Phosphate Conversion Coatings
By Cheryl Hickman – Heatbath Corp. Springfield, MA
Taken from: Products Finishing (pfonline.com) Posted on 9/29/11.
Types of phosphate pretreatments, how to apply them, advantages and disadvantages of each.
CONVERT: To Change into another form, substance or state. In the case if conversion coating, the substrate metal participates in the coating reaction and becomes a component of the coating. Chemical conversion coatings are formed from the surface of the base metal outward. Therefore, the thickness of the coating is dependent upon the porosity of the coating as it forms. Once the surface is sealed from the chemical solution, the reaction stops.
Chemical conversion coatings are an integral part of most finishing operations, serving one or more of the following functions: increase corrosion resistance, absorb lubricants, enhance appearance; promote adhesion and provide wear resistance or facilitate cold forming.
Phosphating is a chemical conversion coating that transforms the surface of the basis metal into a non-metallic crystalline coating. The reaction occurs in an acidic solution containing phosphate ions. Due to the loss of hydrogen at the metal/solutions interface, there is a localized rise in pH and subsequent precipitation of the coating.
Phosphate coatings can be categorized into three main types: zinc, manganese and iron. There are many proprietary formulations available for each, depending on the functional requirements of the part.
Heavy zinc phosphate is usually chosen for its ability to retain rust preventive oils and waxes. The heavy zinc phosphate coating, in the range of 1,000-3,000 mg/sq ft, acts as an absorbent substrate, holding the rust preventative on the surface of the part. This provides corrosion protection to in excess of 200 hours 5% neutral salt spray exposure, depending on the formulation and concentration of the chosen rust preventive.
Heavy zinc phosphate coatings are applied in an immersion process. Parts are run either on racks or in tumbling barrels. The bath is charged at 3-4% by volume (30-40 total acid points) and operates at 175-185°F. The bath is controlled with simple titrations that measure concentration (total acid), aggressiveness (free acid) and iron content.
Iron buildup is usually the limiting factor in the service life of the zinc phosphate bath. When iron levels become higher than zinc, usually due to high metal throughput, coating quality and uniformity are compromised. At this time, a portion of the bath can be decanted, or the bath is charged fresh.
Calcium-modified zinc phosphate is typically used as a base for a paint or other organic coatings. The calcium co-deposits with the zinc phosphate and acts as a built-in grain refiner to form a smooth microcrystalline structure. Coating weights are typically in the range of 150-500 mg/sq ft, which allows for enhanced adhesion properties without being as absorptive as a heavy zinc phosphate coating. In addition, the chemical-resistant nature of calcium-modified zinc phosphate coating confines corrosion to a limited area, often referred to as creepage, if the applied topcoat is damaged.
Calcium-modified zinc phosphate can be applied by spray or immersion. The bath is often a two-component system, with the calcium-rich component added upon start up and infrequently thereafter. Operating temperature is 150°F for spray and 170-180°F for immersion.
Iron content in the bath may interfere with grain refinement and nonuniform coatings may result. Small but frequent additions of a strong oxidizer will precipitate the iron from the solution, resulting in extended bath life.
Cold-forming zinc phosphates are used to facilitate drawing, cold heading, stamping or extruding of the basis metal. These phosphate coatings are designed to retain lubricants under severe conditions of heat and pressure during deformation. The use of the phosphate coating allows increased tool life, faster drawing speeds and more severe reductions of the basis metal. Coating weights for cold forming zinc phosphates can range from 500-2,000 mg/sq ft. The zinc phosphating solution in this application is operated iron-free to ensure a less abrasive zinc phosphate crystal, which will not scratch dies, score or gall. A strong oxidant, commonly nitrite or chlorate, is needed to drop the iron out of the bath in the form of sludge. These operations are always immersion processes, with the optimum operating temperatures of 175-185°F.
Manganese phosphate is most commonly chosen for its wear-resistant properties. The manganese phosphate coating not only prevents metal-to-metal contact between moving parts, such as cylinder liners, camshafts, piston rings and transmission gears, it also has excellent oil retentive properties for both lubricity and corrosion resistance. A grain refining predip is often used prior to the manganese phosphate to ensure a controlled microfinish. The manganese phosphate bath is charged at 10% by volume (12 total acid) and is operated at 195-205°F. The balance of the bath concentration (total acid) when compared to the bath activity (free acid) is critical to control crystal size and coating uniformity. A ratio of 5.5-6.5:1 is recommended and maintained with additions of manganese carbonate. High iron content in the bath can be lowered by treatment with hydrogen peroxide.
Iron phosphate is used as a base for paint or powder coat to enhance adhesion. Unlike both zinc and manganese phosphates, in which the cation is found in the phosphating solution, the cation of the iron phosphate coating is contributed by the basis metal. The iron phosphate solution typically contains alkali metal phosphate and accelerators. Coating weights for iron phosphates range from 25-100 mg/sq ft.
Application is usually in a three- or five-stage spray washer, but there are some immersion processes. For three-stage washers, the iron phosphate has an incorporated detergent system to clean and phosphate in one step. In five-stage washers, the cleaning is done separately in stage one, while the phosphate is applied in stage three. A seal is applied (either chrome or non-chrome) in the final stage to minimize under-film corrosion.
Iron phosphates are easy to control with a simple titration to determine concentration, 1-3% by volume, and pH adjustment, 4.5-5.5 optimum. Operating temperatures are relatively low, 100-130°F.
Phosphate conversion coatings are quite often used to enhance the characteristics of the basis metal allowing for improved function or longer service life of the phosphate coated parts.Return to the List of Articles.