Copper is a metal widely used in modern life, forming pipes, wires, and roofing materials. When new, copper has a distinct reddish-orange hue, but exposure to the environment causes its surface to change color, often turning shades of brown, black, or green. This visible change is a form of corrosion, raising the question: is water the sole cause of this chemical transformation? The answer lies in the specific chemistry of the oxidation process and the impurities found in water.
Understanding Chemical Oxidation
The change observed on the surface of copper is a chemical process known as oxidation. Oxidation is the loss of electrons from an atom or molecule during a chemical reaction, which is always paired with reduction (the gain of electrons). Together, these are known as a redox reaction.
When a metal oxidizes, its atoms lose electrons, often to oxygen, resulting in the formation of a new compound, typically a metal oxide. This resulting compound has a different chemical structure, color, and physical appearance than the original pure metal. This electron transfer is the fundamental mechanism behind all forms of metal corrosion and tarnish.
Pure Water and Copper: The Direct Answer
When considering pure copper and pure water (\(\text{H}_2\text{O}\)), water does not readily oxidize the metal. For a chemical reaction to occur spontaneously, it must be thermodynamically favorable, meaning the reaction must lower the overall energy of the system. In the case of copper reacting with just water, the oxidation is energetically unfavorable.
Experiments conducted with copper in highly purified, de-oxygenated water have shown that the corrosion rate is negligible or non-existent. Any small amount of copper oxide (\(\text{Cu}_2\text{O}\)) that forms is typically attributed to trace amounts of residual oxygen. Furthermore, if water were the primary oxidant, the reaction would release hydrogen gas; studies confirm this hydrogen evolution does not happen, supporting that pure water alone is not a strong enough oxidizing agent for copper.
The Necessary Co-Factors for Real-World Corrosion
While pure water is a poor oxidant, real-world water contains several co-factors that accelerate copper corrosion. The primary driver of oxidation in water is dissolved oxygen (\(\text{O}_2\)), which acts as the electron acceptor in the redox reaction. Oxygen readily receives the electrons lost by the copper atoms, forming copper(I) oxide (\(\text{Cu}_2\text{O}\)) on the surface.
The acidity of the water, measured by its pH level, also significantly affects the corrosion rate. Acidic water, typically with a pH below 7.0, can actively dissolve the protective oxide layer that naturally forms on the copper surface. By breaking down this initial barrier, the acidic water continuously exposes fresh copper metal to dissolved oxygen, speeding up the corrosion process.
Dissolved ions present in tap water also contribute to the corrosion rate. Chloride ions, commonly found due to treatment processes or natural sources, can disrupt the stability of the protective oxide film. These ions increase the water’s conductivity, assisting the electrochemical process and potentially leading to localized pitting corrosion.
The presence of dissolved inorganic carbon (DIC) and the water’s buffering capacity play a complex role in stabilizing the copper surface. These components help form stable, protective layers of copper carbonate that slow down future corrosion. Therefore, the actual corrosion rate of copper plumbing depends on a combination of these factors, including dissolved oxygen level, pH, and the concentration of various ions.
The Appearance of Oxidized Copper
The oxidation of copper is a multi-stage process resulting in a distinct sequence of color changes. The first stage involves copper metal reacting with oxygen to form a thin film of copper(I) oxide (\(\text{Cu}_2\text{O}\)). This initial layer is responsible for the reddish or pinkish tarnish seen on newly exposed copper.
As oxidation continues, the copper(I) oxide is further oxidized by environmental oxygen to form copper(II) oxide (\(\text{CuO}\)). This second compound is black and causes the surface to turn dark brown or completely black, known as tarnish. Although thin, this oxide layer provides a degree of protection against further reaction.
The familiar blue-green color, known as patina or verdigris, develops over many years of exposure to the atmosphere. This color is not simple copper oxide but complex basic copper salts, primarily copper carbonates like malachite (\(\text{Cu}_2\text{CO}_3(\text{OH})_2\)). These compounds form when the underlying copper oxides react with atmospheric carbon dioxide (\(\text{CO}_2\)) and moisture.
In urban or industrial environments, the patina may also contain basic copper sulfates, such as brochantite, formed by reactions with sulfur dioxide (\(\text{SO}_2\)) in the air. The resulting green film is generally stable and serves as a highly effective, self-limiting protective layer that prevents the oxidation of the copper metal underneath.