Naslov
Assessment of CP Effectiveness on PE Tape Coated Buried Pipeline by External Corrosion Direct Examination

Autori
Martinez, Sanja ; Valek, Lidija ; Kapor, Frankica

Vrsta, podvrsta i kategorija rada
Radovi u zbornicima skupova, cjeloviti rad (in extenso), znanstveni

Izvornik
EUROCORR 2008, Managing Corrosion for Sustainability / - Edinburgh, 2008, 1-8

Skup
EUROCORR 2008, The European Corrosion Conference

Mjesto i datum
Edinburgh, Ujedinjeno Kraljevstvo, 07.09.2008. - 11.09.2008

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
cathodic protection ; buried pipeline ; corrosion

Sažetak
The objective of the present study was to: (i) determine severity of corrosion indications obtained from the intelligent pig in-line inspection of the polyethylene tape coated cathodically protected buried pipeline made of X52 steel, (ii) to determine root cause of the corrosion defects and (iii) to assess the ongoing corrosion activity. Prior to excavation, pipe-to-soil and soil-to-soil potentials in longitudinal and lateral directions above the presumed damage were measured. Soil resistivity (by Wenner method) and redox potential were measured at the soil surface and, immediately after excavation, at various depths, horizontally in the ditch. Soil and water samples were collected. The condition of the coating and pipe wall were evaluated, under-film liquid pH was measured and corrosion products were taken for chemical and MIC analysis. Moisture content of soil, sulphide, sulphate and chloride ion concentration, analytical conductivity and pH were determined. Corrosion products were tested for the presence of Fe2+, Fe3+, Ca2+, S2- and CO32-. Polarization experiments were done in as-received soil in order to assess the instantaneous corrosion rate. Bellhole excavations were done at two sites. At first location, microbiologically induced pitting corrosion at electrically shielded area under the disbonded coating was observed. The pits were numerous and clustered, of characteristic cup-like shape, up to 50% of the pipe wall thickness deep. Bacterial analysis showed significant amount of aerobic and anaerobic bacteria, including sulphate reducing bacteria indicating particularly aggressive mixed aerobic – anaerobic conditions, with oxygen concentration diminishing trough the biofilm towards the metal surface. The deduced type of corrosion was supported by the laboratory analysis of corrosion products from the bottom of the pit, showing large amount of Fe3+ ions and small amount of Fe2+ (iron preferably in reduced state under anaerobic conditions) as well as low amount of Ca2+ ions, no CO32- ions and neutral pH (no reach of cathodic protection current). At a neighbouring area of coating damage where a large area of the pipe surface was directly exposed to the soil, a white layer comprised mainly of Ca2+ and CO32- ions and having high pH upon dissolution witnessed the effectiveness of cathodic protection. At the second location, a single pit having depth of 60% of the pipe wall thickness and covered by an extremely thick stratified layer of corrosion products protruding trough a small hole in coating, was found. Mostly aerobic bacteria were identified, while the anaerobic culture did not include sulphate reducing bacteria. Corrosion products were comprised of Fe2+ ions, lacking Fe3+, Ca2+ and CO32- ions. Time dependent measurements of pipe-to-soil potential confirmed the presence of stray currents that were due to the immediate vicinity of the DC traction system and were identified as a probable cause of the observed corrosion damage.

Izvorni jezik
Engleski

Znanstvena područja
Kemijsko inženjerstvo, Temeljne tehničke znanosti

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