Magnesium, Aluminum and Zinc co-substituted Hydroxyapatite: Anti-corrosion Properties

Abstract

The purpose of this study was to investigate the influence of doping ions Mg2+, Zn2+, Al3+ to the structure of hydroxyapatite (HAP; Ca-10(PO4)6(OH)2) and subsequently to evaluate their adaptation in structure and their anticorrosive properties. The substituted hydroxyapatite was synthesized by precipitation method that included the addition of Mg2+, Zn2+ and Al3+ containing precursors to partially replace Ca2+ ions in the hydroxyapatite structure. For precipitation synthesis, three ratios of Ca/P = 1; 1.67; 3 and two values of pH = 7 and 12 were selected. Samples 1 (Ca/P = 1; pH = 7), 2 (Ca/P = 1.67; pH = 7), 3 (Ca/P = 3; pH = 7) and 5 (Ca/P = 1.67; pH = 12) were chosen to monitor the influence of doping ions Mg2+, Zn2+ and Al3+ to the structure of hydroxyapatite and its anticorrosive properties. The chosen synthesis conditions are appropriate for the formation of crystalline HAP substituted by elements Mg, Zn and Al. Only for one sample (1-Mg), two different phases (hydroxyapatite and whitlockite) were identified in the phase composition. On the basis of preliminary corrosion tests, pigments were divided into three groups pursuant to their anticorrosion effectivity: pigments with high corrosion-inhibition efficiency; pigments without anticorrosive properties; and pigments that promote corrosion processes. In addition, no doping effect can be observed except for the sample 1-Mg, which consists of two phases (hydroxyapatite and whitlockite). Preliminary corrosion tests prove that some samples of HAP have extremely high anticorrosive effectivity as effectivity of the commercial pigments. The accelerated corrosion test showed that HAP samples have insufficient corrosion-inhibition properties for coating applications compared with the commercial pigment.