Zinc phosphate coating on magnesium for corrosion resistance application

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Date
2019-07
Authors
Haniaffify Binti Hanisan
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Deposition of zinc phosphate (ZPO) coating on an AZ31 magnesium alloy to against corrosive environment was investigated with different zinc phosphating time and pre-treatment with strontium (Sr) ion. Sample were immersed in acidic solution, alkaline treatment and 0.1M citric acid before immersing in ZPO. Where the ZPO bath compositions are 16g/L H3PO4, 17g/L ZnO, 25ml/L and 0.5 g/L NaF at 65°C (±5°C) and pH 3. As for the pre-treatment ZPO samples, strontium phosphates (SrP) were introduced before the ZPO solution. Sr ion as the pre-treatment can help the ZPO coating depostion to be more uniform on mg substrate. It is because Sr ion can help to slow down the metal dissolution of mg because of its more negativity in standard electrode potential. The surface morphology observations and composition of ZPO coatings were obtained by SEM, EDS and XRD analysis, while the corrosion behaviour of the coating were studied by using immersion test and potentiodynamic polarization. SEM and EDS results show that phosphating time at 30 minutes has the highest coating growth of ZPO crystals. The pre-treated ZPO shows uniform deposition with compact microstructure of ZPO crystal as phosphating time increases to 30 min. XRD results confirm the existence of phosphating coating on the mg for both pre-treated and non-pretreated samples with hopeite as main phase. The formed of 30 min. of ZPO coatings exhibited high corrosion resistance as shown by low weight loss after immersion in 3.5% NaCl solution for 24 hours (4.817 x 10-2 g/cm2 and 0.379 x 10-2g/cm2, for non-pretreated and pre-treated, respectively) compared to 14.973 x 10-2 g/cm2 for bare mg. Polarization measurement showed that phosphate coating with SrP treatment had low corrosion current density and high corrosion resistance.
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