Combination of HAp coating with polymer molecules which act as a nucleus for calcium phosphate deposition is a potential way to enhance the self-healing ability. It was reported that an anionic polymer, sodium polyacrylate (SPA), acted as a nucleus for calcium phosphate deposition . In this study, SPA molecule was combined with HAp coating and the corrosion behavior was examined by a static immersion test and a slow strain rate tensile (SSRT) test in a simulated body fluid .
HAp was coated on AZ31 at 60ºC for 1 hour in a previously reported solution . SPA molecule was dissolved in a phosphate buffer at pH 8 and the HAp-coated AZ31 was dipped at room temperature for 10-30 sec. Scratches were formed on the surface of HAp- and SPA-HAp-coated AZ31 specimens with a knife. The scratched specimens were statically immersed in Hanks’ solution for 7 days. Volume of H2 gas generated was measured. The surface after the immersion was characterized using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). A SSRT test of HAp- and SPA-HAp-coated AZ31 tensile test specimens was performed in Hanks.
Scratched HAp- and SPA-HAp-coated AZ31 showed no significant difference in H2 gas volume generated under the static immersion. Inside the scratches were covered with a calcium phosphate deposit layer regardless of the presence of SPA. The deposit layer with SPA was uniform and prevented the corrosion initiation; whereas, local corrosion initiated from inside the scratches without SPA. Moreover, SPA enhanced the deposition of calcium phosphate granules inside the scratches.
In the SSRT test in Hanks, HAp-coated AZ31 specimen fractured just over the yield strength with small elongation of ca. 1.5%. This result indicates that HAp-AZ31 is susceptible to stress corrosion cracking (SCC). SPA-HAp-coated AZ31 specimen showed larger elongation of ca. 2.4%. Time to failure of HAp-coated AZ31 was ca. 8.4 hours and it was elongated by 45% with SPA to ca. 12.2 hours. Some cracks of SPA-HAp coating were closed at crack mouth, suggesting that SPA enhanced the healing of cracks of HAp coating. It was revealed that the combination of SPA molecules with HAp coating enhanced the self-healing ability of HAp coating and reduced the SCC susceptibility of HAp-coated AZ31.
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