Asking about heat bearing ability after phosphating a job

A. Phosphate coatings undergo marked changes when subjected to thermal treatment. When heated above room temperature, a gradual loss in weight occurs. This can be attributed to a nonstructural weight loss. However, with subsequent increase in temperature, dehydration of the constituent phases of the phosphate crystals occurs. In case of zinc phosphate coating, dehydration of hopeite [Zn3(PO4)2.4H2O] and phosphophyllite [Zn2Fe(PO4)2.4H2O] commences at 80 and 110 °C, respectively. A more pronounced weight loss occurs at approximately 150 °C, where hopeite loses two molecules of water of hydration [Zn3(PO4)2.2H2O]. When heated above 150 °C, both phosphophyllite and hopeite are transformed into their dehydrated forms [Zn3(PO4)2 and Zn2Fe(PO4)2]. The hopeite phase is completely dehydrated at a temperature of approximately 240 °C. The weight loss will considerably increase between 250 and 600 °C. Above 600 °C sublimation of zinc and phosphorous occurs, which would result in complete breakdown of the coating. The change in appearance, colour and morphology of phosphate coatings remain practically unaltered up to 200 °C. However, above this temperature, the grey crystalline phosphate coating changes to a silver grey form and appear dusty. Above 500 °C it becomes brown and above 600 °C, a complete breakdown of coating would occur.

When exposed to service condition such as immersion in an aqueous solution or exposure to high humid atmospheres, the hopeite bihydrated ([Zn3(PO4)2.2H2O] crystals would undergo rehydration to the tetrahydrated [Zn3(PO4)2.4H2O] phase along with the formation of zinc oxide (ZnO). This will increase in volume of the coating and could cause adhesion of the subsequent paint coating.

A. Phosphate coatings undergo marked changes when subjected to variation of temperature. When heated above room temperature, a gradual loss in weight occurs. This was attributed to a nonstructural weight loss. However, with subsequent increase in temperature dehydration of the constituent phases of the phosphate crystals occurs. The dehydration of hopeite [Zn3(PO4)2 .4H2O] and phosphophyllite [Zn2Fe(PO4)2.4H2O] commences respectively at 80 and 110°; however, a more pronounced weight loss occurs at approximately 150°, where hopeite loses two molecules of water of hydration. When heated above 150°, both phosphophyllite and hopeite are transformed into their dehydrated forms. The hopeite phase is completely dehydrated at a temperature of approximately 240°. The weight loss considerably increased between 250 and 600°. Above 600° sublimation of zinc and phosphorous occurs, which results in complete breakdown of the coating. The change in appearance, colour and morphology of phosphate coatings remain practically unaltered up to 200°. However, above this temperature, the grey crystalline phosphate coating changes to a silver grey form and appear dusty. Above 500° the colour changes to brown and above 600° complete breakdown of coating occurs.