Handbook of Modern Coating Technologies

Application of the scanning vibrating electrode technique to characterize modern coatings

SVET measures ionic currents in an electrolyte solution; therefore, it is especially suited for analyzing active coatings or pores and defects in inert coatings applied to active substrates.

Different cases are summarized in Table 1 — 1 and sketched in Fig. 1—7. The first case corre­sponds to nonconductive coatings applied on nonconductive substrates as, for example, a paint on plastic, wood, paper, glass, or ceramic. No currents are developed in these systems; therefore, SVET does not provide any results of interest. Case 2 refers to conductive coatings applied on nonconductive substrates, for example, a metallic film on the substrates of the previous case. The results will be that of the metal film, similar to the bulk metal. The oppo­site case is a nonconductive coating applied on a conductive substrate. The coating can be organic (paint or varnish) or inorganic (chromate conversion film, anodized layer, phosphate layer, and vitreous enamel), and three cases are possible: intact coatings with no activity to be detected by SVET (Case 3);coatings with small pores or blisters, which may or may not be detected by SVET (Case 4);and large pores or defects, easily measured by SVET (Case 5). The last four cases concern conductive coatings on conductive substrates. It is important to distinguish two situations: coating anodic to the substrate (e.g., zinc on steel) and coating cathodic to the substrate (e.g., chromium on steel). For each situation it is also necessary to

consider intact and defective coatings. Thus Cases 6 and 7 are for, respectively, intact and defective coatings anodic to the substrate and Cases 8 and 9 for, respectively, intact and defective coatings cathodic to the substrate. Examples of the application of SVET for each case are now presented.

• Systems with inert coating or inert substrate (Cases 1 -3)

Case 1 corresponds to nonconductive materials in both coating and substrate. No ionic cur­rents are produced to be detected by SVET. Case 2 is for conductive coatings on inert sub­strates. The response is the same as that of the metal constituting the coating. It may be a noble or passive layer, without activity at the surface to be measured. If it is an active metal (hardly chosen as coating material), it will be simply a case of metallic corrosion and will not be presented here. Case 3 refers to a conductive substrate with an intact nonconducting coating. The substrate is isolated from the environment, consequently it does not react and no signals exist to be detected by SVET. Two examples are presented here. Fig. 1—8A shows a sample of the aluminum alloy 2024-T3 with a thin layer (2 pm) of an organic—inorganic film applied by the sol—gel method. No activity was found even after 1 month of immersion in 0.05 M NaCl. Another example—Fig. 1—8B—is a layer of Al₂O₃ applied by plasma deposi­tion on LM24 [119] cast aluminum alloy. After 1 h in 0.1 M NaCl no currents were measured. The maps show an absence of anodic or cathodic currents. The values are not zero, but very

A3)

pA/cm² ■ 0.8 ■ 0.6 0.4 0.2 I 0

0.2 0.4 0.6 0.8 1

small (between 6 1 and 2 pA/cm²), at the noise level of the solution. The values could be taken as the manifestation of many anodes and cathodes (small positive and negative current sources) randomly distributed. To confirm that the signals are indeed noise and not very small currents emanating from the surface, measurements can be repeated closer to the sur­face. If no activity exists, the same noise will be measured in any place of the cell, close or far from the surface. Conversely if currents truly exist, their magnitudes will increase as the maps are acquired closer to the surface. In addition, when corrosion is actually taking place, the activity will continue in subsequent maps, and the cumulative degradation will be visible at later times.

• Coatings with pores or small defects (Case 4)

The absence of signal from SVET in coated active metals (metals with tendency to corrode) is a proof of high-quality coatings. Porosity, or small defects, is enough for the metal/envi- ronment interaction and for corrosion initiation. Examples are now given using the same sys­tems presented for Case 3. This time the sol—gel film was cured for a shorter period (80 min at 120°C compared with 17 h in the previous example), rendering a not completely cured film and consequently not perfectly continuous, permitting the solution to easily reach the substrate. In just 2 days of immersion, one anodic spot was detected and several cathodic regions were developed at the surface of the sample, coincident with the dark regions in the optical picture—Fig. 1—9A. The maps in Fig. 1—9B correspond to the LM24 aluminum alloy

coated with Al₂O₃ layer, where currents were detected after 6 h of immersion in localized points, coincident with the pores in the coating.

