Tricationic phosphating

Tricationic phosphating is a chemical conversion treatment of metal surfaces that has been developed to meet the growing needs for corrosion resistance and surface preparation for subsequent coatings. This process uses a combination of zinc, nickel and manganese phosphates to create a protective, durable coating on metals.

The phosphating process has its origins in the early twentieth century, when the first chemical treatments were developed to improve the corrosion resistance of steel. Zinc phosphating was one of the first techniques used, but as industrial needs evolved, the need arose to further improve the properties of phosphating coatings. In the 1960s and 1970s, tricationic phosphating was developed as an advanced solution, incorporating nickel and manganese along with zinc, to provide an even stronger, higher-performing coating.

Fields of Application

Tricationic phosphating is widely used in various industrial sectors, including:

• Automotive Industry: to treat chassis components, bodywork and engine parts.
• Appliance Industry: to protect metal surfaces exposed to humidity and chemical agents.
• Construction Industry: to prevent corrosion of metal structures and architectural components.
• Military and Aerospace Industry: to improve the durability and corrosion resistance of critical components.

Process

Surface Preparation

Degreasing (Tank 1)

This initial step serves to remove oils, greases and other organic contamination from the metal surface. Alkaline solutions (sodium or potassium hydroxide from 2 to 5%) are used together with surfactants to improve the effectiveness of degreasing.

Rinsing with Water (Tank 2)

Alkaline residues are removed with the use of demineralized water, to prevent contamination of subsequent phases.

Pickling (Tank 3)

Necessary to remove oxides and rust from the metal surface and make it reactive for the subsequent steps. Hydrochloric acid or phosphoric acid (5-15%) is used together with corrosion inhibitors to protect the base metal.

Rinsing with Water (Tank 4)

Acid residues are removed with the use of demineralized water.

Phosphating

Activation (Tank 5)

The surface is prepared for uniform growth of the phosphate coating, using a titanium or molybdate based solution (0.1-0.5%).

Tricationic Phosphating (Tank 6)

The phosphate coating on the surface of the metal is formed, using zinc, nickel and manganese phosphates, in phosphoric acid solution:

• Zinc phosphate: 1-2%
• Nickel phosphate: 0.1-0.3%
• Manganese phosphate: 0.05-0.2%
• Additives: accelerators such as nitrates or chlorates.
• Crystallization modifiers: citric and tartaric acid.

Post-Treatment

Rinsing with Water (Tank 7)

Phosphating residues are removed with the use of demineralised water.

Passivation (Tank 8)

Corrosion resistance and paint adhesion are improved.

A solution based on chromates or silicates is used, in percentages from 0.5 to 2%.

Final Rinse (Tank 9)

Passivation residues are removed with the use of demineralized water.

Drying (Tank 10)

Oven with ventilated air between 80-120°C, for the time necessary for complete removal of humidity.

Innovations

The use of nanotechnology in tricationic phosphating represents an important evolution in this field. These technologies aim to improve the quality, uniformity and functionality of the phosphate coating, as well as reduce the environmental impact of the process. Below are some examples of nanotechnologies used or in development:

Metal Oxide Nanoparticles

Adding nanoparticles of metal oxides, such as titanium dioxide (TiO2) or zinc oxide (ZnO), to the phosphating solution can improve the properties of the coating, in terms of increasing the corrosion resistance of paint adhesion .

The nanoparticles act as nucleants, promoting the formation of smaller, more uniform phosphate crystals, which better cover the metal surface.

Nano-Silica

Nano-silica (SiO2) can be incorporated into the phosphating solution. It improves the hardness and wear resistance of the phosphating coating. The nano-silica particles integrate into the phosphate matrix, creating a more compact and resistant structure.

Cerium nanoparticles

Cerium (CeO2) nanoparticles can be used as additives in phosphating solutions, offering excellent anti-corrosion properties and reducing the need for chromium-based additives, which are toxic. and nanoparticles form an additional protective layer on the metal surface, preventing corrosion from spreading.

Carbon nanoparticles

Inclusion of graphene or carbon nanotubes in the phosphating solution. They improve the electrical conductivity and mechanical strength of the coating by strengthening the phosphate matrix and improve the electrical properties of the coating, making it useful for specific applications.

Conclusions

The tricationic phosphating process is essential for improving the surface properties of metals, ensuring corrosion resistance, paint adhesion and reduced wear. The correct execution of each phase, from the initial degreasing to all the phosphating steps, guarantees a uniform and high quality coating, which allows reaching high standards of resistance in salt spray, managing to exceed even 1000 hours with a single layer. of paint and without the need for primer.

On the other hand, the complexity of the process, the length of the plant, the maintenance, the frequent analytical analyses, the management costs and the dangerousness of the chemicals used make this treatment less and less expanding, thanks above all to the modern nanotechnologies which are rapidly replacing the old pretreatments.