Electrochemically Measured Corrosion Progression Over Time
Non-destructive sweeps were carried out on LAS, 13Cr and S13Cr to understand the electrochemical corrosion rate behaviour with exposure time in CO2 saturated water of varying salinities.
As it can be seen in Fig. 2, the average corrosion rate steadily increases as the salinity of the CO2 solution rises from 0 to 0.16, 0.35 and 3.5% NaCl reaching a peak corrosion rate of 1.31 mm/year at 3.5% salinity. However, further salinity increases to 7, 14 and 28% cause a reduction in the average measured corrosion rate bringing the corrosion down to 0.7 mm/year for 28% which is only slightly greater than the 0.6 mm/year measured at 0.35% salinity.
Significant information can be obtained by observing the corrosion rate progression over time. As seen in Fig. 3, the corrosion rate for the 0, 0.16 and 0.35% salinity remains fairly steady throughout the 1025 mins, but as the salinity increases the corrosion rate trend over time increases as well. The corrosion rate for 3.5, 7 and 14% undergoes a rapid increase in the first 100 min and then continues to increase throughout the test duration at a lower rate. In the case of the 7% salinity solution, the corrosion rate starts at 0.74 mm/year and after only 100 min it has reached 1 mm/year, it then reaches a peak of 1.44 m/year at the 900 min mark. The same trend is also observed for the 28% NaCl solution but the intensity of the increase is significantly lower tends to resemble the steady corrosion rate of 0, 0.16 and 0.35% NaCl solutions.
Unexpectedly, as seen in Fig. 4, the non saline solution produced an average corrosion of 0.011 mm/year for 13Cr, which is greater than those observed in the 0.16 and 0.35% salinity solutions and very close to the 0.012 mm/year corrosion rate of the 3.5% salinity solution. Despite that inconsistency the rest of the results appear to follow the expected trend of increased salinity resulting to increased corrosion rate with a maximum corrosion rate average of 0.04 mm/year achieved for the 28% salinity solution.
The corrosion rate progression trends for 13Cr seen in Fig. 5 are quite different from those seen for LAS. The corrosion rate of 13 Cr in all solutions starts high and drops over time. The corrosion rate in the solutions with 0.16, 0.35, 3.5 and 28% salinity exhibits rapid decrease within the first 150 min of tests and then the reduction slowly tapers off, whereas in the case of the non saline solution the corrosion rate reduction is much more gentle. As an example, the corrosion rate of the 28% salinity stasts at 0.065 mm/year and after 1025 min it goes down to 0.017 mm/year whereas in the saline-free solution the corrosion rate starts at 0.018 mm/year and it goes down to 0.009 mm/year. The starting corrosion rate value of the saline-free solution is the lowest recorded but as the corrosion rate in the other solutions exhibits rapid decrease at the 1025 min mark its corrosion rate is greater than that of 0.16, 0.35 and 3.5% NaCl solutions.
In the case of S13Cr as seen in Fig. 6 the average corrosion rate for the 0, 0.16 and 0.35% salinity solutions do not present any observable differences. The increase of salinity to 0.35% only causes a minute increase in corrosion rate of 0.0006 mm/year but there was a significant increase in corrosion rate to 0.019 mm/year when the salinity was increased to 28%.
The corrosion rate progression over time for the 0, 0.16, 0.35 and 3.5% salinity solutions, seen in Fig. 7, follows the same trend observed for 13Cr. There is initially a high corrosion rate which rapidly decreases in the first 150 min and then continues to decrease in a smoother rate. As it was the case with the corrosion rate average, the curves of 0, 0.16 and 0.35 do not have any discernible differences but the curve of 3.5% salinity although it starts at the same level as the others it does not decrease to the same degree. The curve of 28% salinity does not follow the same trend as the others, there are constant fluctuation with the corrosion rate starting at 0.015 mm/year, reaching a peak of 0.026 mm/year after 623 min and finally dropping down to 0.011 mm/year.
Comparison of Electrochemical Corrosion Rate to Gravimetric Corrosion Rate
In this section, mass loss measurements of the specimens transformed to corrosion rate values are compared to the corrosion rate measurements obtained from the final long-range destructive LPR sweep.
