Corrosion Basics

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.

Forms of Corrosion

General, or uniform, corrosion is usually the least threatening type of attack because associated metal loss is distributed over a wide surface area. Unfortunately, most corrosion encountered in engineering practice is of a more localized type. Localized corrosion can be defined as selective attack at limited special areas or zones on a metal surface in contact with an environment.