Mechanism and Composition of Trivalent Chromium Passivation in Electric Heating Tube Electroplating

Mechanism and Composition of Trivalent Chromium Passivation in Electric Heating Tube Electroplating

The latest third-generation trivalent chromium passivation technology

The sealing agent is directly added to the second generation passivation solution, overcoming the drawback of trivalent chromium passivation without self-healing ability, greatly improving the corrosion resistance of the film layer. The corrosion resistance of the film layer has reached or exceeded the hexavalent chromium passivation process, so it meets the environmental protection and high corrosion resistance requirements of automotive component electroplating. Dipsol Company and EKEM Company have adopted this process. Typical products include Hongzheng's 251 and 252 trivalent chromium colorful passivation, 261 trivalent chromium blue white passivation, and 271 trivalent chromium black passivation. They have been successfully applied in production.

3. Mechanism and composition of trivalent chromium passivation

The traditional passivation film of hexavalent chromium is formed through the dissolution of zinc, the reduction of chromate and the precipitation of trivalent chromium gel. The film contains hexavalent chromium. Therefore, the passivation film has the ability of self repair, also known as self-healing ability. The trivalent chromium film layer forms zinc ions through the dissolution of zinc, and the dissolution of zinc ions causes an increase in the pH value of the zinc surface solution. trivalent chromium directly reacts with zinc ions, hydroxide ions, etc., forming insoluble compounds that precipitate on the zinc surface, forming a passivation film. The specific reaction formula is as follows:

1. Zinc dissolution process: Zn+Ox (oxidant) → Zn2++Ox - ------- (reaction formula 1)

Zn+2H+→ Zn2++H2 ↑ -------- (reaction formula 1a)

2. Film forming process: Zn2++xCr (III)+yH2O → ZnCrxOy+2YH+------- (reaction formula 2)

3. Dissolving process: ZnCrxOy+2yH+→ Zn2++xCr (III)+yH2O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Trivalent chromium passivating agents generally contain the following components:

Trivalent chromium Cr (III): The main component source of the passivation film, and trivalent chromium can be obtained from chromium sulfate, chromium nitrate, chromium chloride, chromium acetate, etc.

Oxidizing agent: Produces zinc ions, promoting membrane formation. Oxidizing agents can be hydrogen peroxide, nitrate, halide, persulfate, cerium tetravalent, etc. During the passivation process using oxidizing agents, due to the automatic increase in pH, trivalent chromium will be oxidized to hexavalent chromium, which will be mixed in the coating, resulting in the coating containing hexavalent chromium. Therefore, the film formed by oxidizing passivating agents has a darker color, but it does not meet environmental requirements.

Other metals: Mainly adjust the appearance color and corrosion resistance, using Mn, Sb, Mo, Ti, Fe, Co, Ni, Ce, and other lanthanide rare earth elements.

Complexing agent: controls the speed of film formation and the stability of the passivation solution. Too strong complexation, slow film formation speed, thin film layer, and even unable to form a film layer; The complexation is too weak, the stability of the passivation solution is poor, and the film layer is dull. Complexing agents include fluorides, organic carboxylic acids, and their mixtures.

Film forming promoter: Adjust the color of the film layer. Choosing different film forming accelerators can create different colors. Available organic and inorganic anions, such as NO3-, SO42-, - PO43-, F -, Cl -, SiO32-, SiF62-, BF4-, RCOOH.

Sealing agent: In order to overcome the corrosion resistance and other challenges of the second generation passivation agent process and meet the environmental protection and high corrosion resistance requirements of automotive component electroplating, EKEM (Hongzheng) company has made breakthroughs in technology through years of research. Its research shows that due to the certain stripes in the film layer of trivalent chromium passivation technology and the lack of self-healing ability, its corrosion resistance is affected. Sealing agent is directly added to the passivation solution, The sealing agent uses particles with a diameter of up to nanoscale to fill the micropores of the passivation layer, making the film more compact, thus greatly improving the corrosion resistance of the film layer.

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