Elsevier

Corrosion Science

Volume 43, Issue 12, December 2001, Pages 2291-2303
Corrosion Science

Inhibitory properties of calcium exchanged silica epoxy paintings

https://doi.org/10.1016/S0010-938X(01)00037-3Get rights and content

Abstract

Epoxy paints with different contents of calcium-exchanged silica were formulated maintaining the ratio PVC/CPVC constant. Steel painted samples were submitted to immersion tests monitored by open circuit potential and electrochemical impedance. The behaviour of calcium-exchanged silica was compared with chromate and red iron oxide. The electrochemical measurements were complemented by water vapour permeability, Auger electron spectroscopy and thermally stimulated currents. The results suggested a film forming mechanism for calcium exchanged silica paintings. In total immersion conditions, they had better performance than the iron oxide, closely approaching the chromate.

Introduction

Replacement of chromates in anticorrosive coatings has led to the development of non-toxic pigments. In general, these pigments have more restrict application than chromates in terms of adequate formulation and start-up of inhibitory properties. Calcium-exchanged silica (Ca/silica) is one example of non-toxic pigment, which is obtained by acid–base reaction between silane groups at silica gel surface and calcium ions in solution. Ca/silica is an alkaline pigment of low specific weight, with relatively high oil absorption when compared with other pigments [1]. This last property allows the use of lower Ca/silica contents in comparison with other pigments to reach a given PVC. Technical bulletins indicate 3–7 wt.% for organic solvent-based paints and 1–4 wt.% for water-based paints as ideal contents [2].

Literature attributes three modes of action to Ca/silica: (i) ion exchange properties which would trap or delay the arrival of aggressive ions to the metal surface, releasing Ca2+; (ii) Ca/silica would interact with the binder, improving crosslinking; (iii) silica and calcium would become mobile in the coating during permeation process and once reaching the metal surface, they would form a protective film [1].

Electrochemical measurements have been applied to the study of the kinetic action of Ca/silica. Armstrong [3], Sarc [4] and Amiradin [5] obtained polarisation curves of steel immersed in aqueous extracts of the pigment. Armstrong and Zhou [3] measured double layer capacitances and suggested Ca2+ anionic adsorption, confirmed by infrared spectroscopy. Fletcher [6] obtained similar capacitance values for polyester paintings pigmented with Ca/silica and chromate. Such behaviour was confirmed elsewhere [7]. On the other hand, for epoxy paintings, Amiradin [8] found much higher capacitance values for Ca/silica than for chromate.

Goldie [1] found low permeability to chloride for alquidic paintings containing Ca/silica. The author reported XPS analysis of a coating/metal interface where Ca and Si were detected as precursors of a dense film.

In this paper the performance of special formulated Ca/silica paintings was monitored by electrochemical measurements and compared with paintings pigmented with red iron oxide and chromate. The main purpose was to search electrochemical evidence for the kinetic action attributed to Ca/silica. The discussion was complemented using water vapour permeability of free-standing films, Auger electron spectroscopy (AES) of the metal coating interface, differential scanning calorimetry (DSC) and thermally stimulated currents measurements. The thermally stimulated polarization (TSPC) and depolarization current (TSDC) techniques were used to search structural changes on the different paintings by dielectric relaxation. The basics for TSDC and TSPC can be found elsewhere [9]. Pébère and co-workers [10] were the first to apply these techniques to paints.

Section snippets

Materials

AISI 1010 mild steel coupons were sandblasted to white metal, acquiring 15μm average roughness profile. Features of the five paints used in this work are given in Table 1. Paints A, B, C, and D were especially formulated, maintaining the same PVC/CPVC ratio in order to compare the behaviour of Ca/silica with red iron oxide. Paint E is a commercial paint considered of excellent performance and it was included as a reference. The samples were coated with two layers of paints A–E (total dry

Thermal analysis and electrical measurements (TSDC/TSPC)

The glass transition temperatures (TG) measured by DSC were 66°C, 69°C and 64°C for A, B and D paintings, respectively. For the experimental conditions employed in this work, the uncertainty on TG is around 0.5°C. As it can be seen, TG is higher for paint B, suggesting that epoxy can interact with the pigment resulting in a more stable structure [10]. However, increasing the Ca/silica content does not enhance this feature (paint D).

In Fig. 1 TSDC curves for coatings A, B and D are shown. For

Conclusions

The total immersion tests showed that epoxy paints pigmented with Ca/silica could have better performance than that pigmented with red iron oxide. Such statement is based on the formulation criterion adopted, maintaining a constant PVC/CPVC ratio. That improvement owes to the addition of good barrier properties and inhibiting action of Ca/silica. The inhibiting action was assessed by electrochemical techniques complemented with water vapour permeability measurements.

Eoc potential monitoring of

Acknowledgements

The authors thank the financial support of Finep, CNPq, FAPERJ, FUNCAP and FUJB Brazilian agencies; Quı́mica Industrial União Ltd. and Akzo Nobel International Coatings Ltd. for paintings formulation; Surface and Thin Films Laboratory in Fed. Univ. of Rio de Janeiro for Auger analyses. The authors are in debt with C.F. Wehmann who made the thermostimulated current measurements.

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