Many factors affect the performance of trivalent chromium passivation films, mainly including passivation solution composition, zinc coating properties (thickness, crystal morphology), galvanizing process, impurities, passivation process, process parameters, and protection and drying of the film after passivation. Among them, passivation solution composition is the main influencing factor. Trivalent chromium passivation solution components include trivalent chromium salt, oxidant, film-forming agent, complexing agent, other metal salts, and sealing agent. Trivalent chromium salt is the key component for film formation, serving as a skeleton to form a network structure in the passivation solution. As can be seen from the film formation process, oxidants and film-forming agents activate the zinc layer and promote the formation of a dense and stable passivation film, while complexing agents, other metal salts, and sealing agents improve the passivation film structure. The influencing factors of trivalent chromium passivation on zinc coatings are discussed below in terms of oxidant, complexing agent, film-forming agent, other metal salts, and sealing agent.
An oxidant is an essential substance in trivalent chromium passivation solutions. Common oxidants include hydrogen peroxide, permanganates, nitrates, halates, persulfates, and cerium(IV) salts. The oxidant used for trivalent chromium passivation must be able to oxidize the zinc layer without oxidizing trivalent chromium in the passivation solution to hexavalent chromium. Hydrogen peroxide, whose reduction product is water, is safe and non-toxic, and is one of the most commonly used oxidants. However, it slowly decomposes during storage and is strongly oxidizing, easily converting trivalent chromium to hexavalent chromium. Similarly, permanganates, halates, persulfates, and cerium(IV) salts are also strongly oxidizing; if used in trivalent chromium passivation, they may oxidize trivalent chromium to hexavalent chromium. Therefore, these oxidants should be avoided as far as possible in trivalent chromium passivation. Nitric acid or nitrates are mostly selected as oxidants for trivalent chromium passivation. Nitric acid has a chemical polishing effect in hexavalent chromium passivation, and it can also improve the brightness of the passivation film in trivalent chromium passivation; however, an excessive content makes the film yellow and iridescent. Nitrates are also strongly oxidizing. If the nitrate concentration is not strictly controlled, NO₃⁻ can be reduced to NH₄⁺ (the standard electrode potential of this reaction is 1.798 V), which also produces hexavalent chromium in the passivation film.
Complexing agents for trivalent chromium passivation mainly include fluorides, organic carboxylic acids (such as oxalic acid, malonic acid, citric acid, tartaric acid, succinic acid, butenedioic acid, malic acid, etc.) and their salts. The inorganic complexing agents used in early days (fluorides, etc.) form relatively stable complexes with trivalent chromium, which affect the deposition of zinc-chromium oxides and result in thinner passivation films with poor corrosion resistance; they have therefore been gradually phased out. At present, organic complexing agents are more commonly used in trivalent chromium passivation. Organic carboxylic acids and their salts not only participate in the formation of mixed complexes of trivalent chromium, which is favorable for trivalent chromium ions to form a uniform and dense passivation film on the zinc coating surface, but also maintain the pH of the passivation solution within a certain range over a long period, making the solution stable, easy to maintain, and longer in service life. Chen Xiaoping et al. used potassium sodium tartrate, tannic acid, methanol, and ethanol as reducing agents to reduce Cr(VI), and obtained four Cr(III) passivation solutions containing different ligands (not specified in the reference) at 25 g/L, with 5 g/L nitrate as the oxidant, and passivated zinc coatings for 20 s. The results showed that the passivation films prepared from the systems reduced by potassium sodium tartrate and tannic acid had better corrosion resistance and appeared bright and uniform blue-white, but the brightness and uniformity of the latter were slightly inferior to those of the former; the passivation films obtained from the other two reducing-agent systems had very poor appearance.
