Corrosion of carbon steel alloys in CO2 environments

Abstract:

Corrosion product films formed on carbon steel in CO2 environments without H2S consist mostly of iron carbonate (FeCO3). The formation rate and morphology of these films depend on environmental conditions such as temperature, water chemistry and pH. When conditions are favorable, dense and adhesive iron carbonate films effectively protect the underlying steel surface against further corrosion. On the other hand, the presence of cathodic depolarizers such as oxygen increases the CO2 corrosion rate of mild steel. This paper discusses the results of a study about the corrosion behavior of carbon steel SAE 1020 in comparison with low alloy steels containing 5%Cr (ASTM A213 grade T5c – UNS K41245) and 9%Cr (ASTM A213 grade T9 – UNS K90941) in CO2 environments, contaminated and uncontaminated with O2 under typical operational conditions of heater exchangers. In the tests, simulating Petrobras’ crude oil heaters (artificial produced water), steel T9 performed much better than others materials, with and without oxygen presence. In the test without oxygen, the corrosion rates decrease with the increase of chromium content, but in the presence of oxygen, SAE 1020 was slightly more resistant than T5. It was also observed that oxygen contamination increases the environmental corrosivity, especially for the T5 and SAE 1020. The corrosion products formed on T5 and T9 were chromium rich and showed cracks, which resulted in localized corrosion. In tests with artificial condensed water (simulating Petrobras’ gas coolers), carbon steel SAE 1020 showed better performance than T5 and T9 which showed similar uniform corrosion rates. In general, the uniform corrosion rates of the three steels in produced water (PW) were higher than in condensed water (CW). These results are probably associated with the temperature of CW (160°C) that allowed the formation of more protective iron carbonate films and reduced the uniform corrosion rate. However, localized corrosion was observed in all steels in the tests with and without oxygen.


Reference:
BRAGAGNOLO, Gislaine Maria; ALMEIDA, Neusvaldo Lira de; FERRARI, Jean Vicente; GUEDES, Flávia Maciel Fernandes; ANDRADE, Cynthia Azevedo; OLIVEIRA, Jefferson Rodrigues de. Corrosion of carbon steel alloys in CO2 environments. In: NACE INTERNACIONAL CONFERENCE, 2014, San Antonio, Texas. Proceedings… 15 p.
 

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