а
b
Figure
4.
Martensite nuclei in the 06Cr1-U (0.025% Nb) steel structure after
deformation of pipes at temperature of the intercritical interval, 10 000 (a)
and dislocation structure of ferrite, 22 000 (b)
3. Corrosion studies of oil and gas line tubes made of 06Cr1-U steel. When
choosing methods of corrosion studies, we used published data on composition of oil
and gas production media and the mechanisms of effect of their corrosive components
on steel as well as requirements of foreign standards to corrosion resistance of oil
country tubes [21].
The study of corrosion kinetics in 06Cr1-U steel tubes by exposure of their
specimens for 1500 hours in a model chloride-acetate solution of 5 % NaCl + 0.5 %
CH
3
COOH which is the basis for standard tests for resistance to SSCC and HC
according to NACE TM 0177 and 0284 has shown that the change in time of corrosion
rate in specimens had an extreme behavior with its maximum (0.23 mm/yr) after 700
hours and subsequent decrease to 0.03 mm/yr after 1000 test hours (Fig. 5, curve 1). In
contrast, the rate of corrosion of tubes of steel 20 simultaneously tested under these
conditions according to GOST 8732 constantly increased to 1.29 mm/yr (Fig. 5, curve
2) which is ≈ 40 times higher than that of tubes made of 06Cr1U steel. Chemical
analysis of the oxide film formed on the surface of 06Cr1-U steel specimens has shown
chromium content 2.5 times higher than in the steel.
High corrosion resistance of 06Cr1-U steel tubes was confirmed by
electrochemical studies of their specimens in comparison with the specimens taken
from the steel 20 tubes (according to GOST 8732) before and after operation in highy
mineralized stratal water at the oil and gas fields of Okhtyrkanaftogaz NGVU. The
study was carried out by the following methods:
- plotting of anode polarization curves (APC) in a solution of 0.1 N NaCl;
- plotting of cathode polarization curves (CPC) in a solution of sulfuric acid and
thiourea (1N H
2
SO
4
+1.5 g/l CS(NH
2
)
2
) which is used in accelerated evaluation of steel
susceptibility to hydrogen saturation and by holding the specimens in this solution at a
- 449 -