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

• Main Author: Zainudin, Abdullah Farhan
• Format: Final Year Project
• Language: English
• Published: UNIVERSITI TEKNOLOGI PETRONAS 2012
• Subjects: TJ Mechanical engineering and machinery
• Online Access: http://utpedia.utp.edu.my/10222/1/Dissertation.PDF; http://utpedia.utp.edu.my/10222/
• Institution: Universiti Teknologi Petronas

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.