The question of whether zinc can “rust” in water requires distinguishing between the specific chemical process of rusting and the broader process of corrosion. Rusting is chemically reserved for the oxidation of iron, which results in iron oxide (\(\text{Fe}_2\text{O}_3\)). Since zinc is not iron, it does not form this specific red-brown compound, meaning it does not rust. However, like nearly all metals, zinc does undergo oxidation when exposed to moisture and oxygen, which is correctly referred to as corrosion.
The Oxidation Reaction of Zinc
When metallic zinc is exposed to water and oxygen, a chemical reaction immediately begins on the surface. Zinc (\(\text{Zn}\)) reacts with oxygen (\(\text{O}_2\)) to form a thin layer of zinc oxide (\(\text{ZnO}\)). This initial layer is typically unstable in the presence of moisture.
The zinc oxide then quickly reacts with water (\(\text{H}_2\text{O}\)) to produce zinc hydroxide (\(\text{Zn}(\text{OH})_2\)). This reaction is part of the overall oxidation process. Unlike the corrosion product of iron, which is flaky and destructive, the zinc hydroxide is the first step in the metal’s protective mechanism.
The presence of carbon dioxide (\(\text{CO}_2\)) in the air or dissolved in the water alters the reaction pathway. This subsequent reaction is why zinc-coated materials, such as galvanized steel, are durable in wet conditions.
The Formation of a Protective Surface Layer
The zinc hydroxide does not remain in that state for long when carbon dioxide (\(\text{CO}_2\)) is present. This gas reacts with the zinc hydroxide to produce a dense, insoluble substance known as basic zinc carbonate, often simplified as zinc carbonate (\(\text{ZnCO}_3\)).
This newly formed carbonate layer is the source of zinc’s exceptional corrosion resistance, a process known as passivation. Unlike iron rust, which is porous and flakes off, the zinc carbonate adheres tightly to the metal’s surface. It forms a compact, non-porous barrier that chemically isolates the underlying zinc from oxygen and water.
This tightly bound layer effectively halts the corrosion process under normal environmental conditions. The integrity of this zinc carbonate patina means the metal essentially seals itself off after initial oxidation. This mechanism is the foundation for using zinc as a protective coating on steel, a process called galvanization.
How Water Conditions Affect Zinc Deterioration
While the zinc carbonate layer is highly protective, certain water conditions can compromise its stability and accelerate the corrosion rate. Water’s acidity or alkalinity, measured by its pH level, plays a major role in the longevity of the protective layer.
pH Level
The zinc carbonate is stable and corrosion is low within a neutral-to-mildly-alkaline range, roughly pH 6.5 to 12. However, in highly acidic (low pH) or highly alkaline (high pH) water, the protective layer begins to dissolve. This dissolution exposes the underlying zinc metal, allowing the oxidation process to restart continuously. The result is a significantly increased rate of zinc corrosion.
Salinity
High salinity, often associated with seawater or industrial brines, is another accelerating factor. High concentrations of chloride ions (\(\text{Cl}^-\)) interfere with the formation and stability of the passive carbonate layer. The presence of chlorides can lead to the formation of less protective compounds, such as zinc hydroxychloride, which accelerates the corrosion.
Temperature
Water temperature also affects the corrosion kinetics, especially in stagnant conditions. The corrosion rate of zinc tends to increase sharply as water temperature rises above approximately \(50^\circ\text{C}\), reaching a maximum around \(70^\circ\text{C}\). This change is attributed to an alteration in the physical character of the corrosion products, which become less adherent and more granular in that range.