Self-healing plasma electrolytic oxidation coatings doped with benzotriazole loaded halloysite nanotubes on AM50 magnesium alloy

Abstract

0 200 -150 0 50 Z (kcm 2 ) Z (kcm 2 )2 Research Highlights  Simple single-step PEO process for production of self-healing coatings on Mg  Benzotriazole loaded halloysite nanotubes incorporated in PEO coatings on Mg  The nanotubes increased mechanical properties of PEO coatings  pH increase at corrosion sites promoted BTA-mediated nucleation of Mg(OH)2  Dense Mg(OH)2 film formed on corrosion sites inhibited pitting corrosion Abstract:Halloysite nanotubes (HNT) and benzotriazole (BTA) loaded HNT were added to a silicatebased electrolyte to produce PEO coatings on AM50 alloy. The coatings were characterised by SEM, EDX and XRD methods. Corrosion behaviour in 3.5 wt.% NaCl was studied by EIS and PDP scans. The HNT increased coating scratch resistance greater than BTA-HNT, although the latter enhanced corrosion resistance due to a self-healing effect. This was triggered by a dense Mg(OH)2 film being formed on corrosion sites following partial coating degradation, preventing pitting corrosion. The single-step process for producing self-healing PEO coatings has good potential in corrosion protection of Mg.

Incorporation of halloysite nanotubes into forsterite surface layer during plasma electrolytic oxidation of AM50 Mg alloy

“…Additionally, the Si also tends to increase as the distance from the substrate increases. This suggests HNTs have been incorporated mostly in the outer parts of the coatings, which is consistent with earlier observations [29].…”

“…The duty cycle was 10% and the studied frequencies were 100, 500, 1000 and 5000 Hz. The typical electrical waveform acquired during unipolar PEO treatments can be found in [29].…”

“…Theses fluctuations are associated to the dissolution and re-precipitation of the corrosion products. When the surface is covered by a compact layer of corrosion products that impede further advance of corrosion, the OCP increases, while these corrosion products are dissolved, the electrolyte has direct access to the substrate leading to a shift towards lower OCP values [29,37]. At longer immersion times, 24 to 72h, all coatings show a stable corrosion behaviour with OCP values in the range of -1.57 and -1.55V vs. Ag/AgCl (Figure 12 b-d).…”

“…The presence of a higher number of smaller pores in the coatings processed at higher pulse frequency obstructs the electrolyte access through the coating since the path to the substrate becomes more tortuous compared to that in the coatings with coarser interconnected porosity. The experimental data was fitted (Table 2) to the equivalent circuit shown in Figure 13e, where R s represents the resistance of the electrolyte; R c and CPE c account for the resistance and capacitive behaviour of the coating and L and R 2 represent the inductive loop [29,68]. In order to account for the heterogeneities in the surface layers, constant phase elements (CPE) were used instead of ideal capacitors.…”

“…HNTs are natural clays composed of aluminosilicates and have already demonstrated utility as nano-containers for different types of species such as corrosion inhibitors [26], anti-oxidants [27] and drugs [28]. Previous studies by Sun et al [29] revealed a partially reactive incorporation of HNTs loaded with benzotriazole, a well-known corrosion inhibitor, into PEO coatings formed on AM50 magnesium alloy, however the process still needs to be optimised to achieve the maximum HNTs uptake with non-reactive incorporation.…”