For decades, copper has been a common material for underground water and wastewater piping. Its popularity is attributed mainly to its numerous desirable properties – the most notable of which is its exceptional corrosion resistance. Copper is also classified as a noble metal, i.e., it is highly resistant to oxidation and various forms of acidic chemical attack.
Copper derives its corrosion resistance from the thin oxide film that forms on the surface of the metal when exposed to oxygen and moisture. This oxide layer, called a patina, firmly adheres to the surface of the metal, acting as a barrier that protects the underlying copper substrate from further corrosion. Most times the patina layer is enough to ensure the longevity of buried copper pipelines and prevent further copper corrosion.
Misconceptions about Copper Corrosion
However, although copper possesses better corrosion resistance than most metals, it has mistakenly gained a reputation for being corrosion-proof. While buried copper pipes can exhibit resilience under relatively moderate conditions, they can indeed corrode and degrade in particularly aggressive environments such as those found in the zone of aeration in soils. (For more information on the myths surrounding copper, read If Copper is a Noble Metal then Why Are My Pipes Corroding?)
What is the Zone of Aeration?
To understand the zone of aeration, it is essential to understand the formation and anatomy of groundwater. Rainwater that comes into contact with the ground gets distributed in several ways. A portion of the water runs off various surfaces into rivers and streams, thus completing the hydrological cycle. Another significant portion is either absorbed by trees and other plant life or evaporated directly into the atmosphere. The remaining rainwater (soil moisture) percolates into the soil until it reaches an impermeable layer where it accumulates.
This accumulation creates a reserve of stored water known as groundwater beneath the ground surface. The layer of soil and rock that is completely filled with water is known as the saturation zone, and its upper portion, or boundary, is termed the water table.
The zone of aeration is the sub-earthen region that exists between the ground surface and the water table. It is composed mainly of porous rock and soil, where the spaces between the soil matrix contain a mixture of air and water. In this layer the air and water interact, leading to the aeration of water (hence its name).
The properties of the zone of aeration depend largely on several environmental factors including the site geology, topography, climate and drainage. These factors ultimately dictate the depth of the water table and, by extension, the height of the zone of aeration and saturation. The zone of aeration can be nonexistent, or it may be hundreds of feet deep.
How the Zone of Aeration Affects Copper Corrosion Rates
The moisture present in the zone of aeration consists of significant amounts of dissolved oxygen. While raw water and treated water also contain dissolved oxygen, the large quantities present in the zone of aeration can cause accelerated rates of corrosion in copper piping.
One of the most common types of corrosion found in underground copper piping and other underground structures is differential aeration corrosion. This type of corrosion occurs when one section of the pipeline is exposed to higher levels of dissolved oxygen than the other. The part of the copper pipe exposed to higher levels of dissolved oxygen becomes a cathode, while the oxygen-deficient section acts as an anode.
At the anode, oxidation occurs as the electrons gravitate towards the cathode. Conversely, electrons are gained at the cathode, resulting in a reduction reaction. During this process a differential aeration cell is produced, whereby the section of the copper that acts as the anode experiences accelerated corrosion while the cathode remains oxygenated, decreasing its corrosion rate.
Differential aeration cells also tend to be autocatalytic, i.e., the ongoing chemical and electrochemical reactions produce conditions that can promote the continuation of the cells and worsen corrosion with time. For example, metal ions released at the anode hydrolyze (chemically react with water) to reduce its local pH and increase its acidity. Reduction in electrolyte pH is known to accelerate corrosion in metals such as copper. Conversely, reduction reactions at the cathode serve to increase the pH of the electrolyte, increasing its alkalinity and improving the protective properties of the oxide layer.
Variation in environmental conditions, such as soil moisture content, dissolved oxygen concentration, depth of pipe installation and oxygen barriers (e.g., paved roads) tend to create conditions ideal for the formation of differential aeration cells. For example, a buried copper pipeline that runs under both paved and unpaved surfaces can be exposed to soils with varying degrees of aeration and dissolved oxygen content, leading to differential aeration cell creation. (Related reading: An Introduction to Soil Corrosion.)
Dissolved oxygen is also known to be significantly more corrosive than carbon dioxide. Studies show that dissolved oxygen in pure water at 65°C (149°F) is six times more corrosive than the equivalent amount of carbon dioxide. Furthermore, the combined corrosive effect of oxygen and carbon dioxide is twice that of these two compounds individually. Oxygen attack, in particular, is known to cause deep pitting in copper pipes.
The zone of aeration is so named due to the mixing of water and oxygen that occurs in this region. This mixing increases the quantity of dissolved oxygen that contributes to the increased rates of corrosion in buried copper pipelines and structures.
Varying dissolved oxygen content (among other soil properties) over the surface of copper pipes can produce differential aeration cells that can lead to localized corrosion such as pitting. This type of corrosion is relatively aggressive, usually overpowering the natural anti-corrosive properties of copper.