Brass is an alloy, a metal blend primarily composed of copper and zinc, prized for its warm, golden color and workability. The common question of whether brass rusts when exposed to water stems from a misunderstanding of chemical definitions. Brass does not rust, but it will change appearance when it encounters moisture and oxygen. This chemical reaction is a form of corrosion distinct from the process that affects iron-containing metals.
Why Brass Does Not Rust
The term “rust” is scientifically specific, referring only to the corrosion of iron and its alloys, such as steel. Rust, or iron oxide, is the reddish-brown, flaky compound that forms when iron reacts with oxygen and water. This resulting compound, hydrated ferric oxide, is unique to iron. Brass cannot rust because it contains no iron; it is a non-ferrous alloy made mainly of copper and zinc. While brass is rust-proof, it is not immune to degradation called corrosion. Corrosion is a generic term describing the deterioration of a material due to a reaction with its environment.
The Chemical Reaction of Brass and Water
Although brass is rust-proof, exposure to water and oxygen initiates tarnishing or oxidation. This process results in a surface layer, known as a patina, which changes the metal’s appearance but often protects the underlying brass. Copper reacts with oxygen, moisture, and carbon dioxide to form copper oxides and copper carbonates. The copper carbonates, often called verdigris, are responsible for the familiar greenish or bluish-green hue seen on aged brass. The zinc component oxidizes simultaneously to form zinc oxide, a white, powdery substance that contributes to a dull surface layer. This resulting layer of mixed copper and zinc oxides is generally thin and tends to adhere to the surface, slowing down further reaction. This protective patina is the key difference between brass corrosion and iron rust; unlike rust, the patina acts as a barrier instead of flaking away. However, in harsh conditions, a more damaging form of corrosion called dezincification can occur. Dezincification selectively leaches zinc out of the alloy, leaving behind a weakened, porous, copper-rich structure that appears dull and coppery.
Environmental Factors That Accelerate Damage
While patina formation is slow under normal conditions, several environmental factors accelerate the tarnishing process. Prolonged exposure to high humidity or constant moisture speeds up the oxidation reaction. This is because water facilitates the electrochemical reactions necessary for the surface to degrade. Water containing high levels of chlorides, such as saltwater or chlorinated pool water, is particularly corrosive. Chloride ions actively break down the protective oxide layer, increasing oxidation and susceptibility to dezincification. High temperatures also cause tarnishing and corrosion to proceed much faster. The acidity or alkalinity (pH) of the environment also plays a role in degradation. Acidic solutions and pollutants like sulfur compounds can react with the brass surface to form corrosive compounds. Frequent handling of unlacquered brass items can also accelerate tarnishing, as the natural oils and salts on human skin speed up the surface reaction.
Protecting Brass from Water Damage
The most effective way to prevent or slow down the effects of water exposure is to apply a protective barrier to the brass surface.
Protective Coatings
Clear lacquers are commonly used, providing a tough coating that seals the metal off from oxygen and moisture. For outdoor items or those in high-humidity areas, a durable, industrial-grade coating is often necessary. For decorative indoor pieces, a less permanent but effective method is the application of a high-quality paste wax or car wax. These products create a thin, sacrificial layer that repels moisture and prevents contact between the metal and the air. Regular reapplication of wax is necessary to maintain this protection.
Maintenance
Simple maintenance routines are also important for reducing water damage and tarnishing. After any exposure to moisture, brass items should be thoroughly dried with a soft cloth to prevent water spots and mineral deposits. Regular cleaning with mild soap and water, followed by complete drying, helps remove surface contaminants that accelerate the corrosive process.