Microstructure and corrosion resistance of modified 2024 Al alloy using surface mechanical attrition treatment combined with microarc oxidation process

doi:10.1016/j.corsci.2010.09.061

Corrosion Science

Volume 53, Issue 1, January 2011, Pages 473-480

https://doi.org/10.1016/j.corsci.2010.09.061 Get rights and content

Abstract

A top ceramic coating was fabricated on the surface mechanical attrition treatment (SMAT) modified nanocrystalline layer of 2024 Al alloy by microarc oxidation (MAO) process. The corrosion resistance of the SMAT-MAO composite coating was studied by EIS. The results show that SMAT-MAO composite coating with 10 μm top ceramic coating exhibited better corrosion resistance, while the SMAT-MAO coating with the thickness of 15 μm showed worse corrosion resistance compared with those simple MAO coatings with same thickness. The formation of a dense passive film at the damaged region caused by the bottom nanocrystalline interface contributed to the improved corrosion resistance property.

Research highlights

► A 20 μm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated on the surface of 2024 Al alloy by means of SMAT. ► A composite coating (named SMAT-MAO coating) was fabricated on the surface mechanical attrition treatment (SMAT) modified nanocrystalline layer of 2024 Al alloy by microarc oxidation (MAO) process. ► SMAT-MAO composite coatings with 10 μm top ceramic coating exhibited better corrosion resistance in comparison with the simple MAO coating with same thickness, while the SMAT-MAO coating with the thickness of 15 μm showed worse corrosion resistance in comparison with that of 15 μm MAO coating. ► The better corrosion resistance of composite coating can be attributed to the formation of a dense passive film at the damaged region caused by the bottom nanocrystalline interface.

Introduction

A relatively novel surface modification technique, conventionally called “microarc oxidation (MAO)”, also commonly called “plasma electrolytic oxidation (PEO) [1], [2]”, is attracting increasing interest in fabricating ceramic coatings on light alloys, with the purpose of providing corrosion- and wear-resistance properties. MAO coatings have been successfully fabricated on the surface of Al [1], [2], [3], [4], Mg [5], Ti [6], [7] and their alloys.

Anti-corrosion properties of MAO coating formed on Al alloy have been extensively investigated [8], [9], [10], [11], [12], [13], [14], [15], [16], and the results show that the corrosion resistance of Al alloy substrate has been improved by the formation of MAO coating. However, MAO coating cannot maintain a stable corrosion resistance in a long-term immersion in corrosion medium due to its porous structure. Barik studied the corrosion performance of MAO coatings, and demonstrated that unsealed MAO coating allows permeation of solution through the pores in the coating [8]. In our previous work [16], it has been proved that the corrosion medium penetrated into the interface between the alloy and the coating through the defects of MAO coating (micro pores or micro cracks in the coating) after a short period of immersion, and caused pitting corrosion.

It has been found that nanocrystalline surface layer has better corrosion resistance compared with that of coarse-grained Al alloys due to the formation of dense passive films [17], [18]. Therefore, if a nanocrystalline layer is fabricated at the bottom of MAO coating, it will be expected to increase global corrosion resistance of coated sample. Surface mechanical attrition treatment (SMAT) [19] is a novel method which can induce grain refinement into the nanometer scale in the surface layer of bulk samples. Using SMAT, nanocrystalline layers in the surface of various materials, such as stainless steel [20], [21], pure Cu [22], pure Fe [23], Al alloy [24], Mg alloy [25] and so on, have been successfully produced.

In this paper, we took SMAT as the pretreatment process for 2024 aluminum alloy, followed by the MAO treatment, in this way the specially modified layers structure with refined grains bottom layer covered by a top conversion ceramic coating by MAO process was fabricated. This work focuses on the influence of the bottom nanocrystalline layer on corrosion resistance of the SMAT-MAO coated aluminum alloy sample, finally understanding the undermined corrosion mechanism.

Section snippets

Materials

The material used in the experiment is 2024 Al alloy with nominal composition shown in Table 1. The specimens, 80 mm × 80 mm × 3 mm in dimensions, were ground with 400#, 800# and 1200# abrasive papers, ultrasonically washed with acetone and distilled water, and dried for SMAT.

Nanocrystalline modified layer preparation by SMAT

SMAT was performed in vacuum using a SNC-1 type machine. A container of glass fiber reinforced plastic (GFRP) was placed inside the steel chamber that was vibrated by a generator. ZrO₂ balls with a diameter of 6 mm, instead of the

Microstructure of nanocrystalline modified surface

The XRD patterns of surface layer of 2024 Al alloy samples before and after SMAT are shown in Fig. 1. It can be found that, compared with 2024 Al alloy, there is an evident broadening of the Bragg-diffraction peaks and a slight right shift in the position of diffractions after SMAT. The Bragg-diffraction peak broadening in the surface layer may be attributed to grain refinement and the shift of the diffraction peaks is due to microstrain development. During SMAT, a nanocrystalline surface layer

Conclusions

(1)
A 20 μm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated on the surface of 2024 Al alloy by means of SMAT.
(2)
On the 20 μm thickness nanocrystalline layer, the top transformed MAO ceramic coatings with 10 and 15 μm thickness were prepared successively.
(3)
During the whole immersion test, the corrosion resistance of the MAO coated alloy decreased gradually, while that of SMAT-MAO coated alloy decreased in the initial state, then increased. The difference can be attributed to the

Acknowledgements

The partial supports from the NSFC Grant Nos. 50701014 and 60776803, the program for New Century Excellent Talents in University of China (NCET-08-0166) are gratefully acknowledged. The authors also thank Prof. T. Zhang for the assistance of corrosion test and analysis.

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Microstructure and corrosion resistance of modified 2024 Al alloy using surface mechanical attrition treatment combined with microarc oxidation process

Abstract

Research highlights

Introduction

Section snippets

Materials

Nanocrystalline modified layer preparation by SMAT

Microstructure of nanocrystalline modified surface

Conclusions

Acknowledgements

Surf. Coat. Technol.

Ceram Int.

Surf. Coat. Technol.

Surf. Coat. Technol.

Appl. Surf. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Electrochim. Acta

Electrochim. Acta

Surf. Coat. Technol.

Corros. Sci.

Corros. Sci.

Corros. Sci.

Mater. Sci. Eng. A

Corros. Sci.

Acta Mater.