What Causes Corrosion

IN THE LAB:

Corrosion is an electrochemical process that occurs due to the natural potential difference (voltage) between connected metals in contact with an electrolyte. The presence of these four basic components create the conditions for corrosion and form what is known as a corrosion cell:

1. Anode - the electronegative electrode

2. Cathode - the electropositive electrode

3. Metallic Path - a common metallic pathway that connects the anode and cathode

4. Electrolyte - a common electrolytic pathway that connects the anode and cathode

   

When all four components are present, electrical current will flow from the negative to the positive electrode through the electrolyte and from the positive to the negative electrode through the metallic path (Figure 1). 

Where current exits the anode into the electrolyte, corrosion occurs. 

IN THE FIELD:

Microscopic surface irregularities create diverse potential differences across even a single piece of metal, resulting in various areas acting as the individual anodes and cathodes required to form a corrosion cell. In the case of a buried pipeline, the areas which exhibit relatively higher negative potentials act as the anodes within the corrosion cell while the soil in which the pipeline is buried acts as the cell’s electrolyte. Variations in the electrolyte can help to drive corrosion for various reasons, including: ion concentration, oxygen concentration, pH, etc. 

Interested in our "Fundamentals: Back To Basics" series? Click here to check it out.