• Coatings with macroscopic defects (Case 5)

Along with pores and small defects, coatings may have much larger defects, either appearing in the course of the service life or deliberately produced to accelerate the degradation of very good coated systems, where changes would take months or years to be noticed. Fig. 1 — 10 shows artificial defects (scribes) produced in an organic coating (polyurethane) applied to 2024-T3 aluminum alloy. They were chosen to demonstrate that it is possible to have defects with no activity detected by SVET (Fig. 1 —10A), or defects with both anodic and cathodic activities, (Fig. 1 —10B), or even with just anodic (Fig. 1 —10C) or cathodic activity (Fig. 1 —10D). In the last two cases, cathodic activity (Fig. 1 —10C) and anodic activity (Fig. 1 —10D) exist but most likely occur under the paint (blisters are observed in some of the optical pictures) and are not detected. The mismatch between anodic and cathodic currents, as well as the not detection of currents that should be there, are examples of the limitations presented in Section 1.4.

When various defects are present, they are often galvanically coupled. This is easily per­ceived in systems with only two defects. While it could be expected that the two worked independently, each one with both anodic and cathodic activities, in practice it has been found a separation of the anodic and cathodic activities in different defects [51,115,120].

FIGURE 1-10 Four examples of the currents that can be measured by SVET above a defect (scribe) on organic coating (Cases 4 and 5 in Table 1 — 1): (A) no currents, (B) both positive and negative currents, (C) only positive currents, and (D) only negative currents. Dashed lines indicate the mapped areas. The units in the scales are pA/cm². SVET, Scanning vibrating electrode technique.

One example is shown in Fig. 1 — 11 with 2024-T3 aluminum alloy coated by a sol—gel film and exposed to 0.05 M NaCl. The film protects the alloy from corrosion except at the defects, and one is positive and the other negative. The same spatial separation of both activities has been found in samples with more defects [116].

• Coatings anodic to the substrate (Cases 6 and 7)

A very common example of metallic coating anodic with respect to the substrate is galva­nized steel. Zinc has higher tendency to corrosion than steel but it corrodes at a slower rate due to the formation of an oxide/hydroxide/carbonate deposit on its surface. As a conse­quence, the zinc coating on steel without defects will corrode slowly and increase the lifetime of the whole material. When the steel substrate is exposed in defects, it will still be protected through cathodic protection conferred by the zinc, which oxidizes preferentially. The way this process takes place can be followed by SVET. Fig. 1—12 shows steel coated with a zinc layer of 7 pm, immersed in 0.1 M NaCl. This corresponds to Case 6. The corrosion of the zinc layer occurs in a localized manner (Fig. 1 —12A) and the ferrous substrate is rapidly reached. In 1 day, steel is exposed in well-defined round spots, but not corroding (no yellow, orange, or brown iron corrosion products) because it is galvanically protected (Fig. 1 —12B). The sample becomes an example of Case 7 (anodic coating with defects). During 1 week, zinc continued dissolving, increasing the exposed substrate area but steel was still protected from corrosion. Eventually zinc will run out and steel will start corroding. Fig. 1 —12D shows the microscopic aspect of the zinc crystals and Fig. 1 —12E gives a detail of one corrosion spot, after 24 h of immersion, with the zinc layer, the exposed steel and corrosion products in the center of the spots. The SVET can be used to monitor a coated system over time, com­pare different systems, or study the influence of environment conditions.

FIGURE 1-11 Example of a typical SVET measurement on a coated sample with two defects: one becomes anodic and the other cathodic. SVET, Scanning vibrating electrode technique.

цЛ/cm²

1120 100 80 60 40 20 0

20 -40

• Coatings cathodic to the substrate (Cases 8 and 9)

A final case is that of a coating cathodic to the substrate. The chosen example is carbon steel with a TiN layer deposited by physical vapor deposition (PVD). This coating is electroactive but passive to corrosion in this medium, therefore no currents are generated at the surface to be detected. Fig. 1 —13A shows the system with intact coating after 4 days immersed in

1. 05 M NaCl and no significant currents are detected by SVET. This is an example of Case 8. 1—13B depicts a similar system but with defects on the TiN film (Case 9). The electrolyte solution is able to contact the ferrous substrate in the defects, leading to localized corrosion. The SVET maps show two intense anodic spots. The remaining area presents negative cur­rents, indicating that the TiN layer is electrochemically active and able to support reduction reactions. The high cathode to anode area ratio leads to high anodic current densities, fast corrosion, and easy visualization of corrosion products in just a few hours.