We investigated several steels containing up to 5% chromium (Cr) with respect to their carbon dioxide ((202) corrosion resistance in oil and gas environments. Steels containing 3% or more Cr showed improved corrosion performance compared to Cr free steels in a wet CO2 environment. This phenomenon was made clear by immersion tests, field tests and loop tests. Furthermore, we have developed a 3% Cr Electric Resistance Weld (ERW) linepipe steel with good Heat Affected Zone (HAZ) toughness in a simulated field weld using laboratory methods.
Plain carbon and low alloy steels used for the linepipe and Oil Country Tubular Goods (OCTG) are known to suffer heavy corrosion in a wet COa environment . There are some previous reports on the effects of small additions of Cr on corrosion rates under flow conditions. In severe wet COa environments, 13% Cr stainless steel has been used for linepipe or OCTG. On the other hand, in mild wet CO 2 environments, carbon steels have been used with inhibitor injections. However, there are several problems from view points of reliability, cost and environment in inhibitor injection. In recent years, the application of 0.5% Cr s t e e l s and 3% Cr containing steels have been tried in mild COa environments without inhibitor injection. We examined the medium Cr containing s t e e l s with improved corrosion resistance compared to carbon steels but with lower cost than 13% Cr containing steel.
In this paper, the effects of Cr content on wet COa corrosion at flow velocities of 0m/s, lm/s, 5.8m/s were studied in a wet CO 2 environments. CO 2 corrosion resistance of 3% Cr containing steel was at least twice greater than that of the carbon steels. In addition, a developed 3% Cr containing steel shows (1) good HAZ toughness in the field weld portion and (2) good properties in ERW portion.
Several types of s t e e l s with different C, Si, Mn, Cr, Nb and Ti contents were used. Table 1 shows the chemical compositions of the steels. These steels were melted in a vacuum furnace, cast into ingots and hot-rolled into 15ram thick plates using laboratory facilities.
Figure 1 for instance shows optical micrographs of the steels with different Cr contents. These s t e e l s were etched by nital etchant (3mass% nitric acid + ethanol). Steel A, B and C consist of bainite, and steel D and E consist of martensite.
Tensile t e s t specimens were taken from 15mm thick plates in the T direction. The gage length was 24ram.
V-notch Charpy impact t e s t specimens with the dimensions of 10mmL× 10mmt × 55mmW were machined in the T direction.
Simulated field welding was performed in the Gas Tungsten Arc Welding (GTAW) method with 3% Cr s t e e l as a welding wire. Table 2 shows the chemical compositions of the t e s t s t e e l and the weld wire. The groove shape used was V-shape type indicated in Figure 2. Table 3 indicated the welding conditions.
The ERW weldability was tested using an ERW simulator. Table 4 shows the chemical compositions of the test steel Table 5 shows the ERW welding conditions.
The simulated HAZ toughness was investigated with the heat pattern of Figure 3. This heat pattern was equal to the lkJ/mm heat imput for a 10ram thick plate. The size of the test specimen was 12mint× 12mmW× 120mmL.
CO2 corrosion t e s t
Immersion t e s t
Laboratory corrosion tests were conducted in a synthetic formation water. The solution was deaerated until the dissolved oxygen was 10 ppb or lower by bubbling nitrogen gas through the t e s t solution before each test. The test conditions are listed in Table 6. The che