Investigating Pipeline Corrosion Failures

Introduction

Failure analysis and root cause analysis (RCA) of pipe and piping system components that have experienced corrosion damage provide operators with valuable information needed to prevent future failures. In many cases, however, the corroded sample is not handled or preserved properly and the ability to diagnose the corrosion mechanism is lost.

Some common examples of incorrect handling and sampling include;

• Buried pipelines that have been cut and flooded with ground water are used to provide samples for microbiological analysis, even after they have obviously been contaminated.
• Cut-out pipe samples are exposed to air, heat, dust and rough handling that oxidize and physically and chemically change corrosion products before they are submitted for corrosion failure analysis.
• Samples have been chemically cleaned with solvents or soaps or have been mechanically cleaned with wire brushes or power washers that alter or remove the corrosion products that are needed to diagnose the cause of the corrosion.
• Foreign objects have been inserted into a leak location to plug the leak or to widen the leak so as to drain product from the pipeline, resulting in contamination of the corrosion products.

Samples that are handled and preserved properly provide reliable, accurate analytical results. In some cases, alteration or contamination of the sample is unavoidable, such as when the failure results in a fire or the failure origin is ejected and submerged in water, mud, or dirt. When a corroded sample has been altered, a representative sample may still be available if corrosion is present at another location on the pipe that was not affected by post-rupture events or contamination. This article discusses processes and procedures that can be used when investigating the cause of corrosion on pipelines and piping systems; particularly emphasizing proper sampling and preservation techniques.

Samples for corrosion investigation are not exclusively provided by leaks or ruptures. Corrosion may be discovered on a pipeline during in-line inspection or direct assessment. In the US, regulated hazardous liquid and natural gas pipelines must be examined for external corrosion whenever they are exposed. The operator must determine the extent of any external corrosion that is found and document the inspection. The need to determine the cause of the corrosion is also included in ASME B31.4, 461.3 (2006).¹ Similarly, regulated pipelines must be inspected for internal corrosion whenever the internal surface is exposed (e.g., during a pipeline repair). If corrosion is found, this provides the operator an opportunity to collect samples and other information to help determine why the corrosion occurred and to determine the appropriate mitigation measures. In plant piping, corrosion may be discovered during routine maintenance or equipment replacement. For example, cleaning and inspection of separators during a plant turn-around may lead to the discovery of internal corrosion under accumulated deposits. Being prepared to take advantage of these “learning opportunities” where corrosion is discovered is important so that evidence of the corrosion mechanism is not lost.

Conditions that Lead to Corrosion

All corrosion is caused by anodic and cathodic electrochemical reactions that occur concurrently on a metal surface that is in contact with an electrolyte (e.g., water) or a material that can hold water (e.g., soil). The electrolyte and any other solid material or biofilm present on the metal surface provide chemical species that may contribute to or facilitate anodic and cathodic reactions. Surface conditions, such as the flow rate of electrolyte across the metal surface, contribute to the distribution and rate of the corrosion damage by removing reaction products that would otherwise accumulate around the cathodic and anodic sites and slow the corrosion reactions. Increasing temperature also tends to increase the rate of aqueous corrosion. Therefore, if the goal of a corrosion investigation is to determine the corrosion mechanism, the chemical and physical environment to which a corroded sample was exposed must be understood. Typically, this understanding comes from analysis of the deposits associated with the corrosion damage, examining the way in which the corrosion is manifested, and analyzing historical information about the environment that was in contact with the metal.