Materials Performance

JUN 2016

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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52 JUNE 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 6 Secondary Cracks Figure 3 shows fewer secondary cracks in the crack propagation zone of the sam- ple in air, and the cracks had a slender shape, large spacing, and no connections. This demonstrated a lack of stress corro- sion tendency for the sample in air. A num- ber of secondary cracks in the sample in solution were larger than those in the sam- ple in air, which indicated corrosion and the formation of holes caused by the NaCl solution. An "occluded cell" 8 appeared in the samples in solution after the secondar y cracks formed. The hydrolysis reaction pro- duced inside the cracks is depicted in Equations (6) and (7). 9 Fe + 2Cl – – 2e – → FeCl 2 (6) FeCl 2 + 2H 2 O → Fe(OH) 2 + 2HCl (7) Equation (7) shows that the HCl caused a drop in the solution pH, promoting corro- sion of the cracks and increasing the SCC sensitivity of the samples. Dimple Morphology and EDS Analysis Figure 4 shows that the dimple mor- phology for the sample in air was com- posed of different sizes and depths, as with small dimples appearing in larger dimples. This illustrates that the fracture mecha- nisms of S355 steel was mainly intergranu- lar fracture and cleavage fracture. The im- purities in the substrate also played a catalytic role in the fracture process. EDS analysis shows that the chemical composi- tion of the dimples (by mass %) was 2.40% C, 0.27% Si, and 97.33% Fe. The main ele- ment of the dimples was Fe, with very low percentages of C and Mn. This illustrates that the SCC sensitivity was minimally af- fected by the impurities. Figure 5 shows that the dimple mor- phology of the sample in NaCl solution was similar to that of the sample in air, but some oxide impurities appeared on the sur- face. EDS analysis shows that the chemical composition ( by mass %) was 2.25% C, 11.72% O, 0.47% Cl, and 85.29% Fe. The frac- ture modes of S355 in solution were stress corrosion fracture and chemical corrosion FIGURE 3 Secondary cracks in the crack propagation zone of the sample in 3.5% NaCl solution. FIGURE 4 Dimple morphology of the fracture zone of the sample in air. FIGURE 5 Dimple morphology of the fracture zone of the sample in 3.5% NaCl solution. Fracture Morphologies Figure 2(a) shows that the fracture mor- phology of the sample in the NaCl solution was comprised of the crack source zone, crack propagation zone, and tearing zone. The oxide layer was thicker and had sepa- rated from the substrate, and a peeling phe- nomenon was observed. There were two reasons for this phenomenon. One reason was anodic dissolution , which began with microcracks appearing locally from stress and the corrosive me- dium. The microcracking was followed by the quick dissolution of metal in front of the microcracks that led to cracks develop- ing longitudinally with the application of stress, and the formation of the crack source zone. Another reason was hydrogen-induced cracking. In the corrosion process, as small bubbles were adsorbed on the fracture sur- face, the following reactions occurred as described in Equations (2) through (4): 7 H 2 O → H + + OH – (2) Fe + 2OH – – 2e – → Fe(OH) 2 (3) 2H + + 2e – → H 2 (4) In the H 2 environment, the H 2 decom- posed into H + , which entered into the sec- ond phase and vacancy cluster of the S355 steel. The H + combined with 2e – to form H 2 , producing strain (P) with a degree that was proportional to the concentration of C H 2 . Since the H 2 was not an ideal gas, the fu gacity ( f ) was replaced as shown in Equa- tion (5): f = (C H /S) 2 = C H 2 exp(–2∆H/RT) (5) When f increases, the atom bonding force (σ th ) forms to break the metal bond, and the cracks appear. Figure 2(b) shows that the dimple depth increased in the crack source zone, and the number decreased. The dimples were at- tached with a layer of cotton wool-like ma- terial, which was primarily composed of iron oxides, illustrating that the dimples were corroded by the NaCl solution. Figure 2(c) shows that a large number of pores appeared in the crack propagation zone, due to the action of Cl – in the NaCl solution. Fe was constantly being corroded and dissolved, so mechanical stress corro- sion and chemical corrosion existed in the zone. No cleavage cracks were observed, because the fracture formed in the region where the Fe atoms were missing before t h e re q u i re d s t re s s c o n d i t i o n s w e re reached for a cleavage fracture. Figure 2(d) shows that the morphology of the tearing zone was larger. A small num- ber of impurities, produced by the oxida- tion of the corrosive solution, appeared in the dimples. MATERIALS SELECTION & DESIGN

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