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Non-chromium passivation

Currently researched non-chromium passivation processes include inorganic passivation (including inorganic salts/oxides of Ti, V, Mo, W, Co, La, Ce, Zr, etc.), organic passivation, oxide passivation (such as silicate phosphating), and organic metal compound passivation, etc.

Passivation with molybdate or tungstate salts

Molybdenum, tungsten, and chromium belong to the same group on the periodic table, exhibiting similar properties. Over the years, salts formed by these elements have been used as corrosion inhibitors in solutions. Wilcox et al. [18] replaced chromate with molybdate/phosphate for passivation treatment, achieving good results and obtaining a patent. The Mo/P ratio in the passivation solution has a significant impact, with the best corrosion resistance observed at Mo/P = 0.6 (pH = 3-5, temperature of 60°C, time of 2 minutes), similar to chromate passivation film.
Formula 1: 10-20g/L sodium molybdate (Na2MoO4), 2-5g/L ethanolamine, pH = 2-5 (adjusted with phosphoric acid), temperature 50-70°C, 20-60 seconds.
Formula 2: 300g/L ammonium molybdate [(NH4)2MoO4], 600ml/L ammonia solution, temperature 30-40°C, time 20-60 seconds, resulting in a black passivation film.
Warwick et al. treated zinc layers in a tungstate salt solution with anodic or cathodic electrochemical processes, forming a conversion film protective layer, but the corrosion resistance was not ideal. However, using cyclic reversing current to obtain the film layer can improve corrosion resistance.
Recently, Sam Path analyzed manganese and chromium’s chemical properties and structures, indicating that their ionic radii and coordination numbers are very close. Thus, the theory proposes using permanganate to replace chromate for passivation treatment.

Silicate passivation

Silicate passivation offers good stability, low cost, easy use, non-toxicity, and no pollution. However, the corrosion resistance of the passivation film is slightly inferior.
Formula 1: 50g/L sodium silicate (Na2SiO3·9H2O), 80ml/L aminotrimethylphosphonic acid, 5g/L thiourea, pH = 2.0-3.5, temperature 20-40°C, time 40-80 seconds, followed by a 3-minute hot water sealing.
Formula 2: 16.2g/L silica dioxide (added in the form of Na2SiO3), 2.4g/L sulfuric acid (H2SO4), 11.7g/L hydrogen peroxide (30% H2O2), 2-7g/L additive (condensation product of hydroxyethyl indole and phosphoric acid), time 20 seconds.
Formula 3: 40g/L sodium silicate (40% Na2SiO3), 3g/L sulfuric acid (98% H2SO4), 40g/L hydrogen peroxide (38% H2O2), pH 2-2.5, resulting in a white passivation film.

Passivation with rare earth salts or oxides

Rare earth compounds such as cerium, lanthanum, and praseodymium can form passivation film layers with zinc and zinc alloys. These layers are oxide or hydroxide deposits, with the best passivation film obtained from cerium salts. Formula: 40g/L cerium chloride (CeCl3), pH 4, temperature 30°C, immersion for 1 minute. A light golden-yellow passivation film can be obtained, exhibiting good corrosion resistance.

Titanium salt passivation

Formula 1: 3g/L titanium oxysulfate [(TiO)SO4·2H2O], 60g/L hydrogen peroxide (38% H2O2), 5ml/L nitric acid, 15ml/L phosphoric acid, 3g/L citric acid, 0.5g/L hydroxyquinoline (stabilizer), pH 1-1.5, temperature at room temperature, time 10-20 seconds. The resulting colored passivation film can withstand a neutral salt spray test for 144 hours, comparable to chromate passivation film.
Formula 2: 2-5g/L titanium oxysulfate [95% (TiO)SO4·2H2O], 50-80g/L hydrogen peroxide (38% H2O2), 8-15ml/L nitric acid, 5-10g/L citric acid, pH 0.5-1.0, temperature at room temperature, immersion time 8-15 seconds, air stop 5-10 seconds. The above processes can produce white passivation films.

Passivation with organic compounds and organometallic compounds

1.Citric acid passivation

Citric acid is non-toxic and soluble in water. It has acidic properties and can dissolve the zinc layer in small amounts. Passivation process: 40g/L citric acid, 5ml/L nitric acid, 20g/L additive (zirconium, fluoride compounds), temperature at 60°C, time for 20 seconds. Initially, the plating is activated in potassium hydroxide solution (pH = 13.5-14, temperature 50°C) for 30-60 seconds, followed by passivation treatment in the above solution. Passivation with 3-5g/L citric acid, temperature at 60°C, time for 30 seconds. This method is simple and cost-effective, and the corrosion resistance of the passivation film is comparable to chromate passivation film, with good adhesion to the coating.

2.Phytic acid passivation

Phytic acid, also known as inositol hexaphosphate, with the molecular formula C6H18O24P6, is non-toxic, soluble in water, and has firm acidity. When phytic acid complexes with metal surfaces, it quickly forms a dense monomolecular protective film on the metal surface, effectively preventing the entry of O2 and slowing down metal corrosion. For example, the surface treatment formula for galvanized sheet: phytic acid (50% solution) 1.6%, silica gel 3.0%, polyvinyl alcohol 5.0%, deionized water 89.4%, (NH4)2TiF6 1.0%. Immersion for 10-20 seconds, dried at 120°C.

3.Epoxy resin passivation

Epoxy resin solution is a gel-like liquid of epoxy resin and inorganic salts. The passivation film obtained from this passivation solution on zinc layers can be compared to chromate passivation film. There are three forms of composition: (1) epoxy resin + sodium metavanadate + calcium borate; (2) epoxy resin + cesium acetate; (3) epoxy resin + sodium metavanadate + cesium acetate + cesium oxalate.

4.Acrylic acid passivation

5. BAT4 and its derivatives passivation

BAT4 and its derivatives are the most promising alternatives to chromate passivation for zinc plating. They can form an insoluble organic complex film on the zinc surface, where the molecules inside the film coordinate with the metal substrate, forming a shielding layer, making the film dense and enhancing its corrosion resistance. In recent years, progress has also been made in passivation treatment using organometallic compounds, but the process is complex, the cost is high, and progress is slow.