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 13O°C), 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 H2S 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 mat-tensitics. 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). Keywords: martensitic stainless steels, standard 13Cr, modified 13Cr, corrosion, interference films, electrochemical testing.