Remarkable enhancement of corrosion resistance of silica coating with acidified halloysite on Mg Alloy

Solely inorganic silica (SiO 2 ) coating cannot provide long-term corrosion protection on metal due to structural defects. Herein, acidified Halloysite (Hal) was proposed and the Hal tubes were incorporated into SiO 2 sol coating for enhancing corrosion resistance on AZ31B magnesium (Mg) alloy. SiO 2 sol originated from the hydrolysis of water glass (Sodium silicate). With improved interaction between Hal and SiO 2 , the cracks and structural defects in cured SiO 2 coating can be reduced. As a result, the impedance modulus at 0.01 Hz (|Z| 0.01 Hz ) of the acidified Hal/SiO 2 composite coating (AG coating) increased to 10 9 Ω·cm 2 , outperforming 2 orders of magnitude higher than pure SiO 2 coating (pure coating) counterpart. The corrosion current density decreased to 9.33×10 –12 A·cm -2 . Besides, the AG coating offered stable corrosion resistance after immersion tests for 30 days, which was comparable to anti-corrosive polymer coating. The release of amorphous SiO 2 from acidified Hal reduced the internal stress. The H + on Hal induced a decrease of pH and adsorption of residual Na + , thereby improving structural integrity and water resistance of SiO 2 coating. Differing from other fillers (CNTs, graphene, MXene), this work proposed a low-cost Hal tube as filler for designing inorganic coating for corrosion protection on Mg alloy. It also shed insight into understanding the underlying corrosion protection mechanism of inorganic SiO 2 coating on metal.