RIASSUNTO
ABSTRACT:
In a case that coating like heat shrinkable sheet (HSS) or cold tape is detached from buried pipeline surface, cathodic protection current can't penetrate into the pipeline surface enough to polarize cathodically due to the impervious characteristic of outer backing layer. Microbiologically induced corrosion can develop seriously at the pipeline inside the failed coating under the favorable environment for sulfate reducing bacteria (SRB) proliferation. A series of works have been performed in order to develop the antibacterial coating against SRB. Minimum inhibitory concentration (MIC) and thermal treatment effect on the antibacterial activity simulating the temperature of production and application were evaluated for the optimal biocide selection. Coating properties as well as antibacterial efficacy of HSS adhesives incorporated with reagent graded agents and industrial biocides were tested. Prototypes of antibacterial HSS system fabricated at factory have shown that the antibacterial coating could be developed to the level of commercial product.
INTRODUCTION
Buried pipelines can be protected from corrosion by coating application combined with cathodic protection for the coating defects¹. Where polyethylene has been chosen as mainline coating, HSS has generally been adopted for field applied coating over girth weld². HSS system consists of an impervious cross-linked polyethylene backing and an adhesive as inner layer. This system has an inherent problem in that impervious backing prohibits the cathodic current to penetrate into the pipeline surface if HSS disbonds. Corrosion as a subsequent step occurs inevitably. Especially serious and substantially rapid microbiologically induced corrosion may proceed if surrounding low resistive clay soil contains abundant SRB, high water content and organic matters for SRB proliferation3,4. Looking into our metal loss cases obtained from the investigations after intelligent pigging, most cases of corrosion at girth weld are involved with typical coating defect (Figure 1) and microbiologically induced corrosion problems without exception. The causes of failure have been presumably attributed to inappropriate application5 (; insufficient pre- and/or post-heat, or surface preparation) or low graded quality in itself assisted by soil subsidence. Soil packing under the confined construction environment like narrow trench and short span of permitted time for construction is unlikely to be achieved thoroughly below the pipeline, which may play a role leading to gradual subsidence and aggravating the coating condition around the position of 6 o'clock at girth weld. Microbiologically induced corrosion has been known as a culprit in pipeline integrity maintenance. As like antifouling coating for marine environment against barnacle, etc, antibacterial adhesive of HSS could play an important role to improve the pipeline integrity if attained. Once the coating is disbonded from the pipeline surface, it can no longer be protective. Therefore, if HSS adhesive contains antibacterial agent, it can start to work its inhibition function right only after the coating fails, keeping biocide in the adhesive without the exposition to the surrounding soil while the coating is intact. However, few trials to develop this kind of antibacterial coating against SRB could have been observed. Commercial water-soluble biocides against SRB such as glutaraldehyde have widely been used for closed-water environment 6.