Key Takeaways & Executive Findings
- •• • Laser surface melting with alternating magnetic field on magnesium alloy (Ref. [60]) significantly improves wear and corrosion resistance, with the magnetic field controlling microstructure and reducing defect density; this matters industrially for components exposed to sliding wear and corrosive environments, such as automotive transmission housings. • • Broad-beam laser cladding of Al–Cu alloy on AZ91HP (Ref. [58]) produces a coating with enhanced microhardness and corrosion potential; the process achieves metallurgical bonding, but dilution from the substrate can compromise corrosion performance if not optimized, affecting durability in marine or deicing salt exposure. • • Electroless nickel plating on AZ61 magnesium alloy (Ref. [56]) provides a uniform barrier layer, but pre-treatment and bath stability are critical; the coating reduces corrosion current density by orders of magnitude, yet galvanic coupling at pores can accelerate localized attack, requiring post-treatment sealing for automotive under-hood applications. • • Laser surface modification of AZ91D (Ref. [62]) refines grain size and redistributes secondary phases, lowering corrosion rate; the refined microstructure reduces galvanic corrosion between α-Mg and β-Mg17Al12 phases, which is essential for biomedical implants where degradation rate must be controlled to match tissue healing.
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Abstract
Magnesium alloys, with a density of approximately 1.74 g/cm³ (two-thirds that of aluminum and one-quarter that of steel), offer high specific strength and excellent damping capacity, making them attractive for automotive, aerospace, and consumer electronics applications. However, their standard electrode potential of −2.37 V versus the standard hydrogen electrode renders them highly susceptible to corrosion in humid atmospheres, chloride-containing media, and mild acidic conditions. This review systematically examines the corrosion mechanisms of magnesium alloys, categorizes corrosion types across different environments, and evaluates surface modification technologies including chemical conversion, electroplating and electroless plating, anodizing and micro-arc oxidation, laser surface treatment, thermal spraying, cold spraying, and organic coatings. These techniques form dense protective layers that isolate corrosive media and enhance corrosion resistance. Key findings from the literature demonstrate that laser surface melting with alternating magnetic fields reduces wear and corrosion rates, while laser-clad Al-Cu coatings on AZ91HP improve surface hardness and corrosion potential. Electroless nickel plating on AZ61 and anodizing treatments provide barrier protection, though coating adhesion and porosity remain challenges. The review also discusses current limitations such as long-term durability, cost-effectiveness, and scalability, and outlines future directions including multi-layer composite coatings and environmentally friendly processes. This work provides a reference for advancing magnesium alloy applications in engineering.
1. Introduction
Magnesium alloys present a persistent engineering paradox: their low density (1.74 g/cm³) and high specific strength enable significant weight reduction in automotive and aerospace structures, yet their standard electrode potential of −2.37 V versus the standard hydrogen electrode drives rapid corrosion in humid, chloride-rich, or acidic environments. Commercial adoption has stalled because unprotected magnesium components fail prematurely in coastal atmospheres, road deicing salt exposure, and industrial cleaning processes. Existing protection methods—such as chromate conversion coatings—face regulatory bans due to hexavalent chromium toxicity, while anodized layers often suffer from porosity and poor adhesion, leading to localized pitting and galvanic corrosion at coating defects.
This review addresses the bottleneck by systematically evaluating surface modification technologies that form dense, adherent barriers. It analyzes chemical conversion, electroplating and electroless plating, anodizing and micro-arc oxidation, laser surface treatment, thermal spraying, cold spraying, and organic coatings. The protocol specifically targets the root cause: the weak, non-protective oxide film on magnesium. By examining processing parameters—such as laser power, magnetic field assistance, and bath chemistry—the review identifies pathways to reduce porosity, enhance coating-substrate bonding, and mitigate galvanic coupling. The findings provide a framework for selecting protection strategies based on environmental severity and component geometry, moving magnesium alloys from laboratory curiosity to reliable industrial materials.
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TANG Rui, LI Chunyan, YANG Longpeng, TANG Yunlong, WANG Xinhua, NAN Hongbing, ZHAO Erxiang, KOU Shengzhong (2026). Corrosion Challenges and Surface Protection Strategies for Magnesium Alloys. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.08.002
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Frequently Asked Questions
What is the primary failure mechanism of anodized coatings on magnesium alloys under chloride-rich conditions?
Anodized coatings on magnesium alloys typically exhibit a porous outer layer and a thin barrier layer. In chloride-rich environments, chloride ions penetrate pores and reach the substrate, initiating pitting corrosion. The barrier layer, often less than 1 µm thick, breaks down at defects, leading to localized attack. Galvanic coupling between the coating and substrate can accelerate dissolution at pore bases. Post-treatment sealing with silanes or polymers is required to block chloride ingress and extend service life.
How does laser surface melting with alternating magnetic field improve corrosion resistance compared to conventional laser melting?
Alternating magnetic field during laser melting induces electromagnetic stirring in the melt pool, promoting uniform distribution of alloying elements and refining grain size. This reduces microsegregation and the volume fraction of cathodic β-Mg17Al12 phase, thereby lowering galvanic corrosion. The magnetic field also helps degas the melt, reducing porosity. The result is a more homogeneous, less defective surface with improved corrosion potential and lower corrosion current density, as demonstrated on magnesium alloys in Ref. [60].
What are the cost and scalability barriers for electroless nickel plating on magnesium alloys in automotive production?
Electroless nickel plating requires multiple pre-treatment steps (degreasing, acid pickling, activation) and tight control of bath temperature (85–90 °C) and pH (4.5–5.5). Bath life is limited by nickel depletion and accumulation of hypophosphite byproducts, necessitating frequent replenishment or replacement. The process is batch-oriented, making continuous inline integration difficult. Costs are further increased by waste treatment for nickel and complexing agents. For high-volume automotive parts, alternative methods like cold spraying or micro-arc oxidation may offer better cost parity.
How do cold-sprayed coatings compare to thermal-sprayed coatings for magnesium alloy protection in terms of porosity and adhesion?
Cold spraying operates at lower temperatures (below melting point), minimizing oxidation and phase transformations. It produces denser coatings with lower porosity (often <1%) compared to thermal spraying, which can have porosity levels of 5–15% due to splat solidification and gas entrapment. Cold-sprayed particles undergo severe plastic deformation, leading to mechanical interlocking and metallurgical bonding, resulting in higher adhesion strength. However, cold spraying requires ductile feedstock powders and high gas pressures, which may limit coating materials and increase equipment cost.
What are the long-term durability concerns for organic coatings on magnesium alloys in outdoor applications?
Organic coatings on magnesium alloys face degradation from UV radiation, thermal cycling, and moisture ingress. Hydrolysis of polymer chains and interfacial delamination can occur, especially if the coating has pinholes or scratches. Once moisture reaches the substrate, corrosion initiates and spreads beneath the coating, causing blistering and loss of adhesion. Long-term durability requires multi-layer systems with a conversion coating or anodized base, a primer with corrosion inhibitors, and a topcoat with UV stabilizers. Field exposure data beyond 5 years are scarce, necessitating accelerated testing that correlates with real-world conditions.
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