The question of whether brass is rust-proof can be answered with a clear negative: brass does not rust. Rust is a specific form of corrosion that only affects iron-containing metals, such as steel, and brass is an alloy without iron. Though it avoids the flaky, reddish breakdown of rust, brass is still susceptible to other forms of corrosion when exposed to air, moisture, and various chemicals. This corrosion manifests as a surface change, which impacts the metal’s appearance and, in some cases, its structural integrity. Brass has been a widely used material for centuries, prized for its durability and aesthetic appeal.
What is Rust and Why Brass Does Not Form It
Rust is the common term for the oxidation of iron, a chemical process that forms iron oxide in the presence of both oxygen and water. This reaction is electrochemical and results in a brittle, reddish-brown material that flakes away, continuously exposing the underlying metal to further decay. The key factor in this destructive process is the presence of iron.
Brass is chemically immune to rust because it is an alloy of copper and zinc, not an iron-based metal. Since the chemical components required for the rusting reaction are absent, brass cannot form iron oxide. This distinction is why brass is often chosen for applications where resistance to environmental degradation is important.
The Composition and Structure of Brass
Brass is a substitutional alloy, a metallic mixture primarily composed of copper and zinc atoms. The ratio of these two elements is varied to create different types of brasses, each with unique mechanical and aesthetic properties. Most common brasses fall within a composition range of 60% to 90% copper, with the remainder being zinc.
For instance, “Cartridge Brass” typically contains 70% copper and 30% zinc, prized for its malleability. “Muntz Metal” uses a 60% copper to 40% zinc ratio, which increases its strength. A higher copper content generally results in a softer, more gold-colored alloy with better corrosion resistance, while increasing the zinc content makes the brass harder and brighter yellow.
How Brass Changes Over Time
Instead of rusting, brass undergoes tarnishing, which is the formation of a thin, dull film on the surface due to reaction with oxygen and sulfur in the air. This initial layer is typically a brown-to-black film composed of copper oxides and sulfides. This tarnish is a mild form of corrosion that can easily be polished away if a bright finish is desired.
With prolonged exposure to moisture and environmental pollutants, brass can develop a patina. This stable, protective layer often appears as a greenish-blue color, known as verdigris, a mixture of basic copper carbonate and other copper compounds. Unlike destructive rust, this patina layer is dense and adheres tightly to the metal, shielding the underlying brass from further, more severe corrosion.
A more aggressive form of degradation is dezincification, which occurs primarily in brasses with a high zinc content (above 15%). This happens when the brass is exposed to specific environments, such as stagnant or acidic water. In this process, zinc is selectively leached out of the alloy, leaving behind a porous, weak, salmon-pink colored mass of copper. This material lacks the original strength and can lead to structural failure.
Protecting and Cleaning Brass Surfaces
To maintain the bright, golden appearance of new brass, protective coatings are often applied to prevent natural tarnishing. Clear lacquers or protective waxes create a physical barrier that isolates the metal surface from the air and moisture. This layer stops the formation of initial tarnish and the subsequent development of patina.
If the brass is unlacquered and a polished look is preferred, tarnish can be removed through simple cleaning methods. Mild abrasive pastes, such as a mixture of salt, flour, and vinegar, or specialized commercial polishes, can gently lift the oxide film. It is important to thoroughly rinse and dry the surface after cleaning to prevent water spots and slow the re-initiation of tarnish.
For long-term maintenance, especially in high-touch areas, a thin application of wax or a periodic reapplication of lacquer can substantially reduce the need for frequent polishing. The decision to maintain a polished surface or allow a protective patina to form depends entirely on the desired aesthetic and the function of the brass item.