Corrosion Performance of Coated Parts

Deposition of a more noble material provides good corrosion protection in the case of a pore-free or defect-free coating. Defects, however, may allow severe local corrosion to occur. As shown schematically in Fig. 2, corrosion of the substrate or interlayers may result in caves underneath the coatings. This effect is further illustrated in Fig. 8 for TiN deposited by physical vapor deposition onto NiP deposited on brass by electrochemical deposition. Already dezincification can be detected. Similar results are found in coatings and thin films deposited by other techniques, such as electroplating or electroless plating, plasma spraying, or chemical vapor deposition.

Fig. 8 Pitting corrosion after salt spray testing of the coating-substrate system TiN deposited by physical vapor deposition NiP electrochemically deposited on brass. Source: Ref 24

Polarization (electrode potential versus current density) curves provide useful information about the corrosion behavior of coating-substrate systems. The more noble and defect-free the coating, the greater the reduction in the measured current density. In general, thicker films exhibit fewer defects penetrating from the surface to the base material. Figure 9(a) shows polarization curves for TiN films deposited by plasma-assisted chemical vapor deposition onto 304 stainless steel. Even at a coating thickness of 10 pm, the behavior of the coated steel does not correspond to that of pure TiN deposited on glass (Ref 25). The deposition parameters also influence the coating morphology; this holds true for all deposition techniques. For PVD coatings, the substrate temperature, among other factors, determines the morphology (porosity) of the coatings (Fig. 9b).

Fig. 9 Anodic polarization curves for selected coating systems. (a) TiN deposited on 304 stainless steel by plasma-assisted chemical vapor deposition. Curves for TiN deposited on glass and for the uncoated base metal are provided for comparison. Environment: 1 M/L HCl. Source: Ref 25. (b) TiN ion-plated onto ball bearing steel. Environment: 1 N H2SO4. Source: Ref 26

Fig. 9 Anodic polarization curves for selected coating systems. (a) TiN deposited on 304 stainless steel by plasma-assisted chemical vapor deposition. Curves for TiN deposited on glass and for the uncoated base metal are provided for comparison. Environment: 1 M/L HCl. Source: Ref 25. (b) TiN ion-plated onto ball bearing steel. Environment: 1 N H2SO4. Source: Ref 26

Polarization curves also show that corrosion resistance can be improved by the deposition of dense electrochemical or physical vapor deposition interlayers, alone or in combination. The polarization curves for reactively magnetron-sputtered

TiN on brass in Fig. 10 demonstrate the marked influence of interlayers. The coating with a 5 pm thick electrochemically deposited 80Pd-20Ni interlayer performs almost as well as pure TiN deposited on glass. The coating with a 2 pm PVD titanium interlayer does not perform quite as well, but much better than the TiN coated on brass without an interlayer. The titanium film was sputter-etched before the TiN was deposited.

Current density, nWcm*

Current density, nWcm*

Fig. 10 Polarization curves for selected coating systems with and without interlayers. Environment: 0.8 M NaCl. Source: Ref 6

References cited in this section

6. H.A. Jehn, I. Pfeifer-Schäller, and M.E. Baumgärtner, Korrosion von Hartstoffschichten, Teil 3: Elektrochemische Korrosionsuntersuchungen an Dekorativen Hartstoffschichten, Galvanotechnik, Vol 84 (No. 12), 1993, p 4059-4064

24. H.A. Jehn and M.E. Baumgärtner, Corrosion Studies with Hard Coating-Substrate Systems, Surf. Coat. Technol., Vol 54/55, 1992, p 108

25. T. Kado, R. Makabe, S. Modizuk, S. Nakajima, and M. Araki, Corros. Eng., Vol 36, 1987, p 503

26. A. Edemir, W.B. Carter, R.F. Hochmann, in Surface Modification Technologies IV, T.S. Gundarsham and D.G. Bhat, Ed., TMS-AIME, 1989, p 261

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