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Structural and thermal studies of the orthophosphates hopeite and phosphophyllite involved in corrosion prevention

Structural and thermal studies of the orthophosphates hopeite and phosphophyllite involved in corrosion prevention
Structural and thermal studies of the orthophosphates hopeite and phosphophyllite involved in corrosion prevention

Modern anti-corrosion coatings in the automotive industry involve the application of coats of zinc based hydrated orthophosphates. The two phosphates involved are hopeite, Zn3(PO4)2.4H2O, and phosphophyllite, Zn2Fe(PO4)2.4H2O. Both are naturally occurring minerals and are structurally related, involving interlinked octahedra and tetrahedra of oxygens filled by phosphorus and metal ions. Hopeite contains zinc in the octahedra holes and phosphophyllite involves iron in these holes.

Hopeite and phosphophyllite were synthesised from solutions of the metal sulphate salts and diammonium hydrogen phosphate; the products are white insoluble crystalline powders. These two materials were characterised using powder x-ray diffraction, to study the structures, and thermal analysis methods to investigate their dehydration processes.

It has been proposed that hopeite exists in two structurally identical forms, different only in their thermal properties, known as α-hopeite and β-hopeite. Both undergo two stages of dehydration equivalent to 2 water molecules each, at 120oC and 300oC for α-hopeite and 120^oC and 160^oC for β-hopeite. Structural analysis revealed one pair of water molecules to be further away from the coordination centre, so are lost first. The structure of the dihydrate formed is not clearly known.

The initial synthesis of hopeite was performed at pH= 0.9, other syntheses were performed at higher pHs. X-ray Rietveld analysis revealed hopeite to be non-stoichiometric, with the octahedral sites being deficient in zinc atoms at the lower pHs. This was reflected in the site occupation and the average Zn-OH2 bondlengths which were seen to increase as the occupation decreased. The thermal study of the variable pH materials showed that low pH precipitations produced α-hopeite, and the higher ones produced β-hopeite. Thus, the type of hopeite formed is dependent on the pH and the difference is caused by the change in the occupation of the ZnO6 octahedra. A natural sample of hopeite gave a β-hopeite like thermal behaviour, but a low occupation of the octahedral site.

University of Southampton
Thomas, Ian Michael
Thomas, Ian Michael

Thomas, Ian Michael (1993) Structural and thermal studies of the orthophosphates hopeite and phosphophyllite involved in corrosion prevention. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Modern anti-corrosion coatings in the automotive industry involve the application of coats of zinc based hydrated orthophosphates. The two phosphates involved are hopeite, Zn3(PO4)2.4H2O, and phosphophyllite, Zn2Fe(PO4)2.4H2O. Both are naturally occurring minerals and are structurally related, involving interlinked octahedra and tetrahedra of oxygens filled by phosphorus and metal ions. Hopeite contains zinc in the octahedra holes and phosphophyllite involves iron in these holes.

Hopeite and phosphophyllite were synthesised from solutions of the metal sulphate salts and diammonium hydrogen phosphate; the products are white insoluble crystalline powders. These two materials were characterised using powder x-ray diffraction, to study the structures, and thermal analysis methods to investigate their dehydration processes.

It has been proposed that hopeite exists in two structurally identical forms, different only in their thermal properties, known as α-hopeite and β-hopeite. Both undergo two stages of dehydration equivalent to 2 water molecules each, at 120oC and 300oC for α-hopeite and 120^oC and 160^oC for β-hopeite. Structural analysis revealed one pair of water molecules to be further away from the coordination centre, so are lost first. The structure of the dihydrate formed is not clearly known.

The initial synthesis of hopeite was performed at pH= 0.9, other syntheses were performed at higher pHs. X-ray Rietveld analysis revealed hopeite to be non-stoichiometric, with the octahedral sites being deficient in zinc atoms at the lower pHs. This was reflected in the site occupation and the average Zn-OH2 bondlengths which were seen to increase as the occupation decreased. The thermal study of the variable pH materials showed that low pH precipitations produced α-hopeite, and the higher ones produced β-hopeite. Thus, the type of hopeite formed is dependent on the pH and the difference is caused by the change in the occupation of the ZnO6 octahedra. A natural sample of hopeite gave a β-hopeite like thermal behaviour, but a low occupation of the octahedral site.

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Published date: 1993

Identifiers

Local EPrints ID: 462507
URI: http://eprints.soton.ac.uk/id/eprint/462507
PURE UUID: 9b024093-cc1a-4985-8118-ccfef73027c0

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Date deposited: 04 Jul 2022 19:10
Last modified: 04 Jul 2022 19:10

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Author: Ian Michael Thomas

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