As seen in Fig. 8 there is an increase in the electrochemical corrosion rate which appears to be relatively proportional to the salinity increase up until 14%. Salinity here, much like in the non-destructive tests, enhances corrosion up to a point and then further salinity increase causes a reduction in corrosion; however, in the destructive tests the salinity effect on corrosion peaks at higher salinity of 14% compared to 3.5% for non-destructive tests. The same effect is also observed on the gravimetric corrosion rate as well; however the values of the electrochemical corrosion rate are significantly lower compared to the gravimetric ones throughout the range of tested salinities.
The electrochemical and gravimetric corrosion rates of 13Cr are a lot more consistent compared to LAS for the majority of the salinities. Surprisingly, the superior chemical composition of 13Cr has only negated the effects of corrosion in the low salinity tests of 0, 0.16 and 0.35% salinity, whereas for 3.5 and 28% salinity, both the electrochemical and gravimetric corrosion rates of 13Cr were higher than the equivalent values for LAS.
The even nobler S13Cr presents slightly lower corrosion rates compared to 13Cr for 0, 0.16 and 0.35% salinity and a significant decrease for 28% salinity where there was a considerable discrepancy between electrochemical and gravimetric corrosion rates. The electrochemical corrosion rate of S13Cr in the 3.5% salinity solution is the greatest recorded followed by 13Cr and finally LAS whereas the gravimetric corrosion rate of LAS is significantly higher than those of S13Cr and 13Cr in the same environment.
Surface Morphology and Composition
It was seen for both 13Cr and S13Cr that pitting had occurred on the surface. EDS analysis highlighted the presence of chromium oxide (Cr2O3) and Cl which aided the opinion that the pitting witnessed was due to this protective chromium oxide layer breakdown in localised regions.
Initially the size and concentration of pitting at 0% NaCl for both materials is negligible An example of the scarce minute pits can been in Fig. 9 under × 1.5 k magnification of pitting was increased for both 13Cr and S13Cr—Figs. 10 and 11, respectively—as a result of growing Cl− ion concentration supplied by increasing salinities. For 13Cr, this agrees with the findings by Li et al. . Which found high Cl− concentration resulted in 13Cr being susceptible to pitting.
LAS showed dark uniform precipitate formed on the exposed surface in the form of FeCO3. As shown previously this resulted in high discrepancies between the corrosion rate witnessed by both gravimetric and electrochemically obtained corrosion rates.
The FeCO3 layer was therefore studied further to ascertain its potential as a corrosion inhibitor. SEM analysis was carried out to analyse the surface morphology of the FeCO3 produced at all salinities as shown in Figs. 12 and 13.
At 0% NaCl a new layer is beginning to form on the surface of an established layer. Evidence of that new layer can be seen at the bottom of Fig. 12a in the form of a darker platelet. The established layer morphology shows high concentration of layer cracks.
As the salinity increases to 0.16% NaCl, the sample is now predominately covered by a protective layer confirmed by EDS to be FeCO3. As seen in Fig. 12b, the oxide layer surface is filed with wide cracks throughout.
As seen in Fig. 12c, a further increase in salinity to 0.35% results in a secondary protective layer (light grey platelets) with much larger plates forming on top of the existing FeCO3 layer. However, the coverage of the secondary FeCO3 layer is poor leaving much of the substrate exposed. The oxide layer’s morphology appears improved over the ones witnessed at 0 and 0.16% NaCl with a lower concentration of cracks.
At 3.5% NaCl the new precipitate forming on the established FeCO3 layer is fairly uniform with little observable cracking, leaving very little of the sublayer exposed as it can be clearly seen at the upper half of Fig. 12d. In order to get a better look at the sublayer, the top layer was carefully removed using a cotton bud soaked in deionised water. The exposed sub layer seen at the bottom half of Fig. 12d, displays a low concentration of cracks.
It is clear for 7% NaCl (Fig. 13e) that a much denser and compact top layer of FeCO3 has formed. There are no major gaps between plate boundaries compared to the FeCO3 layer for 3.5% NaCl.
When salinity is doubled to 14% NaCl, as seen in Fig. 13f, the protective FeCO3 layer shows signs of localised degradation in the form of cracks and flaking of platelets indicated by the lighter grey areas. This is unfavourable as it will result in pockets of high concentration of Cl− which may further aid layer breakdown through to sub-surface and subsequently into the metal at a localised point. This has been confirmed by EDS analysis which has picked up the presence of Cl which was not in lower salinity samples.