Film-forming agents: The main film-forming agents for trivalent chromium passivation are anions such as Cl⁻, SO₄²⁻, F⁻, and PO₄³⁻. These anions promote film formation by accelerating micro-anodic reactions and activating the solid-liquid interface, but their roles differ. Cl⁻: The presence of Cl⁻ in a trivalent chromium passivation solution is conducive to forming a reactive passivation film of a certain thickness and shortening the passivation time. Most current trivalent chromium passivation formulations contain Cl⁻. For example, the formulations proposed by Zhang Wenjing and P. Preikschat et al. both use chromium chloride as the main salt (with Cr³⁺ mass concentrations of 42 g/L and 50 g/L, respectively). In practice, however, the risk of Cl⁻ corroding equipment should also be considered. SO₄²⁻: Sulfate is a necessary component for trivalent chromium colored passivation; it can improve the adhesion between the passivation film and the substrate and often acts synergistically with Cl⁻ in film formation. For instance, the formulation of B.D. Fonte et al. is: chromium salt 25 g/L, NaCl 20 g/L, Fe₂(SO₄)₃ 40 g/L, nitric acid 6% (volume fraction). Fan Yongzhe et al. directly used chromium sulfate as the main salt and added 10 g/L sodium chloride to the passivation solution; under optimal process conditions, the film obtained by passivating the zinc coating showed white rust in neutral salt spray testing only after 84 h. PO₄³⁻ and H₂PO₂⁻: In addition to having effects similar to SO₄²⁻, PO₄³⁻ and HPO₂²⁻ can also serve as passivation film components and are commonly found in black passivation solutions. Research by Li Lizhi and K. Aramaki showed that when the pH of the passivation solution rises to the precipitation point of phosphate (pH 4–5), Zn²⁺, Cr³⁺, and PO₄³⁻ in the solution react on the zinc coating surface to form insoluble zinc phosphate and chromium phosphate, which together with the trivalent chromium skeleton constitute the passivation film. Liu Weizhen et al. passivated galvanized steel sheets with a passivation solution containing 10–13 g/L sodium dihydrogen phosphate (other components: chromium chloride 20–23 g/L, composite organic carboxylic acids 15–18 g/L, sodium nitrate 5–8 g/L, organic sulfur compound 1–3 g/L, ferrous sulfate 4–7 g/L) and obtained a jet-black, glossy passivation film. F⁻: F⁻ can coordinate with trivalent chromium to form relatively stable trivalent chromium fluoro-aqua complexes and slowly release trivalent chromium; it can also inhibit pitting corrosion of iron. However, films obtained from fluorine-containing passivation solutions are relatively thin and have poor corrosion resistance, requiring higher trivalent chromium salt concentrations and higher operating temperatures, and they are less environmentally friendly. Subsequent sealing treatment is generally required to ensure corrosion resistance. Therefore, F⁻ is not as widely used as the previous three film-forming agents.
Other metal salts. Metal salts are mostly added to passivation solutions as additives to improve the corrosion resistance and appearance of the passivation film. They are mainly classified into transition metal salts (such as salts of Mn, Sb, Mo, Ti, Fe, Co, and Ni) and rare earth metal salts. At present, there are not many studies on the mechanism of action of metal ions in trivalent chromium passivation, but the presence of metal ions in the passivation solution does have a positive effect on improving the corrosion resistance of the passivation film. Therefore, finding suitable metal salts or other additives is also an important research direction. Transition metal salts are essential blackening agents for black passivation. Among them, Fe2+ has little effect on the corrosion resistance of the passivation film, but has a relatively large influence on the stability of the passivation solution, and its content should be controlled within 20 mg/L. In addition, Fe2+ is easily oxidized to Fe3+; when the Fe3+ content in the passivation solution exceeds 10 mg/L, the color of the passivation film begins to become uneven. Cobalt salts, as alternative or supplementary blackening agents, can undergo displacement reactions with zinc to form a Zn-Co alloy layer on the coating surface, thereby adjusting and stabilizing the color of the passivation film and improving its corrosion resistance. Research