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CHEMICAL TREATMENT TABLE 2
Chloride content of BHH-11 and acid samples Sample
Concentrated inhibitor Freshly blended red acid
Aged green acid from offshore FIGURE 2
Chloride Content (ppm) 15,700 585 740
Inhibition of Pitting on a Type 316L Stainless Steel Acetic Acid Tank
z Red stock solution: Fresh onshore prepared acid solution
The temperature was kept at 38 ºC and the solution within the cell was kept aerated. A sample of the concentrated bulk
corrosion inhibitor was obtained. This sample was analyzed for chloride content using ion chromatography, along with samples of the "red" acid and the aged "green" acid from offshore (Table 2). The inhibitor is reported as an
imidazoline-based compound, having a bulk chloride content of 15,700 ppm of Cl–
of 5 vol%, the Cl–
; based on the typical dissolution ratio content is reduced to
Photomicrograph of the profile of the exposed surface (at top) near a pit (at right). Note the intergranular character of the attack; light grain boundary etching. Electrolytic oxalic acid etch.
FIGURE 3
700 ppm. From the analysis of the com- position (Table 2) of the green acid blend and the red acid blend, it was found that the former has higher chloride content (740 ppm) than in the fresh red acid blend (585 ppm). The analysis of the two blended acid samples by XRF indicated that the green acid had 29 ppm Fe, 5 ppm Mg, 45 ppm Na, 43 ppm P, 10 ppm Ca, and 6 ppm Si not present in the red acid. Acid strength by titration was 79.2% HAc in the red acid and 77.7% HAc in the green acid. Based on this composition, the differ- ence in the chloride concentration comes from incidental seawater contamination as suggested from the detected Ca2+ Mg2+
and content in the green acid blend from offshore. Results and Discussion
Metallographic Tests Cross sections across the pits were
Cyclic polarization tests of AISI Type 316L SS base metal in laboratory acid with and without 5% (v/v) of inhibitor. No added chlorides.
48 MATERIALS PERFORMANCE April 2012
mounted, polished, and etched. Figure 2 shows a photomicrograph of a typical pit. The attack proceeds in the HAZ imme- diately adjacent to the weld and follows an intergranular pattern. The weld de- posit itself shows less damage but is not completely unattacked, with some weld undercutting observed.
NACE International, Vol. 51, No. 4