DEEP PITTING CORROSION IN SOUR CRUDE OIL TANKERS

Progressively deeper pitting on horizontal surfaces related to depth in the tanks is reported on very large crude carriers. The pits occur in tanks alternating between crude and salt water ballast and are not of the same nature or characteristics as those found in sea water ballast tanks. Pits occur whether or not the surfaces have been coated and whether or not they are cathodically protected. Examination of the pits and analysis of corrosion products within the pits and on adjacent surfaces indicates the accelerated attack is due to both chemical and galvanic attack. Sulfur dissolved in crude and available from hydrogen sulfide, oxygen from sprayed sea water used to clean tanks and from air entering the tanks, reducing environments existing during part of the crude-sea water cycle and unfavorable area relationships between the active pits (anodes) and the surrounding areas covered by corrosion products acting as the cathode, contribute to the attacks. Examinations of tanks showed elemental sulfur deposits on walls and vertical tank members which did not show the accelerated attack observed on horizontal surfaces. pH tests showed residual moisture in tanks ranged from 3 to 6 while chlorine content ranged from 2 to 5 wt percent. Because of the rate of pitting penetration, this corrosion is considered to be a threat to the integrity of the tankers.

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  • Corporate Authors:

    National Association of Corrosion Engineers

    P.O. Box 1499
    Houston, TX  United States  77001
  • Authors:
    • Munger, C G
  • Publication Date: 1976-3

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Filing Info

  • Accession Number: 00137992
  • Record Type: Publication
  • Source Agency: Engineering Index
  • Files: TRIS
  • Created Date: Jul 22 1976 12:00AM