Phosphate Conversion Coatings - Zinc

Transformation means conversion to another form, substance or state. In phosphate conversion coatings, the substrate participating in the metal coating reaction becomes a component of the coating. Phosphate conversion coatings are formed on the surface of the base metal. For this reason, the porosity during coating formation determines the coating thickness. The reaction stops after the surface's contact with the chemical solution is severed. Phosphate conversion coatings bring many important advantages, which can be listed as increasing corrosion resistance, absorbing oils, improving appearance, increasing adhesion, utilizing resistance or facilitating cold forming. The chemical process that transforms the base metal surface into a non-metallic crystalline coating is called phosphating. The reaction takes place in an acidic solution containing phosphate ions. Hydrogen loss at the metal/solution interface causes a local increase in pH and subsequently coating precipitation. Phosphate coatings can be divided into three main types: zinc, manganese and iron. Different special formulations exist for each according to functional requirements.

Heavy Zinc Phosphate

Heavy zinc phosphate is preferred in situations where retention of rust preventive oils and waxes is generally required. Conversion to an absorbent surface in the range of 1,000–3,000 mg/ft² is provided, and this heavy coating prevents rust on the part surface. Depending on the formulation and concentration of the selected rust preventive, more than 400 hours of corrosion protection from 5% neutral salt spray exposure is possible. Heavy zinc phosphate coatings are obtained by immersion. Parts are applied on racks or in drums. The bath is initially filled by volume with 3–4 percent (30-40 total acid number) and operates at 175–185° F. The bath is controlled by simple titrations measuring concentration (total acid), aggressiveness (free acid) and iron content. The factor limiting bath service life is generally iron accumulation. When iron levels reach higher levels than zinc, coating quality and uniformity are endangered. At this stage, part of the bath can be drained or the bath can be refilled fresh.

Calcium-Modified Zinc Phosphate

Calcium-modified zinc phosphate is generally used as a base for paint or other organic coatings. Calcium accumulated with zinc functions as a built-in grain refiner to create a uniform microcrystalline structure. Coating weights typically range from 150-500 mg/ft², allowing enhanced adhesion properties without being absorbent like a heavy zinc phosphate coating. Additionally, the chemical-resistant structure of calcium-modified zinc phosphate coatings limits corrosion to only an area called seeping in case the applied top coat is damaged. Calcium-modified zinc phosphates with wear resistance are mostly used before rubber adhesive application. Calcium-modified zinc phosphate can be applied by spray or immersion. The bath is generally a two-component system, with the calcium-rich component added initially and then subsequently. Operating temperature is 150° F for spraying and 170-180° F for immersion. Iron content in the bath can hinder the grain refining process and uniform coatings may result. The addition of small amounts of a strong oxidizer at frequent intervals causes iron to precipitate in the solution and allows for long bath life.

Cold Forming Zinc Phosphates

Cold forming zinc phosphates can be used to facilitate cold drawing, extrusion, pressing or scratching of the base metal. These phosphate coatings are designed to retain lubricants under the heavy conditions of heat and pressure generated during deformation. The use of phosphate coating aids in longer tool life, faster drawing speeds and more severe reduction of the base metal. Cold forming zinc phosphate weights range between 500-2000 mg/ft². The phosphating solution in this application operates without iron to provide less abrasive zinc phosphate crystals that will not score dies, work or strip. A strong oxidizer, usually nitrite or chlorate, is used to remove iron from the bath as sludge. Additionally, a titanium-based grain refiner pre-immersion can be considered to obtain a softer and more uniform crystalline coating.

Sources • Electrochemical Techniques in Corrosion Science and Engineering. By Robert G. Kelly, John R. Scully, David Shoesmith, Rudolph G. Buchheit, CRC Press, Sep 13, 2002 • Metallic and Other Inorganic Coatings. Phosphate Conversion Coating of Metals. British Standards Institute Staff. B S I Standards, Nov 9, 1917. • Intelligent Coatings for Corrosion Control. By Atul Tiwari, Lloyd Hihara, James Rawlins. Butterworth-Heinemann, Oct 25, 2014. • https://www.pfonline.com/articles/phosphate-conversion-coatings