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Asking about heat bearing ability after phosphating a job



June 23, 2010

Q. Dear,
I just want to ask what will happen if we give heat treatment up to 120 degree? what would be the effect on job? And if it causes bad product then what should be done in that condition.

I have been asked that question by one of my client . Please give me the answer

Shivam Dandage
phosphating - Pune, Maharastra, India



June 27, 2010

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.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at https://advancementinscience.wordpress.com)




To minimize searching & thrashing, multiple threads were merged; please forgive repetition, chronology errors, or disrespect of other responses (they probably weren't there) :-)



Exposing zinc phosphated parts to elevated temperatures

August 27, 2010

Q. MIL-DTL-16232G states that zinc phosphate should not be used in applications where temperatures are above 200 °F. My question is, what happens to zinc phosphate at temperatures above 200 °F? My question pertains to class 2 zinc phosphate with supplemental oil using MIL-PRF-3150 [affil. link or from DLA]. Thanks in advance.

John Parker
Manufacturer - Blue Bell, Pennsylvania, USA



August 31, 2010

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.

T.S.N. Sankara Narayanan
T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note Nov. 2017: The good doctor has a fascinating blog at https://advancementinscience.wordpress.com)


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