Pipeline corrosion

Corrosion, including corrosion assisted cracking, has been identified as the primary mechanism resulting in failures of oil/gas pipelines.^1,2 Pipeline corrosion is a quite complex phenomenon, and the complexity arises as a result of the interaction of multiple reactions and processes occurring simultaneously, which in turn, are very specific to both the material and the environmental interaction. Particularly, the environments where the pipelines are encountered range from highly corrosive ones in upstream oil/gas gathering lines, where high contents of gases such as CO₂ and H₂S are dissolved in multi-phased flow fluids, to mildly corrosive ones at the soil side, where coating properties and performance, cathodic protection (CP), soil properties, microbial activity, etc., affect the corrosivity of the environment. While significant efforts and advances have been made in pipeline corrosion research, there are increasing uncertainties and ignorance to the emerging corrosion phenomena. In recent years, some broadly reported pipeline incidents resulted from corrosion.^3,4 Indeed, corrosion has been a vital issue affecting the pipeline safety, energy transportation, environmental conservation, and even the national policy-making, as seen in the widely watched Keystone XL and Northern Gateway pipeline projects in North America.^5,6

This special issue of Corrosion Engineering, Science and Technology features papers on the topic of pipeline corrosion. The specific focus is to establish a forum for the dissemination and discussion of recent research advances in the pipeline corrosion area with wide-range themes, including internal corrosion, external corrosion, stress corrosion cracking (SCC), coatings and CP, inspection techniques, corrosion modeling, and AC (alternating current) corrosion. The papers included in this issue are from reputed research groups in Australia, Canada, China, Italy and the USA, respectively.

The issue starts with a paper by Eckert,⁷ who reviewed the nature, affecting factors, monitoring and management of internal microbiologically influenced corrosion (MIC), one of the least understood corrosion phenomena in oil/gas pipeline systems. Eckert analysed the current method on enumeration of planktonic for MIC investigation, and concluded that the method is neither representative of sessile microorganisms in biofilms nor indicative of the potential for internal MIC in oil and gas systems. Increased emphasis on understanding the interaction between biofilm and metal is suggested in order to improve current MIC management capabilities for oil and gas assets. It is anticipated that the paper provides essential guides for pipeline MIC research and management. Continuing the theme of internal corrosion, Huang et al.⁸ have looked to corrosion of pipeline steel in H₂S/CO₂ containing environments. Undoubtedly, this is also a difficult research topic, especially at a testing pressure up to 10 MPa with the co-existence of H₂S and CO₂ in the system. They characterised the competitive formation of carbonate and sulfide scales on the steel surface, and discussed the effect of element sulfur deposit on the steel corrosion. Moreover, the inhibitive performance of amine/ammonium salts and imidazoline/pyridine derivatives were evaluated and analysed. The results will cast a ray of light on understanding and management of corrosion of pipelines in high pressure H₂S/CO₂ environments in oil/gas production.

Moving to the theme of pipeline external corrosion, there are two papers investigating corrosion and passivity of pipeline steels in concentrated bicarbonate solutions which are generated under coatings permeable to CP so as to elevate the solution pH. Eliyan and Alfantazi⁹ studied the electrochemical corrosion behavior of X100 steel and its weld, and determined the critical bicarbonate concentration, below which the steel is dominated by anodic dissolution, rather than passivation. Upon formation of a passive film on the steel surface, the metallurgical microstructure shows its role in corrosion by affecting the stability of the film. Lu et al.¹⁰ investigated the negative effect of stress, which is mainly from internal operating pressure of pipelines, on passivity of the steel in bicarbonate solutions. Moreover, the entry of hydrogen into the steel also degrades its passivity.

Three papers are included in the theme of pipeline SCC, a phenomenon used to result in rupture and failures of pipelines due to the synergism of tensile stress, corrosive environment and steel metallurgy.¹¹ Gamboa¹² targeted at inclined SCC occurring in high pH environments, where the majority of stress corrosion cracks are perpendicular to the direction of applied stress. This is an interesting paper providing phenomenal summary and characteristics of this SCC, serving as an important supplement to the present SCC research results. The paper by Shirband et al.¹³ investigated a possible method to create artificial pits on steel surface, simulating the incubators for near-neutral pH SCC on pipelines. The method can be used to shorten the period of crack initiation in lab testing. The last paper in this theme is authored by Fumagalli et al.,¹⁴ who used electrochemical methods to measure the diffusivity of atomic hydrogen in steels featured different microstructures. This work will be helpful to understand and quantify the contribution of hydrogen to cracking of pipelines, especially for those made of high-strength steels which are susceptible to hydrogen induced cracking or hydrogen facilitated SCC. This paper also gave a mini-review about hydrogen diffusion and trapping in steels.

There are two papers categorised in the theme of coatings and CP, the key techniques to protect pipelines from external corrosion. Kuang and Cheng¹⁵ investigated the coating disbondment and its shielding effect on CP in the absence and presence of AC interference. The work is important in advancing the scientific understanding of the CP shielding phenomenon under disbonded coating. Moreover, they measured in-situ the changes of local potential and solution pH under coating as a function of disbonding distance, AC current density and time. The results provide industry recommendations for improved integrity management of pipelines. Li and Castaneda's¹⁶ work furthered the use of electrochemical impedance spectroscopy (EIS) technique in coating performance evaluation. They proposed the parameters of ensemble average phase and ensemble average impedance, which can be derived from the measured EIS, for fast evaluation of coatings used in practical buried pipeline in soil.

There is one paper by Varela et al.,¹⁷ summarising the major methods for inspection and monitoring of pipeline external corrosion. The advantages and limitations of the techniques, including potential survey tools (close interval potential survey, alternating current voltage gradient and direct current voltage gradient), and in-line inspection tools (magnetic flux leakages, ultrasonic transducers, electromagnetic acoustic transducers and eddy current), were analysed. Moreover, the paper overviewed the tools for inspecting non-piggable pipelines, such as pipeline crawlers, external corrosion direct assessment and hydrostatic tests, and the electrochemical and non-electrochemical monitoring techniques for pipeline corrosion.

Corrosion modeling has been a key means for assessment and prediction of pipeline failure and risk management. The paper in this theme is authored by Koch et al.,¹⁸ who used a Bayesian network model that combines physics based models and expert knowledge of oil flow lines to predict corrosion flaws depth and leak probability. This work demonstrates the feasibility to estimate the location-specific probability of pipeline failure, and thus provide input to the prioritisation of inspections and corrosion mitigation.

The final theme contained in the special issue is AC corrosion of pipelines, where two papers are included. Cui et al.¹⁹ investigated the effect of AC voltage on passivity of steel developed in concentrated carbonate/bicarbonate solutions, while the work of Lu et al.²⁰ touches the mitigation of AC corrosion of pipelines. Their results are interesting in that the zinc ribbons used for AC corrosion prevention experience accelerated corrosion by AC, especially when the DC (direct current) decoupler is not applied.

It is evident that there is a broad range of research activities in the pipeline corrosion area. Certainly, further accomplishments, both mechanistically and practically, will be made in the future to improve our management to this important problem.

I hope that you enjoy perusing through this special issue.

Y. Frank Cheng

Guest Editor

Professor and Canada Research Chair in Pipeline Engineering, Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, Canada