Abstract:
The sweet corrosion profiles of X-52 carbon steel in 0.l% NaCl
at 30, 40, 50 and 60°C were analysed. The corrosion tests
lasted for a period of one month. They were performed using a
flow loop under a total gas pressure of 1 atm. containing carbon
dioxide (99.95% & 100 vpm O2).
The corrosion profiles were obtained by scanning the corroded
surfaces with a stylus. The results were analysed and
compared with linear polarisation resistance (LPR) data.
lt has been found that the (LPR) data at 40°C, i the absence
of significant pitting, progressively underestimated the actual
metal loss values. It has been proposed that the above discrepancy
is due to the occurrence of small scale localised corrosion.
The above effect was found to be exaggerated at 60°C,
where extensive pitting occurred. Attempts to find a correlation
between the (LPR) measurements and the actual metal loss distributions
were unsuccessful. Furthermore, although sudden increase
of the instantaneous corrosion rates for passive samples
appeared to signal the onset of localised attack, i the absence
of prior knowledge of the relative spread of the localised corrosion,
it was not possible to quantify the observed errors.
The range of the metal loss distributions was found to increase
with time. This was most notíceable at 60°C. The progress
of localised corrosion therefore, appears to have extended the
distributions towards the extreme values. It has been proposed
that, in the absence of severe pitting, the sweet corrosion
profiles can be approximated by the Weibull statistical function.
For pitted samples, where the localised attack resulted i metal
loss on different scales, the upper 10% of the values were also
found to be approximated by the Weibull function.
The Weibull function was used for predicting the progress of
the metal loss values with time. The analysis of the top 10%
values, for pitted surfaces at 60°C, however, suggested the occurrence
of an accelerating rate of attack. The original model
was therefore improved to provide a time dependent sweet corrosion
model. It has been estimated that the corrosion rate at
60°C increases with time at intervals of approximately 8 days.
The effect of methanol on the sweet corrosion of X-52 carbon
steel in 1% NaCl was also studied under stagnant conditions
at 4, 40 and 60°C. The partial pressure of carbon dioxide was
maintained at 0.3 bar. lt has been found that the corrosion rate
decreases with increasing methanol concentration. The data was
used to develop a predictive model within the range of 10 to 60%
vol. methanol.