Coatings and Their Interaction with Cathodic Protection
Complementary, not alternatives
A coating is the primary barrier; CP protects the holidays and damaged areas the coating cannot cover. Coating quality controls current demand by orders of magnitude. A rough rule of thumb: bare steel in soil may need 1–3 mA/ft² (about 10–30 mA/m²), while good coated pipe may need only 0.01–0.05 mA/ft². That is why coating condition, not pipe area, usually drives rectifier sizing.
What a CP technician must understand
Coating type (fusion-bonded epoxy, liquid epoxy, coal tar, tape, wax) and surface preparation determine adhesion and resistance to cathodic disbondment - the lifting of coating driven by alkalinity generated at the cathode. Holidays concentrate CP current, so a small defect can pull a large current. Disbonded coating can create a shielded region where the electrolyte is stagnant and CP current cannot reach the steel; corrosion can proceed there even though the pipe reads protected. Coating conductance and Pearson-type surveys measure coating quality; DCVG locates defects. Coating resistance is often expressed as ohm-cm² or as conductance per unit area.
Traps
Believing good coating eliminates the need for CP; assuming a pipe reading reflects conditions under a shielded disbondment; ignoring the effect of high CP potentials (overprotection) in accelerating disbondment; forgetting that coatings fail first at welds, bends, and field joints.
Worked example
10,000 ft² of pipe. Bare at 2 mA/ft² requires 20 A. Coated at 0.02 mA/ft² requires only 0.2 A. That 100-fold difference explains why a coating failure changes rectifier loading so dramatically and why current demand is a coating-condition indicator.