Galvanic corrosion behavior in oil and gas environments when the metal specimens were mechanically scratched was investigated in electrochemical method by using material combinations from carbon steels to duplex stainless steels. Galvanic current and coupled potential soared by mechanical scratching and immediately started to decrease and finally returned to the value before scratching in both sweet and sour environments. Therefore, an increase in galvanic corrosion rate by scratching is negligible small for the long term and localized corrosion due to mechanical scratching does not occurrs. The change in coupled potential immediately tier scratching was small in sweet environments and large in sow environments. It is considered that in case of rapid forming corrosion film of less noble metal, galvanic current after scratching decreased rapidly, however, in case of slow forming corrosion film of less noble side, galvanic current after scratching decreased gradually.
In oil and gas wells, the conditions of corrosive environments vary with depth. The usage of a tubing string composed of different tubing materials (combination string) can select the most suitable material required for each depth. Although a combination string would realize cheaper material cost, galvanic corrosion would be a concern. Extensive research for galvanic corrosion has been carried out, however, few researches on galvanic corrosion in oil and gas well environments have been performed 1-5) . We reported galvanic corrosion behaviour for combination of typical tubing material in various simulated oil and gas environments such as sweet and sour environments 7) . The conclusions of this report were as follows. First, an corrosion rate of a galvanic coupled specimen corrosion with equal anode and cathode area was less than twice as large as that of uncoupled. Second, for the specimens with stable, the ratio of anode and cathode area did not affected corrosion rate by galvanic corrosion.
Here, we investigated the effect of scratching on galvanic corrosion. Mechanical scratches can be made inside the surface of tubing by wireline works or so during workover, fresh metal appears by scratching, that may cause localized corrosion’~ . No research on the effect of scratching on galvanic corrosion has been done in oil and gas environments. Therefore, effect of scratching on galvanic corrosion behavior was investigated for combinations of typical tubing materials in various simulated oil and gas environments, such as sweet and sour environments.
Table 1 lists the chemical compositions and the mechanical properties of materials used in this study. All the materials were properly heat treated.
Measurements of Galvanic Current and Coupled Potential
Galvanic current and coupled potential were measured using a high pressure and high temperature autoclave. A pressure-balanced outer Ag/AgC1 reference electrode including 0.1mol/l potassium chloride solution (0.lM KC1) was used. A specimen was coated with silicon resin, leaving an area of 3cm2 for measurement after mechanical polishing with #320 emery paper. Figure 1 shows the test device for scratching. The test device consists of two parts.
