Corrosion Basics

Corrosion Basics: Coating New Construction Seagoing Vessels

Surface preparation and coating application during the new construction of a vessel compose a complex orchestration that requires the coordination of every department in a shipyard. It is also one of the most expensive parts of building a new vessel.

Corrosion Basics: Effects of Coating on Corrosion and Cathodic Protection

The application of a coating can greatly reduce the amount of current required to obtain cathodic protection. In addition, good coating can significantly improve attenuation characteristics along a pipeline.

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).

Corrosion Surveys

Two of the most fundamental and informative field measurements are soil resistivity surveys and pipe-to-soil potential surveys.

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).

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.

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.

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.

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.

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.

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).

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.

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.

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.