Conversion Coating
A complete range of conversion coating chemicals for industrial metal surface treatment.
Phosphating Chemicals
Phosphating (Conversion Coating)
Phosphating is the most widely used metal pretreatment process, where a conversion coating is formed on the metal surface consisting of iron, zinc, or manganese phosphate crystals.
This process involves the deposition of insoluble metal phosphate layers that are chemically bonded to the substrate, providing a stable and uniform surface for subsequent finishing operations. The resulting coating significantly improves paint adhesion and corrosion resistance.
Phosphate coatings provide:
- A clean, oil-free surface
- A corrosion-inhibitive base for powder and liquid coatings
- A non-conductive interface between the base metal and coating
- A chemically stable layer that prevents reaction between metal and paint system
Types of Phosphate Coatings
- Iron Phosphate
- Zinc Phosphate
- Manganese Phosphate
Phosphating is primarily used for iron and steel substrates and can be applied through immersion or spray processes depending on production requirements.
The primary types of phosphate coatings include :
1. Iron Phosphate
Iron phosphating is the most conventional and simplest form of phosphate conversion coating. It provides basic corrosion resistance and is primarily used as a pre-treatment before painting or powder coating applications. Compared to other phosphating systems, its corrosion resistance is relatively lower. The coating typically results in a light bluish film on the metal surface.
Process control parameters such as total acid, pH, temperature, and treatment time are critical to ensure consistent coating quality and performance.
Application Methods
Iron phosphate conversion coating can be applied using the following methods:
- Wipe Iron Phosphating
- Immersion Iron Phosphating
- Spray Iron Phosphating
2. Zinc Phosphate
Zinc phosphating is a conversion coating process used to provide long-term corrosion resistance and improved paint adhesion. It is widely adopted in automotive, hardware, and home appliance industries due to its superior coating performance compared to iron phosphating.
The zinc phosphate bath typically consists of a controlled solution of phosphoric acid and zinc phosphate. Due to its acidic nature, immersion time is generally restricted to avoid excessive base metal attack, typically not exceeding 10 minutes.
Zinc phosphate coatings offer higher corrosion resistance and coating quality; however, the process requires more precise control of operating parameters compared to simpler phosphating systems. It can be applied through both immersion and spray methods.
Types of Zinc Phosphating
- Mono-Cationic System: Contains zinc as the primary cation.
- Di-Cationic System: Contains a combination of zinc with nickel or calcium.
- Tri-Cationic System: Contains zinc, nickel, and manganese for enhanced performance and corrosion resistance.
3. Manganese Phosphate
Manganese phosphating is widely used in piston, defense, and heavy engineering industries where high wear resistance and controlled friction are required. It facilitates smooth movement of components operating under lubricated conditions by reducing metal-to-metal contact and friction.
Compared to zinc phosphate coatings, manganese phosphate exhibits a more stable crystalline structure under high pressure and elevated temperature conditions, making it suitable for demanding mechanical applications.
Process performance depends on strict control of parameters such as temperature, treatment time, concentration, free acid, and total acid values. The process typically operates at 70–98°C, with a bath concentration of 15–20%, and an immersion time of 5–20 minutes, depending on substrate condition and application requirements.
Key Features
- Suitable for piston and defense applications
- Reduces friction in lubricated moving parts
- Stable performance under high pressure and temperature
- Improves wear resistance and service life
- Controlled crystalline phosphate coating