A Comprehensive Comparison of Experimental Techniques to Investigate CO2 Corrosion of Iron and Carbon Steels

Abstract

The objective of this thesis was to undertake a comprehensive study of varying CO2 corrosive conditions analogous to experimental procedures analysed in the literature. The laboratory techniques selected were constructed from potentiostatic controlled polarisation (denoted as Test 1: Part One and Test 1: Part Two) and open circuit potential (Eoc) (denoted as Test 2 and Test 3) based methods, with the bulk CO2-saturated solution chemistry (i.e. pH (6.30 – 6.80), temperature (80 °C) and sodium chloride (NaCl) content (0 – 10 mmol L-1) equivalent to ideal conditions for the formation of protective CO2 corrosion scales. As-received high purity iron (A-R pure Fe) samples were used as reference material, with the emphasis of the study on as-received X65 linepipe (A-R X65) samples. Comparison of the A-R X65 revealed that the increase in average polarisation resistance (Rp) and positive increase in average Eoc was highest for samples under Test 2 conditions. Similar behaviour was observed for samples under Test 3 conditions, with both Eoc based conditions showing the emergence of linear tails at low frequencies for final EIS Nyquist plots, in contrast to Test 1: Part Two procedures. Lower Rp and only slight positive changes in Eoc were recorded for A-R X65 samples under Test 1: Part Two conditions, where SEM and GIXRD analysis showed a lack of FeCO3 crystal coverage with only crystalline Fe2(OH)2CO3 being observed. The A-R X65 surfaces covered with exclusively Fe2(OH)2CO3 crystals were noted to be less corrosion resistant than samples with mixed FeCO3 and Fe2(OH)2CO3 crystal surface coverage. Comparison of the post-immersion GIXRD results for A-R X65 samples under Test 1: Part Two conditions, revealed that the most intense corrosion scale peak represented the {021} Fe2(OH)2CO3 crystal plane. The corresponding Fe2(OH)2CO3 crystal morphologies were also observed to be different between polarisation based and Eoc based procedures. Five samples with varying microstructural features were also examined after 110 h of immersion under Test 3 conditions. Heat treated high carbon (0.68 wt.% C) EN42 spring steel (HT EN42) samples were compared with as-received (A-R) and heat treated (HT) API 5L X65 linepipe samples, with low carbon content (0.10 wt.% C). Results revealed the HT EN42 samples with the highest volume fraction of pearlite (84.5 – 84.6 %) had, on average, a lower corrosion resistance, in contrast to, A-R and HT X65 samples with polygonal ferritic and ferritic-pearlitic microstructures, respectively.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor) & Brian Connolly (Supervisor)