Brass is an alloy primarily composed of copper and zinc, and while exposure to water does not cause it to rust like iron, it does initiate a surface-level chemical reaction. This metal is generally water-resistant but not completely waterproof, meaning that prolonged or repeated wetness will lead to visible changes. Water exposure, especially in the presence of dissolved oxygen from the air, accelerates the natural process of oxidation on the metal’s surface.
How Water Interacts with Brass
Brass’s reaction to moisture is fundamentally electrochemical, driven by the differing potentials of its copper and zinc constituents. When the alloy is exposed to water and oxygen, the copper component slowly undergoes oxidation, forming copper oxide. This initial black layer then reacts further with carbon dioxide and moisture to create copper carbonate, which is the characteristic greenish-blue patina or tarnish.
Unlike iron, which forms a flaky, non-protective iron oxide known as rust, the corrosion on brass is typically a durable surface film that can offer some protection to the underlying metal.
The zinc within the alloy is also susceptible to oxidation, forming zinc oxide, which appears as a dulling, white, powdery substance on the surface. Water can promote a more destructive process called dezincification, especially in harsh or prolonged wet conditions. Dezincification is the selective leaching of zinc from the alloy, leaving behind a porous and structurally weakened material high in copper.
This localized corrosion compromises the mechanical integrity of the brass component over time, which is a particular concern for plumbing and marine fittings.
Environmental Factors That Increase Damage
The speed and specific type of damage inflicted by water exposure depend heavily on the surrounding environment and the water’s chemical makeup. Water with high chloride content, such as saltwater or water heavily treated with chlorine, is highly corrosive to brass. Chloride ions penetrate the natural oxide layer, leading to aggressive, localized damage known as pitting corrosion. High concentrations of chlorine or chloramine, common disinfectants in water systems, significantly accelerate the dezincification process, rapidly weakening the alloy’s structure.
High ambient humidity alone, even without direct immersion, provides the necessary moisture to intensify the electrochemical reactions on the surface. Temperature also plays a significant role in corrosion kinetics, as a rise of 10°C in water temperature can double the overall corrosion rate. The water’s acidity, measured by its pH level, is another major factor, where acidic water (pH below 7) can rapidly corrode brass and accelerate the removal of zinc. Conversely, water that is slightly alkaline (pH 7-8.5) tends to slow general corrosion.
Water hardness provides a complex balance; hard water contains scale-forming minerals that can deposit a somewhat protective layer on the surface, slowing general corrosion. However, soft water lacks these mineral deposits and can promote more rapid corrosion in certain systems, necessitating the use of specialized dezincification-resistant (DZR) brass alloys in plumbing. When brass components are placed near other metals in water, galvanic corrosion can occur, where the brass corrodes at an accelerated rate because it acts as the anode in the electrochemical cell.
Protecting Brass Surfaces from Moisture
Protecting brass from water requires creating a physical barrier to prevent contact with both moisture and atmospheric oxygen. For decorative items not subject to frequent human contact, applying a clear lacquer provides a strong, durable sealant. The surface must be meticulously cleaned before applying lacquer, as any remaining contaminants or fingerprints will cause adhesion failure and allow tarnishing to develop beneath the coating. Lacquer is typically sprayed onto the surface in multiple thin coats rather than brushed, which ensures a uniform and complete barrier against the environment.
For items that are regularly handled or subject to friction, such as door hardware or railings, a microcrystalline paste wax or high-quality furniture wax is often a more suitable choice. Wax offers a softer, more subtle barrier that can be easily reapplied and is ideal for pieces that require periodic buffing to maintain their sheen. Alternatively, rubbing the surface with boiled linseed oil or tung oil provides a simple protective film that retards the tarnishing process. Regardless of the chosen method, for any item that accidentally gets wet, the simplest and most immediate preventative measure is to immediately dry the entire surface with a soft, lint-free cloth.
Removing Tarnish Caused by Water Exposure
When water exposure has already caused tarnish or spotting, remediation focuses on gently dissolving the surface oxides without causing mechanical damage. Mild acidic solutions are highly effective because they chemically break down the copper carbonate compounds that comprise the dark patina. A frequently used household method involves creating a paste from white vinegar and table salt, which combines the acid’s dissolving power with the salt’s mild abrasive quality. The paste is applied to the tarnished area for a controlled period, generally 10 to 20 minutes, which is sufficient for the acid to react.
After application, the surface must be thoroughly rinsed with clean water to neutralize the acid and immediately dried to prevent new water spots. Prolonged soaking in acidic solutions is not recommended, as this can begin to dissolve the brass itself, particularly if the item is thin or brass-plated. Lemon juice, which contains citric acid, can also be used, often mixed with baking soda to create a gentle, slightly alkaline paste. Commercial brass cleaners offer another option, utilizing a formulated mixture of mild acids and ammonia designed to remove tarnish and restore luster. Remember that a thin, natural patina is often protective, and aggressive removal should be avoided if the goal is long-term preservation.