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

Abstract

Corrosion protection systems based on hexavalent chromium are traditionally perceived to be a panacea for many engineering metals including magnesium alloys. However, bans and strict application regulations attributed to environmental concerns and the carcinogenic nature of hexavalent chromium have driven a considerable amount of effort into developing safer and more environmentally friendly alternative techniques that provide the desired corrosion protection performance for magnesium and its alloys. Part I of this review series considers the various pre-treatment methods as the earliest step involved in the preparation of Mg surfaces for the purpose of further anti-corrosion treatments. The decisive effect of pre-treatment on the corrosion properties of both bare and coated magnesium is discussed. The second section of this review covers the fundamentals and performance of conventional and state-of-the-art conversion coating formulations including phosphate-based, rare-earth-based, vanadate, fluoride-based, and LDH. In addition, the advantages and challenges of each conversion coating formulation are discussed to accommodate the perspectives on their application and future development. Several auspicious corrosion protection performances have been reported as the outcome of extensive ongoing research dedicated to the development of conversion coatings, which can potentially replace hazardous chromium(VI)-based technologies in industries.

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

“…Cheaper and technologically easier conversion treatment processes are typically preferred for Mg in engineering applications [ 3 ]. For details on advances in chromate-free conversion coatings for Mg alloys in engineering applications, the reader is referred to of the present review [ 4 ].…”

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

“…This review is the third part of a trilogy on Cr(VI)-free corrosion protection strategies for magnesium alloys. In PART I of the review [ 8 ], conversion coating technologies are reviewed as one of the most widely used methods of protecting metal surfaces. As a controlling step for the performance of a conversion coating, pre-treatment of magnesium surface is also discussed in PART I .…”

“…The latter is deposited as a partially protective layer (e.g., cerium and lanthanum hydroxides), while the former is similar to conversion coating, forming a layer of sparingly soluble magnesium products (phosphates and fluorides). Many of the inhibitors that form precipitation layers on the surface can also be considered as conversion coating precursors (see PART I of this review [ 8 ]). For instance, chromate has been frequently addressed in the literature as either an inhibitor or a chemical used to form conversion coatings.…”

“…This overlap between two categories mostly happens for inorganic substances such as chromate, vanadate, phosphate, and rare earth elements salts. Therefore, here, we briefly provide a review of some inorganic inhibitors studied for magnesium alloys, while the complementary information about conversion coatings formed based on these inhibitors has been provided in PART I of the review [ 8 ].…”

“…Phosphates have been extensively researched as an alternative to chromate-based inhibitors due to their environmentally friendly properties and comparatively high protective properties for magnesium alloys. Numerous phosphate solutions modified with different organic and inorganic additives have been reported, mainly with the aim of achieving a conversion coating on magnesium alloys with higher protective behavior (see PART I of this review [ 8 ]). However, only a few studies have addressed the mechanism of corrosion inhibition caused by phosphate ions in corrosive media.…”