by M. Hara et al. showed that the addition of Co2+ can strengthen the adhesion between the passivation film and the zinc coating. However, Shi Ludan et al., while studying the trivalent chromium blue-white passivation process, found that when the cobalt salt content in a passivation solution of 48 g/L CrCl3·6H2O + 20 g/L NaNO3 + 18 g/L Na3C6H5O7 exceeded 28 g/L, the corrosion resistance of the passivation film decreased and the appearance also deteriorated. Rare earth elements have large atomic radii and active chemical properties; they can refine the grains of the film layer and improve its density, and are increasingly used in trivalent chromium passivation solutions. Huang Qi et al. added 4 g/L Nd(NO3)3·6H2O to a passivation solution of 18.9 g/L Cr2(SO4)3 + 2.2 g/L NaNO3 + 0.7 g/L C6H8O7·H2O + 7 g/L CoSO4·7H2O + 3.4 g/L NaH2PO2·H2O + 0.2 g/L NH4HF2 and found that the presence of Nd3+ in the passivation solution extended the time to white rust in the neutral salt spray test from 60 h to 93 h. Wang Nan et al. added 1 g/L cerium sulfate to a trivalent chromium passivation solution composed of 18–30 g/L chromium chloride, 15 g/L citric acid, 5–15 g/L malonic acid, 1–10 g/L ferrous sulfate, 5–15 g/L thiourea, 5–10 g/L sodium nitrate, and 10–20 g/L sodium dihydrogen phosphate. The results showed that the introduction of cerium sulfate increased the surface roughness and actual surface area of the passivation film, thereby increasing the polarization resistance of the passivation film in 5% NaCl solution, reducing the corrosion current density, and improving corrosion resistance. Liu Yan et al., under the basic process conditions [15–25 g/L Cr2(SO4)3, 20–25 g/L NaNO3, 2.0–5.0 g/L ZnCl2, pH 2.0, room temperature, immersion for 120 s, air pause for 20 s], added 0–16 g/L La2O3, 0–16 g/L Ce(NO3)3·6H2O, and 0–10 g/L Ce(SO4)2·4H2O, respectively, and compared the effects of the three rare earth ions La3+, Ce3+, and Ce4+ on the corrosion resistance of the passivation film. They found that the addition of Ce4+ to the passivation solution could significantly improve the corrosion resistance of the passivation film even without sealing treatment.
Sealing agents: After trivalent chromium passivation, a sealing treatment is usually required, or a sealing agent (also called a pore-sealing agent) is added directly to the passivation solution, so that the corrosion resistance of the passive film can reach or even exceed that of hexavalent chromium passivation. Many publications have summarized the sealing processes for passive films; therefore, this paper mainly introduces the process research on the one-step passivation–sealing method. Sealing agents for trivalent chromium passivation mainly include three categories: inorganic silicon, organic silicon, and water-soluble resins.
1 Inorganic silicon sealing agents
The earliest materials used as sealing agents were inorganic silicon substances, including silicates, nano-SiO2, and fluorosilicon compounds. Guo Xiaofei et al. added Na2SiO3 to a passivation solution containing 60 g/L CrCl3·6H2O, 40 g/L NaNO3, and 15 g/L C6H5O7Na3·2H2O, and found that when its content was 2.0 g/L, the corrosion resistance of the passive film was equivalent to that of a hexavalent chromium passivation film. Chen Yong et al. found that adding 3.5 g/L nano-SiO2 to a passivation solution of 0.1 mol/L CrCl3 + 0.15 mol/L NaNO3 + 0.10 mol/L malonic acid + 1.50 g/L Co(NO3)2 + 1.00 g/L fatty alcohol polyoxyethylene ether sodium sulfate (AES) gave a bright passive film that still showed no rust spots after 120 h of neutral salt spray testing. Wang Chao et al. added nano-fluorosilicon compounds with a particle size of 10–20 nm to a trivalent chromium black passivation solution, and the results showed that the corrosion resistance and heat resistance of the passive film were best when the dosage was 2.0 g/L. Their base formulation was: 0.1 mol/L Cr3+ (composed of chromium trichloride and chromium nitrate at a molar ratio of (3–5):1), organic acid ligands (a combination of 2–3 substances selected from citric acid, tartaric acid, lactic acid, malonic acid, succinic acid, gluconic acid, maleic acid, and glycolic acid) at a molar ratio to Cr of 1.6, cobalt sulfate 5.0 g/L, nickel sulfate 3.0 g/L, copper sulfate 0.5 g/L, potassium dihydrogen phosphate 10.0 g/L, acetic acid 5.0 g/L, and AES 1.0 g/L.
