Corrosion Basics

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 by Water

The concentrations of various substances present in water in dissolved, colloidal, or suspended form are typically low but can vary considerably. The importance of these concentrations depends on the particular substance as well as the alloy, configuration, and function of the metallic structure with which the water comes into contact.

High-Temperature Corrosion

High-temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte. In this corrosion mechanism, metals react directly with gaseous atoms in the atmosphere rather than ions in solution. Sometimes, this type of damage is called “dry corrosion” or “scaling.” The first quantitative analysis to oxidation behavior was made in the early 1920s with the postulation of the parabolic-rate theory of oxidation by Tammann and, independently, by Pilling and Bedworth.

Corrosion Testing and Monitoring in Manufacturing Plants

Metals and their alloys are used in a multitude of changing conditions, and corrosion failures are not uncommon. The limited predictability of metal performance is the main reason why corrosion tests and corrosion monitoring programs are so important. Properly conducted, these tests can provide significant savings. In many cases, they are built-in requirements.

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.

Corrosion 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 to acids, cooling waters, steam, and many other environments—either continuously or intermittently—to reduce the intensity of corrosion that might otherwise threaten the intended design life of a structure.

Protective Coatings

Applying an organic or metallic barrier coating between a corrosive environment and the material to be protected is a fundamental method of corrosion control. In fact, coatings and linings represent the most widely used method of protecting most metals.

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.