Corrosion costs the world $2.5 trillion (that’s trillion with a T) according to NACE, roughly 3.4% of global GDP. Yet corrosion isn’t widely understood by the average person.
Corrosion is actually a chemical or electrochemical reaction between metal and its environment. The smelted iron and other metals used in buildings, equipment, and vehicles have a higher energy content than their counterparts in nature, such as iron oxide. As a result, metals like iron and steel are unstable and have a tendency to corrode to their natural state by combining with other elements. Overtime they combine with oxygen to form iron oxide, i.e. rust or corrosion.
But what actually causes corrosion?
Causes of corrosion
Four ingredients must be present for the electrochemical process of corrosion to occur, i.e., the corrosion cell. All four of these elements must be present:
- Anode: The site of metal where oxidation (corrosion) occurs
- Cathode: The site of metal where corrosion does not form
- Metallic pathway: Provides the flow of electrons from anode to cathode
- Electrolyte: A liquid that contains charged particles and allows ions to flow from cathode to anode
The interaction between these four components is known as the corrosion cell.
Three components of corrosion are already present in most metals: the anode, the cathode and the metallic pathway. All that’s missing is the electrolyte that comes from the atmosphere or chemicals. Once the electrolyte is present, electrons flow from the anode to the cathode, and ions flow from the cathode to the anode through the electrolyte, completing the electrical circuit. The ions flowing from the cathode to the anode cause the anode to corrode and the metal to become compromised.
Electrolytes are typically sodium chloride and calcium compounds. Chloride compounds are usually found in seawater, rain, mist and fog, as well as road salts and cleaning chemicals. Not quite as common, but sulfur and nitrogen compounds can be found in fuel oil and coal.
All this said, water is the most common culprit of corrosion.
Corrosion prevention by design
Corrosion is prevented by removing one of these components from from the corrosion cell. As long as one of these components is missing, the electrical circuit cannot be completed, and corrosion cannot occur.
There are several methods of corrosion control. Ideally corrosion prevention is built into the design of whatever’s being built.
Certain materials are immune to corrosion, such as gold and platinum. Other alloys like stainless steel, nickel, chromium and copper resist corrosion by forming natural films that act as barriers between the metal and the environment. Selecting materials that are inherently resistant or immune to corrosion will reduce the costs and damages associated with corrosion.
Geometric design issues can also cause corrosion. The most common geometric design issues include:
- Water traps
- Sharp edges
- Inaccessible areas
- Sheared edges
- Stray currents
While designing for corrosion prevention is always recommended, corrosion is inevitable in any application. Inspection and maintenance are required to prevent structural damage to buildings, vehicles and equipment that are susceptible to corrosion.