Borate’s effects on coatings by PEO on AZ91D alloy

ABSTRACT

This research is to produce corrosion protection coatings on magnesium alloys (AZ91D) with good performance and less time and less power consumption by optimising the electrolytes used in plasma electrolytic oxidation processes. Coatings were produced in a NaAlO₂ solution and two composite electrolytes, namely NaAlO₂/NaBO₂ and NaAlO₂/NaBO₃. The effects of borate additives on the coating formation processes, phase structure, microstructure, and corrosion property are discussed. The results indicate that borate additives can greatly increase the coating growth rate and improve the corrosion property by strengthening the micro-arcing growth process and accelerating coating’s fuse and re-crystallisation. The coating can have a thickness of about 40 μm with NaBO₃ additives in NaAlO₂ electrolyte after 50-min treatment. The coating thickness is only 8 μm using NaAlO₂ electrolyte. Specifically, the coating produced in NaBO₃–NaAlO₂ solution achieves the best corrosion resistance among all the coatings due to its uniform, dense, and thick structure.

KEYWORDS:

• Borates
• PEO
• coating
• magnesium
• corrosion
• formation
• phase
• microstructure

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Herbert DaCosta http://orcid.org/0000-0001-8501-3764

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (CN) under Grant No. 51301153 and No. 51501165; Zhejiang Provincial Natural Science Foundation of China under Grant No. LY17E010002.