Every year, the federal government and state Departments of Transportation (DOTs) in the United States allocate billions of dollars toward the rehabilitation and restoration of bridges. This investment is essential due to the deterioration of infrastructure, with one of the primary culprits being the corrosion of steel bridges. As steel is a fundamental material in bridge construction, understanding how corrosion affects these structures is critical for timely interventions and prolonging their service life.
The need for efficient bridge inspections is crucial for identifying corrosion early, allowing for necessary repairs before the damage becomes irreversible. To properly assess the corrosion status of steel bridges, inspectors must be familiar with the various forms of corrosion that can occur. Each type of corrosion manifests differently, requiring tailored approaches to inspection and restoration.
Understanding Corrosion in Steel Bridges
Corrosion is the deterioration of metal due to its reaction with the environment, resulting in the metal being converted into an oxide form. This process typically occurs when steel is exposed to elements such as oxygen, water, and various pollutants. The rate and progression of corrosion depend on numerous factors including environmental conditions, surface protection, the presence of pollutants, and the presence of bacterial or stress conditions.
It is important to note that the rate of corrosion will vary across different bridge structures based on their specific environmental exposures. For example, steel bridges located in coastal regions are more prone to corrosion due to the salty, moist air, while those in arid regions may exhibit different corrosion patterns.
Types of Corrosion in Steel Bridges
The corrosion of steel bridges manifests in several forms. Each type presents unique challenges, and understanding these can guide effective maintenance strategies.
A. Uniform Corrosion
Uniform corrosion refers to the overall thinning of metal, often observed as a uniform rust layer across the steel surface. This type of corrosion is easily identifiable with the naked eye and typically occurs on steel surfaces that are directly exposed to the atmosphere. Steel bridges in arid areas, where large surface areas of metal are exposed to the elements, often experience uniform corrosion. Common locations include vertical guest plates, girder webs, and truss verticals and diagonals. These bridge components dry quickly, which prevents the onset of other forms of corrosion.
B. Galvanic Corrosion
Galvanic corrosion happens when two metals with different compositions come into contact in an electrolytic environment, such as when a steel bridge element is in contact with other metals, like aluminum or copper. This form of corrosion occurs due to a potential difference between the metals, creating an electrochemical reaction where one metal becomes the anode and the other the cathode. This causes one metal to corrode more rapidly than the other. In steel bridges, galvanic corrosion can be found in areas where components such as electrical conduits, handrails, or light poles come into contact with steel parts. Insulating materials are typically used to prevent this form of corrosion by separating the metals.
C. Crevice Corrosion
Crevice corrosion is localized corrosion that occurs in confined areas of a steel bridge that are not exposed to the outside environment, such as between gaps or small openings. It is often seen where there are differences in material thickness, alignment issues, or unsealed portions of the structure. The high concentration of chloride or hydrogen ions in these confined spaces can break down the protective oxide film on steel surfaces, leading to corrosion. Crevice corrosion is one of the most common types of corrosion found in steel bridges and requires close inspection, especially in confined, difficult-to-reach areas.
D. Deposit Attack
Deposit attack is a specific form of crevice corrosion that occurs when foreign materials or debris accumulate on the steel surface, creating a confined area that retains moisture. Common culprits for deposit attack include bird nests and excrement, which contain acids that can cause severe damage to steel members and protective coatings. These deposits can act as breeding grounds for corrosion, trapping moisture and other substances that accelerate the corrosion process. Proper cleaning and maintenance are critical to prevent this form of corrosion.
E. Underfilm Corrosion
Underfilm corrosion occurs under a damaged or defective paint film. When protective coatings such as paint are compromised, the steel beneath is exposed to corrosive elements, leading to the formation of rust under the surface. This form of corrosion is visible through cracking, blistering, or peeling of the paint. It highlights the importance of regular maintenance of protective coatings to prevent the exposure of steel to the environment.
F. Pitting Corrosion
Pitting corrosion is characterized by the formation of small, deep holes or “pits” on the steel surface. It typically occurs in areas where the steel has imperfections in its metallurgy, protective coating flaws, or foreign material deposits. Pitting corrosion is dangerous because it can rapidly lead to structural weakening without being immediately noticeable. It can also be difficult to detect since the corrosion is localized and may not be visible over large areas.
G. Intergranular Corrosion
Intergranular corrosion attacks the grain boundaries of the metal, causing the grains to deteriorate and eventually fall apart. This type of corrosion can be challenging to detect because it occurs along the microscopic boundaries between the metal grains. Once the grain boundaries are weakened, the structural integrity of the metal is compromised. Intergranular corrosion requires specialized diagnostic methods for detection and is often found in high-performance steel alloys used in bridge construction.
H. Erosion Corrosion
Erosion corrosion occurs when steel is exposed to high-velocity fluid flow, such as water or air, carrying abrasive particles. This flow can wear away protective coatings and accelerate the corrosion process. In steel bridges, erosion corrosion is most commonly seen in the piles that are submerged in water. The particulate matter in the fluid acts like an abrasive, gradually wearing down the steel surface and contributing to its degradation.
I. Stress Corrosion
Stress corrosion cracking is a type of corrosion that occurs when steel bridge elements are subjected to tensile stresses in combination with a corrosive environment. This form of corrosion is particularly prevalent in industrial or marine environments where both mechanical stresses and corrosive agents like chlorides are present. Stress corrosion can lead to catastrophic failure if left undetected, as it causes cracks to propagate in the steel structure, weakening it over time.
Conclusion
Corrosion is a serious threat to the longevity and safety of steel bridges. From uniform corrosion to stress corrosion cracking, each type of corrosion presents unique challenges that require careful monitoring and specialized maintenance strategies. Regular inspections, early detection, and proper repair techniques are essential in managing these corrosion issues and ensuring the structural integrity of steel bridges. As bridge rehabilitation costs continue to rise, understanding and addressing the various forms of corrosion can help extend the life of our bridge infrastructure, ultimately protecting both public safety and financial resources.