Pipeline Corrosion

Introduction

The Pipeline Toolbox is home to many tools and calculators. The PLTB User’s Guide presents information, guidelines and procedures for use during design, construction, operations and integrity tasks for field or office applications.
It has been recognized within the pipeline industry that some sections of high-pressure pipelines, particularly those with long service histories, may experience corrosion.

It has also been recognized, through theoretical analysis, scientific research and testing, and industry operating experience, that some amount of metal loss due to corrosion can be tolerated without impairing the ability of the pipeline to operate safely. In 1984, ASME published the first edition of the B31G Manual for Determining the Remaining Strength of Corroded Pipelines. The B31G document provided pipeline operators with a simplified evaluation method based on the results of analysis and tests. The application of B31G has enabled pipeline operators to reliably determine safe operating pressure levels for pipe affected by corrosion, and to determine whether repairs are necessary in order to continue operating safely.

B31G continued to be reissued by ASME with only minor revisions over time, although other corrosion evaluation methods had evolved since B31G’s initial publication. A majority of these other methods are based on the same theoretical model from which the original B31G method was derived but may offer some refinement in accuracy. Subsequently, an effort was undertaken to update the B31G document to recognize certain other corrosion evaluation methods that have proven sound and that have seen successful use in the pipeline industry.

Module/Application

• ASME B31G Evaluation of MAOP in Corroded Areas
• ASME B3IG Maximum Allowable Longitudinal Extent of Corrosion
• Rate of Electrical Current Flow Through the Corrosion Cell
• Relationship Between Resistance and Resistivity
• Electrolyte Resistance from Surface of an Electrode to any Distance
• Ohm’s Law of Corrosion Current
• Electrical Resistance of a Conductor

References

• ANSI B31.G Calculations
• PRCI – A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe, Database for Corroded Pipe Tests and Continued Validation of RSTRENG
• NACE – Electrical Resistance and Resistivity Calculations

Appendix of Definitions

ANION – A negatively charged ion that migrates through the electrolyte toward the anode under the influence of a potential gradient.

ANODE – The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. Corrosion usually occurs, and metal ions enter the solution at the anode.

BITUMINOUS COATING – An asphalt or coal-tar compound used to provide a protective coating for a surface.

CATHODE – The electrode of an electrochemical cell at which reduction is the principal reaction. Electrons flow toward the cathode in the external circuit.

CATHODIC PROTECTION – A technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.

CATION – A positively charged ion that migrates through the electrolyte toward the cathode under the influence of a potential gradient.

COATING – A liquid, liquefiable, or mastic composition that, after application to a surface, is converted into a solid protective, decorative, or functional adherent film.

CORROSION – The deterioration of a material, usually a metal, that results from a reaction with its environment.

CORROSION INHIBITOR –  A chemical substance or combination of substances that, when present in the environment, prevents or reduces corrosion.

CURRENT – A flow of electric charge. The amount of electric charge flowing past a specified circuit point per unit time, measured in the direction of net transport of positive charges. (In a metallic conductor, this is the opposite direction of the electron flow.)

CURRENT DENSITY – The current to or from a unit area of an electrode surface.

DEEP GROUNDBED – One or more anodes installed vertically at a nominal depth of 15 m (50 ft) or more below the earth’s surface in a drilled hole for the purpose of supplying cathodic protection.

ELECTRODE – A conductor used to establish contact with an electrolyte and through which current is transferred to or from an electrolyte.

ELECTROLYTE – A chemical substance containing ions that migrate in an electric field.

EROSION – The progressive loss of material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or solid particles carried with the fluid.

FATIGUE – The phenomenon leading to fracture of a material under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material.

GROUNDBED – One or more anodes installed below the earth’s surface for the purpose of supplying cathodic protection.

HOLIDAY – A discontinuity in a protective coating that exposes unprotected surface to the environment.

IMPRESSED CURRENT – An electric current supplied by a device employing a power source that is external to the electrode system. (An example is direct current for cathodic protection.)

ION – An electrically charged atom or group of atoms.

IR DROP – The voltage across a resistance in accordance with Ohm’s Law.

MODULUS OF ELASTICITY – A measure of the stiffness or rigidity of a material. It is actually the ratio of stress to strain in the elastic region of a material. If determined by a tension or compression test, it is also called Young’s Modulus or the coefficient of elasticity.

OXIDATION – Loss of electrons by a constituent of a chemical reaction. Corrosion of a metal that is exposed to an oxidizing gas at elevated temperatures.

PITTING – Localized corrosion of a metal surface that is confined to a small area and takes the form of cavities called pits.

POLARIZATION – The change from the open-circuit potential as a result of current across the electrode/electrolyte interface.

RESISTIVITY – The resistance per unit length of a substance with uniform cross section. A measure of the ability of an electrolyte (e.g., soil) to resist the flow of electric charge (e.g., cathodic protection current). Resistivity data are used to design a ground bed for a cathodic protection system.

STRAY-CURRENT CORROSION – Corrosion resulting from current through paths other than the intended circuit, e.g., by any extraneous current in the earth.

STRESS CORROSION CRACKING – Cracking of a material produced by the combined action of corrosion and tensile stress (residual or applied).

VOID – A holiday, hole, or skip in a coating. A hole in a casting or weld deposit usually resulting from shrinkage during cooling.

YIELD STRENGTH – The stress at which a material exhibits a specified deviation from the proportionality of stress to strain. The deviation is expressed in terms of strain by either the offset method (usually at a strain of 0.2%) or the total-extension-under-load method (usually at a strain of 0.5%.)

Related Links