UNDERSTANDING THE HIGH TEMPERATURE CORROSION BEHAVIOUR OF MODIFIED 13 Cr MARTENSITIC OCTG

摘要

Recent efforts by manufacturers of OCTG have led to the development of several new grades of modified or 'supermartensitic' tubulars. The main feature of these products is improved corrosion performance at high temperatures (above 130℃), i.e. in conditions which standard 13Cr, based on type 420 chemistry, corrodes at a rate too high to permit its use. In order to study these new materials, laboratory corrosion tests have been conducted, in conditions in which the standard 13Cr (API 5CT L80) suffers severe corrosion. The H_2S partial pressure in these tests was in the range 0.01 - 0.1 bar (1 -10 kPa). It was found that the general corrosion rate of the modified alloy was approximately one-tenth of that of the standard 13Cr, and the pitting rate was reduced by a factor of 3 - 4. Observation of the samples after test revealed the presence of coloured interference films on the modified materials, whereas the standard 13Cr was different in having a corrosion product on its surface. This was less adherent and protective than the film on the modified martensitics. Electrochemical testing has confirmed that the corrosion behaviour of the modified martensitics falls between that of the standard 13Cr (which tends to corrode generally) and that of 22Cr duplex in the same conditions (which is fully passive).