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.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys—A Review: PART I—Pre-Treatment and Conversion Coating

“…These two treatments must be addressed together, as the pre-treatment step that aims to modify the magnesium surface properties plays a critical role in the performance of the subsequent conversion layer against corrosion. PART II of the review [ 5 ] focuses on Plasma Electrolytic Oxidation (PEO) coating as one of the highly developed methods to protect magnesium surface in the recent years. PART III [ 6 ] reviews corrosion inhibitors for magnesium and approaches to incorporate them into coating systems.…”

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys—A Review: Part III—Corrosion Inhibitors and Combining Them with Other Protection Strategies

“…As a controlling step for the performance of a conversion coating, pre-treatment of magnesium surface is also discussed in PART I . PART II of the review [ 9 ] focuses on Plasma Electrolytic Oxidation (PEO) coating as one of the highly developed methods to protect magnesium surface in the recent years. This part of the review ( PART III ) focuses on corrosion inhibitors for magnesium and approaches to incorporate them into coating systems.…”

“…However, a further increase in 8HQ concentration results in more heterogeneous morphology, with some regions porous and others compact ( Figure 20 c,d) [ 234 ]. A comprehensive review of the effects of organic/inorganic additives to the PEO electrolytes for Mg alloys has been provided in PART II of this review [ 9 ]. One of the very first attempts to incorporate inhibitors into a PEO layer during the coating formation on a magnesium alloy in a PEO bath was carried out by Blawert et al [ 239 ].…”

“…Sealing of the PEO pores after the inhibitor loading is essential to prevent the wasteful release of the inhibitors into the corrosive medium. Sealing of the pores can be achieved via different surface post-treatments that are described in detail in PART II of this review [ 9 ]. Some chemicals known for their inhibition effect on Mg alloys, such as rare earth element salts [ 240 , 241 , 242 , 243 , 244 , 245 , 246 , 247 , 248 , 249 ], phosphates [ 242 , 250 ], stannate [ 240 ], and surfactants [ 240 ], can be used primarily for the sealing post-treatment.…”

In-situ estimate of coating by equivalent circuit for PEO of AZ31B

“…Magnesium alloy has poor corrosion resistance and strong chemical activity, it needs to be improved by surface treatment [1,2]. Among various surface treatment methods, plasma electrolytic oxidation (PEO) has the advantages of a strong strengthening effect, non-pollution, and simple process [3–5]. However, the plasma discharge process of magnesium alloy involves a complex combination of physical processes, chemical reactions and electrochemical reactions [6–8].…”