The Formation Behavior of Corrosion Protective Films of Low Cr Bearing Steels in CO2 Environments

ABSTRACT
Corrosion protective film is important to prevent CO 2 corrosion. The formation behavior of corrosion protective film on C-Mn steel and low Cr bearing steels with martensitic microstructure was investigated in a CO2 environment at 60°C. Immersion tests were performed with test periods of 24, 48, 96 and 720 hours. At each of these periods the corrosion protective film was examined using a scanning electron microscope (SEM) and an X-ray diffraction analyzer. Quantitative analysis of the Cr element in the corrosion protective film was carried out by using energy dispersive X-ray analysis (EDX). Three measures were used to estimate the C-Mn and low Cr bearing steels' resistance to CO2 corrosion: (1) weight loss in test specimens; (2) thickness of the corrosion protective film on the test specimens; and (3) Cr enrichment of the corrosion protective film. In C-Mn steels, it was found that J55 steel, with ferritic- pearlitic microstructure, lost weight at higher rate than N80 steel, with martensitic microstructure, in the first 48 hours of immersion (the initial stage of corrosion). In contrast to N80 steel the rapid weight loss in J55 steel resulted in the test solution reaching FeCO3's solubility earlier, which in turn led to FeCO3 film forming on J55 earlier than N80 steel. Therefore, J55 steel has better resistance to localized corrosion. The low Cr bearing steels, 3CR and 5CR steels had better resistance to CO2 corrosion than 1CR steel. Weight loss in both 3CR and 5CR steels remained almost constant after 96 hours of immersion. Cr enrichment in the corrosion protective film on 5CR steel was greater than the other low Cr steels, with approximately 50 mass %. The corroded surface of the 5CR steel test specimens was smoother than that of 3CR steel test specimens.

INTRODUCTION
The CO2 corrosion of C-Mn and low Cr bearing steels, known as "Sweet Corrosion", has been one of the most important problems in the oil and gas industry because of both a high corrosion rate and severe localized corrosion. The severity of CO2 corrosion to the alloyed steel depends on temperature, CO2 partial pressure, material characteristics and various other factors 2,3,4.

For C-Mn steel, CO2 corrosion behavior is related to the formation behavior of FeCO3, which is a corrosion product in CO2 environments. CO2 corrosion is classified into three types: (1) a general corrosion type which occurs at temperatures below 60°C; (2) a deep pitting and a ringworm corrosion type which occurs at temperatures around 100°C; and (3) a corrosion resistance type which occurs at temperatures over 150°C and is due to the formation of protective FeCO3 film. CO2 corrosion can be understood from the formation behavior of FeCO35.

For the prevention of CO2 corrosion, two methods concerning corrosion protective film are well known. One is the usage of Cr bearing steels with martensitic microstructure 6. The corrosion rate of the Cr bearing steels decreases as the Cr content in the steel increases. As Ueda et al explain, Cr bearing steels show good corrosion resistance because of the amorphous Cr enriched-oxide film. The effect of Cr enriched-oxide film on corrosion is as follows:

The Cr enrichment arises in the corrosion product formed on the Cr bearing steels in CO2 environments. Then, the Cr enrichment makes amorphous Cr enriched-oxide film on the steel. After that, the corrosion is prevented by the amorphous Cr enriched-oxide film that acts as corrosion protective film 3

The other method is a control of microstructure. The microstructure of the steel affects the formation of FeCO3 film. FeCO3 film formation on the steel is described below.

At the initial stage of corrosion, ferrite phase dissolv