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Corrosion Basics

Water Constituents

The concentrations of various substances in water in dissolved, colloidal, or suspended form are typically low but can vary considerably. A hardness value of up to 400 ppm of calcium carbonate (CaCO3), for example, is sometimes tolerated in public supplies, whereas 1 ppm of dissolved iron would be unacceptable. In treated water for high-pressure boilers or where radiation effects are important, as in some nuclear reactors, impurities are measured in very small units such as parts per billion (ppb).

Steam Generation

The greatest use of high-temperature water and steam is in electrical power generation. 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 are different in many ways; however, they share a reliance on technically advanced water treatment and control for successful operation.

High-Temperature Corrosion by Sulfidation

The major constituent in flue-gas corrosion that differentiates it from common high-temperature atmospheric corrosion is the sulfur content. In general, oxidation by sulfur, or sulfidation, is a considerably more destructive form of high-temperature corrosion than oxidation by oxygen. Sulfide scales tend to crack and spall more readily than oxides, which can remain continuous and provide some degree of corrosion protection. In some cases, depending on the form in which sulfur is present in the atmosphere, continuous sulfide scales cannot form, so attack will proceed linearly; that is, the scale will afford no protection. The melting points of metallic sulfides usually are lower than those of the corresponding oxides.

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.

Corrosion Behavior of Cast Irons

Cast iron is a generic term that applies to high-carbon/iron alloys containing silicon. The common ones are designated as gray cast iron, white cast iron, malleable cast iron, and ductile or nodular cast iron.

Cautionary Use of Test Data

The corrosion behavior of materials under conditions of service should be an important consideration during the design of a system. Although it is relatively straightforward to obtain quantitative information on physical and mechanical properties such as tensile strength, yield strength, impact values, fatigue limit, effect of temperature on properties, and so forth, truly representative corrosion data are often much more difficult to obtain.

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).

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).