Location of Thesis Examination

Room 4185 Support Services Building

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

David W. Shoesmith

Abstract

This thesis reports a series of investigations examining external corrosion processes along gas transmission pipelines. TransCanada PipeLines Ltd. (TCPL) has developed six proposed corrosion scenarios to describe external pipeline corrosion, based primarily on corrosion products and corrosion rates (CRs) observed at field sites. The six proposed corrosion scenarios can be divided into two groups: abiotic and biotic. The three abiotic corrosion scenarios are (1) anaerobic corrosion, (2) aerobic corrosion, and (3) anaerobic corrosion turning aerobic; while (4) anaerobic corrosion with microbial effects, (5) aerobic corrosion turning anaerobic with microbial effects, and (6) anaerobic corrosion with microbial effects turning aerobic comprise the three biotic corrosion scenarios.

The primary objective of this project was to develop a series of laboratory-based corrosion mechanisms in order to validate the six proposed corrosion scenarios. The approach used was to electrochemically monitor the evolution in corrosion behaviour of steel exposed to simulated groundwaters anticipated near exposed pipelines. The primary electrochemical techniques used were corrosion potential (ECORR) measurements to monitor redox conditions, linear polarization resistance (LPR) measurements to calculate CRs, and electrochemical impedance spectroscopy (EIS) measurements to monitor changes in steel/film properties. A secondary objective was to match corrosion products (i.e., “rusts”) produced electrochemically to those reported from field investigations using surface analytical techniques. Scanning electron microscopy (SEM) and optical microscopy were used to analyze corrosion product morphologies, while elemental composition and iron phase identification were examined by energy dispersive X-ray (EDX) spectroscopy and Raman spectroscopy, respectively.

Reports indicate that seasonal variations in the water table can alter the distribution of anaerobic and aerobic sites, which can be particularly damaging to pipeline integrity. Anaerobic corrosion is associated with wet soil conditions and is the most prevalent corrosion scenario observed in the field. To understand steel-oxygen interactions, the influence of traces of dissolved oxygen on pretreated steel was investigated. Adventitious oxygen was found to initiate a slow evolution in film properties on pretreated steel leading to a drastic film transition (i.e., a rapid rise in ECORR coupled to a large increase in CR).

Should soils dry, oxygen tends to diffuse more readily through the soil to the pipe surface and drives steel corrosion. To better understand the effect of changes to redox conditions on pipelines, long term corrosion measurements were made by monitoring the changes in corrosion behaviour of pretreated carbon steel under alternating anaerobic-aerobic cycles. With increasing cycle number the corrosion process becomes localized at a small number of locations, consistent with the formation of tubercles. As expected, periods of aerobic corrosion were associated with more positive potentials and higher CRs; whereas anaerobic corrosion yielded more negative potentials and lower CRs.

Microbially induced corrosion (MIC) is particularly damaging to pipeline integrity as soils containing microbes generate sulphide, which can significantly alter the groundwater chemistry. Difficulties in simulating complex field conditions in the laboratory have meant that only a limited number of experiments have accurately characterized the complex chemical and biological interactions between bacteria and steel. Several biological microcosms were investigated to further understand how variations in nutrient levels affect steel-microbe interactions. Raman analyses indicated mackinawite (Fe1+xS) was the dominant iron sulfide phase formed both by microbes and inorganic sulphide.

In order to avoid biological complications, and to improve our understanding of Fe-S interactions, inorganic HS was used as a model system to aid in the interpretation of steel undergoing MIC. A series of five corrosion potential measurements investigated the effect of variations in redox conditions and inorganic sulphide concentration on the corrosion behaviour of pretreated and untreated carbon steel. Field reports have indicated the highest CR expected for external corrosion occurred when pipelines, initially under anaerobic MIC control, transformed to aerobic corrosion. The laboratory results support field sampling methods as the highest CRs observed occurred when both sulphide and oxygen were present. In addition, under anaerobic conditions with sulphide, CRs were lower for freshly exposed steel compared to anaerobically pre-corroded steel. This suggests pre-corroded surfaces prevent the formation of passivating iron sulphide films.