Cathodic Protection

Hot Oil Aboveground Storage Tank Bottom Corrosion Failure and Cathodic Protection Upgrade—Part 2

Several types of anode installations for tank bottoms are possible, but the methods selected do not always produce the desired results. This two-part article discusses a case history in which existing cathodic protection (CP) was ineffective and testing methods did not identify system deficiencies. This led to the premature failure of the tank bottom. Part 1 covered the findings of an investigation conducted to identify the cause of the corrosion. Part 2 describes the remedial approach taken to enhance CP for effective corrosion control.

Hot Oil Aboveground Storage Tank Bottom Corrosion Failure and Cathodic Protection Upgrade—Part 1

There are several types of anode installations that distribute protective current to a tank bottom. In some cases, however, the methods selected do not always produce the desired results. This two-part article discusses a case history in which existing cathodic protection (CP) was ineffective and the methods used to verify the performance of CP did not identify system deficiencies. This led to the premature failure of a tank bottom. Part 1 covers the findings of an investigation conducted to identify the cause of the corrosion. Part 2 discusses the remedial approach taken to enhance the CP for effective corrosion control.

Special Cathodic Protection Requirements for Specific Pipeline Applications

Most pipeline cathodic protection (CP) applications involve either galvanic anode or impressed current CP (ICCP) systems installed in earth for protection of external surfaces. Of the galvanic anode installations in neutral soils, magnesium is the most commonly used anode material. Rectifiers are the most common source of direct current power for impressed current systems.

Close-Interval Potential Surveys

The principle of a close-interval potential survey (CIPS or CIS) is to record the pipe-to-soil (P/S) potential profile of a pipeline over its entire length by measuring potentials at intervals that do not significantly exceed the depth of the pipe (often ~1 m).

Corrosion Surveys

Two of the most fundamental and informative field measurements are soil resistivity surveys and pipe-to-soil potential surveys.

Polymeric Materials

Polymers are complex molecules formed by chains of duplicated groups of atoms (monomers); these groups are typically linked by covalent bonds along a “backbone” of carbon or silicon atoms. Important polymeric materials related to corrosion include plastics and synthetic rubbers (elastomers).

Stray Current Effects

Before preparing a cathodic protection (CP) design, the possible presence of stray currents must be considered. Stray currents are defined as those which follow a path other than the one intended. Where stray currents discharge from a structure into the electrolyte environment in order to return to the source, corrosion will occur.

Effects of Coating on Corrosion and Cathodic Protection

The four basic elements of a corrosion cell are an anode, a cathode, and the metallic and electrolytic pathways between them. Corrosion control can be achieved by eliminating (or reducing) any of these elements. One such method is to modify the electrolytic pathway by introducing a barrier between the threatened metal surface and the corrosive medium (i.e., by applying some kind of coating).

Cathodic and Anodic Protection

A metallic structure in contact with an electrolyte (typically soil or water) usually includes anodic sites, where oxidation (corrosion) occurs, and cathodic sites, where reduction (protection) occurs. Cathodic protection (CP) is a technique to reduce the corrosion of a metal surface by making that entire structure the cathode of an electrochemical cell—that is the derivation of the term. This is typically accomplished by discharging current from an external anode so that current will flow through the electrolyte to, instead of away from, the original anodic sites on the structure surface.

Polarization

As is the case with other chemical reactions, the driving force of a corrosion reaction is related to the difference in energy between an initial equilibrium that is higher in energy than the final equilibrium. As corrosion action proceeds, this difference in energy tends to decrease as a result of the effects of the products of anodic and cathodic reactions in the vicinity of the corrosion sites. The cathodic reaction, and with it the overall corrosion reaction, would slow down if, for example, the hydrogen product of the cathodic reaction were not removed by evolution as gas or some reaction involving oxygen. This slowing down is said to be the result of cathodic polarization.

Principles of Electrochemistry Applied to Corrosion

Although corrosion can take several forms, the mechanism of attack in aqueous environments involves some aspect of electrochemistry. There is a flow of electricity from certain areas of a metal surface to other areas through a solution capable of conducting electricity, such as seawater or fresh water. The term anode is used to describe that portion of the metal surface that is corroded and the term cathode is used to describe the metal surface from which current leaves the solution and returns to the metal.