Manganese phosphate coatings

A zinc phosphate solution tends to produce coatings more quickly than iron or manganese phosphate solutions, and dissociation of primary zinc phosphate proceeds rapidly through reaction 15.2 to 15.3 or directly to tertiary zinc phosphate via reaction 15.4. Even so, a processing time of 30 min is usual with the solution near boiling. 

Manganese phosphate coatings heated in the absence of air lose their corrosion resistance at between 200 and 218°C. At these temperatures, between 75 and 80% of the water of hydration is lost and it is assumed that this results in a volume decrease of the coating which causes voids and thereby lowers the corrosion resistance. Fig. 15.4 shows the loss of water of hydration from zinc, iron and iron-manganese phosphate coatings. 

Fig. 15.4 Effecl of heating on phosphate coatings for 16 h at various temperatures, showing loss of water of hydration. Curve A zinc phosphate, B iron phosphate and C iron manganese phosphate (courtesy y./.S./., 170, II (1952))...

Manganese phosphate coatings are not used in coil coating operations because they are relatively slow in forming and, as such, are not amenable to the high production speeds of coil coaters. 

Nitriding of steels is not in itself beneficial to bonded film performance, but Kawamura et al showed an interesting synergistic effect of soft nitriding and manganese phosphating on the wear life of a bonded molybdenum disulphide coating on an annealed steel. 

Figure 10-20. Phosphorus (I) and sulfur (II) in an interaction film on hardened cast iron under dynamic conditions. (a) Effect of load and running time 1% zinc dialkyldithiophosphate in SAE 10 base oil. (b) Effect of change from compounded oil to base oil. Treated parts furnished with surface coating of manganese phosphate. Data by Loesser, Wiquist and Twiss [70].

Chem. Descrip. Iron, zinc, or manganese phosphate coating Uses Protective coating for ferrous and nonferrous metals K-Flex 128 [King Ind.]... 

This process is applied mainly to steel components which are sprayed or dipped in a phosphoric acid solution containing iron, zinc or manganese phosphate. The most common processes use zinc or manganese phosphate and are employed in the treatment of car bodies and domestic appliances before painting. Where long-term corrosion protection is required, the phosphate coating must be sealed with oil lacquer or paint... 

A buildup of Al + in the zinc phosphating baths will restrict the amount of coating formed unless these ions are ranoved by addition of fluoride or borate. An advantage of zinc phosphating, however, is that it avoids the use of toxic hexavalent chromium compounds. Some simple bath compositions for zinc and manganese phosphating of aluminium are... 

When a metal surface is treated with a weak phosphoric acid solution of iron, zinc, or manganese phosphate, phosphate layers are formed. These phosphate coatings are applied to iron and steel, zinc, aluminum, and their alloys. 

Inorganic (microdiscontinuous), chrome flash, hard chromium, nickel-iron Chemical Conversion Coatings Zinc phosphate, manganese phosphate, iron phosphate, black oxide, anodic oxide (aluminum). 

Manganese phosphate is used as a coating on steel acting as a lubricant carrier and improving wear resistance. 

Medium to heavy zinc phosphate coatings, and occasionally, heavy manganese phosphate coatings are used for corrosion resistance when... 

Heavy manganese phosphate coatings (10.8 to 43.0 g/m, or 3.5 X 10 to 0.14 oz/ft ), supplemented with proper lubrication, are used for wear-resistance applications. Parts that are manganese phosphate coated for wear resistance are listed in Table 1. 

Table 1 Parts immersion coated with manganese phosphate for wear resistance...

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