Manganese Phosphating

Phosphate Conversion Coatings

Manganese Phosphate Coating

Manganese phosphating is extensively employed to improve the sliding properties of engine, gear, and power transmission systems. The use of manganese phosphated coatings for improved corrosion resistance can be found in virtually all branches of the metal working-industry. Typical examples mentioned here include motor vehicle components in brake and clutch assemblies, engine components, leaf or coil springs, drill bits, screws, nuts and bolts, washers, anti-vibration washers, tools, magnet cores, casting interiors and many other small items.

Manganese phosphate coating has the highest hardness and superior corrosion and wear resistances of general phosphate coatings. Manganese phosphate coatings for conferment of good corrosion resistance, whether a post-treatment such as oil application is to be used or not, are invariably applied by the immersion method.

Machined Metal Workpieces With and Without Manganese Phosphate Coating

Hardened Metal Parts With and Without Manganese Phosphate Coating

The processing sequence can be summarized as follows:
      1. Degreasing and Cleaning,
      2. Water Rinse,
      3. Pickling In Mineral Acid (Where Necessary)
      4. Water rinse (Only After Pickling)
      5. Activation
      6. Manganese Phosphating
      7. Water Rinse,
      8. Final Oven Drying (Optional)
      9. Lubricating With Special Oils or Emulsions.

The degreasing and cleaning are usually done with strongly alkaline cleaners at concentrations of 1-5% and temperatures 65-95°C. Treatment times range from 5-15 minutes.

Activating pre-rinse has been developed for manganese phosphates which permits alkaline cleaning and pickling of the work, without the penalty of coarse-crystalline phosphate formation. This is based on a finely-dispersed manganese phosphate at concentrations 1-2 g/l.

Manganese phosphating is mainly by immersion. Treatment times range from 5-20 minutes, the optimum time depending on the surface condition. The bath operating temperature is around 95 °C and only in special cases can satisfactory coatings be formed at temperatures around 80°C.

The phosphated components, after drying, are immersed in the oil or lubricant baths for 0.5-2 minutes, allowed to drain. The thickness of the resulting oil film depends on the oil used and its concentration.

Manganese phosphating as a wear protection measure is widely used in the auto industry. Gearwheels in the gearbox, crown and pinion gears in the differential, cam-shafts, valves and valve-steams as well as pistons in larger diesel engines are frequently treated in this way. In other industries, the process is used to treat components in refrigerator compressors or oil pumps and their associated hydraulic rams for vehicle assembly plants.

Operating Parameters For Some Thick Phosphating Process

Phosphating System Accelerator Concentration (total acid points) Temperature (°C) Treatment Time (min.) Coating Weight (g/m2)
Zn-Ph None 20-50 90-98 30-60 20-45
Zn-Ph Nitrate 50-80 70-98 5-15 10-35
Mn-Ph None 20-50 90-98 30-60 20-45
Mn-Ph Nitrate 30-60 90-98 5-15 8-30

The coating weight and crystal size of the manganese phosphate coatings are influenced to an even greater extent than in zinc phosphating by the mechanical, thermal, and chemical pretreatment of the workpiece surface prior to phosphating. For example, cleaning in alkaline aqueous cleaning agents or pickling in acids produces coatings with a much coarser texture. Even after such treatments, however, fine crystalline phosphate coatings are still obtainable if the workpieces are prerinsed in an activation rinse prior to the phosphating.