**1. Introduction**

156 Corrosion Resistance

[24] V. Shankar Rao, V.S. Raja, Anodic polarization and surface composition of Fe-16Al-

[26] G.K. Gomma, Corrosion of low-carbon steel in sulphuric acid solution in presence of

[27] C.F. Zinola, A.M. Castro Luna, The inhibition of Ni corrosion in H2SO4 solutions containing simple non-saturated substances, Corro. Sci. 37 (1995) 1919-1929. [28] M.R.F. Hurtado, P.T.A. Sumodjo, A.V. Benedetti, Electrochemical studies with a Cu-5 wt.% Ni alloy in 0.5 M H2SO4, Electrochimica Acta 48 (2003) 2791-2798. [29] F.M Reis, H.G. de Melo, I. Costa, EIS investigation on Al 5052 alloy surface preparation for self-assembling monolayer, Electrochimica Acta 51 (2006) 1780-1788. [30] T.M. Yue, L.J. Yan, C.P. Chan, C.F. Dong, H.C. Man, G.K.H. Pang, Excimer laser surface

[31] I. Epelboin, C. Gabrielle, M. Keddam, H. Takenouti, Achievements and tasks of

[32] M. Metikoš-Huković, R. Babić, S. Brinić, EIS-in situ characterization of anodic films on antimony and lead-antimony alloys, J. Power Sources 157 (2006) 563-570. [33] A.R. Trueman, Determining the probability of stable pit initiation on aluminium alloys

[34] Y.M. Tang, Y. Zuo, X.H. Zhao, The metastable pitting behaviours of mild steel in

[35] R.T. Foley, T.H. Nguyen, The chemical nature of aluminum corrosion, J. Electrochem.

[36] S. Van Gils, C.A. Melendres, H. Terryn, E. Stijns, Use of in-situ spectroscopic

[37] H. Nakazawa, H. Sato, Bacterial leaching of cobalt-rich ferromanganese crusts,

[38] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-

[39] A.A. Hermas, M. Nakayama, K. Ogura, Formation of stable passive film on stainless

[40] K. Varga, P. Baradlai, W.O. Barnard, G. Myburg, P. Halmos, J.H. Potgieter,

[41] R. Wang, An AFM and XPS study of corrosion caused by micro-liquid of dilute sulfuric

hydrochloric acid solutions, Electrochimica Acta 42 (1997) 25-35.

acid on stainless steel, Appl. Surf. Sci. 227 (2004) 399-409.

electrochemical engineering, Electrochimica Acta 22 (1975) 913-920.

treatment of aluminum alloy AA7075 to improve corrosion resistance, Surface and

using potentiostatic electrochemical measurements, Corros. Sci. 47 (2005) 2240-

ellipsometry to study aluminium/oxide surface modifications in chloride and

Ray Photoelectron Spectroscopy, 1st ed., Perkin-Elmer Corporation, Minnesota,

steel by electrochemical deposition of polypyrrole, Electrochimica Acta 50 (2005)

Comparative study of surface properties of austenitic stainless steels in sulfuric and

, Corros. Sci. 50 (2008) 989-994.

0.14C alloy in 0.25 M sulfuric acid, Corros. 59 (2003) 575-583. [25] L. Young, Anodic Oxide Films, 1st ed., Academic Press, London, 1961.

Coating Technology 179 (2004) 158-164.

bicarbonate and nitrite solutions containing Cl-

sulfuric solutions, Thin Solid Films 455 (2004) 742-746.

International Journal of Mineral Processing 43 (1995) 255-265.

2256.

1979.

3640-3647.

Soc. 129 (1982) 464-467.

pyrazole-halides mixture, Mater. Chem. & Phys. 55 (1998) 241-246.

Well-known the electrochemical nature of most processes of corrosion, the technology of anticorrosive coatings is oriented in the direction of making products that control the development of electrode reactions and that generate the isolating of metal surface by applying films with very low permeability and high adhesion (Sorensen et al., 2011).

The zinc-rich coatings and those modified with extenders and/or metal corrosion inhibitors display higher efficiency than other coatings. A problem that presents this type of primers is the extremely reactive characteristic of metallic zinc; consequently, the manufacturers formulate these coatings in two packages, which imply that the zinc must be incorporated to the vehicle in previous form to coating application (Jianjun et al., 2008 & Lei-lei & De-liang, 2010).

Considering the concept of sacrificial anode (cathodic protection), coatings that consist of high purity zinc dust dispersed in organic and inorganic vehicles have been designed; in these materials, when applied in film form, there are close contacts of the particles among themselves and with the base or metallic substrate to be protected.

The anodic reaction corresponds to the oxidation of zinc particles (loss of electrons) while the cathodic one usually involves oxygen reduction (gain of electrons) on the surface of iron or steel; the "pressure" of electrons released by zinc prevents or controls the oxidation of the metal substrate. Theoretically, the protective mechanism is similar to a continuous layer of zinc applied by galvanizing with some differences because the coating film initially presents in general a considerable porosity (Jegannathan et al., 2006).

