Stearic acid coating on magnesium for enhancing corrosion resistance in Hanks' solution

Magnesium is potentially a desirable metallic material for degradable orthopedic implants if its corrosion rate can be reduced so that the implants can maintain sufficient mechanical integrity in the initial period of bone fracture healing. To achieve this aim, an organic coating was prepared on magnesium samples by a two-step process: (i) hydrothermal treatment to form a thick hydroxide layer, followed by (ii) immersion coating using stearic acid (SA, CH3(CH2)16COOH, an endogenous long-chain saturated fatty acid which is the main component of fat). The hydroxide layer formed in the first step provided sites to anchor the SA coating in the subsequent step, with the formation of magnesium stearate, which would enhance adhesion of the coating to the substrate. The corrosion behavior of the coated samples in Hanks' solution (a simulated body fluid) was studied by electrochemical methods. The corrosion resistance of the coated samples was much higher than that of bare magnesium, especially in the initial period, due to the barrier effect provided by the coating. Degradation of the coating occurred due to cracking in long-term immersion, with the formation of magnesium hydroxide and apatite.