Failure of cathodic protection for storage tank bottom plates

Storage tanks typically rest on annular reinforced concrete foundations. The tank bottom is usually underlain by bedding layers—such as sand, gravel, and asphalt—and is equipped with protective coatings and cathodic protection systems. Given the large surface area of ​​crude oil tank bottoms, bedding layers often feature seams from segmented installation, and factors such as improper construction can lead to through-thickness cracks. Under cyclic loading conditions (alternating between empty and full states), soil heterogeneity and compaction errors during foundation construction can cause localized settlement of the underlying soil, thereby altering the stress state of the tank bottom plate. This deformation leads to bending of the bottom plate, creating air gaps or voids. Moisture and rainwater from the atmosphere, along with water, oxygen, and salt ions from the underlying soil, can migrate through the bedding layers and accumulate in these voids beneath the plate, creating conditions conducive to corrosion.

Cathodic Protection Defects
1. Non-uniform protection potential distribution
The distribution of the anodic electric field can cause excessive current to flow toward the edges of the tank bottom—particularly if the bottom is uneven—creating a potential difference between the center and the edges. This results in under-protection at the center and over-protection at the edges. Excessive protection potential not only wastes electrical energy but also leads to hydrogen evolution, potentially causing hydrogen embrittlement in the steel.
2. Non-uniform potential distribution
The uniformity of the cathodic protection potential across the tank bottom plate is influenced by factors such as soil resistivity, pH, moisture content, oxygen levels, current density, the degree of polarization at the tank bottom, and the shape, size, and distribution of the anodes.
3. Coating blistering
When coating blistering occurs, the ability of the cathodic protection current to penetrate the coating and cathodically polarize the substrate beneath the failed coating becomes a critical factor affecting corrosion; this requires assessment via Electrochemical Impedance Spectroscopy (EIS) coating degradation testing.
4. Cathodic disbondment
Cathodic disbondment can be viewed as an adverse effect resulting from the combined use of coatings and cathodic protection. Under the influence of cathodic protection, coatings can easily lose adhesion to the metal substrate, leading to the formation of cracks that become filled with corrosive media; this necessitates EIS-based coating degradation testing.