Production Precautions

 (a)Chromic AnhydrideImpact of Concentration

        General Plating SolutionChromic AnhydrideThe concentration of the plating solution varies within the range of 190 to 250 g/L. As the concentration of chromic anhydride increases, the conductivity of the plating solution improves, and the covering ability also increases, while the cathodic current efficiency decreases. After adding certain additives, the impact of concentration becomes secondary. Typically, the concentration of the plating solution can be determined by the specific gravity method, and the relationship between chromic anhydride content and Baume degree is shown in the figure.

     Currently, a new automatic chromic anhydride replenishment device has been developed to address the difficulties in replenishing chromic anhydride and controlling concentration. Its quality and performance have been validated by the market and received high recognition and praise from users. The functional advantages are as follows:

1. Mechanical stirring (time controllable) to avoid impurities being introduced into the solution;

2. Replenishment control by the plating tank PLC to avoid inaccurate ampere-hours;

3. Liquid level display and alarm replenishment (quantitative replenishment is convenient and intuitive);

4. Convenient replenishment of chromic anhydride (can replenish 6 barrels in a few minutes);

5. Distribution drug box and replenishment box, close the valve during replenishment to avoid uneven solution;

6. The dosing pump is powerful, accurate, and durable, can be replenished remotely, with a one-year warranty;

7. The tank design is precise, with a 10cm liquid level for each barrel of chromic anhydride, making it easy to calculate the replenishment amount of chromic anhydride and pure water.

 (b) Impact of Sulfuric Acid Concentration

         In the chromium plating process, sulfuric acid acts as a catalyst, dissolving the basic chromate gel film, allowing chromium to precipitate smoothly. The concentration of sulfuric acid has a significant impact on the quality of the chromium layer, and the important factor is the ratio of chromic anhydride to sulfuric acid, rather than the absolute content of sulfuric acid. When CrO₃/SO₄^2-=100, the current efficiency is highest; when CrO₃/SO₄^2-=95 (i.e., H₂SO₄ content is slightly high), the smoothness and density of the chromium layer are good, but the current efficiency and covering ability decrease, and the sides of the plating drum may turn white; when CrO₃/SO₄^2- >100 (i.e., H₂SO₄is slightly low), the covering ability is better, but the smoothness of the chromium layer decreases, resulting in mottling and roughness. The impact of sulfuric acid on the cathodic reaction is shown in the figure.

If the sulfuric acid content in the plating solution is too high, barium carbonate (BaCO₃) can be used to precipitate excess sulfuric acid, with the reaction as follows: H₂SO₄ + BaCO₃ →BaSO₄↓ + CO₂↑ + H₂O

21 gram of BaCO₃ can precipitate 1 gram of H₂SO₄.

(c) Impact of Trivalent Chromium

In addition to the necessary chromic anhydride and sulfuric acid, the chromium plating solution should also contain a certain amount of trivalent chromium. The plating solution must be electrically treated to generate a certain amount of trivalent chromium. Generally, the cathode used by plating companies employs carbon brushes, while the anode uses lead-antimony alloy bars, and some use titanium-platinum alloys. It is recommended to use theGerman sintered titanium-based platinum-coated anode from Maituo Company.This anodeuses THE-process sintering technology,which belongs to high-end insoluble anodes, with a uniform coating, low internal stress, no voids, good toughness, and excellent adhesion.Its performance reaches the best quality effect currently available internationally,and the vast majority of high-end customers use this anod

In production, the area ratio of the anode to the cathode is controlled at 2:1 or 3:2 to maintain the stability of trivalent chromium. When the trivalent chromium content is low, a large cathode and small anode electrolysis is used; when the trivalent chromium content is high, the anode area should be increased and the cathode area decreased for electrolysis. At a temperature of 50-60°C, a current density of 5-10 A/dm is applied.²The current is applied for 2-4 hours until the trivalent chromium reaches the process requirements. In medium concentration plating solutions, some companies standardize the trivalent chromium content to 1-5 g/L, while others set it at 2-5 g/L. Practice shows that low trivalent chromium content results in slow chromium deposition speed, soft plating layer, and poor coverage ability; high trivalent chromium content leads to a dull, rough plating layer, poor brightness, and a reduced range of bright current density. When the trivalent chromium content is too high, a fine iron rod is used as the cathode, making the anode area about 10-30 times that of the cathode, with an anode current density of 1.5-2 A/dm.²until the trivalent chromium content is reduced to the specified range.

 (d) Two methods for producing trivalent chromium

① Electrolytic treatment method: At a temperature of 50-60°C, a small current (7-10 A/dm) is applied to a waste plate roller for 30 hours to produce trivalent chromium ions.²② Chemical treatment method: Some companies add an appropriate amount of formaldehyde (analytical grade, 3000 g) to a 1000L main tank at a temperature of 50-60°C and circulate for half an hour; other companies add an appropriate amount of hydrogen peroxide (1000-1500 g) to the 1000L main tank; generally, 1 gram of alcohol can generate 1.8 grams of trivalent chromium.