Localized corrosion behaviour of carbon steel has been studied in long-term flow loop tests with total CO2 and H2S partial pressures of around 5 bar and various CO2/H2S ratios. The corrosion layers (iron sulphide) offered good protection against general and localized corrosion on some parts of the exposed areas while on other areas continuous breakdown of the corrosion layers took place apparently with wide localized attacks as a result. The findings are discussed based on recent knowledge on iron sulphide growth and breakdown mechanisms and flow effects.