Corrosion: The destruction of a material by electrochemical reaction to the environment (or the destruction that can be detected by rust formation). Can result in lowering weight limits of structures, costly steel replacement, and/or collapse of a structure.
Corrosion requires four elements:
- An Anode – The electrode where corrosion occurs.
- A Cathode – The other electrode needed to form a corrosion cell.
- A Conductor – A metallic pathway for electrons to flow.
- An Electrolyte – A liquid that can support the flow of electrons.
Steel, being a heterogeneous material, contains anodes and cathodes. Steel is also an electrical conductor. Therefore, steel contains three of the four elements needed for corrosion, while moisture provides the fourth element (an electrolyte).
Salt, from deicing or a marine environment, accelerates corrosion of steel bridges and reinforcing steel in concrete.
Methods for Corrosion Resistance
Three mechanisms by which coatings provide corrosion protection:
1. Barrier Coatings – Isolate the steel from the moisture. Low water and oxygen permeability.
2. Inhabitive Primer Coatings - Contain passivating pigments. Low-solubility pigments that migrate to steel surface when moisture passes through the film to passivate steel surface.
3. Sacrificial Primers (Cathodic Protection) – Contain pigments (such as zinc), which gives up electrons to the steel, becomes the anode, and corrodes to protect the steel.
No comments:
Post a Comment