Corrosion and scale are both intricate and complex processes that are influenced by several factors – water characteristics, metals used and any stray electrical current – to name a few. In this blog, we’ll look at the difference between corrosion and scale and dissect a few of their main causes.
Corrosion is the rusting of metal in pipes or tanks due to the corrosive action of either water or soil. Scales are deposits caused by the deposition of calcium carbonate on the inner surface of the water treatment equipment pipes or tanks. Stable water is the water that tends to be neither corrosive nor scale-forming. Corrosive water, also known as aggressive or unstable water, will tend to corrode (rust) metal in the pipes or tanks it passes through. Scale-forming water will tend to deposit calcium carbonate scale on the inner surfaces of these pipes or tanks.
Corrosive and scale-forming waters are at the opposite ends of a spectrum. A variety of water characteristics combine to influence water’s location along this spectrum. The goal of the corrosion control in water treatment is to render the water stable, which will neither corrode pipes nor form scale.
The chemical characteristics, such as alkalinity, hardness and pH of the water flowing through a pipe will influence corrosive reaction. The table below summarizes characteristics of corrosive water and of scale-forming hard water.
The graph shown below is the Baylis Curve. In it, you will see the relationship between alkalinity and pH, which in turn affects the water stability. Stable water is found in the white area about corrosive water and below what would be scale-forming.
Oxygen, carbon dioxide and dissolved solids found in water also influence the corrosion process. Oxygen causes corrosion by reacting with hydrogen gas at the cathode. This reaction speeds up the corrosion process by causing depolarization, which means water with highly dissolved oxygen will tend to be corrosive. Other oxidizing agents, such as nitrate and chlorine, can perform the same function, although they are less common.
Carbon dioxide causes corrosion by combining with water to form carbonic acid. This lowers the pH of the water and a low pH promotes corrosion. Dissolved solids are also present in water as ions that increase the electrical conductivity of the water, making the electrolyte more effective and increasing the rate of corrosion.
In addition to the chemical properties of water, physical characteristics, such as temperature and velocity of flow, will influence corrosion. Temperature has a complex effect on corrosion. A high water temperature promotes scale formation and slows corrosion by reducing the solubility of calcium carbonate in water. Temperature alters corrosion by forming pits and tubercles in cold water, while hot water promotes uniform corrosion. High temperature is more beneficial since it can slow corrosion.
How corrosion is influenced by the flow velocity is complex. Moderate flow rates allow the formation of scale without breaking loose tubercles, which is the most beneficial. Corrosion is increased at low velocities and, due to the prevalence of oxygen concentration cell corrosion, tends to be in the form of tuberculation. High velocities cause abrasion of the water against the pipe and tend to wear the pipe away in a very different form of corrosion. They can also increase corrosion by removing protective scale and tubercles, which increase the contact of the pipe with oxygen.
Bacteria can cause and accelerate the rate of corrosion. Corrosion is accelerated below bacterial colonies on pipe walls due to oxygen cell concentration, causing increased pitting and tuberculation. Additionally, bacteria can produce carbon dioxide, which turns into carbonic acid when met with water and then causes corrosion to accelerate. The two main types of bacteria carry specific traits needed to identify them. Iron bacteria use ferrous iron and produces slime in high velocities, causing red water and a foul odor. Sulfate-reducing bacteria rely on sulfate in water to produce hydrogen sulfide, which reacts with metal in the water and causes corrosion. It also produces a rotten egg smell.
The chemical, physical and biological aspects of the water flowing in your pipes must all be considered when assessing the condition of a distribution system for maintenance, repair or replacement.