Handbook of Modern Coating Technologies

Review of published work using the scanning vibrating electrode technique to characterize coatings performance

After an overview of the responses that can be found by SVET with the various types of coat­ings, a short review of the literature is now presented. The first published study with SVET and coatings dates from 1987, in which Isaacs described the use of the vibrating probe tech­nique to detect the presence of defects in ion vapor—deposited aluminum on steel [58].

FIGURE 1-13 Corrosion of steel coated with TiN. Example of a cathodic coating applied to an anodic substrate intact and with defects (Cases 8 and 9 in Table 1 -1). (A) Steel sheet coated with 2 pm thick TiN layer deposited by PVD, after 4 days of immersion in 0.05 M NaCl. (B) Same coated system with defects in the TiN layer. The numbers 1, 2, and 3 represent, respectively, an optical image of the sample with the indication of the scanned area (dashed lines),

2D current density map measured by SVET, and the same map in 3D. PVD, Physical vapor deposition; SVET, scanning vibrating electrode technique.

A nonaggressive borate solution was chosen to prevent any significant damage to the coating during defect location. The experimental set-up involved polarization measurements on spe­cific areas of the surface. The results showed that the defects were not from the exposed steel but were caused by the presence of inclusions in the coating. Worsley et al. combined SVET and micrographic analysis to study the mechanisms of corrosion on the surface and on the edges of hot dip galvanized steel and also the effect of the cooling rate and gauge on total zinc loss and anodes lifetimes [59]. The defects induced by mechanical forming of galvanized steel was also analyzed by SVET [60,61]. Another subject with intensive study using SVET was the corrosion at cut-edges [36,106-108,111,121-127].

Of all types of coatings, organic coatings (paints) are the largest group with many different compositions and possible responses. They were also studied by SVET. One of the first stud­ies with organic coatings was reported by Isaacs [62], where the technique was used to detect defects of a 20-pm thick epoxy paint applied on phosphated zinc-plated steel or pure zinc sheet, immersed in 10 mM sodium chloride or sodium sulfate. A deconvolution method was used for obtaining the current densities at the metal surface. Not long after, Sekine and cow­orkers [63,64] used SVET to study several coating systems (epoxy, polyurethane, polyester, polyvinyl chloride, and alkyds) and found a linear correlation between the current density measured by SVET, the film resistance measured by EIS, and the coating lifetime (the higher the current density, the shorter was the coating lifetime). They also found that the current density for the same coating system depended on the testing solution, increasing in the order

NaCl < Na₂SO₄ < K₂CO₃ < H₃PO₄ < HCOOH. Later, SVET was used to study chromate-epoxy primers applied on steel and aluminum [65] and contributed to the elucidation of the sacrifi­cial effect of magnesium rich primers developed as replacement to chromate based pigments [65—67]. Another study involved weldable primers (very thin films incorporating conductive particles to provide electrical continuity through the coating) for welding of coated metal panels in the automotive industry [68].

Conductive polymers have been investigated for corrosion control, directly as pretreat­ments or as modifiers of fillers and pigments in paints. Several works used SVET to charac­terize the action of conductive polymers, principally in defective areas [69—74].

• Self-healing coatings

A new line of investigation in coatings research is the search for self-healing systems [75—79,128—135]. "Self-healing” can be defined as the autonomous recuperation of the ini­tial properties of a coating that were lost after a negative interaction with the surrounding environment (corrosion, pH change, mechanical impact, thermal shock, chemical attack, UV irradiation, water ingress, etc.). "Smart” is a related term applied to a coating capable of sensing alterations in the environment and able to respond in a preestablished manner. It may be the self-recovery of properties lost after an external aggression (self-healing) or the indication (change in color for example) that a critical threshold (temperature, humidity, radiation, pressure, etc.) has been reached (in the coating or in the environment), prompting human action to fix the problem.

In the field of coatings for corrosion protection, self-healing is currently being implemented either by responsive prepaint layers or by formulations containing self-sealing components or incorporating micro/nanoreservoirs of corrosion inhibitors for later release [128—135]. The loading of corrosion inhibitors into micro/nanoreservoirs and their incorporation in paint for­mulations as additives or replacement of pigments is the most common approach. The reser­voirs are dispersed in the coating and the inhibitor stays inside until an external stimulus activates the liberation of inhibitors, and, sometimes, the capture of aggressive species like chloride ions. The stimulus can be the level of moisture, temperature, UV radiation, mechani­cal impact, change in local pH (at anodes or cathodes), rise of ionic strength, or increase in the concentration of metallic ions (from metal oxidation) or of aggressive ions (e.g., Cl^_).

SVET can monitor the self-healing capability of surface coatings by means of maps, lines, or measurements on a single point above artificial defects. If indeed self-healing is operating, after an initial increase, the corrosion current should decrease until the degradation process is stopped. The possible responses of current versus time are typified in Fig. 1 — 14. In a nor­mal situation corrosion will start at the defect and grow in size with an increasing current over time (Fig. 1—14a). Alternatively the current will stabilize at a certain level or it may decrease (Fig. 1 —14b and c). This is sometimes considered an indication of self-healing because the system is able to prevent the progress of corrosion or even minimize its effect. The ideal situation is represented in Fig. 1 —14d with the complete suppression of corrosion and the ability to keep healing whenever further defects are made.

For this type of investigation it is necessary to discard other reasons for the decrease in SVET signal before concluding for self-healing: (1) the defects can be covered by corrosion products which may hinder or deviate the current pathway; (2) corrosion might move from the defect to areas below the coating or out of the scanned area, not measurable by SVET; and (3) the anodic and cathodic areas may become so small and uniformly distributed that the current lines between them flow close to the surface and below the plane of measure­ment. In all these cases, corrosion exists but is not sensed by SVET.

A few examples from the literature are now presented. In 2004 Khramov et al. [75] studied a 2024-T3 aluminum alloy covered by a hybrid organoinorganic film containing corrosion inhibi­tors. Films as thin as 1 pm thick were able to heal the localized corrosion on artificial defects. With mercaptobenzimidazole corrosion was not detected during the testing period, while with mercap- tobenzothiazole the current measured by SVET increased at the beginning but then it declined to values that remained negligible during the rest of the testing period. Healing was also detected when the inhibitors were loaded into nanocontainers dispersed in the sol—gel film [77]. EIS showed evidences of healing but the corrosion suppression was better visualized by SVET. In another study nanoparticles with a layered double hydroxide structure incorporating benzotria- zole (BTA) were mixed in epoxy coating together with nanosized bentonite clay with Ce³¹ [79]. The coating was applied to 6061 aluminum alloy in contact with carbon fiber—reinforced plastic (CFRP). To accelerate the degradation one defect was produced on the metal part and another one on the CFRP part. The degradation was followed by SVET. Oxidation appeared on the defect in aluminum, and reduction on the defect in the conductive plastic. The SVET currents were noticeably smaller in these samples compared with samples coated with the epoxy without nano­containers (reference system). The decrease in corrosion was attributed to the synergistic action of BTA and Ce³¹, the former mainly at the anode and the later at the cathode.