For the highest salinity of 28% NaCl (Fig. 13g) the surface is cluttered with cracks and appears to be more porous. Areas of top and sub-surface FeCO3 appear together much more frequently, as opposed to defined areas of sub layer and new top layer as seen for lower salinities. EDS analysis confirms both the layers to be FeCO3. Also, EDS in the sub-surface demonstrated high concentrations of iron, chromium and oxygen which show the presence of ferrous oxides and Cr2O3. This may indicate layer breakdown has reached and exposed the surface of the metal.
Corrosion Performance of Pre-corroded LAS
As seen above LAS in the high salinity environments developed films that appear stable. In order to further examine the stability and corrosion protection potential of those films, LAS samples previously pre-corroded (PC) at salinities 7, 14 and 28% NaCl were subjected to tests once again following the same protocol. The secondary set of tests was conducted in 3.5% NaCl solution. The corrosion performance results of the PC specimens were compared to that of a clean surface specimen.
The following results in Fig. 14 show the progression of corrosion rate of a clean surface specimen and PC specimens in a 3.5% salinity environment. Effect on the corrosion rate of a metal in 3.5% NaCl CO2 saturated water when it has been pre-corroded in CO2 water of higher salinities 7, 14 and 28% NaCl.
For the pre-corroded LAS specimens in the 7 and 14% NaCl solutions, the measured corrosion rate is negligible and indistinguishable between the two in the plotted graph. The 7 and 14% NaCl pre-corroded exhibit an average corrosion rate of 2.91 × 10–7 and 2.42 × 10–6 mm/year, respectively—a vast reduction compared to the 1.29 mm/year average for an untreated sample—achieving near complete protection.
Surprisingly, pre-corroding the LAS specimen in the 28% NaCl CO2 saturated solution, caused an increase in corrosion rate to an average of 2.65 mm/year. Furthermore, as seen in Fig. 14 the corrosion rate was volatile for the first 300 min in contrast to the other specimens which further indicates the lack of passivation. A stable state is then reached after 300 min exposure time and follows the same behaviour as the untreated sample although at an increased rate—indicating a level of protection has been achieved by the exposure to the 3.5% NaCl solution.
Gravimetric corrosion rates were obtained for samples post-test and compared to the corrosion rates obtained from the broad-range destructive LPR sweeps.
As seen in Fig. 15 there are major discrepancies between the gravimetric and electrochemical data, with gravimetric corrosion rate being consistently higher. Such discrepancy can be attributed to the removal of fragile film during the post-test rinsing or simply due to the inaccuracy of the potentiostat when it comes to measuring corrosion rate where thick films are in place.
Both the 7 and 14% PC specimens yield a significantly lower corrosion rates in comparison to the untreated LAS specimen.
Based on the gravimetric results, the 7 and 14% PC specimen showed the best performance by lowering the corrosion rate from 12.54 mm/year, for the untreated sample, to 2.18 mm/year and 2.9 mm/year, respectively.
By contrast, the corrosion rate for the 28% PC specimen increased to 16.6 mm/year for the untreated sample. However, what is very impressive is that the corrosion resistance of the 7 and 14% PC LAS specimens was not only superior to the clean LAS specimens but also superior to 13Cr and S13Cr (see Fig. 8). Namely, the gravimetric corrosion rates of 13Cr and S13Cr were 5.7 and 4.4 times higher than the 7% PC and 4.3 and 3.3 times higher than the 14% PC corrosion rates, respectively.
The extremely low corrosion rates recorded for the 7 and 14% NaCl pre-corroded samples in 3.5% NaCl solution tests warranted a closer inspection. The specimens were cut and mounted in Bakelite with the cut face polished to a mirror finish in preparation for cross-sectional analysis.
The SEM analysis shows large disruption to the FeCO3 layer, indicated by the dark grey areas, and LAS surface, indicated by the light grey areas, for both specimens due to cutting and polishing, seen in Fig. 16a, c. However, even with the disruption some interesting observations can be made. As seen in Fig. 16b at × 5.0k magnification, in the 7% NaCl PC sample, the FeCO3 layer, as confirmed by EDS, close to the metal surface appears to have a dense structure and it only becomes porous as we move further away from the LAS surface.
For the 14% NaCl PC sample, seen in Fig. 16d, the protective FeCO3 layer appears uniformly porous. The more porous appearance of the FeCO3 layer, indicative of poorer mechanical properties and adhesion, may be a result of the reduced availability of CO2 during film formation caused by the higher salinity and therefore greater salting out effect.