Influence of Heat Treatment and Microstructure on the Corrosion Performance of Carbon Steel Line Pipe in CO2 Environment

Hydrocarbon-transporting pipelines are intensively exposed to CO2 corrosion. Due to
high cost of corrosion-resistant alloys (CRA), carbon steel is seen to be the best option
for pipelines material. During fabrication, the pipelines undergo specific heat treatments
to achieve desired mechanical properties prescribed by the users. These heat treatments
affect the microstructure of the pipelines. Previous studies done have discovered that the
microstructure of the carbon steel offers significant influence on its corrosion behavior.
The main objective of this project is to characterize the relationship between
microstructures and corrosion performance of carbon steel in CO2 environment.
Meanwhile, the microstructure of different heat treatments is observed as well. The
project focuses on the common materials used in pipeline construction. Pipe samples are
collected from pipeline manufacturers and Centre for Corrosion Research (CCR),
Universiti Teknologi PETRONAS (UTP). The samples go through sample preparation
which includes sectioning, mounting, grinding, polishing and etching before examined
under a microscope. The microstructure is recorded according to its corresponding heat
treatment. Each sample is then put through corrosion test using glass cell in 3% NaCl
solution at 50°C, pH 4, 1 bar CO2, for two weeks. The result shows that sample with
ferrite/bainite/tempered martensite microstructure, and fine ferrite/pearlite microstructure
have the lowest corrosion rate. Banded ferrite/pearlite microstructure has the highest
corrosion rate. Iron carbonate scale is found the most on fine ferrite/pearlite
microstructure and covers most of the surface area. Fine grains anchor scale better than
coarse structure does. Iron carbide or cementite influences scale adherence on the steel
surface. In banded structure, the segregated distribution of cementite causes poor
performance in terms of localized corrosion. In other microstructures, cementite is more
evenly distributed. For hydrocarbons transportation, fine ferrite/pearlite microstructure,
or ferrite/bainite/tempered martensite microstructure is recommended for better corrosion
resistance.