Corrosion Fundamentals and Protective Coating Theory
Why steel pipelines corrode
Buried and submerged pipeline steel corrodes through an electrochemical cell with four elements: an anodic reaction (iron dissolution), a cathodic reaction (usually oxygen reduction or hydrogen evolution), an electrolyte (soil moisture or water), and a metallic return path. Remove any element and corrosion stops. Coatings work mainly by excluding the electrolyte, while cathodic protection (CP) supplies electrons so exposed steel at defects becomes the cathode.
What a coating must do
A protective coating must be a barrier to water, oxygen, and ions; adhere strongly; resist mechanical damage and soil stress; and remain compatible with CP. Adhesion is critical: a disbonded coating creates a crevice where corrosion concentrates and where CP current may be shielded. Questions often test the misconception that a coating is a complete barrier. All coatings have holidays and some permeability; CP is required at defects, and coating quality controls CP current demand.
Traps
Watch for “the coating prevents all corrosion,” “CP protects through a disbonded coating,” and confusion among barrier, inhibitive, and sacrificial mechanisms. Also distinguish general corrosion from localized attack driven by differential aeration, galvanic coupling, CO2/H2S, or microbes.
Rule of thumb
Coating is the primary barrier and CP is the backup at defects; neither substitutes for the other. Current demand scales roughly with bare area, so a coating with 1% defects greatly reduces CP current, but the remaining defects still need adequate potential.
Worked example
A line has 0.5% coating damage and receives CP. If the CP system was sized for bare steel, it is oversized; if sized assuming zero defects, it is undersized. Assess coating condition and defect area, not just “coated” status.