Short CommunicationCorrosion protection afforded by praseodymium conversion film on Mg alloy AZNd in simulated biological fluid studied by scanning electrochemical microscopy
Introduction
The rapid corrosion of magnesium and its tendency for localized corrosion in biological environments [1] are serious drawbacks that significantly limit its biomedical applications [2]. One effective method to overcome these issues is chemical modification of the magnesium surface. Chemically formed conversion coatings that occur via reaction of an active species (e.g. a rare earth element (REE)) can be more inert with respect to corrosion. Conversion coatings based on lanthanum (La), cerium (Ce) and praseodymium (Pr) has been shown to provide levels of corrosion protection to the underlying metal substrate and their protective properties have been studied for number of Mg alloys such as WE43 [3] AZ31 [4], AZ91, AM50 [5], AZ63 [6] and WE43 [7]. Majority of these studies have shown effective corrosion protection afforded by the REE conversion coating in the short term (e.g. under 24 h) that tends to deteriorate as exposure time to corrosive environment increases. The corrosion inhibition mechanism of REE is often attributed to the deposition of an insoluble passive RE oxide/hydroxide film at cathodic domains [4], [8], facilitated by the alkaline pH which arises from the reduction of water and/or oxygen [9]. The failure mechanism of these conversion coatings have been generally attributed to penetration of corrosive species (e.g. chloride) through the conversion coating towards the metal substrate. However, the role of coating inhomogeneities and localized defects in corrosion mechanism has not been fully understood due to the lack of studies with localized electrochemical methods.
The robust data collection modes of SECM present a unique localized electrochemical technique that combines capabilities of physically assessing insulating/conducting domains of a surface with highly sensitive chemical detection [10]. SECM in AC mode allows for localized detection of active/passive domains and defective areas of corrosion protective films on an active metal [11], [12]. In generation/collection (G/C) mode of SECM, electrochemically oxidisable or reducible species produced at the substrate can be sensed via an electrochemical reaction at the SECM probe. Applying an oxidative potential at the SECM, flux of H2 evolution can be sensed and implemented to evaluate corrosion behaviour on Mg surface with a localized approach [13], [14], [15]. In comparison to other well-trusted corrosion measurement methods such as weight loss measurement and hydrogen collection [16], [17], SECM presents the unique capability of studying spontaneous corrosion phenomena while cumulative methods such as weight loss and hydrogen collection can only affords corrosion rate data after a certain period of immersion.
We investigate here a Pr surface treatment to impart corrosion protection to AZNd. In biomedical application, use of RE compounds should also be considered from the perspective of their potential cytotoxicity. Anti-carcinogenic properties of REs has been shown in number of studies [18], [19], [20], [21], however some studies have also reported cytotoxic and hepatotoxic effects at high dose [22], [23], [24], [25]. It has been shown that rare-earth alloying elements including Ce, Nd, Y, and Yb have no adverse effect on growth of living cells but they can induce inflammatory effects at high concentration [26]. In vitro studies of cytotoxicity of REs on primary human and mouse cells demonstrated highest relative cytotoxicity of La and Ce while good cell viability was achieved in presence of Eu, Nd and Pr [27]. A recently advanced [13], [15] electrochemical approach using SECM was used to evaluate H2 evolution as a measure of corrosion protection afforded by the surface treatment. Degradation of the Pr conversion layer on AZNd is also studied at a local scale using SECM in AC mode. The aim of this study is to help for better understanding of the degradation mechanism of a Pr conversion layer in a simulated biological environment.
Section snippets
Material
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. XRF analysis measured the composition as Al 7.3%, Zn 0.32%, Mn 0.024%, Nd 0.63% and the balance Mg. The first composition detailed above is the actual quantities of elements used during the casting of the AZNd alloy. The XRF analysis was performed to investigate if contaminants entered the alloy composition during the casting process. The mismatch
Sample preparation
All specimens (5 mm × 5 mm) were abraded successively using 600, 1200 and 2400 grit emery and ultrasonically cleaned in isopropanol before exposure to electrolyte. Specimens were then immersed in 0.2 M NaOH solution for 2 min. Specimens were then rinsed and 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. This method produced a PrOx film with 700 nm to 1 μm thickness confirmed by SEM analysis at
Methods
Scanning electrochemical microscopy (SECM) was performed using CH instruments SECM model 920D. A 25 μm Pt ultra-micro-electrode (UME) with RG > 10 was used as working electrode. A Pt mesh and a standard Ag/AgCl (3 M KCl) served as reference and counter electrodes, respectively. An area of 1000 × 1000 μm was examined according to the experimental procedure described in reference [13]. A schematic representation of the SECM set-up and data collection principle used in this study is shown in Fig. 2. The
Results and discussion
SEM micrographs shown in Fig. 3a and b illustrate the nano-porous structure of the Pr treated AZNd surface. EDX analysis (Fig. 4 and Table 1) identified the chemical composition of surface layer as Pr2O3. The cracks in the Pr2O3 film shown in Fig. 3a were not observed under optical microscope examination (Fig. 3g) before the SEM analysis and therefore appearance of these cracks were attributed to the dehydration of the conversion film in vacuum chamber during SEM analysis.
