Corrosion Under Insulation (CUI) is one of the most significant corrosion issues in oil and gas production facilities. One way to address this issue is to use insulation materials with corrosion inhibitors. In this study, under the simulated CUI conditions in the laboratory, the performances of some corrosion inhibitors were evaluated on carbon steel. Immersion tests of polished metal surface specimens and corroded ones were conducted in the solutions controlled 18,000 ppm chloride ions (3.0 wt.% NaCl) at 50°C, and the mass loss values were compared. In addition, potentiodynamic polarization measurements were performed to presume these inhibition mechanisms. One nonflammable inhibitor known to contain phosphates and nitrites showed good anti-corrosion performance on both the polished metal surface specimens and the corroded ones. The primary inhibition mechanism was considered to be the formation of the protective film on the steel surfaces. On the other hand, it was deemed that localized corrosion was a possible concern in spots where the inhibitor did not act sufficiently. Therefore, applying corrosion inhibitors to insulation materials is expected to be one of the corrosion control techniques for preventing further CUI as a temporary expedient until other permanent repairs are carried out.
Corrosion Under Insulation (CUI) is the corrosion of piping or equipment under insulation that occurs when moisture ingresses the interface between insulation and piping or equipment, helping to form corrosion cells. CUI is one of the costliest problems shared by the oil and gas industries. One reason this problem has been a perennial challenge is that CUI is difficult to detect because it occurs under the insulation. And since it occurs regardless of the type of fluid in the pipe, every part of the plant would be included in the monitoring scope. Complete insulation removal and thorough inspection of piping and equipment are time-consuming and costly. Much of the maintenance cost is spent on external piping inspection, removal and replacement of insulation, painting of piping, and piping repair.1 The root cause of CUI is the presence of moisture and the enrichment of corrosive species on the piping or equipment surface under the insulation. When atmospheric oxygen dissolves into moisture, it leads to corrosion. The restricted shape of insulation accumulates moisture that can develop into electrochemical cells on the metal surface, resulting in a corrosion attack. The insulation promotes severe crevice corrosion attack, which is exacerbated by high degree and long duration of wetting.2-5 For example, API RP583 states that CUI can occur in the temperature range of –4 °C to 177 °C, with 77 °C to 110 °C being more severe conditions.6 CUI is more likely to occur when wet and dry conditions are repeated, such as in cyclic service with temperature fluctuations. Chlorides and sulfides are the most likely contaminants and generally increase corrosion rates. In the case of carbon steel piping or equipment, water trapped under insulation combines with chlorides, sulfates, and concentrates as it evaporates, forming corrosion cells. One effective solution to the CUI problems is to apply an organic protective coating on the outer metal surface. It can physically prevent water from contacting piping or equipment. However, this may not be a perfect measure, as various factors can cause coating defects and holidays. We have also experienced severe localized CUI at on-site coated sections. This problem also poses another issue in the coating repair works; it is challenging to prepare the metal surfaces with power tools when repairing the anti-corrosion coating during the plant operation period. Therefore, further measures by vapor phase corrosion inhibitors and corrosion inhibitor impregnated tape, which apply corrosion inhibitor technology, are still being actively investigated and proposed. 7-9 In this study, we focused on the applicability of a corrosion inhibitor applied to the inner surface of the thermal insulation materials and verified their effectiveness in extending the service life of piping and equipment. While API RP 571 3rd edition states that "Insulation with added corrosion inhibitor is available.", it does not say much about what to keep in mind.10 Therefore, we examined whether the targeted inhibitor could be effective on corroded metal surfaces and what concerns might exist if the effectiveness is incomplete.
