The effect of volatile organic acids and CO2 on the corrosion rate of carbon steel from a Top-of-Line-Corrosion (TLC) perspective

S B Gjertsen,A Palencsar,T H Hemmingsen,M Seiersten

doi:10.1088/1757-899x/1201/1/012079

S B Gjertsen, A Palencsar + Show 2 more

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https://doi.org/10.1088/1757-899x/1201/1/012079

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Abstract

Models for predicting top-of-line corrosion (TLC) rates on carbon steels are important tools for cost-effectively designing and operating natural gas transportation pipelines. The work presented in this paper is aimed to investigate how the corrosion rates on carbon steel is affected by acids typically present in the transported pipeline fluids. This investigation may contribute to the development of improved models. In a series of experiments, the corrosion rate differences for pure CO2 (carbonic acid) corrosion and pure organic acid corrosion (acetic acid and formic acid) on X65 carbon steel were investigated at starting pH values; 4.5, 5.3, or 6.3. The experiments were conducted in deaerated low-salinity aqueous solutions at atmospheric pressure and temperature of 65 °C. The corrosion rates were evaluated from linear polarization resistance data as well as mass loss and released iron concentration. A correlation between lower pH values and increased corrosion rates was found for the organic acid experiments. However, the pH was not the most critical factor for the rates of carbon steel corrosion in these experiments. The experimental results showed that the type of acid species involved and the concentration of the undissociated acid in the solution influenced the corrosion rates considerably.

Highlights

Over the last two decades, the interest in top-of-line corrosion (TLC) has increased in the oil and gas industry [1]
This is in agreement with the conclusion made by Garsany et al that the concentration of weak acids in the solution is more critical for the corrosion rate than the pppp value [8]
The pppp value is not unessential for the corrosion rate because with decreasing pppp values, the acid equilibrium reactions 2, 3, and 4 shift towards the left according to Le Chatelier's principle

Summary

Introduction

Over the last two decades, the interest in top-of-line corrosion (TLC) has increased in the oil and gas industry [1]. TLC is a form of corrosion occurring typically in submerged wet gas transportation pipelines. The ocean's cooling effect on the pipeline's flowing fluid causes transported vapor to condense onto the pipe wall. In a stratified flow regime, typical for wet gas transport, droplets will condense from the vapor at contact with the colder steel surface. The wetting of the internal pipe wall establishes electrochemical cells at the steel surface, and condensed water together with acids plays the role of corrosive medium. A pppp below 4 is typical before the condensed water is affected by the iron ions (Fe2+) dissolving from the corrosion reaction [3]. Hinkson et al reported a pppp of 3.3 in freshly condensed water from modeling the condensate composition in a pipeline scenario [4].

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