Abstract

This work analyzes the effects of ultrafine aluminum (Al) grains on the anodizing coating reaction and anticorrosion performance of anodized industrial pure Al. Equal-channel angular pressing (ECAP) was applied to cast pure Al continuously for 16 passes at room temperature, and its average grain size was dramatically refined to about 1.5 μm. The ultrafine-grain (UFG) pure Al was further anodized with a cast sample via a parallel anodizing circuit at a constant total input current. Benefited by the higher volume fraction of grain boundaries and higher internal energy of the UFG substrate, the anodizing process of the ECAP-processed pure Al was significantly accelerated, showing a more intense initial anodizing reaction, a faster initial coating thickening, and much earlier porous-layer formation compared to the cast sample. As the anodizing reaction continued, the newly formed thicker coating of the ECAP-coated sample significantly hindered the diffusion process, weakening the thermodynamic advantage and decreasing the anodizing current of the ECAP-processed sample. During the entire anodizing duration, the ECAP-processed pure Al experienced gradually decreased anodizing current, while the cast sample experienced increased anodizing current. Because of the more total reaction, the ECAP-coated sample always maintained a relatively thicker coating and better anticorrosion performance during the entire anodizing duration.

Highlights

  • Industrial pure aluminum (Al) has the distinct advantages of low density, excellent plasticity, good thermal/electrical conductivity, and good corrosion resistance in the atmospheric environment [1,2,3].wide application of industrial pure Al as a structural metallic material is limited significantly because of its extremely low absolute strength [4]

  • Based on the optical microscope (OM) observation, one can deduce that dramatic grain refinement may be achieved in the ECAPed sample

  • Grain size, and detailed intragranular microstructure characteristics of the ECAPed sample were revealed by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) observation

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Summary

Introduction

Industrial pure aluminum (Al) has the distinct advantages of low density, excellent plasticity, good thermal/electrical conductivity, and good corrosion resistance in the atmospheric environment [1,2,3]. Many researchers have investigated and reported the scientific and engineering issues with anodizing Al and Al alloys based on the conventional coarse-grain Al matrix, such as optimization of anodizing parameters, evolution of anodizing reaction, and microstructure and anticorrosion performance of the anodizing coating [33,34,35,36]. Because the Al matrix is involved in the anodizing reaction, the special microstructure characteristics of the UFG Al matrix may have great impact on the anodizing reaction, as well as the coating’s microstructure characteristics and anticorrosion performance It is of great scientific and practical significance to investigate the effect of Al’s UFGs on the anodizing process and coating performance of anodized pure Al. In this study, cast industrial pure Al was processed by a multi-pass ECAP process to achieve. Effects of UFGs in the substrate achieved by ECAP on the accelerated anodizing reaction were systematically revealed, including a faster coating thickening and thicker, more corrosion-resistant anodizing coating of the ECAP-processed pure Al

Processing Route
Microstructure Characterization
Electrochemical Corrosion Test
Microstructure Characteristics
Electrochemical Corrosion Behavior in NaCl Medium
Effect of UFG Al Matrix on Accelerated Anodizing Reaction
Conclusions

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