Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: Part II-PEO and Anodizing

Abstract

Although hexavalent chromium-based protection systems are effective and their long-term performance is well understood, they can no longer be used due to their proven Cr(VI) toxicity and carcinogenic effect. The search for alternative protection technologies for Mg alloys has been going on for at least a couple of decades. However, surface treatment systems with equivalent efficacies to that of Cr(VI)-based ones have only begun to emerge much more recently. It is still proving challenging to find sufficiently protective replacements for Cr(VI) that do not give rise to safety concerns related to corrosion, especially in terms of fulfilling the requirements of the transportation industry. Additionally, in overcoming these obstacles, the advantages of newly introduced technologies have to include not only health safety but also need to be balanced against their added cost, as well as being environmentally friendly and simple to implement and maintain. Anodizing, especially when carried out above the breakdown potential (technology known as Plasma Electrolytic Oxidation (PEO)) is an electrochemical oxidation process which has been recognized as one of the most effective methods to significantly improve the corrosion resistance of Mg and its alloys by forming a protective ceramic-like layer on their surface that isolates the base material from aggressive environmental agents. Part II of this review summarizes developments in and future outlooks for Mg anodizing, including traditional chromium-based processes and newly developed chromium-free alternatives, such as PEO technology and the use of organic electrolytes. This work provides an overview of processing parameters such as electrolyte composition and additives, voltage/current regimes, and post-treatment sealing strategies that influence the corrosion performance of the coatings. This large variability of the fabrication conditions makes it possible to obtain Cr-free products that meet the industrial requirements for performance, as expected from traditional Cr-based technologies.

Keywords: Cr(VI)-based coatings; coating; magnesium; micro-arc oxidation (MAO).

Figure 1

Voltage and current vs. time plots and evolution of discharge sparks. Reprinted from [11] with permission from Elsevier.

Figure 2

Schematic of current–voltage diagram with the corresponding metal oxide film formation steps during plasma electrolytic oxidation treatment (a) in the near-electrode area and (b) in the dielectric film on the electrode surface. Reprinted from [10] with permission from Elsevier.

Figure 3

Simplified timeline for the development of PEO coatings on Mg alloys.

Figure 4

Schematic depiction of the sequence of events during a single discharge, showing: (a) initial electrical breakdown, (b) the development of the plasma channel through the coating thickness, (c) initial bubble growth and formation of oxide in the plasma, (d) bubble expansion and heating of the region around the discharge, (e) shrinkage and cooling as the plasma resistance rises, causing the current to fall, and (f) final quenching and the expulsion of some liquefied oxide from the channel [47].

Figure 5

Scanning electron micrographs of two-layer (a,b) [49] and three-layer (c–e) [53] PEO coatings on Mg alloys. Schematic diagrams of two-layer and three-layer PEO coatings (f). (a,b) Reprinted from [49], (c–e) reprinted from [53] with permission from Elsevier.

Figure 6

General overview of the most frequently used electrolytes components and phases formed in the PEO process on Mg. * Phase formed in a presence of K₂TiF₆.

Figure 7

(a) X-ray diffraction pattern [71] and (b) backscattered electron cross-sectional micrograph of DOW17 coating on AZ91 alloy [72], the latter produced by applying constant current with the voltage rising to 200 V.

Figure 8

Magnesium test plates after TAGNITE-8200, HAE, and Dow 17 treatments and exposure to salt spray (ASTM B117) for 168 (Types I) and 336 (Types II) hours. Adapted from [79].

Figure 9

Cross-section SEM image of a 25 µm-thick Keronite coating on die cast AZ91D. Reprinted from [86] with permission from Elsevier.

Figure 10

Schematic diagrams of (a) PEO coating, (b) simplified model of pores, and (c) corrosion initiation.

Figure 11

SEM microphotographs showing the types of corrosion morphologies of PEO-coated Mg alloys: (a) general undercoating corrosion [49,117], (b) localized corrosion [118], and (c) coating dissolution [116]. Reprinted (a) from [49], (b) from [118], (c) from [116] with permission from Elsevier.

Figure 12

Schematic of soluble organic and inorganic substances added to PEO electrolytes and their effect on the resulting coatings.

Figure 13

(a) Surface morphology; (b) cross sectional morphology; (c) EDS spectrum, along with the chemical composition. (d–i) X-ray elemental mapping of the MAO coating deposited on Mg in two stages, sequentially using the alkaline silicate electrolyte in the first stage, followed by the acidic zirconate electrolyte in the second stage, both at 300 V and for 3 min each stage [180]. Reprinted from [180] with permission from Elsevier.

Figure 14

Potentiodynamic polarization curves of coatings formed in electrolyte without (Bath 1) and with KMnO₄ (Bath 2); (a) samples coated for 60 s; (b) samples coated for 120 s; (c) samples coated for 300 s [175]. Reprinted from [175] with permission from Elsevier.

Figure 15

Overview of the insoluble particle types added in situ into the PEO electrolyte and their effect on the formed phases.

Figure 16

Oxide films formed on Mg alloys in organic electrolytes: (a) barrier type [261]; (b) nanotubular type [260]. Reprinted (a) from [261] and (b) from [260] with permission from Elsevier.

Figure 17

Cross-sectional secondary (a–c) and planar view (d–f). Electron micrographs of PEO coating on AZ31B alloy formed in ethylene glycol and NH₄F electrolyte: (a) polished morphology and EDS line-scanning, (b) fractured morphology, (c) enlarged view of (b); morphology of as-coated surface (d), the coating polished to 11.2 μm (e) and 5.7 μm (f); (g) porosity statistics [262]. Reprinted from [262] with permission from Taylor & Francis.

Figure 18

Salt spray testing results of Cr(VI)-based commercial coating and varied PEO covered with three-component epoxy primer, evaluated according to ASTM D 1654 standard [135].

Figure 19

NSST results for full systems protection with scribed defect (in the form of a cross in the center): (left side) CCC/chromated primer system; (right side) inhibitor loaded PEO coating with chromate-free primer (SiPF/4MSA/primer) [136].

Figure 20

Surface appearance of (a) Ti/Zr + polymer and (b) PEO + polymer coatings after impact + adhesion tests [63]. Reprinted from [63] with permission from Elsevier.

Figure 21

The surface appearance of AZ31 specimens before and after accelerated corrosion tests: (a) untreated, (b) PEO, (c) Ti/Zr + polymer, (d) PEO + polymer, (e) scribed Ti/Zr + polymer and (f) scribed PEO + polymer [63]. Reprinted from [63] with permission from Elsevier.

Figure 22

BSE cross-sections of AZ31 specimens after exposure to ASTM B117 and VDA tests [63]. Reprinted from [63] with permission from Elsevier.