Ca-based sealing of plasma electrolytic oxidation coatings on AZ91 Mg alloy
Introduction
Plasma electrolytic oxidation (PEO) is an environmental-friendly surface treatment process that has shown advantages to enhance both corrosion and wear resistance of Mg alloys [[1], [2], [3], [4], [5], [6]]. The technique involves polarization at high voltages (i.e. above dielectric breakdown) resulting in short-lived micro discharges and development of ceramic-like layer. The phase composition of the coating is mainly originated from the substrate and electrolyte [[7], [8], [9]]. However, porosity formed due to gas evolution at the location of the high-intensity discharges severely compromises the coating corrosion performance [10,11].
Sealing of the porous layer by post-treatment has been explored as an alternative to improve the long-term corrosion resistance of PEO coated Mg alloy. Sol-gel [[12], [13], [14]], epoxy [[15], [16], [17], [18]] and inorganic coatings [[19], [20], [21], [22], [23]] are examples of strategies aimed at forming additional layers or blocking the coating pores and defects. As a general rule, when compared with organic sealings, inorganic based post-treatments are easier to apply, more environmentally friendly and show better thermal and mechanical integrity [24]. The adhesion of polymer based top layers also tends to be higher in case of inorganic sealings, although new hybrid systems based on organic sealings can provide a good adhesive based for organic coatings [18,25,26]. As can be observed in Table 1, sealing of PEO coated Mg by means of immersion into inorganic solutions appears to be an efficient and facile way for industrial application. It was recently found that optimized Ce-based treatment can improve the corrosion performance of PEO coatings by sealing the open pores with precipitation product. However, the acidic electrolyte used during post-treatment process may lead to excessive coating dissolution and deterioration of the inner barrier layer [19,20,22]. Mingo et al. [27] investigated the sealing effect of three different post-treatment processes on the corrosion property of PEO coatings on AZ91 Mg alloy. It was reported that coating sealed by octodecylphosphate acid (ODP) showed the best corrosion performance because of the hydrophobic property.
Corrosion inhibitors for Mg alloys are being studied extensively due to their promising results [[28], [29], [30], [31], [32]]. In this sense, it is worth noting that the high porosity of PEO coatings can be regarded as a natural container for storing corrosion inhibitors [14,31,33,34]. For instance, Yang et al. [33] added 3-methysalicylate via low vacuum environment and subsequently sealed the PEO coating by dip-coating in an epoxy resin. They found that the electrochemically active areas and corrosion current density were reduced to relatively low levels after incorporation of corrosion inhibitor. Therefore, loading of corrosion inhibitors into the porous PEO layer is feasible and may constitute a viable means to achieve active corrosion protection.
In the present study, Ca-based sealing post-treatment was applied to seal the porous PEO coating formed on AZ91 Mg alloy for structural applications. Since the dissolution/precipitation reaction during sealing is of vital importance for the coating properties, the effects of pH during post-treatment were studied on the morphology, composition and corrosion resistance of PEO coatings. Sodium dodecyl sulfate (SDS), which was previously found to be an effective corrosion inhibitor for AZ91 Mg [35,36], was evaluated as an additive for improving the sealing performance.
Section snippets
Materials and coating preparation process
AZ91 Mg alloy in size of 30 × 15 × 5 mm was ground up to 2000 grit SiC paper before fabrication of PEO coatings. The chemical composition of the alloy can be found in our previous study [37]. The PEO specimens were produced in an alkaline phosphate containing electrolyte (30 g/L Na3PO4·12 H2O, 4 g/L KF·2 H2O and 2 g/L NaOH) under a galvanostatic current mode (2 A/cm2) for 15 min. The applied frequency was 500 Hz and duty ratio was 30%. The post-treatment solution was composed of 70 g/L Ca(NO3)2
Coating characterization
Fig. 1, Fig. 2 demonstrate the surface and cross section of the coatings before and after the different sealing treatments. Open pores in size of 2–5 μm and cracks can be observed on the coating surface, which is typical microstructure for PEO coated Mg (Fig. 1a) generated due to gas evolution and discharges during coating formation process [38]. It is apparent that the surface of all PEO coatings is partially covered by newly formed precipitates after the sealing post-treatments. Precipitation
Conclusions
- 1)
The corrosion resistance of PEO coatings on AZ91 Mg alloy can be enhanced by performing sealing post-treatment process. Corrosion inhibitor (SDS) is added into the post-treatment electrolyte to further improve the corrosion performance of the coatings.
- 2)
Newly formed Ca containing precipitate, i.e. hydroxyapatite, is deposited on the original coating surface, which is balanced by the dissolution and redeposition speed during sealing post-treatment process.
- 3)
The existence of SDS provides
CRediT authorship contribution statement
Xiaopeng Lu: Conceptualization, Investigation, Writing – review & editing, Funding acquisition. Jirui Ma: Conceptualization, Methodology, Investigation, Writing – original draft. Marta Mohedano: Conceptualization, Investigation, Writing – review & editing. Borja Pillado: Conceptualization, Investigation, Writing – review & editing. Raúl Arrabal: Conceptualization, Investigation, Writing – review & editing. Kun Qian: Conceptualization, Writing – review & editing. Yan Li: Investigation, Writing –
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (NO. 52071067 and 51531007), the Fundamental Research Funds for the Central Universities (N2002009), RTI2018-096391-B-C33 (MCIU/AEI/FEDER, UE) and M. Mohedano is grateful for the support of RYC-2017 21843.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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