Influence of coating layers and immersion time on adhesion force and corrosion resistance of DCPD coatings on pure magnesium

The widespread use of magnesium (Mg) in the medical implant industry has been limited by its rapid degradation rate, which surpasses tissue healing and causes a loss of mechanical strength before complete healing occurs. In this study, an immersion coating technique based on a wet chemistry approach was developed to deposit a homogeneous and dense layer of dicalcium phosphate dihydrate (DCPD) on pure Mg substrates to improve corrosion resistance. By optimising the phase composition, immersion time, and number of coating cycles, the coating achieved the desired thickness and improved adhesion, forming an effective barrier in Hank’s solution. This study demonstrated that the deposition and growth of DCPD on pure Mg are influenced by its solubility behaviour in relation to pH, which in turn determines the optimal immersion time. The results showed that extending the immersion time from 2 to 24 h led to the deposition of homogeneous coatings with increased thickness and adhesion force. However, increasing the immersion time to 48 h did not achieve further increases in coating thickness and resulted in lower coating adhesion force due to severe cracks, uneven surfaces, and pores. Increasing coating cycles caused continuous hydrogen evolution, which destabilised the structure and led to uneven formation of the coating, cracking, delamination, and reduced adhesion strength. Electrochemical tests confirmed that a 24-hour, single-cycle coating formed a dense, adherent layer that effectively limited the ability of Hank's solution to penetrate the coating and provided robust protection to the underlying Mg.