Corrosion passivation of magnesium alloy with the duplex coatings of plasma electrolytic oxidation and tetrafluoroethylene-based polymers

Abstract

In view of insufficient protection against corrosion provided by plasma electrolytic oxidation (PEO) coating on magnesium alloys, we have prepared and analyzed the duplex PEO coatings using the polytetrafluoroethylene (PTFE)-based polymer. Four duplex coatings have been prepared to compare with the two PEO coatings with and without nickel hydroxide sealing. Without the protection of PTFE, the PEO coating cracks with hairline fissures after 50 h immersion in 3.5% sodium chloride solution. With the sealing treatment of nickel hydroxide, the fissure width becomes slimmer, yet the fissure number is still plenty. The PTFE overlay prevents the cracks and offers a much improved corrosion protection and surface hydrophobicity. The improvement is evidenced by an impedance leap of two orders of magnitude. The overlay of colloidal Teflon dispersion raises the contact angle from 76.5° (oxide coating) to 129.9° (duplex coating); on the other hand, Zeffle of PTFE copolymer provides a thinner and less hydrophobic coating with contact angle 91.4°. Among the four duplex coatings, protection of Teflon (SY3) and Zeffle (SY5), without prior sealing, outperforms SY4 and SY6 in potentiodynamic polarization measurements. When the soak time is 50 h, the corrosion currents of SY3 and SY5 are practically the same, 1.69 × 10−7 A cm−2. When the soak time is 150 h, SY5, with corrosion current 2.09 × 10−6 A cm−2 and polarization resistance 6.85 × 104 Ω cm2, surpasses SY3 in protection durability.