Corrosion Basics

Power Plant Corrosion

Power plants for the generation of electricity are among the most complex sites related to corrosion, involving many different corrosion mechanisms and requiring a full range of corrosion control approaches. The core activity in almost all electrical power plants is the generation of steam to drive turbines that, in turn, spin dynamos. Historically, fossil fuels (i.e., wood, coal, gas, and oil) were used almost exclusively to heat water and make steam until the introduction of nuclear power steam generators in the second part of the 20th century. The two types of power plants have many operations and components in common, but each presents specific corrosion control challenges.

Types of Corrosive Atmospheres

Although atmospheres can be classified into four basic types, most of them are mixed and present no clear lines of demarcation. Furthermore, the type of atmosphere may vary with the wind pattern, particularly where corrosive pollutants are concerned.

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.

Corrosion by Water and Steam

The most familiar uses of water are for potable service and fire control purposes. Other major uses of water in industry are the transfer of heat and the production of steam. There is extensive use of cooling water in almost every manufacturing process, in commercial air conditioning, and even a substantial percentage in domestic air conditioning. Fossil and nuclear fuel steam plants are encountered in the heating and power-generating fields.

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.

Why Metals Corrode

The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. To reach this metallic state from their occurrence in nature in the form of various compounds (ores), it is necessary for them to absorb and store up the energy required to release the metals from their original compounds for later return by corrosion. The amount of energy required and stored varies from metal to metal.

Corrosion and the Environment

The fact that corrosion does occur should not be cause for surprise. Almost all materials should be expected to deteriorate with time when exposed to the elements. Corrosion is a perfectly natural process, as natural as water flowing downhill. If water flowed uphill or remained stationary on a hillside, there may be cause for surprise, yet our human ingenuity can accomplish this by putting water in a closed container (pipe) and closing the bottom end, or merely freezing it. Similarly, if iron or steel were exposed to air and water, rust would be expected to develop within a matter of hours.

Special Considerations When Using Inhibitors

An inhibitor is a substance that slows down a chemical reaction (in the present context, a corrosion reaction). Corrosion inhibitors are commonly added in small amounts, either continuously or intermittently, to control serious corrosion in aggressive environments such as acids, cooling waters, and steam. While they can be highly effective, many inhibitors are also toxic, particularly in the concentrations suitable for shipping and storage. It is important to employ precautions to ensure personnel safety, environmental protection, and uninterrupted operation of equipment.

Surface Preparation

Proper surface preparation is an essential preliminary step for any coating application. It is false economy to skimp on surface preparation in the belief that the coatings applied will compensate for surface deficiencies. This is especially true of high-performance coating materials.

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.

Failure Analysis and Design Considerations

Ideally, the corrosion mechanisms and other factors that can affect the reliability of machinery can be anticipated and minimized during the original design process. Nevertheless, system failures and subsequent failure investigations have become increasingly important in our modern societies. Besides liability issues, an important reason for conducting a failure investigation is to identify the mechanism(s) and cause(s) of a problem to prevent its recurrence.

Engineering Materials

The corrosion literature is filled with data on the performance of various materials in myriads of chemical environments. While modern electronic search techniques can provide ready access to a wealth of constantly updated information, the sheer volume of data can be overwhelming. Engineers must constantly be on guard when considering such information to be certain not only that the chemical environment is adequately defined, but also that the particular alloy (including its heat treatment) and the character of attack are fully described and understood.

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).

Corrosion Damage in Reinforced Concrete

Steel-reinforced concrete is a very important material of construction. One reason for the success of this combination is the similarity of thermal expansion properties of carbon steel and concrete. Another is that the extremely high pH of the cement content (typically pH > 13 for new concrete) passivates the steel surfaces against corrosion activity.