The Electrochemical Magic of Magnesium
The defining feature of ZAM steel is its ability to protect cut edges without paint. This is due to the unique dissolution and re‑precipitation chemistry of magnesium‑containing corrosion products.
Self‑Healing Mechanism Step‑by‑Step
Initial Exposure: At a sheared edge, bare steel and the Zn‑Al‑Mg coating are exposed to moisture and oxygen.
Galvanic Corrosion: Zinc and magnesium corrode preferentially, releasing Zn²⁺ and Mg²⁺ ions. The steel cathode is protected.
pH Rise: The cathodic reaction (oxygen reduction) generates OH⁻ ions, locally raising the pH at the cut edge.
Mg(OH)₂ Precipitation: Mg²⁺ reacts with OH⁻ to form a gel‑like magnesium hydroxide layer.
Simonkolleite Formation: Zn²⁺ ions, Cl⁻ from the environment, and water react to form Zn₅(OH)₈Cl₂·H₂O (simonkolleite), a dense, highly insoluble, and stable corrosion product.
Physical Barrier: This simonkolleite/Mg(OH)₂ mixture cements over the cut edge, blocking further ingress of oxygen and moisture. The corrosion effectively stops.
Why GI Can’t Do This: Standard zinc corrosion products (ZnO, Zn(OH)₂, basic zinc carbonate) are porous and non‑protective, allowing corrosion to continue undercutting the paint or coating.