Since the late 1970s, Chinese researchers, represented by those from Harbin Institute of Technology, began to study trivalent chromium electroplating processes, mainly conducting experimental research and theoretical discussions on formate, glycine, and acetate systems.
In the 1980s, the formate-acetate bath was applied to small-scale trial production and achieved results in two respects: first, the cathode process characteristics were measured using a micro-antimony electrode, and chromium deposits nearly 20 μm thick were obtained by pulse plating; later, a chromium-nickel alloy was produced using a trivalent chromium bath.
After the 1990s, research mainly focused on developing new decorative chromium plating processes, improving bath stability, modifying anodes, and improving the appearance, color, and thickness of the deposits.
In decorative chromium plating, Guangzhou Erqing Research Institute, after years of research and effort, achieved breakthroughs in both sulfate trivalent chromium electroplating and titanium-based DSA coated anodes. It was the first to pass pilot testing at an enterprise in Zhejiang and has since been put into batch production, achieving good economic and social benefits.
1. Advantages of sulfate trivalent chromium plating
(1) Low toxicity.
The trivalent chromium ion content (4~6 g/L) in a trivalent chromium plating bath is only one-seventh or even less than the chromium ion content in a hexavalent chromium bath, and its toxicity is only one-hundredth that of hexavalent chromium. Wastewater treatment is also easier; simply raising the pH above 8 causes Cr(OH)3 precipitation.
Due to the low concentration, chromic acid mist is greatly reduced, and drag-out losses during operation are also significantly decreased.
(2) The bath has good throwing power and covering power, improving product yield, especially for complex-shaped products, where the first-pass quality rate is greatly increased.
(3) The bath has a wide operating temperature range, generally 15~60°C, with 45~55°C being optimal for the sulfate trivalent chromium system.
(4) Intermittent plating capability. Interrupting power during plating or removing the product from the tank for inspection and then returning it does not affect coating adhesion, although the decorative coating surface may develop a haze.
(5) Wide current density range, from 0.54 A/dm2 to 100 A/dm2, and the chromium deposit does not burn even at high current densities. Current efficiency can reach up to 50%~60%.
2. Disadvantages of the trivalent chromium electroplating process
(1) As a decorative coating, its appearance is somewhat blackish and yellowish, close to the color of stainless steel, and cannot achieve the silver-white appearance with a light blue tint typical of hexavalent chromium deposits.
(2) The bath contains many components, making it difficult for on-site operators to control.
(3) The bath is sensitive to various impurities and has low tolerance to them.
If impurities exceed the following levels, they will severely affect and damage the bath performance, making it impossible to produce qualified deposits: Ni ≤ 50 mg/L; Cu ≤ 10 mg/L; Zn ≤ 30 mg/L; ferric iron ≤ 10~30 mg/L; hexavalent chromium ≤ 5 mg/L.
(4) Decorative chromium coatings are difficult to thicken.
The typical plating time for decorative chromium plating is 2~5 min, with a thickness of only a few tenths of a micron. The coating has low hardness and poor scratch resistance, which also causes poor corrosion resistance in actual products. Therefore, passivation sealing or immersion in an organic coating is generally required after plating.
(5) Insoluble materials are used for anodes.
When the anode coating is damaged, hexavalent chromium is easily generated in the anode zone under current. When the hexavalent chromium content exceeds 5 mg/L, bath performance deteriorates, making it impossible to produce qualified coatings or even any chromium deposit.
3. Application of the sulfate trivalent chromium plating process
The author's former company, Stanley (Zhongshan) Hardware Co., Ltd., introduced the McGean-Canning sulfate trivalent chromium plating process in 1998 and set up a 2500 L chromium tank, mainly producing various series of door hinges.
Because there was no practical experience in this field at that time, almost no qualified products could be plated. With accumulated experience, by 2000 the company expanded to a trivalent chromium circular automatic line with about 4500 L of chromium plating solution, which could basically run stable, continuous production.
At present, the author's company mainly uses various wires, tubes, and sheets as raw materials to produce fan guards, kitchen and bathroom products, garden products, daily necessities, and similar items. Since 2004, it has successively configured a total of more than 20,000 L of sulfate trivalent chromium plating solution. It now has 2 manual trivalent chromium lines and 2 fully automatic trivalent chromium production lines, which have run stably and well since 2004.
4. Process and production maintenance
The author has been engaged in the application and maintenance of sulfate-system trivalent chromium electroplating on the production front line for many years and has a fairly comprehensive understanding of both trivalent chromium processes based mainly on imported raw materials (such as the sulfate system from Meijian Chemical) and those using entirely domestic materials (such as the BH-88 sulfate system from Guangzhou Erqing Research Institute). The following points are summarized from practical experience.
Conductive salt provides solution conductivity, with a mass concentration of 240~350 g/L and an optimum of 310~340 g/L. In actual production, for simple-shaped iron wire or sheet products, the conductive salt is controlled at 240~320 g/L. This not only produces a good decorative chromium coating but also reduces drag-out losses and lowers production cost. For complex-shaped products, however, the conductive salt concentration should be controlled at 300~350 g/L so that the bath has good covering power, thereby improving the first-pass rate.
