Zinc phosphate coatings

The zinc phosphate conversion coating process used today converts the metal surface into a non-metallic, polycrystalline coating containing iron, manganese, nickel, and zinc phosphates. Since the zinc phosphate pretreatment process converts the metal surface, the coating provides an excellent base for paint adhesion. As zinc phosphate conversion coatings incorporate additional metal ions into the coating to increase the alkaline resistance, it makes an excellent paint base offering superior adhesion and durability.

The zinc phosphate pretreatment process is applied by either spray or immersion applications depending on the industry and can be used on a wide variety of substrates including steel, hot-dipped and electrogalvanized, zinc-nickel alloy, and aluminum. Each of these processes incorporates three to four chemical stages.

Once the zinc phosphate conversion coating has been successfully applied to the metal surface, the coating provides an excellent base for powder paint, electrocoat, and liquid paint, water or solvent based. The final zinc phosphate coating and paint application creates a finished surface with excellent adhesion and corrosion resistance.

The first active step in the zinc phosphating process is cleaning the incoming surfaces of all contamination. This ensures the proper deposition on the zinc phosphate coating. The second active step is applying a surface conditioner prior to the zinc phosphate stage. Using a surface conditioner provides nucleation sites for the zinc phosphate, thus promoting a finely dense microcrystalline zinc phosphate coating. 

After conditioning, the surface enters either a spray or immersion zinc phosphate bath. The zinc phosphate bath changes the topographical and chemical nature of the surface by depositing a dense polycrystalline zinc phosphate conversion coating. Depending on the metal substrate being processed, the zinc phosphate coating will contain iron, manganese, nickel, and zinc phosphate. Incorporating these additional metal ions into the zinc phosphate coating changes the chemical nature of the surface increasing alkaline resistance and paint durability.

The final active step in the process, which is optional, is the application of a post-treatment or sealer. The post-treatment/sealer chemistry reacts with the zinc phosphate coating, reducing the porosity of the surface and increasing its corrosion resistance.

The use of aluminum is increasing, especially in automotive  manufacturing, as OEM and manufacturers continue to mix metals and reduce the weight of vehicles while maintaining federal safety standards. This increase of aluminum through the zinc phosphate pretreatment process presents some unique challenges. For the pretreatment process of these lightweight and mixed metal substrates, Henkel has developed our innovative Zinc Phosphate 2-Step Process. 

The 2-step process employs the same cleaning and surface conditioning steps described above, however, the conversion coatings change depending on the substrate being processed. In the case of steel, hot-dipped or electrogalvanized or zinc-nickel (Zi/Ni) alloyed steel the traditional zinc phosphate spray or immersion coating is deposited on the surface. In the case of the aluminum being processed, the zinc phosphate spray or immersion bath chemistry is modified to reduce the etching of the surface to prevent the zinc phosphate coating from depositing on the aluminum surface. The conversion coating deposited on the aluminum comes from the passivation or sealer stage. This stage deposits a cutting-edge nanoceramic  conversion coating on the aluminum surface and does not affect the traditional zinc phosphate coating on the other substrates.

Like traditional zinc phosphate coatings, the conversion coatings deposited during the zinc phosphate 2-step process offer a high level of paint adhesion and corrosion resistance and provide a perfect foundation for subsequent coating with electrocoat, powder coating, or water/solvent-based paint processes.

Henkel’s BONDERITE^® Zinc Phosphate Spray or Immersion process is a tri-cationic pretreatment process containing zinc, nickel, manganese, or cobalt that deposits a polycrystalline conversion coating on steel, galvanized, alloyed coated (Zn/Ni) steel, and aluminum. The subsequent zinc phosphate coating offers excellent adhesion and corrosion protection that provides an excellent surface for subsequent painting or organic coating. 

At Henkel, we supply a range of BONDERITE^® Zinc Phosphate products, including traditional tri-cationic, the 2-Step process, and nickel-free conversion coatings - all specially formulated to promote adhesion and corrosion protection for specific metal preparation applications prior to paint applications.

Surface conditioners directly precede the zinc phosphate solution and help promote a dense, uniform zinc phosphate crystalline structure. By changing some of the surface conditioner operating parameters, such as concentration, pH, or time, the final zinc phosphate conversion coating properties will be affected. For example, by using a higher concentration of the conditioner, finer zinc phosphate crystals are allowed to form, thereby reducing the total coating weight. To ensure perfect adhesion for zinc phosphate conversion coatings, it is necessary to prepare the surface prior to treatment. This preparation removes oil and grease, using aqueous or solvent-based cleaning solutions, creating the ideal bonding surface for subsequent treatment.

Henkel supplies a range of BONDERITE^® surface conditioners specially formulated (either titanium or zinc-based) for preparing metallic surfaces for zinc phosphate coating, thereby ensuring a tough, long-lasting, and high-quality surface finish.

Further treatment following zinc phosphate coating, especially in the case of aluminum, has been proven to have a significant beneficial effect on long-term performance. Post-treatment/seals will reduce the porosity of the surface, thereby improving its overall long-term adhesion and corrosion resistance.

Like other zinc phosphate coating suppliers, Henkel offers a broad portfolio of BONDERITE^® products specially formulated for the post-treatment after the application of zinc phosphate conversion coating.

The zinc phosphate coating process applied by spray or immersion is a trusted pretreatment designed to prepare metal surfaces for painting by providing an excellent foundation that promotes paint adhesion and corrosion protection.