Phosphate Coatings for Metal Protection
Phosphate coatings are inorganic conversion layers formed by a controlled chemical reaction between a metal surface – typically steel or iron – and a solution containing phosphate salts. Unlike deposited coatings, phosphating transforms the outer surface of the metal into a crystalline phosphate layer.
Coating thickness is influenced by solution formulation, concentration, pH, temperature and immersion time. Accurate process control ensures a consistent, predictable finish suitable for engineering applications.
During phosphating, metal ions dissolve from the substrate and combine with phosphate ions, forming a tightly bonded crystalline structure. Crystal size and uniformity are critical to coating performance.
Fine-grained coatings offer better adhesion, consistent torque–tension values and stable mechanical characteristics – essential for fasteners and precision assemblies. All phosphate systems used within the Anochrome Group include grain refiners to achieve optimum coating structure and repeatability.
Phosphate coatings provide limited corrosion resistance on their own, but their porous structure makes them ideal for absorbing supplementary oils, waxes and sealants. These post-treatments significantly enhance corrosion protection and reduce wear. Phosphate also acts as an excellent pretreatment, promoting strong adhesion for paints, polymer coatings and dry film lubricants, improving component life in demanding environments.
Zinc phosphate is the most widely used variant and typically appears light to dark grey depending on coating weight and post-treatment. Surface preparation – such as alkaline cleaning, blasting or activation – also affects finish quality.
Phosphate Types
Heavyweight Zinc Phosphate (10–25 g/m²)
Produces a thick, coarse crystal layer. Used with oils or waxes for durable, low-cost corrosion protection on fasteners, military hardware and parts exposed to handling or storage.
Lightweight Zinc Phosphate (3–7 g/m²)
Features a fine crystal structure ideal for bonding high-performance coatings such as Xylan® and molybdenum disulphide. Common in automotive, aerospace and industrial applications requiring precise friction control and torque stability.
Phosphating offers a reliable, economical and versatile surface treatment. At Anochrome, stringent process control ensures consistent quality across all applications.
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