Does zinc rust?
Like all metals, zinc corrodes when exposed to air and moisture. However, this element does not rust like most other metals.
Iron, for example, reacts with water and oxygen in the atmosphere to form hydrated iron (III) oxide on the metal's surface. This corrosion product, commonly called "rust," is known for its reddish-brown color and its damaging effects. Hydrated iron oxides are brittle and flake away easily, thus exposing more of the underlying metal to the atmosphere for further corrosion. This cycle repeats itself until the iron is fully consumed by the corrosion process.
Zinc, on the other hand, reacts with oxygen to form a thin oxide layer. This layer then reacts with moisture to produce zinc hydroxide, which further reacts with carbon dioxide to create zinc carbonate. Unlike iron oxides, which flake off easily, zinc carbonate is resilient, chemically stable, and adheres firmly to the metal's surface. This layer acts as a protective barrier that prevents air and moisture from contacting the underlying substrate and prevents further corrosion and deterioration.
While the carbonate layer has protective properties, zinc is a reactive metal and will slowly corrode over time. Zinc's corrosion rate is 1/30 that of steel. However, when sulfur-type pollutants are present, they will react with the zinc carbonate and moisture to form a water-soluble zinc sulfate. In these cases, the corrosion rate can be as much as four times greater than in coastal environments without such pollutants. The amount of pollutants present will impact the corrosion rate. (For more on corrosion rate, read: Corrosion Rate Conversion: Simple Ways to Convert Data Between Common Corrosion Units.)
Therefore, zinc metals and components coated with zinc-based products can have a significantly longer service life than other metals when exposed to the atmosphere. Zinc-rich coatings are used in a processes known as hot-dip or cold galvanizing to prevent corrosion.
Hot-dip galvanizing of steel is accomplished by immersing the part continuously, or in batches, in a bath of molten zinc. The coating's structure consists of successive layers that are enriched in zinc until the outer layer is pure zinc. The outer layer is relatively thin and thus the corrosion resistance is limited.
Cold galvanizing involves applying zinc paint, either by brushing it on or by spraying zinc dust/oxide mixed with a binder. (For more about galvanizing, read: Hot-dip vs Cold Galvanizing: What’s the Difference?)
Zinc-bearing paints can be used as a primer or finish coating. This type of coating provides the least long-term corrosion protection but is the easiest to apply in the field. Thus, they make repairs fairly straightforward. Zinc-bearing paints can also be used as a coating on galvanized steel structures in marine environments or for those immersed in seawater for added corrosion protection.
Other common zinc coating processes include electrogalvanizing and zinc plating. Electrogalvanizing usually produces a thinner coating with less corrosion protection. A thin, adherent coating of zinc is accomplished by electrodeposition.
Service life is a linear function of zinc's thickness. The inner layers of iron-zinc provide some additional corrosion protection once the outer zinc layer has been corroded away. Requirements for galvanizing iron and steel products along with pertinent ASTM standards can be found in ASTM Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products A123 and ASTM A525 Standard Specification for General Requirements for Steel Sheet, Zinc-coated (Galvanized) by the Hot-Dip Process. (For more on ASTM standards, read: 6 Tests to Assess Intergranular Corrosion Using ASTM A262.)
Zinc is often electroplated onto steel screws, nuts and other small parts. The article to be plated is made the anode in an electrolytic cell containing a zinc salt or electrode. When current is passed through the cell, zinc is plated onto the part. According to the ASTM B633 Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, the maximum plating thickness is about 25µm.
Zinc may also exhibit a type of corrosion known as white rust. White rust occurs when zinc surfaces have not had enough time to fully develop the protective carbonate layer. Instead, the surface remains with a zinc hydroxide layer, which has a white, powdery appearance. This typically occurs due to a lack of free-flowing air over the zinc surface and forms from the reaction of moisture and carbon dioxide with the zinc hydroxide layer. However, white rust is usually considered to be superficial and can be remedied by exposing the rust deposits to free-flowing air or by removing it with light brushing. White rust that forms, for example, on freshly zinc-galvanized surfaces is also referred to as wet-storage stain.
Variables affecting the corrosion rate and life expectancy of various zinc coatings include:
- Airborne salinity.
- Sulfur dioxide level.
- Relative humidity.
- Sheltering condition.
- Coating thickness.
Although zinc has excellent corrosion resistance, it is less noble than many metals and thus has been used as a sacrificial anode for cathodic corrosion protection of structural metals in corrosive environments such as for seawater applications. A galvanic cell is created when the most active metal, such as zinc, preferentially corrodes to protect the cathode metal, such as steel of a ship hull or pier. (For more on cathodic protection, read: The Basics of Cathodic Protection.)Other metals that form protective oxide layers similar to zinc include aluminum, copper, chromium and titanium.
More Q&As from our experts
- Why do zinc rich primers sometimes leave a powdery topcoat?
- Does aluminum rust?
- How can I determine and measure steel's endurance limit?
- Zinc Carbonate
- Zinc Hydroxide
- Zinc Anode
- Zinc Oxide
- Galvanic Coating
- White Rust
- Oxide Layer
- Hot-Dip Galvanizing
- Cold Galvanizing
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