The Effect of Change in Solution Chemistry of Bulk Solution on Crevice Corroson Propagation of Stainless Steel

Once crevice corrosion initiates, the hydrolysis of dissolved metal ions occurs to induce the acidification of solution inside the crevice and the migration of Cl^- ions into the crevice from solution bulk. This Cl^- ions migration promotes subsequent active dissolution and a further decrease of pH in the crevice. Therefore, the reduction of the concentration of Cl^- ions is expected to be effective for suppressing crevice corrosion propagation. In this study, the influence of changes in solution chemistry on the propagation of crevice corrosion of stainless steel was investigated. After crevice corrosion was induced, in a chloride-containing solution the test solution was exchanged to chloride-free or chloride-containing solutions and corrosion propagation and repassivation behavior were monitored under the potentiostatic control.

An austenitic stainless steel (0.017%C, 0.58%Si, 0.84%Mn, 0.028%P, 0.001%S, 12.15%Ni, 17.43%Cr, 2.08%Mo) was used to fabricate a crevice specimen schematically shown in Fig. 1. The steel was cut into 20 mm × 20 mm × ca. 5 mm, and the surface was polished down to 1 μm diamond paste. A through-hole with a diameter of 6 mm was formed at the center of the surface. Ultrasonic cleaning was carried out in ethanol, and then the specimen was passivated in 30 mass% HNO₃ at 323 K for 1800 seconds. After that, the electrode surface area of the passivated specimen was polished down to 1μm diamond paste again. A lead wire was soldered to the electrode. An artificial crevice was formed between the electrode surface and a 20 mm × 20 mm × 2 mm polycarbonate sheet fixed by a bolt and a nut. The specimen was covered by insulating rubber except for the electrode surface. During the fabrication of the crevice, the electrode surface was kept being in contact with a NaCl solution (pH 5.0) dearated by N₂ gas. The concentration of Cl^- was adjusted to 20000 ppm.

Potentiostatic polarization measurement at 0.3V (vs. Ag/AgCl) was conducted for 1000 seconds in a 20000 ppm Cl^- containing NaCl (pH 5.0) at 298K to initiate crevice corrosion of the specimen. After that, the polarization was stopped and the test solution was exchanged to a chloride-free solution (test A) or a chloride containing-solution (test B), respectively. A Na₂SO₄ solution was selected as a chloride-free solution_. The concentration of SO₄^2- was adjusted to 20000 ppm. A chloride containing-solution was the same as the test solution used for the initiation of crevice corrosion. Subsequently, the potentiosstatic polarization at 0.3V was restarted and the change in current was monitored. After the test, the specimen surface was rinsed with deionized water and observed using an optical microscope.

Figure 2a shows the time variation of current measured in the crevice corrosion tests. Figure 2b shows an enlarged view of the first 1000 seconds, that is, the current transient of the specimens in the NaCl solution. During this initial period, the currents gradually increased up to c.a. 1000 μA. It is therefore presumed that crevice corrosion occurred and propagated under the potentiostatic polarization at 0.3 V in the NaCl solution. After the solution was changed to the Na₂SO₄ solution, the current value increased to c.a. 700 μA until 4000 seconds passed and then decreased gradually to c.a. 30 μA. On the other hand, after the solution was exchanged to the NaCl solution, the current increased rapidly and remained at a high level until the end of the crevice corrosion test. These results suggest that the removal of Cl^- ions from the bulk solution suppressed the propagation of crevice corrosion of stainless steel.

Caption list:

Figure 1 Schematic illustration of the specimen for crevice corrosion tests.

Figure 2 (a) The time variation of current measured during the crevice corrosion tests at 0.3 V

              (b) An enlarged view of the first 1000 seconds of the crevice corrosion test.