Evaluation of corrosion protection properties of additives for waterborne epoxy coatings on steel
Introduction
The introduction of strict regulations in the use of volatile organic compounds (VOCs) has brought about the development of solvent-free coating technologies like powder coatings, electrocoatings, UV curable coatings and waterborne systems. Waterborne coatings have gained increasing importance for many applications. These coatings usually contain different additives such as extenders, dispersion agents, defoamers, fillers and corrosion inhibitors. The choice of a functional additive requires a good knowledge of its interaction with other additives and pigments, its compatibility with the binder and the substrate and its influence on the adhesion properties [1]. Additives can interact with waterborne paint system in different ways with corresponding consequences for the corrosion protective properties. For example, a previous study performed in our laboratory showed that the selection of a suitable dispersion agent was crucial for the performance of coated steel samples subjected to corrosion tests [2], the reason being that it affects the wetting properties of the binder emulsion with respect to the substrate and the pigments.
The degradation of a polymer-coated metal occurs after water penetrates at the coating–substrate interface, where failure mechanisms such as osmotic blistering, cathodic delamination and anodic undermining may be initiated [3], [4]. Corrosion inhibiting additives are designed to react at the metal surface by improving adhesion and slowing down the electrochemical reactions. In modern coatings traditional inorganic inhibitors such as chromates, lead oxides, etc. are being replaced by less harmful inorganic [5], [6] and organic compounds [7], [8]. Inorganic pigments like phosphates, molybdates, vanadates, silicates and borates are commonly used for the protection of solvent-borne and waterborne coatings. Environmentally friendly organic inhibitor formulations specifically designed for waterborne coatings are an attractive alternative [9].
The development of corrosion inhibiting additives requires test methods which yield significant results in a time as short as possible. Salt spray and different cyclic exposure tests are widely used in the coating industry, but these tests yield essentially qualitative information. More recently, electrochemical methods, especially electrochemical impedance spectroscopy (EIS), have found increasing interest for testing of coatings, because these methods are capable to yield mechanistic data which can be quantified by using suitable models [10], [11]. On the other hand, EIS data acquired in long term exposure tests, often scatter considerably and require, especially in the case of damaged coats, parallel experiments on several samples and a statistical evaluation [12], [13]. The adhesion properties of coatings are crucial for corrosion protection and durability of a coating system. Corrosion cells can deteriorate the adhesion of organic coatings on steel because the rise in pH at cathodic locations on the surface can lead to cathodic disbonding and blistering. Several authors have studied the mechanism of cathodic delamination and disbonding using different methods [14], [15], [16], [17]. In our laboratory scanning acoustic microscopy (SAM) has recently been used for monitoring the blister formation due to the loss of adhesion during open circuit immersion tests [2]. For the development of corrosion inhibiting additives it is important to be able to measure by what mechanism a given additive affects the coating performance and on which property of the metal-coating system has the most effect. The goal of the present study is to evaluate the effect of two model corrosion inhibiting additives on the performance of a waterborne epoxy coating using a variety of electrochemical and non-electrochemical methods which measure different properties.
Section snippets
Materials and samples preparation
The fully formulated paint system utilized in this work is composed of a waterborne amine-modified epoxy binder, additives and pigments as reported in Table 1. The effect of two different inhibitors was evaluated. Inhibitor (A) was a proprietary organic inhibitor (IRGACOR 287, Ciba Specialty Chemicals) containing carboxylic acid for corrosion inhibition [18] neutralized with a polysiloxane base for improving solubility and adhesion properties [19]. Inhibitor (B) was a commercial zinc–aluminum
EIS results
Plots of water uptake, paint resistance, double layer capacitance and charge transfer resistance for the blank coat, and the coats containing (A) and (B) inhibitor, respectively, are reported in Fig. 1. Standard deviation plots are also included. In the first 30 h the water uptake values (see Fig. 1a) increase steeply for all the tested coats, denoting a fast diffusion of water into the coatings. The initial period was followed by a slower almost linear increase for the remaining test duration.
Conclusions
The present study demonstrates the usefulness of applying a variety of experimental methods for characterizing the effects of corrosion inhibiting additives on the performance of waterborne coating systems. The obtained data show that corrosion inhibiting additives drastically modify properties such as adhesion, water uptake or blistering, and they also provide evidence that different additives affect these properties differently. The experimental approach described here allows one to identify
Acknowledgements
The authors thank A. Braig, D. Renoux and M. Frey from Ciba Specialty Chemicals Inc., Basel Switzerland, for providing the paint systems and the additives. Dr. A. Kulik kindly permitted the use of the SAM at the Institut de Génie Atomique, EPFL Lausanne. Financial support from CTI Bern is gratefully acknowledged.
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