Self-healing characteristic of praseodymium conversion coating on AZNd Mg alloy studied by scanning electrochemical microscopy

doi:10.1016/j.elecom.2017.01.004

Electrochemistry Communications

Volume 76, March 2017, Pages 6-9

https://doi.org/10.1016/j.elecom.2017.01.004 Get rights and content

Highlights

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Self-healing mechanism of praseodymium conversion coating on magnesium is studied.
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Scanning electrochemical microscopy was used for monitoring defective site.
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The conversion coating shows self-healing characteristic in presence of Pr^3 +.
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The mechanism is associated with high pH at the defect caused by corrosion of Mg.
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Formation of insulating layer and reduction in H₂ evolution was observed.

Abstract

Application of rare earth conversion coatings as a surface treatment for magnesium has been the subject of several studies revealing the potential to act as an effective passivating layer. Herein a mechanistic study is presented on the formation of a rare earth conversion layer based on Pr(NO₃)₃ on AZ80X magnesium alloy in simulated biological (buffered) solution. Scanning electrochemical microscopy (SECM) was used to investigate the insulating properties and degradation behaviour of the Pr conversion layer. The self-healing properties of the conversion layer in the presence of Pr^3 + were also studied using SECM. Results revealed the self-healing characteristic of the Pr conversion film in the presence of active, Pr^3 +, species. The Pr conversion layer provided passivation in the short term by producing an electrochemically inert and insulating layer. SECM results in potentiometric mode elucidated the role of near surface pH in the formation of the conversion coating.

Introduction

Conversion coatings based on lanthanum (La), cerium (Ce) and praseodymium (Pr) have been shown to provide levels of corrosion protection to the underlying metal substrate and their protective properties for a number of Mg alloys such as WE43 [1] AZ31 [2], AZ91, AM50 [3], AZ63 [4] and WE43 [5] have been studied. The majority of these studies have shown effective corrosion protection afforded by the rare earth element (REE) conversion coating in the short term (e.g. under 24 h) that tends to deteriorate as exposure time to the corrosive environment increases. The corrosion inhibition mechanism of REE is often attributed to the deposition of an insoluble passive rare earth (RE) oxide/hydroxide film at cathodic domains [2], [6], facilitated by the alkaline pH which arises from the reduction of water and/or oxygen [7]. In the present study, a recently advanced [8] electrochemical approach using SECM is used to evaluate H₂ evolution as a measure of corrosion protection afforded by the Pr surface treatment. Insulating characteristics of the Pr conversion layer on AZNd were also studied at a local scale using SECM in AC mode. The aim of this study is to provide a better understanding of the film formation and self-healing mechanism of a Pr conversion layer in a buffered solution.

Section snippets

Materials and methods

AZNd was supplied by Boston Scientific with the approximate composition of Al 7.26%, Zn 0.59%, Mn 0.10%, Nd 0.66% (all in wt%) and the balance Mg [9]. Simulated biological fluid (SBF) was prepared according to the recipe outlined in ref. [10]. Praseodymium conversion layers were formed by immersing the AZNd coupons in 0.2 M Pr(NO₃)₃ solution for 30 s. Specimens were then rinsed with DI water and dried with N₂.

SECM was conducted using a 25 μm Pt ultra-micro-electrode (UME) with RG > 10 as working

Results and discussion

The profilometry of the surface with artificial defect (partially conversion coated) is shown in Fig. 1a. Statistical parameters of the surface profile were R_a = 138 nm, R_q = 170 nm and R_z = 1.69 μm. The negatively profiled domains (blue areas) indicate the domains attacked by oxidizing agent (NO₃⁻), during deposition of the PrOx conversion coating. Elemental (Fig. 1d) and structural analysis (Fig. 1c) confirmed formation of a nano-porous layer with chemical composition of Pr₂O₃ and the film thickness

Conclusion

It was shown that Pr^+ 3 serves as an active corrosion inhibitor for Mg with self-healing characteristics. Defective areas of Pr conversion coating can be replenished if active inhibitor is present in the environment resulting in a dynamic deposition of Pr oxide/hydroxide species at highly alkaline domains.

Acknowledgement

Funding from the Australian Research Council Centre of Excellence Scheme (Project Numbers CE0561616 and CE 140100012) and Linkage Grant with Boston Scientific (LP0990621) is gratefully acknowledged.

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¹: Current address: Faculty of Science, Engineering and Technology, Swinburne University of Technology, VIC, Australia.

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Self-healing characteristic of praseodymium conversion coating on AZNd Mg alloy studied by scanning electrochemical microscopy

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusion

Acknowledgement

Corros. Sci.

Appl. Surf. Sci.

Corros. Sci.

Surf. Coat. Technol.

Surf. Coat. Technol.

Corros. Sci.

Corros. Sci.