Materials Performance Supplements

CORTEC 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

Issue link: http://mp.epubxp.com/i/684495

Contents of this Issue

Navigation

Page 21 of 23

The high corrosion rates from the ER probes were confirmed by the actual status of the steel plates. Upon removal of the steel plates from the unprotected tanks, it was observed that the internal surfaces were covered with sand and corroded, especially at the center area (Figure 6). ICCP pro- tected tanks (TK-04, TK-05, and TK-06) showed an average instant-off potential of -1,024 mV (Figure 7), satisfying the -850 mV instant-off protection criteria. Corro- sion rate data from ER probes installed in control tanks showed an average corrosion rate of 3.2 mpy as calculated per Equation (1) from the data in Figure 8. The low corrosion rate is in line with the fact that protection criteria were achieved. However, visual inspection of the underside surface of the plates revealed considerable levels of corrosion (Figure 9). Despite meet- ing the -850 mV instant-off protection cri- teria, the actual status of the underside sur- faces showed other wise. The corrosion morphology looked similar to the unpro- tected tanks. This might be attributed to the fact that th e C P syst em was not commi ssion ed during and after the construction of tanks for a period of about two weeks. Similar challenges, even on a larger scale, exist in real life, where tanks take from several months to years to be boxed up and their CP systems commissioned. Tank bottom plates are usually left without any protec- tion during this time. In other cases, lack of availability of a power supply hinders acti- vation of the CP system for several years at the job site. Post-Injection Phase After injection of VCI slurry through the dispensing ring, a noticeable effect was observed on the metal loss of ER probes in both unprotected (Figure 10) and ICCP pro- tected tanks (Figure 11). The average corro- sion rate of ER probes installed in TK-02 and TK-03 reduced from 13.1 mpy to 0.66 mpy, with an average percentage reduction of 95%. However, the corrosion rate in TK-01 didn't reflect the same level of effect after VCI application where the corrosion rate was reduced from 15.44 to 6.39 mpy, a 59% reduction only. For ICCP-protected tanks, the average corrosion rate of ER probes went from 3.2 mpy to 0.3 mpy, with an aver- age percentage reduction of 90%. It is worth- while to note that the introduction of VCI slurry under the tank plate helped maintain an average corrosion rate under 1 mpy in all tanks, excluding TK-01. Table 1 summa- rizes the corrosion rates of the individual ER probes before and after VCI application. It is worthwhile to note that the reduction in the corrosion rate of all ER probes not only confirmed the ability of VCI molecules to diffuse through a compacted sand layer over a short period of time and protect the underside of the tank floor, but also diffused through the corrosion product layer on the tank floor and hence reduced the corrosion rate of pre-rusted steel. It was noticed that VCI slurry shifted the average potential of unprotected tanks from -550 mV to -500 mV (Figure 12). For ICCP tanks, each tank reacted differently to the VCI slurry (Figure 13). In TK-05, the average instant-off potential shifted tem- porarily from -1,020 mV before injection, to -1,205 mV for the first 19 days before it started to go back to the original value through the end of the experiment. TK-06 also showed a transient behavior, where its instant-off potential shifted in the negative direction from an average of -1,000 mV to reach a value of -1,300 mV on day 16 after injection. However, the instant-off potential shifted in the positive direction to stabilize at an average of –1,200 mV until the end of the experiment. Prior to injection of VCI slurry, TK-04 showed an average instant-off potential of -1,004 mV for about 36 days. A sudden shift in the negative direction of the instant-off potential was noticed on day 37 and contin- ued for seven days before injection of VCI slurry to reach -1,318 mV. After the intro- duction of VCI slurry, no clear change was noticed until the end of the experiment. However, if the average instant-off poten- tial for all tanks was considered before and after injection in Figure 13, it can be con- cluded that an overall shift of 150 mV in the negative direction occurred. Although the findings might not be conclusive in terms of an exact value of the potential shift and whether this shift is permanent or tran- sient, the CP operator can expect a shift in the instant-off potential of the protected tank. Therefore, a longer study should be conducted to answer such queries. Conclusions Soil-side corrosion on ASTs, including those protected by CP, can present a chronic challenge to operating companies. There is a growing industrial awareness about the importance of finding a viable solution to supplement the performance of the afore- mentioned technique. One promising solu- tion is the use of an amine carboxylate-based VCI. This experiment was designed to assess the effectiveness of an amine carboxyl- ate-based VCI system on the protection of AST bottoms against this type of corrosion as standalone and in combination with an ICCP system. The experiment also looked into the effect of VCI slurry on the instant-off potential and in turn the protection criteria of an ICCP system. The obtained results led to the following conclusions: • Despite having a CP system satisfying the protection criteria of -850 mV instant-off potential, the tanks showed signs of soil-side corrosion. This might be partially attributed to the CP system not being commissioned as soon as the tanks were constructed, allowing the corrosion process to start. Due to the spontaneous protection mechanism of an amine carboxylate VCI system, it might be advantageous to introduce amine carboxylate VCI material into the tank sand pad to provide protec- tion of the underside of tank bottom plates during construction and until the CP system gets commissioned. • ER corrosion rate probes can be used to evaluate the corrosiveness of the environment under an AST and indi- cate the effectiveness of VCI in reduc- ing and controlling soil-side corrosion. • VCI slurry can be effectively introduced and distributed through a designed online injection system under existing and new ASTs. • VCI slurr y alone showed the ability to reduce the corrosion rate by 82.5%, which makes it a viable solution to protect against soil-side corrosion , especially for tanks without a CP sys- tem or when the existing CP system is deficient. • VCI slurry in combination with ICCP showed a synergetic effect on the cor- rosion rat e and h elp ed maintain it below 0.5 mpy, with an average reduc- tion of 89.7%. This suggests that sup- plementing new and existing CP sys- tems with VCI material is therefore advantageous to operating companies. The introduction of VCI slurry may have VAPOR PHASE CORROSION INHIBITORS 22 JUNE 2016 MATERIALS PERFORMANCE CORTEC SUPPLEMENT TO MP

Articles in this issue

Archives of this issue

view archives of Materials Performance Supplements - CORTEC 2016