Electrochemical testing, aluminum coatings

At an electrolyte pH of 8, the passivation region extended for over 0.5 V. Electrochemical tests were performed on acid etched, bare A1 2024-T3 panels with artificial pits as a control experimental to determine if the increased passivation with increasing electrolyte pH was a result of self-passivation of the bare aluminum alloy surface (Fig. 6.12(b)). The polarization curves for the bare A12024-T3 showed no appreciable passivation and no significant difference with pH, indicating that the passivation observed in the primers was not a result of self-passivation of the substrates, but activity from the primer. This behavior provides important clues not only to the inherent electrochemical properties of the initial coatings, but also the mechanisms responsible for corrosion protection. 

The corrosion widths of Prohesion salt spray-tested alloys are calculated and summarized in Figure 32.14. E-coated IVD controls (CC/E), i.e., the combination coating systems of chromate conversion coating with nonchromated E-coat, showed very large corrosion widths for all the IVD Al-coated aluminum alloys. This combination did not provide good corrosion protection, which could be taken as proof that the two completely different approaches (electrochemical corrosion protection and corrosion protection by barrier adhesion principle) should not be mixed. 

In previous work (1), we examined the impact of surface impurities on electrochemical reactions beneath polysiloxane encapsulant coatings. Test specimens consisted of aluminum combs prepared on Si02 by photolithography. Leakage currents were measured as functions of temperature, surface impurity levels, and relative humidity. In all cases, increasing relative humidity, RH, produced a monotonic rise in leakage current, corresponding to increased water... 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

Figure 15.4 Mapping electrochemical material loss against mechanical erosion rates for a nonpassivating surface carbon steel (AISI1020) along with two potentially passivating surfaces of nickel aluminum bronze (NAB) one that has been thermally sprayed by high-velocity oxy-fuel deposition as a coating on carbon steel ( j and another which has been cast (A.). These results were obtained from jet impingement erosion-corrosion tests. Reprinted from Ref. [7]. Copyright (2007) with permission from Elsevier.

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