Galvanized coating, as a common anodic protective layer, is widely used in the production of fasteners.
However, it is prone to corrosion in air or humid environments. To enhance its corrosion resistance, chromate post-treatment is commonly employed.
This process is low-cost, simple, and has good passivation effects. However, hexavalent chromium is toxic, which limits its application. Trivalent chromium salt passivation has low toxicity and similar properties to hexavalent chromium salt, thus attracting extensive attention.
Nevertheless, due to the differences in the film-forming process, the protective performance of its passivation film still has certain differences from that of hexavalent chromium passivation film.
1. The composition of trivalent chromium passivator
Trivalent chromium passivator is composed of trivalent chromium salts, oxidants, complexing agents, stabilizers, film-forming accelerators, rare earth metals and sealing agents, etc.
1.1 Trivalent chromium salts
Trivalent chromium salts are the main agents for trivalent chromium passivation films. There are many types of trivalent chromium salts, mainly including the following:
a. Chromium nitrate, also known as chromium nitrate nonahydrate, is a purple-red monoclinic crystal and soluble in water.
b. Chromium sulfate comes in various products due to different water content, and their colors vary significantly. Some are soluble in water while others are not (and thus unusable). Anhydrous chromium sulfate is a purple or red powder and insoluble in water or acid. Chromium sulfate pentadecahydrate is a dark green flaky compound and soluble in water. Chromium sulfate octadecahydrate is a purple cubic crystal and soluble in water.
c. The color of chromium chloride also varies. Anhydrous chromium chloride is a rose-purple prismatic crystal and almost insoluble in water. Chromium chloride hexadecahydrate comes in three colors: dark green, emerald green and purple, and is soluble in water. In aqueous solution, it turns into a mixture of dark green and purple.
d. Chromium vanadate comes in three types: potassium chromium vanadate, ammonium chromium vanadate and sodium chromium vanadate. All of them contain 24 water molecules of crystallization and appear as green or purple cubic crystals, and are soluble in water.
1.2 Oxidizing Agent
Oxidizing agents dissolve the zinc coating to produce zinc ions, which can promote the formation of a passivation film. Oxidizing agents include hydrogen peroxide, nitrates, halogenates, persulfates, and tetravalent cerium, etc.
Any oxidizing agent that can oxidize trivalent chromium to hexavalent chromium is not environmentally friendly and thus cannot be used. Currently, hydrogen peroxide is no longer used in trivalent chromium passivators.
1.3 Complexing Agent
The role of complexing agents in solution is to control the film-forming rate and the stability of the passivation solution. If the complexing ability is too strong, the film-forming rate will be very slow, the film layer will be thin, and even no film can be formed; if the complexing ability is too weak, the stability of the passivation solution will be poor, and the film layer will be dull.
Complexing agents include fluorides, organic carboxylic acids and their mixtures, such as malonic acid, oxalic acid, glutaric acid, maleic acid and malic acid, etc. In fact, most complexing agents are also stabilizers. Selecting appropriate complexing agents is a key step to obtain high-quality passivation films and stable passivation solutions.
1.4 Stabilizer
Stabilizers can stabilize the valence state of trivalent chromium in the passivation solution (for instance, prevent the originally purple solution from turning green), and can also be used to stabilize the pH value of the passivation solution, keeping it within the range of pH 1.8 to 2.3. Some carboxylates with short carbon chains also have a good ability to stabilize the pH value.
1.5 Film-forming Promoter
Film-forming accelerators can adjust the color of the passivation film. By choosing different film-forming accelerators, films of different colors can be formed. Organic or inorganic anions such as NO3-, SO42-, PO43-, F-, CI-, SiO32-, SiF63-, BF4-, and RCCO- can be used.
1.6 Other Metals
Other metals mainly adjust the appearance and corrosion resistance of the passive film. Available metal elements include Ag, Cu, V, Ti, Y, Nb, Ta, Al, Ga, In, Mn, Sb, Ti, Fe, Mo, Co, Ni, Ce and lanthanide rare earth elements, etc.
1.7 Sealer
To overcome the problem of poor corrosion resistance of trivalent chromium passivation films, nano-sized particles are directly added to the passivation solution as sealing agents to fill the micro-pores in the passivation film layer, thereby significantly enhancing the corrosion resistance of the passivation film. Hydrolyzed organosilicon can be used as a sealing agent.
Although trivalent chromium passivators are composed of the above seven major components, not all passivators contain these ingredients. They can be increased or decreased, but trivalent chromium salts are indispensable, as they are the main agent of trivalent chromium passivation films.
2. Quality Issues and Causes of Trivalent Chromium Passivation Film
2.1 Quality Issues of Trivalent Chromium Passivation Film
Classification of types of substandard trivalent chromium passivation film quality:
① The galvanized layer is too thin, not meeting the requirement of ≥0.06μm;
② The corrosion resistance area of the zinc coating does not meet the standard of ≤5% white precipitate in salt spray test;
③ The color of the zinc coating after white passivation is too light yellow and there is a color film;
④ The surface color of the zinc coating after color zinc passivation is too light and the color zinc is blurred;
⑤ The surface color of the zinc coating after black passivation turns brown and is not lustrous;
⑥ There are defects such as spots and stains on the surface sealing layer of the zinc coating after passivation;
⑦ Hexavalent chromium still remains on the surface of the zinc coating after passivation;
⑧ The threads of the zinc-coated products are bumped or scratched.
