How Corrosion Is Layered

The most common types of corrosion in steel structural components include the following:

• Uniform Corrosion This is the most prevalent form of corrosion in steel bridge structures. When a metal surface is exposed to the atmosphere, a uniformly distributed rust layer (oxide) forms. The rate of uniform corrosion can be effectively reduced by limiting the contact area between the metal and the atmosphere.
• Electrochemical Corrosion When two metal materials with different corrosion potentials are in contact and exposed to a corrosive environment containing an electrolyte, an electric current flows, leading to corrosion damage.
• Pitting Corrosion As one of the typical forms of localized corrosion, pitting corrosion creates holes (pits) of varying depths on the steel surface. Pitting corrosion is prone to occur when steel components have defects or accumulate surface dirt, resulting in cracks on the metal surface.
• Crevice Corrosion This localized corrosion phenomenon arises from differences in the environment inside and outside narrow crevices, particularly due to variations in ion concentrations in different areas.
• Erosion Corrosion When a fluid impacts a metal surface at a relatively high velocity, it can damage the protective film on the surface, thereby accelerating the corrosion process.
• Stress Corrosion Under the combined action of a corrosive environment and applied tensile stress, metal materials can experience brittle cracking.
• Corrosion Fatigue In a corrosive environment, repeated loading can lead to stress concentration, ultimately causing fatigue cracks in the metal.
• Fretting Corrosion When two surfaces are in close contact under load, the wear of the surfaces by oxides can occur.
• Intergranular Corrosion This type of corrosion occurs at the grain boundaries of steel, significantly affecting the material's mechanical properties.

In addition to the corrosion types mentioned above, the rate and progression of corrosion are influenced by various factors, primarily including:

•Environmental conditions: temperature, humidity, atmospheric pollutants (e.g., sulfides and chlorides)

•Material properties: different grades and types of steel

•Protective measures: application of protective coating systems

•Structural factors: presence of crevices, stress states, contaminants

To effectively prevent corrosion, test standards for protective coating systems are established through long-term material weight loss tests and surface degradation tests conducted in various corrosive environments. ISO 12944 also classifies environments based on their corrosivity.

See the table below: