Comparison Analysis of Galvanized, Aluminized Zinc, and Zinc-Aluminum-Magnesium Coatings

In the field of metal corrosion protection, coated steel plates are widely used due to their cost-effectiveness and enhanced durability. Among them, pure zinc (GI), zinc-aluminum (AZ), and zinc-aluminum-magnesium (ZM) coated steel plates represent three distinct generations of corrosion-resistant materials. This article provides a comprehensive comparison of their composition, corrosion resistance mechanisms, mechanical properties, and application scenarios.

Composition and Microstructure

Pure Zinc Coating (GI)

Pure zinc coatings contain no less than 99% zinc in the molten bath. The microstructure consists of a dense zinc layer with occasional zinc flowers (spangles), which do not significantly affect corrosion resistance. The coating adheres firmly to the steel substrate through iron-zinc alloy layers, forming a physical barrier against environmental corrosion.

Zinc-Aluminum Coating (AZ)

AZ coatings typically comprise 55% aluminum, 43.4% zinc, and 1.6% silicon. The alloy forms a honeycomb-like structure where aluminum particles encapsulate zinc, creating a dual-layer protection mechanism. Aluminum oxidizes to form a dense Al₂O₃ film, while zinc provides sacrificial cathodic protection. This structure enhances corrosion resistance compared to pure zinc.

Zinc-Aluminum-Magnesium Coating (ZM)

ZM coatings introduce magnesium (1–3%) into the AZ alloy, forming a ternary Zn-Al-Mg phase structure. Key phases include Zn₂Mg₃ and MgZn₂, which contribute to a more compact microstructure. Magnesium promotes the formation of stable corrosion products (e.g., hydroxide and chloride compounds) that self-repair scratches and cut edges, addressing a critical limitation of AZ coatings.

Corrosion Resistance Mechanisms

Pure Zinc Coating

Relies solely on zinc’s sacrificial cathodic protection. In neutral salt spray tests, red rust appears after 120 hours for a Z180 coating (180 g/m²). Corrosion resistance is proportional to coating thickness but offers limited protection for cut edges and scratches.

Zinc-Aluminum Coating

Combines aluminum’s physical barrier (Al₂O₃ film) with zinc’s cathodic protection. Corrosion resistance is 2–4 times higher than pure zinc, with red rust appearing after hundreds of hours in salt spray tests. However, cut edges remain vulnerable due to aluminum’s weaker sacrificial behavior compared to zinc.

Zinc-Aluminum-Magnesium Coating

Exhibits triple protection: zinc’s cathodic action, aluminum’s barrier effect, and magnesium’s self-healing capability. In salt spray tests, red rust appears after over 1,000 hours, with corrosion depth reduced to 1/4 of pure zinc’s. Cut edges form a dense protective film via magnesium-induced corrosion product flow, eliminating the need for additional edge sealing.

Mechanical and Processing Properties

Pure Zinc Coating

Weldability: Excellent, suitable for resistance welding without electrode adhesion.

Formability: Good ductility allows deep drawing and bending without coating damage.

Hardness: Lower hardness (HV 60–80) makes it prone to scratches during handling.

Zinc-Aluminum Coating

Weldability: Poor due to molten aluminum-zinc alloy adhesion to electrodes, requiring frequent electrode maintenance.

Formability: Moderate; bending may cause edge cracking if not properly controlled.

Hardness: Higher than pure zinc (HV 100–120) but still inferior to ZM coatings.

Zinc-Aluminum-Magnesium Coating

Weldability: Comparable to pure zinc, with minimal electrode wear during resistance welding.

Formability: Superior to AZ coatings, enabling deep drawing with reduced mold cleaning frequency.

Hardness: Highest among the three (HV 140–160), resisting scratches and abrasion effectively.

Application Scenarios

Pure Zinc Coating

Cost-sensitive applications: General-purpose structural components, automotive outer panels, and construction frameworks.

Limitations: Unsuitable for harsh environments (e.g., coastal or chemical zones) due to edge corrosion vulnerability.

Zinc-Aluminum Coating

Mderate corrosion environments: Building roofs, walls, automotive exhaust systems, and home appliances (e.g., refrigerator back panels).

Trade-offs: Requires edge sealing for cut sections, limiting use in high-humidity areas.

Zinc-Aluminum-Magnesium Coating

Extreme environments: Offshore wind farms, photovoltaic (photovoltaic supports), chemical plants, and underground pipelines.

Advantages: Long lifespan (20–30 years), reduced maintenance costs, and compatibility with harsh conditions (e.g., salt spray, high humidity).

Cost: Higher initial investment but offsets long-term expenses through durability.