The insulating
Conclusion
AC–SECM results showed that Pr treatment using nitrate salt solution results in formation of an inconsistent PrOx conversion layer on the AZNd Mg alloy with a variation of insulating properties across the surface. Sensing H2 evolution by means of SECM in SG/TC mode was used as a method of examining the corrosion behaviour of Mg and obtaining comparative measures of corrosion rate between a non-treated and a Pr treated Mg surface. It was shown that corrosion resistance of AZNd during a short
Acknowledgments
The authors thank the Australian Research Council (ARC) for continuing financial support. GGW and MF thank the ARC for their Laureate Fellowships and SEM thanks the ARC for his QEII Fellowship.
References (35)
- et al.
Acta Biomater.
(2011) - et al.
Acta Biomater.
(2011) - et al.
Corros. Sci.
(2000) - et al.
Appl. Surf. Sci.
(2007) - et al.
Corros. Sci.
(2005) - et al.
Surf. Coat. Technol.
(2003) - et al.
Surf. Coat. Technol.
(2006) - et al.
Corros. Sci.
(2010) - et al.
Corros. Sci.
(2012) - et al.
Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy
Electrochim. Acta
(2003)
Applications of scanning electrochemical microscopy (SECM) for local characterization of AZ31 surface during corrosion in a buffered media
Corros. Sci.
J. Electroanal. Chem.
Semin. Cancer Biol.
Toxicol. Appl. Pharm.
Fund. Appl. Toxicol.
J. Rare Earths
Acta Biomater.
Cited by (40)
Development of anti-corrosion and hydrophobicity of a nanostructured Ce-La film via the PDA post-treatment modification
2023, Journal of Alloys and CompoundsOxygen reduction reaction monitored during corrosion by means of pumped-micropipette delivery/substrate collection (Pumped-MD/SC) mode of scanning electrochemical microscopy
2023, Electrochimica ActaCitation Excerpt :Iron (Fe2+)[9,10], Nickel (Ni2+)[11], Copper (Cu2+)[12,13], Magnesium (Mg2+)[14,15,16], and Zinc (Zn2+)[17] ions flux coming out from the anodic sites were successfully monitored. On the other hand, the cathodic sites were also mapped mainly by the redox competition (RC) and tip generation/substrate collection (TG/SC) modes of SECM to monitor O2 reduction reaction (ORR)[10,18,19,20,21,22], and the substrate generation/tip collection (SG/TC) mode of SECM to monitor H2 evolution[23,24], respectively. However, the visualization of cathodic sites based on amperometric probes either by the RC or TG/SC modes present several limitations.
Corrosion resistance and mechanisms of Nd(NO<inf>3</inf>)<inf>3</inf> and polyvinyl alcohol organic-inorganic hybrid material incorporated MAO coatings on AZ31 Mg alloy
2023, Journal of Colloid and Interface ScienceInvestigating local corrosion processes of magnesium alloys with scanning probe electrochemical techniques: A review
2022, Journal of Magnesium and AlloysPreparation and characterization of cerium-based conversion coating on a Fe<inf>50</inf>Mn<inf>30</inf>Co<inf>10</inf>Cr<inf>10</inf> dual-phase high-entropy alloy
2021, Applied Surface ScienceCitation Excerpt :However, chromate conversion coating is strictly forbidden by international legislation because the hexavalent chromium compounds used in the process are toxic and carcinogenic and pollute the environment [46]. Various conversion coating methods involving molybdate [47], zirconium [48], stannate [39], titanate [49], vanadate [50,51], and rare-earth metal salts [52–56] comply with the Restriction of Hazardous Substances directive and have grown in popularity for their low toxicity and small environmental impact. The cerium-based conversion coating method for Mg and Al alloys has developed steadily [56–61] since the pioneering study of AA7075 by Hinton et al. [62].
The chemical treatment of the AZ31-Magnesium alloy surface by a high-performance corrosion protective praseodymium (III)-based film
2021, Materials Chemistry and Physics