Materials Performance

DEC 2014

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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42 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 FIGURE 3 Knoop microhardness (0.1 and 0.25 N) of uncoated AISI 1015 mild steel and on the cross section of shot-peened Al and Al-Mg IVD coatings. FIGURE 4 Variation in OCP vs. immersion time for shot-peened Al and Al-Mg IVD coatings with and without hexavalent chromate conversion treatment in an aerated 0.6 M NaCl solution. COATINGS & LININGS reference electrode, and a platinum elec- trode as the counter electrode. The immer- sion duration of OCP measurements was 100 h. Prior to starting the potentiody- namic polarization scans, the test elec- trode was allowed to stabilize for ~60 min. Immediately following the stabilization period, the test electrode was polarized at a scan rate of 1.667 mV/s from an initial potential of −1 V (vs. O CP) to the final potential of +2 V. The corrosion potential (E corr ), corrosion current density (CD) (I corr ), and anodic/cathodic Tafel slopes (β A and β C ) were calculated from these tests using proprietary software. Then, based on the approximate linear polarization at the cor- rosion potential (E corr ), polarization resis- tance (R p ) values were determined using the relationship in Equation (1): 6 R 2.3 i ( ) p A C corr A C = β × β × β + β (1) Results and Discussion Ion Vapor Deposition Coating Characterization and Microhardness Measurements It is known that an Al IVD coating, as deposited, has an open columnar struc- ture. 7 Figure 1(b) shows a SEM image of the Al-Mg IVD coating surface before the shot peening process. The Al-Mg IVD coating has a similar open columnar structure to that of the Al IVD coating. After the shot peening procedure, the microstructure of the Al IVD coating is dense with the pres- ence of a limited number of pores (in a size range of 2 to 20 µm) on the surface (Figure 1[c]), while the microstructure of the Al-Mg IVD coating showed many pores (10 to 30 µm) over the entire coating surface (Figure 1[d]). This is because the Al-Mg IVD coat- ing is significantly less ductile and does not respond to the shot-peening procedure as effectively as the Al IVD coating. The result- ing residual porosity makes the Al-Mg IVD coating more susceptible to corrosion than the Al IVD. Typical XRD patterns of Al and Al-Mg IVD coatings are shown in Figure 2. The Al IVD coating shows a strong peak, which can be related to FCC Al [111]. The namic polarization scans. The test elec- trode was immersed in aerated 0.6 M (3.56 wt%) sodium chloride (NaCl) solution; and a standard circular area (0.28 cm 2 ) of the specimen was exposed to the solution at room temperature. The electrochemical measurements were performed using a conventional three-electrode cell, where the IVD coating was the test electrode, with a saturated calomel electrode (SCE) as the

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