Materials Performance

MAY 2017

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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37 NACE INTERNATIONAL: VOL. 56, NO. 5 MATERIALS PERFORMANCE MAY 2017 ronments; their anticorrosion performance in deep sea environments has not yet been thoroughly investigated . This study at- tempts to address this situation. Experimental Procedures Sample Material and Preparation Solid Al wire (2.0-mm diameter) with 99.9% Al was sprayed on a steel substrate, using a spraying voltage of 32 to 34 V, spray- ing current of 120 to 140 A, spraying dis- tance of 180 to 200 mm, and atomization air pressure of 0.7 MPa to obtain a coating thickness of ~200 μm. The selected speci- mens had dimensions of 10 by 10 by 4 mm and were prepared for electrochemical tests, polished with grit paper (200, 400, 600, 800, and 1,000 grit size), dusted with acetone, and rinsed with distilled water. Rust Detection Rust morphologies were observed using s c a n n i n g e l e c tr o n m i c r o s c o py ( SE M ) (Quanta 200 † ). The compositions of corro- sion products were identified using x-ray diffraction (XRD) (D08 † ) and Fourier trans- form infrared spectra (FTIR) (S-V70 † ). Electrochemical Measurements Electrochemical measurements of high HP were conducted using the authors' assembled equipment, with a test HP value of 30 atm (simulating the HP at 300-m water depth). All tests were executed for 72 h at 283 °K in 3.5% sodium chloride (NaCl) solution. Potentiodynamic polarization cur ves were measured using an IM6ex † electrochemical workstation, with the test sample as the working electrode, platinum (Pt) counter electrodes, and silver/silver chloride (Ag/AgCl) reference electrodes. Potentiodynamic polarization curves were analyzed using C-View 2 † . Results and Discussion Corrosion Morpholog y Analysis Samples with Al coatings were im- mersed for 72 h at 1 and 30 atm, respec- † Trade name. FIGURE 1 Al coatings SEM morphologies after 72 h immersion: (a) at 1 atm, (b) at 30 atm. FIGURE 2 XRD patterns of Al coatings. tively. After 72 h of immersion at 1 atm, Al coatings were lightly corroded, as shown in Figure 1(a), and only a few white corrosion products appeared on the surface. In con- trast, after 72 h at 30 atm, Al coatings were corroded heavily. Moreover, the powder-like corrosion products were loose, as shown in Figure 1(b). At high HP, the corrosion prod- ucts cannot protect the Al coatings, which experience accelerated corrosion. Rust Composition Analysis Figure 2 shows XRD patterns of these two specimens. These Al coatings show similar XRD graphs in different corrosion conditions; therefore, it can be inferred that the corrosion products are homologous. FTIR analysis is shown in Figure 3. From the graph of 1 atm, only the peak of 1,496 cm –1 —the bending vibration peak of OH – — is sharp; the other peaks are ver y weak, indicating that only a few functional group molecules reacted with the corrosion prod- ucts. However, from the graph of 30 atm, the broad absorption band (the shift zone from 3,600 to 2,750 cm –1 is the stretching vibration peak of OH – , and the central peak is ~3,300 cm –1 ), and two sharp peaks appear

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