Chromate passivation of steel is a surface treatment process that immerses the steel workpiece in a chromate solution and uses a chemical reaction to form a passivation film with chromate as the main component on its surface, thereby improving the corrosion resistance of the steel. This passivation film is usually 0.5-2μm thick and has an amorphous or crystalline structure. It can effectively isolate the steel substrate from contact with air, water and other corrosive media, significantly improving its corrosion resistance, especially in atmospheric environments. The chromate passivation process is easy to operate and has low cost. It has been widely used in the anti-corrosion treatment of steel products, such as post-treatment of galvanized steel plates and short-term protection of mechanical parts.
The principle of chromate passivation of steel is based on the multivalent chemical properties of chromium. The chromate solution contains hexavalent chromium ( CrO₄²⁻ or Cr₂O₇²⁻ ). Under acidic conditions, the hexavalent chromium is reduced to trivalent chromium by the iron on the steel surface, while the iron is simultaneously oxidized to divalent or trivalent ferrous ions. These ions interact on the steel surface, forming a passivation film primarily composed of trivalent chromium and iron oxides, hydroxides, and chromates. The formation of the passivation film can be divided into three stages: dissolution, redox, and film deposition. First, the steel surface undergoes slight dissolution in the acidic chromate solution, producing ferrous ions. Next, the ferrous ions reduce the hexavalent chromium to trivalent chromium. Finally, the trivalent chromium and ferrous ions (or ferric ions) combine with chromate ions in the solution and deposit on the steel surface to form the passivation film.
The chromate passivation process parameters of steel have a significant impact on the quality of the passivation film, mainly including the concentration of chromate solution, pH value, treatment temperature and treatment time. The concentration of chromate solution is usually 2%-10%. If the concentration is too low, it is difficult to form a complete passivation film, while if the concentration is too high, it will increase costs and environmental pollution. The pH value is generally controlled between 1-3. Acidic conditions are conducive to promoting dissolution and redox reactions on the steel surface, but too low a pH value will cause the film layer to dissolve, while too high a pH value will slow the reaction rate. The processing temperature is mostly between room temperature and 60°C. Increasing the temperature can accelerate the reaction speed and shorten the processing time, but exceeding 60°C may cause the film layer to become loose. The processing time is usually 10-60 seconds, which is adjusted according to the thickness requirements of the film layer. If the time is too short, the film layer will be incomplete, and if it is too long, the film layer will easily fall off.
The chromate passivation process for steel is relatively simple, primarily consisting of pretreatment, passivation, and post-treatment. Pretreatment involves removing oil, rust, and scale from the steel surface. Degreasing and pickling are typically employed to ensure a clean surface and create conditions for the uniform formation of the passivation film. Passivation involves immersing the pretreated workpiece in a chromate solution and treating it according to pre-set process parameters. Stirring the solution during this process can improve film uniformity. Post-treatment includes rinsing and drying. The rinsing step removes residual chromate solution from the surface to prevent subsequent corrosion. Drying is performed at room or low temperatures to prevent cracking of the film. For applications with higher requirements, sealing treatments (such as oil immersion) can be performed to further enhance corrosion resistance.
Although chromate passivation of steel can significantly improve corrosion resistance, due to the strong toxicity and carcinogenicity of hexavalent chromium, it poses great harm to human health and the environment. This process is gradually being replaced by environmentally friendly passivation processes, such as chromium-free passivation (using compounds such as zirconium, titanium, and silicon) and trivalent chromium passivation. Although the chromium-free passivation process is slightly inferior to traditional chromate passivation in corrosion resistance, it has excellent environmental performance and has been widely used in the automotive, home appliance and other industries. In the future, as environmental protection requirements continue to increase, steel passivation treatment technology will continue to develop towards chromium-free and low-toxicity directions. At the same time, the performance of the passivation film will be continuously improved through process optimization to meet industrial corrosion protection needs.
