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

Coated steel plates are commonly used in metal corrosion protection because they are cost-effective and provide greater resistance to corrosion. Of these, pure zinc (GI) and zinc-aluminum (AZ) coated steel plates, as well as zinc-aluminum-magnesium (ZM) coatings, represent three generations of corrosion-resistant materials. This paper reviews their characteristics in terms of compositions, corrosion performance, mechanical characteristics, and application settings.

Composition and Microstructure
Pure Zinc Coating (GI)
The molten bath contains no less than 99% zinc in pure zinc coatings. The microstructure is comprised of a dense layer of zinc coat and zinc flowers (spangles), which barely contribute to corrosion resistance. Its iron-zinc alloy layers help to attach the coating to the steel substrate and form a physical barrier to corrosion against any environmental agents.

Zinc-Aluminum Coating (AZ)
AZ coatings are usually composed of 55% aluminum, 43.4% zinc, and 1.6% silicon. The aluminum particles encapsulate zinc to create a honeycomb structure of the alloy, which offers a dual-layer protection. Aluminum oxidizes into a dense Al₂O₃ film, and zinc offers sacrificial cathodic protection. Such a structure provides greater corrosion resistance than zinc without any additives.

Zinc-Aluminum-Magnesium Coating (ZM)
ZM coatings add 1–3% magnesium to the AZ alloy creating a ternary Zn-Al-Mg phase structure. Key phases include Zn₂Mg₃ and MgZn₂, resulting in a more compact microstructure. Magnesium aids in the production of stable corrosion products (for instance, hydroxide and chloride compounds) that self-heal scratches and cut edges, overcoming a significant shortcoming of AZ coatings.

Mechanisms for Corrosion Resistance
Pure Zinc Coating
Uses zinc only for its sacrificial cathodic protection. In neutral salt spray tests, red rust is observed after 120 h of a Z180 coating (180 g/m²). Corrosion resistance has to do with coating thickness with little protection for cut edges or scratches.

Zinc-Aluminum Coating
It combines aluminum’s physical barrier (Al₂O₃ film) and zinc’s cathodic protection. Its corrosion resistance is two to four times higher than pure zinc; after hundreds of hours when subjected to salt spray tests, it will produce red rust. But cut edges still are susceptible because aluminum is a weaker sacrificial material, while zinc serves similarly as a sacrificial agent.

Zinc-Aluminum-Magnesium Coating
Has triple protection: zinc’s cathode, aluminum barrier effect, and magnesium self-healing capability. Within more than 1,000 hours, red rust appears in salt spray tests, and corrosion depth decreases to 1/4 of the pure zinc's. Cut edges create a dense protective film through magnesium-induced corrosion product flow and thus necessitate no extra edge sealing.

Mechanical and Processing Properties
Pure Zinc Coating
Weldability: Good, for resistance welding without electrode adhesion.
Formability: Good ductility enables deep drawing or flexing without penetrating the coating.
Hardness: Less hardness (HV 60–80) can leave it prone to scratches on handling.

Zinc-Aluminum Coating
Weldability: Poor as molten aluminum-zinc alloy sticks to electrodes and must be maintained regularly.
Formability: Moderate; edge cracking can be introduced if bent if not accurately handled.
Hardness: Higher than pure zinc (HV 100–120) but still inferior to ZM.

Zinc-Aluminum-Magnesium Coating
Weldability: Relatively similar to pure zinc with minimal electrode wear for resistance welding.
Formability: Better than AZ coatings, allows deep drawing at low mold cleaning rate.
Hardness: The highest of them (HV 140–160), well-resisting scratches and abrasion.

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.