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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41 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 FIGURE 1 Surface SEM images of (a) a non-shot-peened Al IVD coating, (b) a non-shot-peened Al-Mg IVD coating, (c) a shot-peened Al IVD coating, and (d) a shot-peened Al-Mg IVD coating. FIGURE 2 XRD patterns obtained for shot-peened Al-Mg IVD coatings deposited onto AISI 1015 mild steel substrates. Technolog y utilizing similar deposition parameters to those used to deposit com- mercially pure Al IVD coatings in aero- space applications. Two kinds of IVD coat- ings were deposited: Class I (≈21 µm) Al IVD coatings from commercially available pure Al (≥99.9%) wire and Class I (≈18 µm) Al-Mg IVD coatings from Al-5wt% Mg wire. When deposited via IVD, Al has an open and columnar surface and low density (Figure 1[a]). Because of this open struc- ture, Al IVD coatings are extremely suscep- tible to corrosion as deposited. Thus, it is standard practice to use glass bead peening to densify the coating. Following the glass bead peening, a hexavalent chromate con- version treatment is required to obtain the desired corrosion resistance. A JEOL-6400 † scanning electron micro- scope (SEM) was employed in this work to study the surface morphology. A Siemens D5000 † x-ray diffractometer (XRD) with Cu Kα radiation (at 40 kV acceleration voltage and 30-mA filament current) was used for surface phase analysis of the IVD coatings. A scan program was set up in a standard aq:2q mode to detect the surface phase composition of the IVD coating. The sam- ples were scanned in the 2q range from 20 to 90 degrees, with a 0.02-degree step size, for phase identification of Al and Al-Mg IVD coatings. The Knoop microhardness measure- ments were conducted on the uncoated substrate (AISI 1015 mild steel) in addition to the Al and Al-Mg IVD coatings. In order to avoid a substrate influence on the hard- ness measurements for the coatings, the hardness measurements were taken on the cross section of the coatings at very low loads of 0.1 and 0.25 N. Electroless nickel coatings were deposited onto the IVD coat- ing surface to retain and support the coat- ing when applying the indentation loads. The corrosion behavior of pure Al and Al-Mg IVD coatings deposited onto AISI 1015 mild steel substrates with and without hexavalent chromate post-treatment were studied by means of open-circuit potential (O C P) m easurem ents and p ot entio dy- † Trade name.

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