Schutzmaßnahmen gegen interkristalline Spannungsrißkorrosion von Gashochdruckleitungen aus Stahl bei erhöhter Betriebstemperatur

Protection of high-pressure steel pipelines for the transmission of gas against stress corrosion cracking at high temperature Grade X 60 line pipe sections were capped to pressure vessels and were placed in concrete pits where they were exposed to controlled corrosion conditions. The corrosive conditions were represented by a sand environment containing NaHCO₃, a wall temperature of 70°C and pipe-to-soil potentials of U_H = ? 0.43, ? 0.55 and ? 0.68 V. The pipes were polyethylene coated to DIN Standard 30 670 without prior surface treatment (no mill scale removal) or after the removal of the mill scale by brushing or sandblasting. The coating included holidays of different sizes to allow disbonding. These pressure vessels were tested by pressure cycling. The strain rate for the cycles was 10^?7 sec^?1 whereas strain release was much faster. The test programme included 4800 cycles between 44 and 87% YS, 1390 cycles between 66 and 87% YS and 5000 cycles between 79 and 87% YS. The total duration of the test was 9830 hrs. Following the completion of the pressure cycles, the pipes were examined. No corrosion defects were discovered in the holiday areas. In the cases of the pressure vessels where mill scale had been removed by sandblasting or brushing, no corrosion was found below the disbonded coating, either. Minor crack formation which was not revealed by conventional test methods was, however, established below the disbonded coating of the pressure vessels where the mill scale had not been removed. The scanning electron microscope showed corrosion pitting type strain induced cracking of a depth of up to 0.2 mm. In a limited number of cases these cracks could be considered intercrystalline stress corrosion cracking. The paper discusses what action must be taken to prevent such stress corrosion cracking. It concludes that stress corrosion cracking in West German high-pressure gas transmission pipelines is impossible if the pipelines are designed, built and operated in accordance with existing standards and codes of practice.