of temperature; 3. It increases with the increase of current density.
2. In the chromium plating solution, a certain amount of anions, such as SO42-, must be added to achieve the normal deposition of metal chromium.
3. The dispersion ability of the chromium plating solution is very low. For parts with complex shapes, pictographic anodes or auxiliary cathodes are required to obtain a uniform chromium plating layer. The requirements for hangers are also relatively strict.
4. Chromium plating requires a higher cathode current density, usually above 20A/dm2, which is more than 10 times higher than general plating. Due to the large amount of gas released from the cathode and anode, the resistance of the plating solution is large, the tank voltage increases, and the electroplating power supply is required to be high. A power supply greater than 12V is required, while other plating types can use a power supply below 8V.
5. The anode of chromium plating does not use metallic chromium, because chromium is very easy to dissolve in the plating solution, making the anode current efficiency greater than the cathode efficiency, resulting in an increasing consumption of chromic acid. Therefore, an insoluble anode is used. Lead, lead-antimony alloy and lead-tin alloy are usually used. The chromium consumed in the plating solution needs to be supplemented by adding chromic anhydride.
6. The operating temperature of chromium plating has a certain dependence on the cathode current density. Changing the relationship between the two can obtain chromium coatings with different properties. In order to increase the bonding strength between the chromium plating layer and the substrate, the plate roller can be preheated.

The cathode (roller surface) reaction principle during gravure chrome plating:

The chrome plating solution mainly exists in the form of chromic acid (CrO42-) and dichromic acid (Cr2O72-). When the pH value is less than 1, (Cr2072- has 2 negative charges and 7 oxygen atoms) as the main form; when the pH value is 2-6, Cr2O72- and CrO42- exist in the following equilibrium, that is, Cr2072- +H20===2CrO42- + 2H+. It can be seen that the ions present in the chrome plating electrolyte include Cr2O72-, H+, CrO42- and SO42-. Except SO42-, other ions can participate in the cathode reaction. The four processes of electrochemical reaction at the cathode (roller surface):

Stage 1: As the electrode potential rises, the current density rises. The electrode reaction is 2H→ H2 Reaction 2
Stage 2: As the electrode potential continues to rise, the current density turns to decrease. This is the process of forming an alkaline cathode film. (The formation of the alkaline cathode film is due to the consumption of a large amount of H+ by two reactions ①② on the cathode surface). Reaction 1, Reaction 2
Stage 3: When the chromium precipitation potential is reached, chromium is plated on the plate roller surface. As the electrode potential continues to rise, the current density turns to increase again. The electrode reaction is Cr6→Cr 2H→H2 Reaction 1, Reaction 4

Cathodic film theory and its influence on quality during chrome plating:

During the chromium plating process, an alkaline cathode film is formed on the surface of the plate roller. This dissolution first occurs locally and gradually expands, thereby exposing a small area of the substrate, the real current density is very high, and the polarization effect is large. Only then can chromium plating (reaching the chromium precipitation potential) proceed at a certain speed. A colloidal film will be generated on the surface of the new chromium layer, and the dissolution and generation of the colloidal film will be repeated, playing an important regulatory role.

Although SO42- in the plating solution and the trivalent chromium generated during the cathode process do not directly participate in the electrode reaction, their presence and content are crucial to the quality of the chrome plating layer.
1. If the trivalent chromium content is low, the colloidal film is difficult to form or thin and porous, and sulfuric acid can easily dissolve it. At this time, the exposed substrate area is large, and the area with low current density cannot reach the precipitation potential of chromium, so the chromium covering ability is poor.
2. If the trivalent chromium concentration is high, the colloidal film is thick and dense, and sulfuric acid is difficult to dissolve. The chromium layer can only grow on the original grains, resulting in rough crystallization and dark and dull coating.
3. The sulfuric acid content is high, it is easy to dissolve the colloidal film, and there is no chromium layer in the low current density area, which is the same as the situation when the trivalent chromium is low. If the sulfuric acid is insufficient, the chromium layer will be rough, just like the situation when the trivalent chromium is high.
4. Therefore, their content must be strictly controlled in chrome plating, especially the ratio of chromic anhydride to sulfuric acid

The influence of impurity ions in rotogravure chromium solution and removal methods:
The harmful impurities in chromium plating electrolyte mainly include iron, copper, zinc, nickel, etc. Among them, when any metal ion accumulates to a certain content, it will bring harm to the chromium plating process, such as the reduction of the bright range of the coating, the reduction of the dispersion ability of the electrolyte, and the deterioration of conductivity. When the metal ion content in the electrolyte is high, the electrolyte must be treated. Treatment with low current density can achieve certain results. However, chromium liquid is highly corrosive, and some impurities are dissolved after electrolysis. When the content of iron ions is too high, ion exchange is used for treatment. During treatment, the chromium plating solution is first diluted so that the content of chromic acid does not exceed 120g/L, and then injected into the exchange column. The chromium plating solution treated in this way can be reused. To extend the service life of the resin, it is necessary to avoid direct contact between the concentrated chromium plating solution and the cationic resin, so as to prevent the resin from being destroyed by oxidation. The cation exchange method has the same effect on copper ions and trivalent chromium, but it is complicated and time-consuming.

Effects of trivalent chromium in gravure chromium solution and removal methods:
Generally, the increase of trivalent chromium is treated by electrolysis with a large anode and a small cathode. If the sulfuric acid content is high, it is best to reduce the sulfuric acid to normal before electrolysis. Excessive sulfuric acid will seriously affect the electrolysis effect, making it difficult to reduce trivalent chromium. There are generally several reasons for the increase of trivalent chromium:
1. The anode area is too small. The anode area should be 2-3 times the cathode area.
2. The content of metal impurities in the plating solution is too high.
3. Anode oxidation causes part of the anode to be non-conductive.

Introduction to the working principle of gravure printing chromium mist inhibitor:

During the chromium plating process, due to the use of insoluble anodes and low cathode current efficiency, a large amount of hydrogen and oxygen are precipitated. When the gas escapes from the liquid surface, it carries a large amount of chromic acid, forming chromium mist and causing serious pollution hazards. There are currently two methods to suppress chromium mist.

1. Floating body method: Put foam plastic pieces or fragments on the surface of the plating solution. These floating bodies can block the escape of chromium mist.
2. Add foam inhibitor: Foam inhibitor is a surfactant that can reduce the surface tension of the plating solution and produce a stable foam layer (similar to laundry detergent water, with countless small bubbles floating on the surface of the plating solution).

The foam layer formed by the chromium mist inhibitor in the plating solution tightly covers the surface of the plating solution. When the hydrogen and oxygen containing chromic acid evaporate, they come into contact with the foam layer on the surface, and countless tiny chromic acid mists combine into larger droplets. Due to the effect of gravity, they will return to the plating solution when they rise to a certain height, while the hydrogen and oxygen continue to rise until they leave the liquid surface, thus achieving the removal of gas and effective suppression of chromium mist.