Brass is an alloy composed primarily of copper and zinc, and it resists water well under normal conditions. This composition allows brass to withstand moisture exposure better than iron-based metals, which are susceptible to rusting. However, brass is not waterproof, and exposure to water will inevitably lead to surface changes. This process is generally a slow form of corrosion known as tarnishing, which causes the material to lose its bright, golden luster. The degree of change depends heavily on the specific environmental conditions and the brass’s composition.
The Basic Chemistry of Brass and Water
Brass’s interaction with moisture is an electrochemical process driven by the presence of oxygen. The copper component oxidizes to form copper oxides and sulfides, creating a thin layer of surface corrosion called tarnish or patina. Unlike the deep flaking of rust on steel, tarnish is a self-limiting surface phenomenon that forms a protective barrier, slowing down further corrosion of the underlying metal. This patina shifts the brass color from its original shiny yellow to shades of dull brown, red, or sometimes blue-green verdigris. Water acts as an electrolyte, significantly accelerating the chemical exchange, meaning consistent exposure quickens the development of the patina.
Factors Accelerating Corrosion and Tarnish
While simple water exposure leads to tarnish, certain environmental inputs dramatically speed up the corrosive process. High humidity, especially in coastal or marine environments, increases the concentration of moisture and salt, acting as a stronger electrolyte. The presence of chlorine, such as in swimming pools, is highly corrosive and aggressively attacks the metal. Acidic environments (low pH) also accelerate the chemical reactions that cause tarnishing. Additional factors that quicken deterioration include high temperatures (which increase reaction energy), atmospheric pollutants (like sulfur compounds), and human contact, which transfers oils and salts to the surface.
Understanding Dezincification
A more severe form of corrosion, particularly relevant in plumbing and industrial applications, is dezincification. This selective leaching occurs when the zinc component (common in alloys over 15% zinc) is preferentially dissolved out by a corrosive medium, typically water. The electrochemical process removes the more reactive zinc atoms, leaving behind a porous, copper-rich residue that appears reddish or pinkish. Although the component retains its shape, the internal structure is weakened and brittle, leading to structural failure. Dezincification is categorized into two types: layer-type (uniform reduction in wall thickness) and plug-type (deep, localized holes or cracks), which are significant concerns for components like valves and pipe fittings.
Maintaining and Protecting Brass from Moisture Damage
To preserve the appearance and structural integrity of brass, preventative measures are necessary, especially in moisture-rich settings. For existing surface tarnish, a mild abrasive or specialized brass cleaner can restore the metal’s original luster, followed by a thorough rinse with clean water and complete drying. The most effective method for long-term protection is applying a protective coating that creates a barrier against moisture and oxygen. Clear lacquers provide a durable, long-lasting seal for items exposed to the elements, preventing the natural oxidation process. Alternatively, a specialized wax offers a softer barrier for decorative items, requiring regular reapplication to maintain the protective seal.