Corrosion Fundamentals

Why Corrosion Fundamentals Matter

Cathodic protection is not just a field-testing procedure. It is a method of controlling electrochemical corrosion. If the corrosion cell is misunderstood, then CP readings, anode behavior, current flow, polarization, and protection criteria will also be misunderstood.

In CP work, the most important corrosion concept is that metal loss occurs at anodic areas. Cathodic protection is designed to reduce that anodic activity by forcing protective current onto the structure.

A technician or designer who does not understand corrosion fundamentals may incorrectly assume that a coating alone is protection, that a voltage reading alone proves protection, or that corrosion only occurs when a structure is visibly damaged.

Core Concept

The corrosion cell

For electrochemical corrosion to occur, four elements must be present: an anode, a cathode, an electrolyte, and a metallic path. If any one of these elements is removed or controlled, corrosion can be reduced or interrupted.

Anode

The anode is the area where oxidation occurs and metal loss takes place. At an anodic area, metal atoms give up electrons and enter the electrolyte as ions. This is the location where corrosion damage occurs.

Cathode

The cathode is the area where reduction reactions occur. In a corrosion cell, the cathodic area is protected relative to the anodic area because metal loss is not occurring there in the same way.

Electrolyte

The electrolyte is the conductive environment that allows ionic current flow. In cathodic protection work, common electrolytes include soil, water, seawater, concrete pore solution, and other conductive environments surrounding the structure.

Metallic path

The metallic path allows electrons to move between anodic and cathodic areas. On a single steel structure, the structure itself provides the metallic path. Between connected structures, bonds, piping, grounding systems, or other metallic connections may provide the path.

Oxidation and Reduction

Corrosion involves oxidation and reduction reactions. Oxidation occurs at the anode, where metal gives up electrons. Reduction occurs at the cathode, where electrons are consumed by reduction reactions in the environment.

A common way to remember this is “OIL-RIG”: oxidation is loss of electrons, and reduction is gain of electrons. In a corrosion cell, oxidation occurs at anodic areas, which is where metal loss occurs.

Cathodic protection reduces corrosion by shifting the protected structure in the cathodic direction. The intent is to reduce the tendency for the protected metal surface to act as an anode.

Corrosion on a Steel Structure

A buried or submerged steel structure does not corrode uniformly in every situation. Different areas of the same structure can become anodic or cathodic because of coating defects, oxygen concentration differences, soil changes, moisture variation, stray current, dissimilar metals, welds, or other environmental differences.

At anodic areas, steel dissolves into the electrolyte. At cathodic areas, reduction reactions occur. The electrical and chemical balance between these areas determines the corrosion behavior.

Cathodic protection changes this balance by supplying protective current to exposed metal surfaces. The structure is forced to behave more cathodically, reducing corrosion at areas that would otherwise be anodic.

Field Application

Corrosion fundamentals are used during almost every CP task. When a technician measures a structure-to-electrolyte potential, they are evaluating electrochemical behavior. When a rectifier output is adjusted, current distribution and polarization are being changed. When an anode is installed, it becomes part of a corrosion-control circuit.

These concepts also explain why coatings and CP are commonly used together. Coatings reduce the amount of exposed metal surface. Cathodic protection supplies current to the exposed metal that remains at coating defects, holidays, damaged areas, or bare surfaces.

Corrosion fundamentals are also necessary for troubleshooting. Low potentials, rapid anode consumption, interference, shorts, shielding, and poor current distribution are all easier to diagnose when the corrosion cell is understood.

Common Mistakes

1. Thinking corrosion occurs at the cathode.
   Why it is wrong: Metal loss occurs at anodic areas. Reduction reactions occur at cathodic areas.
2. Ignoring the electrolyte.
   Why it is wrong: The electrolyte allows ionic current flow. Soil, water, concrete, and seawater conditions strongly affect corrosion and CP behavior.
3. Assuming coated steel cannot corrode.
   Why it is wrong: Coatings can have holidays, damage, disbondment, aging, or installation defects. CP is often used to protect exposed steel at coating defects.
4. Assuming corrosion is always uniform.
   Why it is wrong: Corrosion can be localized because of environmental differences, coating defects, dissimilar metals, oxygen concentration differences, or stray current.
5. Confusing electron flow and ionic current flow.
   Why it is wrong: Electrons move through metallic paths. Ions move through the electrolyte. A corrosion or CP circuit involves both.

Field Example

A coated buried steel pipeline has a small coating holiday. Soil contacts the exposed steel at that defect. Because the soil acts as an electrolyte, corrosion can occur at exposed anodic areas on the steel surface.

If cathodic protection current reaches that exposed steel, the current can polarize the exposed surface in the cathodic direction and reduce corrosion. If the coating defect is shielded, electrically isolated from the CP current path, or located in a high-resistance environment, the area may remain underprotected.

This is why CP evaluation must consider more than a single voltage reading. The technician must understand where current is flowing, what surface is exposed, and whether the measurement represents the area being evaluated.

Practice Questions

1. What are the four required elements of an electrochemical corrosion cell?
2. At which part of the corrosion cell does metal loss occur?
3. What role does the electrolyte play in a corrosion cell?
4. Why are coatings and cathodic protection commonly used together?
5. Why can corrosion be localized instead of uniform across an entire steel structure?

Related Pages

• What Is Cathodic Protection?
• How Cathodic Protection Works
• Galvanic Cathodic Protection
• Impressed Current Cathodic Protection
• Anodes
• Structure-to-Electrolyte Potential