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CHEMICAL TREATMENT FIGURE 4
Cyclic Polarization Tests The effect of the inhibitor mixture on
the pitting resistance is depicted in the cyclic polarization curves (Figure 3), ob- tained using the synthetic acid and stock solutions with no added chlorides. As shown in Figure 3, the presence of the 5% (v/v) inhibitor in the HAc solution induces a strong positive hysteresis that is charac- teristic of pitting corrosion. This suggests that the presence of pitting in the blended acid solution is due to the presence of 700 ppm chlorides. This is consistent with the threshold
concentration of chlorides found to pro- duce pitting in the base metal Type 316L specimens as obtained from the cyclic polarization tests performed in the labora- tory acid solution with different concentra- tions of chloride ion. Chloride content over 10 ppm induces positive hysteresis behavior, which is associated with pitting. The pitting tendency is ameliorated by
a relatively high repassivation potential, which should aid in repassivation of any pits that form. The results indicate that the welded Type 316L is more susceptible to pitting even at no chloride content. At high chloride content (as in the laboratory blended acid solution with 5% inhibitor mixture), the increased susceptibility is revealed by the lower repassivation poten- tial when compared to the base metal condition. The relative effect of the welding on the
pitting rate is also revealed in potentio- static tests performed to assess the pit propagation kinetics (Figure 4). The results indicate that in the weld and HAZ, the pits propagate much faster at comparable ap- plied polarization. The description of the pitting suscepti-
bility and growth behavior in the as- welded and base metal Type 316L is vali- dated by repeating the previous tests using green acid blend. The results in green acid blend suggest that both types of steel samples show
NACE International, Vol. 51, No. 4
Chrono-amperometric curves in laboratory blended acid with 5% inhibitor solution, showing the effect of the welding on the pitting susceptibility of the Type 316L. The blue curve is the pitting current as a function of time for the weld/HAZ sample. The red curve shows the pitting current vs. time for the Type 316L SS base metal.
similar pitting susceptibility since the pit- ting and repassivation potentials are similar. In the case of the red acid blend, it appears that the weld-containing sample exhibits a more stable pitting propagation (less tendency to repassivate) than the base metal specimen, since it exhibits a larger hysteresis and a lower pit repassivation potential.
Potentiostatic Tests To better understand the relative dif-
ference in the pitting severity between the fresh and aged acid blends, a potentio- static test was devised where weld/HAZ specimens of the Type 316L exposed to each acid blend are polarized in discrete increments of the applied potentials at 50 mV for each step. The results indicate that the green acid
blend is more aggressive in terms of the pit propagation rate. This behavior also is consistent with a lower pitting potential than what can be expected in the red acid. From the previous results obtained on the testing with the laboratory acids, it is clear that the increase in the chloride content within the HAc solution is associated with the increase in the pitting susceptibility in both the base metal and the weld-contain- ing specimens of the Type 316L SS.
Cathodic Protection The cathodic protection (CP) scenario
was evaluated considering that due to operating conditions no contamination with cations in acid solution is acceptable since that could bring scaling problems downhole in the producing wells. Conse- quently, an impressed current system is proposed since the use of inert anodes is ZMY]QZML \W N]TÅTT \PQ[ WXMZI\QVO ZMY]M[\ In this context, a test sequence was
developed where Type 316L weld/HAZ specimens are exposed in the aged green acid blend and initially polarized at a suitable potential that will produce the nucleation and stable propagation of pit- ting (150 mV above open circuit poten- tial) and allowed to grow for up to 2 h. Immediately afterward, a galvanostatic X]T[M Q[ IXXTQML I\ I [XMKQÅML K]ZZMV\ density (CD) and the subsequent chrono potentiometric response recorded.