2.2 Causes of Quality Issues in Trivalent Chromium Passivation Film
Due to the influence of various factors, there are many reasons for the quality problems of trivalent chromium passivation films, mainly including the following types.
① Lack of understanding of the performance of the passivator and failure to conduct necessary process verification tests before production;
② Insufficient passivation time;
③ The passivation solution was not prepared with distilled water or deionized water;
④ The volume of the passivation solution was small, prone to aging, with high iron and zinc ion content, and was not adjusted or partially replaced in a timely manner;
⑤ The pH value of the passivation solution was too high or too low, and was not detected and adjusted in time;
⑥ The temperature of the passivation solution was not strictly controlled (too low or too high), deviating from the process specifications;
⑦ The trivalent chromium ion content in the passivation solution was low, and the replenishing agent was not added in time;
⑧ The content of the brightener solution was low or it had aged, or it was contaminated by iron ions, and nitric acid was not added or the solution was not replaced in time during operation;
⑨ The passivation solution and the workpieces were not stirred and turned over sufficiently during passivation;
⑩ Too much material was loaded at one time during passivation, the turning was uneven, a large amount of passivation solution was carried out, and the composition of the passivation solution changed greatly.
3. Countermeasures for Quality Issues of Trivalent Chromium Passivation Film
① The appearance of the coating should have fine and dense crystallization and good luster. Trivalent chromium is low-chromium acid passivation, and the passivation solution has no chemical polishing effect. Therefore, the quality of the galvanized layer must be good and uniform. Brightening must be carried out before passivation.
② The temperatures of trivalent chromium passivation solutions provided by different enterprises are not exactly the same. Generally, the passivation temperature should be controlled at 15 to 35℃.
If the temperature is too high, the passivation film will be thick but loose, with poor adhesion; if the temperature is too low, the film formation will be slow, and the color will be pale and the film will be thin. If it is on an automatic production line, the temperature is best controlled at around 25℃ to ensure that the same color and passivation film quality are obtained within the same period of time.
③ During passivation, the galvanized parts should move relatively to the passivation solution. This is conducive to the convection and diffusion of the solution, ensuring a uniform and consistent film layer and preventing the parts from sticking together. Intense stirring with compressed air must be adopted; otherwise, it is difficult to guarantee the quality of the passivation film.
④ The passivation time is determined by the chromic acid content in the passivation solution, the concentration of the activator, the pH value and the temperature of the workpiece. It is particularly important on an automatic production line and should be maintained within the required process range.
When other conditions are the same, the passivation time should be appropriately shortened in summer when the temperature is higher and appropriately extended in winter.
⑤ After passivation is completed, thorough cleaning is necessary. If there is any residual chromic acid, the galvanized layer will be corroded quickly, resulting in the defect of “white spots”.
⑥ After drying, the passivation treatment is preferably followed by baking and aging to enhance the corrosion resistance and adhesion of the film. However, the aging temperature should not be too high.
For trivalent chromium, the suitable temperature is 80 to 100 degrees Celsius, and for hexavalent chromium, it is 60 to 80 degrees Celsius. If the temperature is too high, the passivation film may crack due to dehydration, and the corrosion resistance will significantly decrease. The aging time should also not be too long, and should not exceed 15 minutes; otherwise, the corrosion resistance will decline.
⑦ The dedicated lines for trivalent chromium passivation and water washing tanks should be isolated from the flowing water tanks and hot water tanks to prevent hexavalent chromium solution from being carried in.
⑧ Before passivation, check the color of the passivation solution and the water washing tank. If there is any reaction, take measures.
⑨ Formulate a classified management plan to prevent the aging of the passivation solution and control the content of iron and zinc ions.
⑩ In the trivalent chromium passivation of commodity-grade products, a sealer is directly added to address the drawback of trivalent chromium’s lack of self-healing ability.
4. Conclusion
Passivation process is one of the most crucial steps in the electro-galvanizing procedure. The use of high-quality trivalent chromium passivator is a prerequisite for ensuring quality. Of course, it is also closely related to the quality of pre- and post-treatment processes during galvanizing.
Some details must be paid attention to, such as when adding nitric acid, it should be diluted and slowly added to the passivation tank under stirring. Reasonable QC control sheets and operation instructions should be compiled, and operators should be trained in trivalent chromium passivation process.
A dedicated cleaning line should be set up for cleaning after trivalent chromium passivation, and the cleaning water should be pollution-free clean tap water to prevent splashing into chromic acid solution. It is also stipulated that the cleaning water must be replaced in time every shift.
The trivalent chromium passivation film is relatively thin compared to the chromate passivation film, but it is thicker on zinc alloys. The film has no self-repairing ability, and the corrosion resistance of the passivation film on the zinc layer is not as good as that of the chromate passivation film, but the passivation film on zinc alloys is often better than that of the chromate passivation film.
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