Handbook of Modern Coating Technologies

Critical account on the application of the scanning vibrating electrode technique to study coatings

The examples presented in the last section show the application of SVET to a great number of coated systems. This was possible, despite the limitations discussed in Section 1.4.6. A coating brings further limitations. It is a barrier between electrode and electrolyte solution, blocking the current path. Often, only part of the actual processes taking place at the surface is accessible to SVET. It is very important to be aware of these limitations when studying organic coatings. Fig. 1 — 15 summarizes the most common problems. The first example is a thick coating. Fig. 1—15A shows 1-mm thick silane—based clear coat, applied on AZ31 mag­nesium alloy, after 20 days of immersion. The sample is still not much attacked, just a dark area adjacent to the scribe. The small current involved in the corrosion process is not strong enough to be detected by SVET so far from the surface. Similarly a large blister (Fig. 1 —15B) also does not show any signals detectable by SVET. The large distance to surface and the blocking effect of the polymeric film makes difficult the detection of any current. Smaller blisters will also not be well resolved, as demonstrated in Fig. 1 —15C. In this case, two defects were produced on an epoxy clear coat applied on pure zinc sheet. Due to an incom­patibility of this coating formulation with the substrate, it rapidly delaminated from the surface, in the form of many blisters. The purple color inside the blisters is due to phenolphthalein (pH indicator) added to solution, permitting to identify the alkalinization associated with the cathodic reactions and evidencing a case of cathodic delamination [13]. The activity at the defects was easily detected by SVET but not the currents at the blisters. The map shows small cathodic currents that may be assumed to be emanating from the blis­ters. However, given the small magnitude, those signals can be just noise. In any case, the level of blistering in the sample (number, size, and shape of blisters) could not be resolved. A last case selected to show the limits of SVET with organic coatings, is a scribe on epoxy clear coat applied to electrogalvanized steel (Fig. 1 —15D). In this example, SVET only detects positive currents at the defect. The coating is transparent making possible to see the degra­dation of the sample, with a large area of zinc layer dissolved and the dissolution front well away from the defect. It is evident that the degradation of the sample was not properly char­acterized by the SVET. The currents were flowing mainly below the paint film, not crossing it to be detected by the probe.

The corrosion of the substrate beneath a metallic coating may also be not well charac­terized by SVET. Fig. 1 — 16 shows an aluminum alloy coated with Inconel 625 (corrosion resistant NiCr-based alloy) applied by HVOF, after 6 days immersed in 0.1 M NaCl. A single point of strong anodic activity dominates the SVET map. In the optical picture it coincides with a crack on the Inconel layer. The crack is surrounded by a reddish color, attributed to a copper deposit formed by reduction of copper ions that come from the oxidation of the aluminum alloy (which contains 2% of copper). The Inconel does not corrode and the cur­rent is due to the dissolution of the aluminum alloy taking place beneath it. The extent of the attack is only visible after removal of the Inconel layer (Fig. 1 —16C), which reveals that corrosion progressed in depth and laterally, even outside the mapped area. SVET (or any other technique) is unable to anticipate the morphology of the substrate dissolution beneath the metallic coating. However, the volume of lost metal could have been predicted by monitoring the peak current, then use Eq. (1.8) to relate the current in solution mea­sured by SVET with the current emanating from the crack and, finally, using the Faraday laws of electrolysis to obtain the mass loss. From the mass, knowing the density, the corre­sponding volume is obtained.

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FIGURE 1-16 Cast LM24 aluminum alloy coated with a layer of Inconel 625, after 6 days of immersion in 0.1 M NaCI, with evidences of anodic undermining: (A) optical picture, (B) SVET map, and (C) aluminum substrate after removal of Inconel layer (white line shows the limit of the SVET map). SVET, Scanning vibrating electrode technique.

SVET is normally used with active samples, where the spontaneous redox reactions of the corrosion process originate the currents to be measured. For conductive substrates or coat­ings, when no spontaneous currents are generated (because corrosion is not likely to occur), it is still possible to pass current through the sample and monitor the response by SVET. By this way it is possible to identify conductive regions, especially pores and defects in a non­conductive coating, and the electroactive ability of the conductive surface (substrate and/or coating) to sustain electrochemical processes. Fig. 1 —17A shows the surface of coated tin­plate (steel sheet with a very thin—400 nm—layer of tin covered by ~ 1 pm hybrid silane film) after 48 h of immersion in 0.05 M NaCl. Steel is corroding in a few points, identified as brown spots in Fig. 1 —17A. When a positive or negative current of 2 pA is applied to the sample (in an organic solvent—absolute ethanol saturated with NaCl—to avoid the corrosive effect of aqueous solutions), more active points are visible in the SVET maps. Many points are common in maps (C) and (D) but some are only active for the cathodic or for the anodic processes, which might give indication about the electroactivity of those points. Fig. 1—17E and F shows a similar approach to reveal the porosity of an Al₂O₃ layer applied on an alumi­num alloy (same system presented in Figs. 1—8B and 1—9B). The objective was to rapidly detect the porosity of the sample without promoting the corrosion of the substrate [117]. Instead of organic solvents, this analysis could have been done in aqueous solutions of low corrosivity, for example, buffered borate solutions [58]