Self-healing characteristic of praseodymium conversion coating on AZNd Mg alloy studied by scanning electrochemical microscopy
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 Ra = 138 nm, Rq = 170 nm and Rz = 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.
References (14)
- et al.
The corrosion protection afforded by rare earth conversion coatings applied to magnesium
Corros. Sci.
(2000) - et al.
Characterization of rare-earth conversion films formed on the AZ31 magnesium alloy and its relation with corrosion protection
Appl. Surf. Sci.
(2007) - et al.
Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys
Corros. Sci.
(2005) - et al.
Cerium-based chemical conversion coating on AZ63 magnesium alloy
Surf. Coat. Technol.
(2003) - et al.
Influence of a silane treatment on the corrosion resistance of a WE43 magnesium alloy
Surf. Coat. Technol.
(2006) - et al.
Electrochemical techniques for practical evaluation of corrosion inhibitor effectiveness. Performance of cerium nitrate as corrosion inhibitor for AA2024T3 alloy
Corros. Sci.
(2010) - et al.
Dissolution of cerium from cerium-based conversion coatings on Al 7075-T6 in 0.1 M NaCl solutions
Corros. Sci.
(2012)
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Current address: Faculty of Science, Engineering and Technology, Swinburne University of Technology, VIC, Australia.