Corrosion is defined as a naturally occurring chemical or electrochemical reaction that causes materials (primarily metals) to deteriorate. Like other natural hazards, corrosion can be potentially destructive, resulting in extensive damage to numerous objects and structures, from vehicles and home appliances to bridges and buildings.
However, while natural disasters are largely caused by forces beyond our control, some corrosion processes can occur due to neglectful or careless human activity, such as improper construction methods, lack of maintenance and inappropriate material specification.
In this article, we will look at the various types of corrosion failures and explain the role of human activity in their formation.
Threat #1: Galvanic Corrosion
One of the most common types of corrosion caused by human neglect is galvanic corrosion. This type of corrosion, also known as bimetallic corrosion, occurs when two dissimilar metals are in direct or indirect contact with each other in the presence of an electrolyte (usually water). Galvanic corrosion is generally observed as accelerated corrosion in one metal while the other remains unaffected.
Galvanic corrosion arises due to the difference in electrode potentials between the contacting metals. This difference causes electrons to flow from one metal to the other, thus forming a galvanic cell or bimetallic couple. As a result, the corroding metal becomes the anode, while the other becomes the cathode. (Learn more in the article Why Do Two Dissimilar Metals Cause Corrosion?)
Galvanic corrosion is common in numerous industries, particularly in situations where persons specifying materials fail to consider the incompatibility between the two connecting metals. The interfaces near metallic connections and fasteners are common sites for galvanic corrosion. (You may be interested in: High Pressure Fastener Coating Practices Under Fire: Ian MacMoy Speaks Out.)
This type of corrosion can be prevented by carefully assessing the properties of the contacting metals during the early stages of the design process.
Threat #2: Weld Corrosion
Weld corrosion, as its name suggests, is a type of corrosion that is caused by improper welding technique. At welded joints, several physical, chemical, mechanical and metallurgical transformations occur, some of which can cause the joint to become less corrosion resistant.
One of the mechanisms by which weld corrosion can occur is galvanic corrosion (explained previously). If the weld metal differs from the parent metal, a galvanic cell can form, where one metal becomes the anode and the other becomes the cathode. Therefore, contractors and welders should consider the properties of the metals being joined.
Another, more common weld-related defect that contributes to corrosion is hot shortness. (These and other causes are examined in the article Causes and Prevention of Corrosion on Welded Joints.) Hot shortness, also known as hot tearing, solidification cracking or hot cracking, is characterized by cracking along the grain boundaries of the metals as it cools and solidifies after welding.
The grain boundaries of metals represent paths of high corrosion susceptibility because the segregation of impurities at these locations can result in weak passivation. The tensile stresses induced by heating and subsequent cooling during welding serves to open cracks at these weakened boundaries. This process is known as stress corrosion cracking (SCC).
SCC due to hot shortness can be prevented by:
- Selecting materials with low levels of impurities, such as phosphorus and sulfur
- Reducing the heat during welding
- Ensuring that the weld joint is free from oil, grease and other contaminants
- Using an appropriate filler metal
Threat #3: Corrosion Under Insulation (CUI)
Corrosion under insulation (CUI) is an aggressive form of localized corrosion that forms in metallic equipment insulated with fibrous materials such as mineral wool, calcium silicate and expanded perlite. CUI arises when moisture (usually rainwater) is allowed to contact and penetrate the thermal insulation down to the level of the metal substrate. Corrosion is initiated when the absorbed moisture contacts the steel in the presence of oxygen.
Traditionally, the insulation that surrounds the metal structure is encased in a jacket to prevent the intrusion of moisture. However, these jackets can become damaged, exposing the underlying insulation to adverse weather conditions such as rain.
While wind and debris are usually responsible for causing damage to the jacket structure, human activity such as maintenance operations and construction can also result in dents, scrapes and punctures that may compromise the jacket. Routine inspection, maintenance and repair can ensure that the jacket remains intact and functions as intended.
Threat #4: Erosion Corrosion
Erosion corrosion is the accelerated rate of deterioration of a metal's surface due to the relative motion of a corrosive or abrasive fluid against the metal. The fluid typically moves at a velocity high enough to cause the surface to experience mechanical wear. The metal is usually removed in the form of dissolved ions or solid corrosion products. This type of corrosion is typically characterized by the formation of holes, grooves, waves and gullies in the direction of fluid movement.
Erosion corrosion can be caused by neglect in several areas, such as:
- Incorrectly specifying the right pipe material to carry the transported fluid
- Failing to maintain filters and other devices that are intended to prevent the presence of solids in the liquid
- Improper monitoring of fluid velocities
- Unnecessary and sharp bends in the piping design
(Various methods to address erosion corrosion are discussed in Erosion Corrosion: Coatings and Other Preventive Measures.)
Threat #5: Uniform Corrosion
Uniform corrosion attack is one of the most common types of corrosion. It is characterized by the formation of corrosion products over the entire exposed area (or a significantly large area) of the metal's surface. The corrosion products flake off and reduce the thickness of the metal until it fails. Uniform corrosion is usually electrochemical in nature, but it may also be caused by chemical reactions.
Uniform corrosion may be caused by neglect during the application of protective paints and coatings. Improperly applied coatings can leave metallic surfaces vulnerable to corrosion due to the lack of barrier protection. As the coating is applied, measures should be taken to reduce the occurrence of failures such as blistering, peeling, bleeding, running and sagging.
Care must also be taken to ensure that paints and coatings are applied uniformly using techniques that are appropriate for the given project and environment.
While corrosion is mostly considered to be the result of natural phenomena, it can also be a consequence of neglectful human behavior. Several preventative techniques, such as careful planning, proper material specification and hiring qualified personnel, can go a long way to ensure that corrosion failure is not caused by adverse human interaction.