No abstract

In this work, the corrosion performances of micro‐arc oxidation (MAO) coating on AZ31B cast alloy were examined. MAO coating synthesized at a low current density of 34 mA/cm2 has a uniform and compact microstructure with very fine micro‐pores and few structural defects; thus, robust protection is provided for the AZ31B substrate in corrosive solutions. The long‐term corrosion and corrosion fatigue behaviors of the MAO coatings formed at the optimal current density for 5 and 10 min were characterized. The results show that the MAO coating formed for 10 min provides greater protection in a salt fog environment than the coating formed for 5 min does because it is thicker. The fatigue life and fracture analysis of bare, MAO‐coated and E‐paint MAO‐coated AZ31B indicate that the MAO coating reduces corrosion fatigue life by 55% due to the micro‐pores and micro‐cracks on its surface providing initiation sites under stress conditions and accelerating crack propagation. However, E‐paint MAO‐coated specimens show improved corrosion fatigue strength in 107 cycles at ∼60 and ∼50 MPa under the same conditions due to the sealing of the micro‐pores and micro‐cracks in the MAO coatings.

In this paper, we report the application of zinc phosphate electrostatic-painting top coating on cold sprayed AA7075 leading to a significant improvement in corrosion-fatigue performance. High strength AA7075 powder was sprayed on AZ31B substrate, followed by the application of the top coating. The electrochemical corrosion and corrosion-fatigue tests of the coated and uncoated specimens were performed in 3.5% NaCl solution. Transmission electron microscopy (TEM) analysis showed that a continuous nanolayered mixture of Mg/Al was formed at the cold spray coating/substrate interface leading to high bonding strength. The results showed that the combined coatings improved the corrosion resistance remarkably, and significantly increased the fatigue life, with a fatigue strength of 80 MPa at 107 cycles, as compared to the as-cast specimen. Surface topographic analysis of the corrosion-fatigue-tested specimens demonstrated the presence of deep macro-pits on the cold sprayed AA7075 coating after 3.7 million cycles, while there were no such pits on the top-coated specimens, even after 107 cycles when tested at 30 Hz. The fractographic analysis of the fatigue-fractured specimens showed that the formation of pits allowed the NaCl solution to penetrate in the AZ31B substrate, creating localized corrosion pits resulting in premature failure, which eventually reduced the fatigue life.

In the present study, corrosion-protective microarc oxidation (MAO) coatings were prepared on AZ31B, AZ80, and ZK60 cast magnesium alloy substrates in an alkaline silicate electrolyte. The corrosion performances of the uncoated and MAO-coated alloys were investigated using electrochemical and salt spray chamber corrosion tests. The microstructure characterization and experimental results show that among the three alloys studied, the ZK60 Mg alloy exhibited the best and AZ31B the least corrosion resistance under the salt spray conditions. The MAO coating provided robust corrosion protection of the Mg substrates and resulted in a significant decrease in the corrosion rate of the alloys by 3-4 orders of magnitude. The MAO coating on ZK60 alloy showed better corrosion protectiveness than that on the AZ series alloys due to the incorporation of different alloying elements in the coating, especially the Zn and Al elements, which are from the Mg substrate. The corrosion performances and mechanisms of the uncoated and MAO-coated Mg alloys are interpreted in terms of the microstructure and phase/chemical compositions of both the substrates and coatings. K E Y W O R D S electrochemical testing, magnesium alloys, microarc oxidation, salt spray corrosion test