Make-up solution provides trivalent chromium ions, with a volume fraction of 80~120 mL/L and an optimum of 100 mL/L. If drag-out losses are large, in addition to replenishing according to analysis results, the make-up solution can be used once per shift in place of the replenisher during daily maintenance to maintain the chromium salt concentration and stabilize bath performance.
Auxiliary agent The auxiliary agent is the carrier of the brightener and helps obtain a bright chromium deposit. Its volume fraction is 7~12 mL/L, with an optimum of 9~10 mL/L.
During daily maintenance, it should be added in small amounts but frequently. When a single addition reaches 0.5 mL/L, electrolysis treatment at 1 A/dm2 for more than 10 min is required; otherwise, the chromium surface may become hazy.
Wetting agent The volume fraction is 2~3 mL/L, reducing the surface tension of the bath to 29~34 dyne/cm (i.e., 29~34 mN/m), wetting the product surface, preventing pinholes, and promoting a uniform, bright coating. In production, an appropriate amount can be added each shift. When regularly removing organic impurities, activated carbon (0.15~0.35 g/L) can be continuously filtered and adsorbed in the filter to keep the bath clean, but it will adsorb some wetting agent, so 20%~40% of the wetting agent should be replenished in time.
During production, DSA anodes inevitably suffer natural damage to their coating under strong current to some extent, or artificial damage to the plate coating due to improper operation such as short circuit between the anode and cathode, which accelerates the generation of harmful hexavalent chromium ions and destabilizes the plating bath. Therefore, to avoid artificial damage to the anode during production, the anode plate can be wrapped with an acid- and alkali-resistant insoluble nylon mesh. This does not affect the conductivity of the plate and effectively prevents cathode products from contacting the DSA anode, preventing short-circuit arcing. In addition, the anode should be periodically removed to inspect the condition of the coating and replace damaged anodes with new plates in time.
Buffers are used to stabilize and maintain the pH of the plating bath within the process range. Boric acid is a good buffer and can be replenished based on periodic laboratory analysis results.
pH range: 3.0–3.7, optimum control at 3.3–3.4. Too low pH affects the covering power. When pH is high, burning occurs in high current density areas, that is, the coating at tips appears blue or yellow. pH must never exceed 3.8; otherwise, the bath can easily fail and cannot be adjusted.
Working temperature is usually 40–55°C, with optimum at 48–52°C. Within this temperature range, a higher current density can be used to improve the covering power of the bath and increase the first-pass yield.
Chromium plating time: for decorative chromium plating, the time is generally controlled at 2–5 min. Too short a time results in a thin coating with low hardness and poor scratch resistance; too long a time can cause a blue haze on the coating surface and an unclear appearance. Post-plating treatment: Although research data show that the special lattice structure of trivalent chromium coating gives it more advantages than hexavalent chromium coating, in actual production the corrosion resistance of trivalent chromium coating is far inferior to that of hexavalent chromium coating.
Therefore, after trivalent chromium plating, the products must undergo necessary sealing treatment to enhance corrosion resistance and service life. Common methods are passivation treatment or immersion in organic coating.
5. Effects of impurities on the bath and treatment measures Effects of impurities: Metal ions such as copper, nickel, and iron easily cause faults in trivalent chromium plating, such as poor throwing power, darkening in low current density areas, difficulty in thickening the coating, and discoloration after plating.
Organic impurities easily cause defects such as hazy coatings and a narrowed bright range. Hexavalent chromium deteriorates bath performance to the point where a chromium layer cannot be deposited.
Treatment measures: Although trivalent chromium plating solution is sensitive to impurities and has low tolerance, daily maintenance requires frequent use of activated carbon to adsorb organic impurities and low-current electrolysis to remove metal impurities, which is relatively tedious. However, as long as an optimized process flow is ensured and regular electrolytic impurity removal is performed, it is easy to maintain normal continuous operation of the trivalent chromium process.
Fortunately, electroplating researchers have never stopped in-depth research and exploration of trivalent chromium technology. A metal "impurity remover" can effectively remove nickel ions, copper ions, iron ions, and other impurities that affect bath performance. After adjustment and verification by Hull cell testing, add the impurity remover and stir thoroughly, then filter and electrolyze, and the bath can be restored to normal.
The best way to prevent bath contamination is to strictly follow the electroplating process flow, strictly operate according to work instructions, and keep containers and measuring cups dedicated to each process to prevent cross-contamination. Develop the good habit of fishing out products that fall into the plating tank every day, and enforce it through system rules. Only in this way can the bath be protected from human contamination; otherwise, treatment will be quite time-consuming and laborious.
Conclusion: The key to stable operation of trivalent chromium electroplating lies in designing a reasonable process flow to reduce human factors; strictly controlling the operation steps to prevent cross-contamination; and strictly complying with and controlling process conditions.
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