Trivalent Chromium Plating Process

For a long time Chromium plating typically uses hexavalent chromium electroplating solutions. In recent years, due to the environmental and other adverse impacts caused by hexavalent chromium, research on trivalent chromium plating has been intensified. In fact, studies proposing the substitution of hexavalent chromium with trivalent chromium have been ongoing for quite some time. Compared with hexavalent chromium plating, trivalent chromium plating for decorative chrome exhibits numerous superior characteristics; however, in practical applications, certain issues still remain, and its platability is somewhat limited. Consequently, functional chromium plating using trivalent chromium has not yet been widely adopted in industrial practice. This article also introduces the underlying mechanisms of trivalent chromium plating and discusses future prospects, while identifying several key research questions that warrant further investigation.

Hexavalent chromium is highly toxic and poses a serious threat to environmental pollution. Chromium anhydride is extensively used in chrome-plating solutions, making it the primary source of chromium-containing wastewater in the electroplating industry. This issue has drawn widespread public attention, and governments around the world have stepped up legislative measures to address it—for instance, in the United States, the emission standards for hexavalent chromium have been tightened from... The concentration of hexavalent chromium has been reduced from 0.05 mg/L to 0.01 mg/L and has been enforced since 1997. Another issue is the low current efficiency and poor coverage capability of hexavalent chromium plating solutions. To fundamentally reduce pollution and improve both current efficiency and coverage capability, trivalent chromium plating processes have increasingly gained popularity.

Trivalent chromium plating by itself Since BUNSEN published his first paper in 1854, more than a century has passed. However, progress has been relatively slow due to the persistence of certain technical challenges that have proven difficult to overcome. By the 1970s, with advances in science and technology, an increase in the availability of chemical raw materials, and growing public awareness of environmental protection, research on trivalent chromium plating once again rose to the top of electroplaters' agendas. In 1974, the United Kingdom introduced the ALECRA-3 trivalent chromium plating process and, in 1975, filed a patent for a trivalent chromium plating process using chromic chloride as the primary salt—known as ALECRA-3000. In 1981, the UK developed the ENVIR0-CHOME trivalent chromium plating process, which uses a sulfate-based electrolyte and is environmentally friendly. This process employs a selective ion-exchange membrane to separate the cathode and anode regions, thereby preventing the harmful effects of hexavalent chromium formed by oxidation at the anode from contaminating the trivalent chromium plating bath. Almost simultaneously, the U.S. company HARSHA0 also developed the TRI-CHROME trivalent chromium plating process.

Plating solution composition
The main components of the trivalent chromium plating solutions currently being researched and used are as follows:
(1) Main salt: Currently, the trivalent chromium electroplating systems mainly include chloride-based systems, sulfate-based systems, and mixed sulfate-chloride systems.
Chloride plating solutions have good conductivity, low voltage, and high leveling, coverage, and current efficiency. However, the anode produces toxic fumes. CL2, which causes relatively severe corrosion to equipment; during chromium sulfate electroplating, the anode produces non-toxic oxygen, resulting in zero pollution. However, the plating solution has lower conductivity than chloride solutions, and its throwing power, coverage capability, and current efficiency are also lower.
(2) Complexing agents: Typically hydroxycarboxylic acids and their salts, such as formate and acetate, aminoacetic acid, oxalic acid and its salts, citric acid and its salts, thiocyanate, and tartrate, among others.
(3) Conductive salts: Reduce energy consumption. These are mostly chlorides and sulfates of sodium, potassium, and ammonium.
(4) Buffer agents: Maintain the stability of the plating bath’s pH value. These typically include boric acid, acetates, aluminum salts, and citrates, among others.
(5) Stabilizer: A composite reducing agent that inhibits and reduces the formation of hexavalent chromium. Common components include methanol, sodium sulfite, and halides.

Process conditions
(1) Temperature: It is best to carry out the process at room temperature; the operating temperature is generally controlled within the range of 15–55°C. An increase in temperature will reduce concentration polarization and shift the deposition potential of Cr³⁺ toward more positive values, while also intensifying hydrogen evolution, which is detrimental to chromium deposition.
(2) The pH value should be between 1 and 4. At low pH values, hydrogen evolution is severe, leading to a decrease in cathodic current efficiency. At high pH values, hydroxy bridging reactions are more likely to occur, causing the coating to darken.
(3) The current density ranges from 3 to 100 A/dm². The cathode current density has a relatively wide range; however, at lower values, the chromium deposition rate will be affected, while excessively high cathode current densities can compromise the coating performance.
(4) The current efficiency is 10% to 25%. Gentle stirring is permissible.
In addition, trivalent chromium plating can be carried out either in a single-tank or a two-tank system. In the single-tank system, the anode material is a graphite rod, just like in other conventional electroplating processes. The double-tank method uses an internal anode tank and allows the use of a lead-tin alloy anode shield. Additionally, dilute sulfuric acid is used as the base electrolyte for the anode.

Advantages of trivalent chromium plating
Trivalent chromium electroplating has the following advantages:
(1) Low toxicity and minimal pollution. The plating bath rinse water contains no hexavalent chromium, and the wastewater can be discharged after only slight treatment. Moreover, the electroplating process does not generate toxic chromic acid mist.
(2) The plating solution has a low concentration—only one-tenth that of hexavalent chromium plating—yet exhibits excellent dispersing and covering capabilities, leading to an improved yield rate.
(3) The electroplating process is unaffected by current interruptions and does not require stripping.
(4) The cathodic current efficiency can reach 21–25%, which is higher than that of hexavalent chromium plating, thereby improving productivity.

Summary and Outlook
It has become an inevitable trend for environmentally friendly trivalent chromium plating to replace hexavalent chromium plating. In recent years, trivalent chromium plating has developed rapidly and has already been applied in decorative chrome plating as well as adopted for industrial production in many countries; however, it has yet to be industrialized for functional chrome plating.
Currently, our country has also achieved considerable progress in the research on the application of trivalent chromium plating. However, further efforts are needed to strengthen studies on both process performance and theoretical foundations. In particular, attention should be paid to the following aspects:
(1) Improve and enhance the appearance and thickness of coatings, and strengthen research on the application of functional coatings.
(2) Study stable trivalent chromium plating solutions.
(3) Currently, the cost of trivalent chromium plating solutions is higher than that of hexavalent chromium plating solutions, so it is necessary to reduce costs.

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