**4. Final considerations**

170 Corrosion Resistance

The incorporation of conducting reinforcement fibers seems to have favored the conductivity, which leads to reduction of the efficient PVC, according to the abundant amount of zinc corrosion products visually observed, the results of the electrode potentials and those obtained in the salt spraying (fog) chamber. Figures 7 and 8 display the primer films based on binder A (water-based nano lithium silicate of 7.5/1.0 silica/alkali molar ratio), microzinc I (fine, 4 µm) and fiber 2 (graphite) in level c (2.0% w/w) for 57.5% and 70.0% PVC values respectively, after finishing the accelerated aging test. The analysis of the cited figures reveals that despite having larger distance between the particles of microzinc in the case of 57.5% PVC compared with that formulated with 70.0% PVC, the galvanic activity in the two primers is significant in both cases (as evidenced by the amount of white zinc salts). In addition, results of figures show that the conductive reinforcement fibers linked electrically the microzinc particles each other, even in the primer of less PVC (all particles, despite having no direct contact between them, demonstrated activity like sacrificial

Fig. 7. SEM micrograph of primer A.I.2.c formulated with 57.5% PVC.

Fig. 8. SEM micrograph of primer A.I.2.c formulated with 70.0% PVC.

anodes).

To explain the great tendency of zinc particles to corrode at the film surface of water-based nano lithium silicate primers as comparing with those solvent-based, partially hydrolyzed tetraethyl orthosilicate, it is necessary to consider that the first ones are based on binders, as mentioned, with a higher superficial tension. The last one implies inferior wetting, that means lower adhesion, penetration and spreading during metal zinc incorporation previous to application; consequently, they wet with more difficult the zinc particles while the second ones do it in a better way (more reduced interfacial tension).

The above-mentioned characteristic explains the great porosity of zinc-rich nano lithium silicate films and their high cathodic protective activity as comparing with zinc-rich tetraethyl orthosilicate films.

With regard to average diameter of zinc particle, size diminution increases significantly the surface area for a given weight. Since all surfaces have a given level of free energy, the ratio of surface energy to mass in small particles is so great that the particles adhered strongly themselves. For this reason, a lower particle size in a poor dispersion originates a greater flocculates (a high number of unitary particles are associated), which lead to zinc-rich

Reinforcement Fibers in Zinc-Rich Nano Lithiun Silicate Anticorrosive Coatings 173

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primer films of high porosity and because of good cathodic protective activity. Moreover, a lower particle size could lead to films with a higher electrical contact (better packaging ability); since the current density is inherent to the chemical nature of zinc dust and the operating conditions of the corrosion cell, the increase in specific area elevates not only the current of protection but also generates a better superficial distribution (more efficient primers). During immersion test in 0.1 M sodium chloride solution, visual inspection of plates protective with zinc-rich primers (both types of binders) showed a more localized steel attack when zinc dust of the higher particle size was used.

Concerning incorporation of reinforcement fibers, the conductive or non-conductive characteristic was a very important variable. The first ones improved notably the primer performance since they increased the electrical contact between particles and with the metallic substrate, particularly in the higher levels in the formulations; the performance is correlated with the higher useful zinc in the film. On the other hand, non-conductive reinforcement fibers did not modify the primer efficiency as compared with reference panels (without reinforcement fibers) and for this reason their incorporation is not justified from technical and economical viewpoints.

Referring to PVC values (zinc content in dry film), previous results of laboratory tests demonstrated that a higher amount of microzinc leads to a longer useful life of primers. Nevertheless, it is important to mention that the choice of zinc content must be made by considering the physical characteristic of the primer film required for each particular case. When pigment volume concentration exceeds largely the CPVC, film properties such as adhesion, flexibility, abrasion resistance, etc. are drastically reduced while when the percentual level is slight under the critical value the efficiency is also considerably diminished.

In the case of primers, which have got incorporated conductive reinforcing fibers, results allow concluding that it is possible to reduce appreciably the PVC without affecting significantly the efficiency in service. In addition, it is important to mention that the quoted diminution of zinc content in the film is direct proportional to decrease the primer cost since it is the most expensive component of the composition.
