Corrosion Basics

Corrosion Basics: Close-Interval Potential Surveys

The principle of a close-interval potential survey is to record the pipe-to-soil potential profile of a pipeline over its entire length by measuring potentials at intervals that do not significantly exceed the depth of the pipe.

Corrosion Management and the Significance of Regular Reporting

Regular inspection, monitoring, sampling, and chemical treatment activities are needed to mitigate corrosion. Data are not always adequately reported on a regular basis, however, which could gradually render an asset integrity management system less effective.

NACE International’s Annual Corrosion Career Survey Results for 2015

Corrosion professionals in North America and Europe have experienced another year of continued growth in career opportunities and salary levels, according to the 2015 Corrosion Career Survey conducted by Materials Performance magazine.

Wanted: Technical Articles for MP

To successfully communicate the wide variety of corrosion-related issues affecting corrosion professionals today, MP is actively encouraging corrosion control professionals worldwide to submit technical articles to share their corrosion-related experiences with over 36,000 NACE International members around the globe.

Intergranular Corrosion

Intergranular corrosion is a form of localized surface attack in which a narrow path is corroded out preferentially along the grain boundaries of a metal. It initiates on the surface and proceeds by local cell action in the immediate vicinity of a grain boundary. Although the detailed mechanism of intergranular corrosion varies with the metal system, its physical appearance at the microscopic level is quite similar for most systems.

Application of Organic Coatings

Organic coatings include latex paints, plastics, asphaltic materials, rubbers, and elastomers. The specific material selected for a coating job must have characteristics that would allow its proper application under all conditions existing during the coating process. Proper identification of the physical or other properties of the film expected from the application must be specified.

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).