In immersion conditions, the time of protection depends on the zinc content in the film and on its dissolution rate. The mechanism is different for films exposed to the atmosphere, because after the cathodic protection in the first stage, the action is restricted substantially to a barrier effect (inhibition resistance) generated by the soluble zinc salts from corrosion by sealing the pores controlling access to water, water vapor and various pollutants. Due to the

Reinforcement Fibers in Zinc-Rich Nano Lithiun Silicate Anticorrosive Coatings 159

*Water-based nano lithium silicate of 7.5/1.0 silica/alkali molar ratio*. Previous experiences with these solutions on glass as substrate allowed infer that as silicon dioxide content in the composition increases the film curing velocity also increases and that in addition the

For this study, a commercial colloidal lithium silicate (3.5/1.0 silica/alkali molar ratio in solution at 25% w/w) was selected; with the aim of increasing the silica/alkali ratio, a 30% w/w colloidal alkaline solution of nanosilica was used (sodium oxide content, 0.32%). The aim was to develop a system consisting of an inorganic matrix (alkaline silicate) and a nanometer component (silica) evenly distributed in that matrix with the objective of determining its behaviour as binder for environment friendly, anticorrosive nano coatings. *Solvent-based, partially hydrolyzed tetraethyl orthosilicate.* The tetraethyl orthosilicate is synthesized from silicon tetrachloride and anhydrous ethyl alcohol. This product commercializes as condensed ethyl silicates and usually contains approximately 28% w/w of SiO2 and at least 90% w/w monomer. The additional purification removes waste products of low boiling point (mainly ethanol) and the dimmers, trimmers, etc.; in some cases, this treatment allows obtaining pure tetraethyl silicate conformed by 99% w/w

Theoretically, the complete hydrolysis of ethyl silicate generates silica and ethyl alcohol. Nevertheless, the real hydrolysis never produces silica in form of SiO2 (diverse intermediate species of polysilicates are generated). Through a partial hydrolysis under controlled conditions, it is possible to obtain a stable mixture of polysilicate prepolymers. The stoichiometric equation allows calculating the hydrolysis degree X (Giudice et al., 2007 &

The pure or condensed ethyl silicate does not display good properties to form a polymeric material of inorganic nature. In this paper, ethyl silicate was prepared with 80% hydrolysis degree in an acid medium since catalysis carried out in advance in alkaline media led to a

The empirical equation of ethyl silicate hydrolyzed with degree X was used to estimate the weight of the ethyl polysilicate and the hydrolysis degree, through the calculation of the necessary amount of water. The weight was obtained replacing the atomic weights in the

The percentual concentration of the silicon dioxide in the ethyl polysilicate is equal to the relation molecular weight of SiO2 x 100 / weight of the ethyl polysilicate; consequently, SiO2, % = 60 x 100 / (208-148 X). On the other hand, to calculate the water amount for a given weight of tetraethyl orthosilicate and with the purpose of preparing a solution of a predetermined hydrolysis degree, the equation weight of water = 36 (100 X) / 208 was used. Finally, the amount of isopropyl alcohol necessary to reach the defined percentual level the silica content was calculated. It is possible to mention that after finishing the first hydrolysis

mentioned empirical formula; the result indicates that it is equal to 208-148 X.

**2. Materials and methods** 

**2.1.1 Film-forming materials** 

dissolution rate decreases.

monomer.

Hoshyargar et al., 2009).

fast formation of a gel.

**2.1 Characterization of main components** 

above, it is necessary to find the appropriate formulation for each type of exposure in service (Hammouda et al., 2011).

With regard to zinc corrosion products, they are basic compounds whose composition varies according to environmental conditions (Wenrong et al., 2009); they are generally soluble in water and can present amorphous or crystalline structure. In atmospheric exposure, zinc-based coatings that provide amorphous corrosion products are more efficient since these seal better the pores and therefore give a higher barrier effect (lower permeability). Fortunately, zinc-rich coatings of satisfactory efficiency in outdoor exposure display in the most cases amorphous corrosion products.

The durability and protective ability depends, in addition to environmental factors, on the relationship between the permeability of the film during the first stage of exposure and the cathodic protection that takes place (Xiyan et al., 2010). The protection of iron and steel continues with available zinc in the film and effective electrical contact; therefore, particularly in outdoor exposure, the time of satisfactory inhibitory action may be more prolonged due to the polarizing effect of the corrosion products of zinc (Thorslund Pedersen et. al., 2009).

A cut or scratch of the film applied on polarized panel allows again the flow of protective electrical current: metallic zinc is oxidized and the film is sealed again. A substantial difference with other types of coatings is that the corrosive phenomenon does not occur under the film adjacent to the cut (undercutting).

With respect to spherical zinc, the transport of current between two adjacent particles is in tangential form and consequently the contact is limited. With the purpose of assuring dense packing and a minimum encapsulation of particles, the pigment volume concentration (PVC) must be as minimum in the order of the critical pigment volume concentration (CPVC).

The problems previously mentioned led to study other shapes and sizes of zinc particles. The physical and chemical properties as well as the behaviour against the corrosion of these primers are remarkably affected by quoted variables and in addition, by the PVC; thus, for example, it is possible to mention the laminar zinc, which was intensely studied by the authors in other manuscripts (Giudice et al., 2009 & Pereyra et al., 2007).

The objective of this paper was study the influence of the content and of the nature of reinforcement fibers as well as the type of inorganic film-forming material, the average diameter of spherical zinc dust and the pigment volume concentration on performance of environmentally friendly, inorganic coatings suitable for the protection of metal substrates. The formulation variables included: (i) two binders, one of them based on a laboratoryprepared nano solution lithium silicate of 7.5/1.0 silica/alkali molar and the other one a pure tetraethyl silicate conformed by 99% w/w monomer with an appropriate hydrolysis degree; (ii) two pigments based on spherical microzinc (D 50/50 4 and 8 µm); (iii) three types of reinforcement fibers used to improve the electric contact between two adjacent spherical zinc particles (graphite and silicon nitride that behave like semiconductor, and quartz that is a non-conductor as reference); (iv) three levels of reinforcement fibers (1.0, 1.5 and 2.0% w/w on coating solids) and finally, (v) six values of pigment volume concentration (from 57.5 to 70.0%).
