Each year, the Federal Government and the State Departments of Transportation (DOTs) in the United States spend billions of dollars for bridge rehabilitation. The main reason behind the need for bridge rehabilitation and restoration is the corrosion of steel bridges.
To properly inspect the corrosion of steel bridges and provide early remedies, a bridge inspector must be aware of different forms of corrosion observed in steel bridges.
Corrosion of steel bridges is the deterioration of metal due to its reaction with the environment. It is a reaction where the metal is converted into its oxide form in the presence of oxygen, water, and other agents.
The rate and progression of corrosion on steel depend on environmental conditions, surface protection, presence of pollutants, deposits, and bacterial and stress conditions. Due to these factors, each bridge structure would have a different corrosion rate.
Different Forms of Corrosion in Steel Bridges
Corrosion in steel bridges is known to appear in different forms and is classified based on how the corrosion attacks the metal. The eight forms of corrosion in steel bridges are:
- Uniform corrosion
- Galvanic corrosion
- Crevice corrosion
- Deposit attack
- Underfilm corrosion
- Pitting corrosion
- Intergranular corrosion
- Erosion corrosion
- Stress corrosion
1. Uniform Corrosion
Uniform corrosion involves the overall thinning of metalwork and is observed as uniform rust over the entire surface. Uniform corrosion can be identified by the naked eye. It is commonly observed in all the bare metals exposed to the atmosphere.
Steel bridges in arid areas exhibit uniform corrosion. They are observed on plates and shapes with large surface areas that can be attacked or oxidized uniformly, like vertical guest plates, girder webs, truss verticals, and diagonals. These members dry quickly and do not invite other forms of corrosive attacks.
2. Galvanic Corrosion
Galvanic corrosion occurs when steel bridge elements are in an electrolytic environment. It happens when light poles, electrical conduits, handrails come in contact with bare steel or weathering steel. It is often visible to the naked eye.
This corrosion occurs when metals with different compositions are placed together in the presence of an electrolyte. The variation in metal composition creates a corrosive potential difference, making one metal an anode and the other cathode followed by their oxidation. Hence, this is also called as dissimilar metal corrosion.
In most cases, insulating materials are placed between these dissimilar metals to prevent galvanic corrosion.
3. Crevice Corrosion
Crevice corrosion is localized corrosion observed in confined locations of steel bridges that are not exposed to the outside environment. It is one of the most common forms of corrosion found in steel bridges. Crevice corrosion in steel bridges can be visually observed with the naked eye.Â
It occurs between small gaps and openings in steel bridges, where differences in thickness, alignment, unsealed portion, etc., are common. Steel surfaces with oxide film protection are more susceptible to crevice corrosion. The high chloride or hydrogen ions concentration in these crevices destroys this film and corrodes the area.
4. Deposit Attack
Deposit attack is a type of crevice corrosion frequently occurring in steel bridges at the location of debris deposits. This debris has foreign materials deposited in the steel surfaces and creates a confined space that behaves like a crevice. In addition, these deposits hold moisture and other corrosion-triggering substances.
One of the most annoying types of deposit comes from bird nests and bird excrement. These deposits contain acids that can damage steel members and protective coatings.
5. Underfilm Corrosion
Underfilm corrosion is a type of crevice corrosion that is visible under physically damaged paint or any defects on the paint film. This corrosion results in the debonding of paint from the metal surfaces.
Underfilm corrosion is seen in a structure as cracking, blistering, or peeling of the paint film.
6. Pitting Corrosion
Pitting is a type of localized corrosion that causes the formation of deep, narrow penetrations into the steel surfaces. It is observed in metals with imperfections in the metallurgy, paint protection flaws, foreign material deposits, etc.
7. Intergranular Corrosion
Intergranular corrosion is corrosion seen on the boundaries of the metal grains. Once the grain boundaries deteriorate, the grains fall out, and the metal disintegrates.
Intergranular corrosion requires precise examination methods for their proper diagnosis.
8. Erosion Corrosion
When parts of steel bridges are exposed to high-velocity fluid flow (streams), the particles in the fluid abrade the metal surface. The flow wears away the surface coating on protective corrosion products and is called erosion corrosion.
Erosion corrosion is dangerous when the flowing stream carries particulate matter that erodes steel piling.
9. Stress Corrosion
The bridges exposed to adverse environmental conditions, like industrial and marine environments, are subjected to stress corrosion cracking.Â
Stress corrosion cracking is caused by the simultaneous occurrence of tensile stress concentrations in steel bridge elements accompanied by a corrosive environment.
FAQs
The eight forms of corrosion in steel bridges are uniform corrosion, galvanic corrosion, crevice corrosion, deposit attack, underfilm corrosion, pitting corrosion, intergranular corrosion, erosion corrosion, and stress corrosion.
Stress corrosion cracking is caused by the simultaneous occurrence of tensile stress concentrations in steel bridge elements accompanied by a corrosive environment.
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