For duplex coatings (plating + PVD) several studies have shown the criticality of rather inaccessible zones like small holes and undercuts for the corrosion damage. Also sharp edges are reported as dangerous zones for their capability of stress localization and, therefore, as cause of premature coating cracking due to inappropriate local stress distribution. In the present study the effect of holes and undercuts present on a component in AA 6060 alloy (fig. 1) obtained using four different mechanical finishing (wet vibratory finishing, dry vibratory finishing, manual polishing and mechanical polishing respectively) and then Ni plated and PVD coated using two different types of coating (Ni/Cr/ZrCN and Ni/Cu/Ni/Cr/ZrCN) have been evaluated in terms of corrosion resistance. The samples characterized by different mechanical finishing have been analyzed by Scanning Electron Microscopy to investigate the surface morphologies next to or inside the holes (fig. 2, fig. 3, fig. 4 and fig. 5). Then the specimens have been coated by decorative Chromium plating (Ni +Cr - nominal depth 20 μm) and by a ZrCN PVD layer (0.4 μm). The so obtained samples have been investigated by a neutral salt fog testing using 5% sodium chloride solution and a pH range of 6.5 to 7.2; the temperature of the salt spray chamber was controlled to maintain 35 ± 2°C according to the ASTM B 117 standard (fig. 6 and fig. 7). The duration of the test ranged from a minimum of 2 hours to a maximum of 24 hours. The Cu interlayer characterizing the Ni/Cu/Ni/Cr/ZrCN coating showed a significant improvement in the corrosion resistance of the coated part. The coating architecture, and in particular the introduction of an interlayer having a coefficient of thermal expansion intermediate between the Al value and the Ni data affected the residual stress distribution as measured using X-ray technique and, therefore had also an effect on the mechanical behavior of the coating as resulted by nanoindentation test. The four different polishing procedures used did not influence the coating adhesion; no particular effect can be also reported in terms of corrosion resistance.
Effect of chromium plating and decorative PVD coatings on the corrosion resistance of AA6060 components
Gelfi M.;La Vecchia G. M.
2011-01-01
Abstract
For duplex coatings (plating + PVD) several studies have shown the criticality of rather inaccessible zones like small holes and undercuts for the corrosion damage. Also sharp edges are reported as dangerous zones for their capability of stress localization and, therefore, as cause of premature coating cracking due to inappropriate local stress distribution. In the present study the effect of holes and undercuts present on a component in AA 6060 alloy (fig. 1) obtained using four different mechanical finishing (wet vibratory finishing, dry vibratory finishing, manual polishing and mechanical polishing respectively) and then Ni plated and PVD coated using two different types of coating (Ni/Cr/ZrCN and Ni/Cu/Ni/Cr/ZrCN) have been evaluated in terms of corrosion resistance. The samples characterized by different mechanical finishing have been analyzed by Scanning Electron Microscopy to investigate the surface morphologies next to or inside the holes (fig. 2, fig. 3, fig. 4 and fig. 5). Then the specimens have been coated by decorative Chromium plating (Ni +Cr - nominal depth 20 μm) and by a ZrCN PVD layer (0.4 μm). The so obtained samples have been investigated by a neutral salt fog testing using 5% sodium chloride solution and a pH range of 6.5 to 7.2; the temperature of the salt spray chamber was controlled to maintain 35 ± 2°C according to the ASTM B 117 standard (fig. 6 and fig. 7). The duration of the test ranged from a minimum of 2 hours to a maximum of 24 hours. The Cu interlayer characterizing the Ni/Cu/Ni/Cr/ZrCN coating showed a significant improvement in the corrosion resistance of the coated part. The coating architecture, and in particular the introduction of an interlayer having a coefficient of thermal expansion intermediate between the Al value and the Ni data affected the residual stress distribution as measured using X-ray technique and, therefore had also an effect on the mechanical behavior of the coating as resulted by nanoindentation test. The four different polishing procedures used did not influence the coating adhesion; no particular effect can be also reported in terms of corrosion resistance.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
