Materials Performance

DEC 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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48 DECEMBER 2018 W W W.MATERIALSPERFORMANCE.COM It can be seen from Table 2 that the max- imum tensile strength of the base metal shows no significant changes for the two corrosive solution exposures compared with the one in the air. However, elongation and breaking time show the clear changes. It is shown from I b that the SCC sensitivity of base metal in 3.5% NaCl solution is higher than that in 0.052% NaHSO 3 . Generally, there is an obvious tendency toward SCC when the SCC I b of a research system is higher than 35%. When I b is between 25% and 35%, there is a tendency only toward SCC. Meanwhile, there is no obvious ten- dency when I b is less than 25%. 8 As a result, 09CuP CrNi weath ering steel show s no obvious tendency toward SCC in those two corrosive solutions. Thereare majorchanges of the mechan- ical properties of welded joints exposed to the two corrosive solutions compared with the one in the air, especially in NaHSO 3 solu- tion. The formation of a hardened micro- structure makes the tensile strength of welded joints (514.8 MPa) higher than that in the air (495.0 MPa), but the elongation decreases sharply ( from 14.40% to 9.91%). In the meantime, I b is up to 31.18%, close to the 35% limit of "obvious tendency toward SCC," which is far above the value of the base metal in 0.052% NaHSO 3 . However, the SCC sensitivity of the welded joint in 3.5% NaCl is only a little higher than that of the base metal. It indicates that the material- medium system consisting of 09CuPCrNi welded joint and 0.052% NaHSO 3 solution shows the higher SCC sensitivity. Analysis and Discussion The difference of SCC behavior in the two corrosive mediums is attributed to dif- ferent corrosion mechanisms. In 3.5% NaCl, Cl – is notdirectly involved in the corrosive reaction. Rather, because its radius is so small that it can pass through the corrosion products easily and weaken the acting force between the underlying steel matrix and corrosion product, it prevents the formation of the protective cohesive oxide, which pro- motes the corrosion process. In addition, Cl – reduces the surface tension and increases the diffusivity of the solution, which makes corrosion tendency higher. 2 The defects of the material surface will be those weak spots that become preferen- tialcorrosion locations and become origins of SCC in combination with tensile stress and Cl – . The occlusive area formed at the crack becomes the anode of the electro- chemical reaction. The hydrolysis of Fe 2+ in the occlusive area brings down the pH value, which promotes the dissolution of the crack tip and facilitates the growth of the crack. However, in 0.052% NaHSO 3 solution, the medium is faintly acid and the cathode reac- tion has bothreactions of hydrogen separa- tion and oxygen absorption. The hydrogen atom created by the hydrogen separation reaction can spread inside the metal, which has an important effect on SCC. But the hydrogen-induced cracking (HIC) mecha- nism is so complicated that as of yet there is no agreed and recognized theory. This research shows that in both the 3.5% NaCl and 0.052% NaHSO 3 solutions, the sensitivity of the welded joint to SCC is higher than that of the base metal, but the difference is not as obvious in 3.5% NaCl. The reason is that general corrosion of 09CuPCrNi dominates in 3.5% NaCl. There TABLE 2. RESULTS OF SSRT FOR 09CuPNiCr BASE METAL AND WELDED JOINTS Material Medium Tensile Strength, m (MPa) Elongation, b (%) Failure Time, T (h) SCC Susceptive Index, I b (%) Base metal Air 497.6 25.02 75.65 — 3.5% NaCl 482.9 19.32 53.71 22.78 0.052% NaHSO 3 483.0 21.67 60.23 13.39 Welded joint Air 495.0 14.40 41.01 — 3.5% NaCl 435.2 10.97 30.51 23.82 0.052% NaHSO 3 514.8 9.91 27.53 31.18 FIGURE 2 Stress-strain curves of 09CuPNiCr base metal (a) and welded joints (b). MATERIALS SELECTION & DESIGN

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