However, there are many shortcomings in using inorganic silicon substances as sealing agents. Oversized particles, excessive content, or charge neutralization by metal cations can cause inorganic silicon to agglomerate and lose effectiveness. Although halide ions and coordinating groups have a certain anti-agglomeration effect, this defect cannot be fundamentally eliminated, making this type of sealing agent difficult to promote.
2 Organic silicon sealing agents
Currently, the most studied are silane coupling agents. Hydrolysis or alcoholysis of a silane coupling agent generates Si–OH, which can combine with active hydroxyl groups on inorganic materials; through dehydration condensation, a three-dimensional network structure similar to Si–O–Si can be formed on the metal surface, providing a good pore-sealing effect. These substances can be used not only for sealing treatment after passivation, but also as sealing agents to improve the performance of passive films.
Zou Jinguang et al. added γ-aminopropyltriethoxysilane to a trivalent chromium passivation solution. The resulting passive film was uniform blue and could pass the 72 h neutral salt spray test. The specific formulation and process conditions were: CrCl3 0.090 mol/L, NaNO3 0.200 mol/L, Co(NO3)2 0.010 mol/L, γ-aminopropyltriethoxysilane 14 g/L, organic carboxylic acid CX 0.090 mol/L, pH 1.4, temperature 29 °C, time 30 s.
3 Water-soluble resin sealing agents
Resins are often used as organic paint layers for sealing treatment after passivation. Recent studies have found that adding water-soluble resin materials to trivalent chromium passivation solutions can also improve passive film performance. Fan Yongzhe et al. compared the sealing effects of waterborne acrylic resin, sodium silicate, and silane coupling agent, and found that acrylic resin had the best sealing effect, with a neutral salt spray resistance time of 120 h. The optimal formulation was: chromium chloride 10 g/L, sodium nitrate 20 g/L, sodium fluoride 10 g/L, nickel sulfate 40 g/L, acrylic-based emulsion 30 g/L, pH 2.7, temperature 50 °C, time 60 s. Ren Yanping et al. also found that, in a passivation solution containing 7–11 g/L Cr2(SO4)3 and appropriate amounts of sodium hypophosphite, nitric acid, sulfuric acid, and boric acid (pH 6.5–7.0), the corrosion resistance of the passive film was optimal when acrylic resin was added at a volume ratio to water of 1:3.
Compared with adding an additional subsequent sealing step, this method simplifies the operation and improves work efficiency, but problems with the adhesion between the organic paint layer and the passive film, as well as its effect on appearance, still exist.
Conclusion
As the zinc passivation process with the greatest market potential at present, trivalent chromium passivation still has more or less problems: its corrosion resistance is not as good as that of hexavalent chromium passivation, the tones of colored and blue-white passivation are relatively light, and various performance-enhancing additives often have to be added, which increases production costs. By reviewing the effects of passivation solution components on trivalent chromium passive films, the following suggestions are proposed for the future development of trivalent chromium passivation: (1) Develop new complexing agents, or try the combined use of multiple complexing agents, to improve the film formation process. (2) Find one or more metal salt additives with lower cost and better effect, so that the passive film can meet application requirements without sealing, thereby omitting the subsequent sealing treatment. (3) On the basis of optimized processes, continue to explore new two-in-one passivation–sealing solutions, with special attention to silane coupling agents.