For a long time, the hexavalent chromium plating solution is usually used for electroplating chromium. In recent years, due to the pollution of hexavalent chromium to the environment, the research on trivalent chromium electroplating has been stepped up. In fact, researches proposing the replacement of hexavalent chromium with trivalent chromium have been around for a long time. Compared with hexavalent chromium plating, decorative chromium plating with trivalent chromium has many excellent characteristics, but there are also some problems in practical application, and its platability is limited to a certain extent. Therefore, electroplating functional chromium with trivalent chromium has not been widely used in practice. The mechanism and prospect of trivalent chromium electroplating are also introduced, and the problems to be further studied are proposed.
Hexavalent chromium is highly toxic and causes serious environmental pollution. Chromium anhydride is widely used in chrome plating solution, which is the main pollution source of chrome-containing wastewater in electroplating industry. This issue has attracted widespread attention, and governments around the world have also strengthened legislative management. For example, the US emission standard for hexavalent chromium has been reduced from 0.05MG/L to 0.01MG/L, and has been implemented since 1997. Low current efficiency and poor coverage of hexavalent chromium baths are also a problem. In order to fundamentally reduce pollution and improve current efficiency and coverage, trivalent chromium plating is more and more popular.
Trivalent chromium plating has a history of more than 100 years since the first paper was published by BUNSEN in 1854. Because some technical problems are difficult to break through, the progress is relatively slow. In the 1970s, with the development of science and technology, the increase of chemical raw materials, and the further enhancement of people’s awareness of environmental protection, the research on trivalent chromium plating was mentioned on the agenda of electroplating workers. In 1974, the United Kingdom published the ALECRA-3 trivalent chromium plating process, and in 1975 applied for a trivalent chromium plating patent using chromium trichloride as the main salt, namely ALECRA-3000. In 1981, the United Kingdom developed sulfuric acid The trivalent chromium plating process of salt-based environment-friendly chromium (ENVIR0-CHOME). The process uses a selective ion separator to separate the cathode area and the anode area, which can avoid the harm caused by the oxidation of hexavalent chromium on the anode plate to the trivalent chromium plating solution: almost at the same time, the American HARSHA0 company also developed TRI-CHROME Trivalent chrome plating process.
Bath composition
The main components of the trivalent chromium plating solution currently researched and used are as follows:
(1) Main salt: The current trivalent chromium plating system mainly includes chloride system, sulfate system and sulfate-chloride mixed system.
The chloride plating solution has good conductivity, low voltage, and high dispersion ability, coverage ability and current efficiency of the plating solution, but the anode will precipitate toxic CL2, and it will corrode the equipment seriously; Pollution, but the conductivity of the bath is not as good as that of chloride, and the dispersion ability, coverage ability and current efficiency of the bath are lower.
(2) Complexing agent: usually hydroxycarboxylic acid and its salt, such as formic acid and acetate, glycine, oxalic acid and its salt, citric acid and its salt, thiocyanate and tartrate, etc.
(3) Conductive salt: reduce the consumption of electric energy. Mostly sodium, potassium, ammonium chloride and sulfate.
(4) Buffer: keep the pH value of the plating solution stable. Most of them are boric acid, acetate, aluminum salt and citrate.
(5) Stabilizer: composite reducing agent, inhibits and reduces the production of hexavalent chromium. Mostly methanol, sodium sulfite, halides, etc.
Process conditions
(1) Temperature: It is best to carry out at room temperature, and the working temperature is generally controlled at 15 to 55 °C. The increase of temperature will reduce the concentration polarization and make the precipitation point of CR3+ move positively, meanwhile, the hydrogen evolution is serious, which is not conducive to the deposition of chromium.
(2) PH value is 1～4. When the pH value is low, the hydrogen evolution is serious, and the cathode current efficiency is reduced. When the pH value is high, the hydroxyl bridge reaction is easy to occur, and the coating becomes dark.
(3) The current density is 3～100A/DM2. The cathode current density range is wide, but the lower value will affect the chromium deposition rate, and the high cathode current density will affect the coating performance.
(4) The current efficiency is 10% to 25%. Light stirring is possible.
In addition, trivalent chromium chrome plating has a single tank method and a double tank method. The anode material in the single tank method is graphite rod, which is the same as other ordinary electroplating; the double tank method uses an anode inner tank, and a lead-tin alloy anode cover can be used. Dilute sulfuric acid was used as the anode base liquid.
Advantages of trivalent chrome plating
Trivalent chromium plating has the following advantages:
(1) Low toxicity and low pollution. The cleaning water of the plating solution does not contain hexavalent chromium, and the waste water can be discharged after a little treatment, and the electroplating process does not produce toxic chromic acid mist;
(2) The concentration of the plating solution is low, only 1/10 of that of hexavalent chromium plating, the dispersion ability and coverage ability are good, and the yield is improved;
(3) The electroplating process is not affected by current interruption and does not need to be stripped;
(4) The cathode current efficiency can reach 21-25%, which is higher than that of hexavalent chromium electroplating, which improves the productivity.
Summary and Outlook
It is an inevitable trend to replace hexavalent chromium plating with environmentally friendly trivalent chromium plating. In recent years, trivalent chromium electroplating has developed rapidly and has been applied in decorative chrome plating and used in industrial production in many countries, but functional chrome plating has not yet been industrialized.
At present, our country has also made a lot of achievements in the application of trivalent chromium plating, but we must strengthen the research on process performance and theoretical basis. The following aspects should be paid attention to:
(1) The appearance and thickness of the coating are improved, and the application research of functional coatings is strengthened.
(2) Research on stable trivalent chromium plating solution.
(3) At present, the cost of trivalent chromium electroplating solution is higher than that of hexavalent chromium electroplating solution, and it is necessary to reduce the cost.