Keywords

• ACET;
• NSS;
• Organophilized zinc molybdate phosphate;
• Standard zinc phosphate;
• Equivalent circuit;
• Epoxy coatings

1. Introduction

Pigments [1] and [2] (normally inorganic crystalline solids) are one of the basic components of primer formulations. They act as inhibitors, increasing the resistance of steel surfaces to corrosion [3]. The inhibitor action takes place in different ways. Thus some inhibitor pigments act by improving the barrier properties of the coating. These kinds of pigment have a physical activity which is the separation of the metallic substrate from the corrosive medium. Other pigments show a chemical activity and can be classified principally in four groups according to their mechanism of action [4]. First, some pigments decrease the rate of anodic reaction by binding the metal ions produced by corrosion reactions, and as a consequence, coordination compounds are formed at anodic zones. Other pigments decrease the activity of both anodic and cathodic processes by liberating passivating ions. A third kind of pigment reacts with the resin, giving rise to compounds that decrease metal corrosion. The last kind of pigment acts by increasing the concentration of OH⁻ ions in water diffusing through the porous paint. In steels, this increase of pH decreases the activity of metallic corrosion processes [4] due to the formation of no soluble species (oxides and hydroxides). In general, chemically active pigments are soluble substances that dissolve continuously in the water that permeates through the coating, giving rise to an inhibitive solution that reaches the coating–metal interface. Among this kind of pigments, those most extensively used are red lead and zinc chromate, because of their efficiency. However, these compounds are highly toxic and cause serious environmental pollution [5]. Hence, in recent decades, the surface finishing industry has undergone fundamental changes regarding the development and promotion of environmentally friendly pigments for primer formulation [6], [7] and [8]. In this context, zinc phosphate has been proposed as an alternative pigment. This compound has a level of toxicity much lower than chromates [1].

For some decades, zinc phosphate has been the standard non-toxic pigment used instead of chromates [9], [10], [11], [12], [13], [14], [15], [16] and [17]. In order to improve the protective behaviour of these zinc phosphates, a “second-generation” pigment based on zinc phosphate has been developed by adding elements such as molybdenum or aluminium [18], [19], [20] and [21]. Improved behaviour has been observed from primers pigmented with basic zinc molybdenum phosphate hydrate, basic aluminium zinc phosphate hydrate and basic zinc phosphate hydrate with an organic pre-treatment. This second generation of pigments has been developed aiming to combine, in a synergistic way, the corrosion inhibition properties of phosphates and molybdates, so they have been called phosphomolybdate pigments. Zinc molybdenum phosphate seems to be a good possibility and is claimed to have equal or superior anticorrosive behaviour than chromate and zinc phosphate alone [16]. It is basically composed by zinc phosphate modified with zinc molybdate up to 1% (expressed as MoO₃). According to the literature (not much is available), it is claimed to have anticorrosive properties when employed in anticorrosive paints applied on metals [22] and [23]. Bittner and co-workers [17] and [24] reported the behaviour of zinc molybdenum phosphate in alkyd paints, compared with zinc phosphate and zinc chromate. The active inhibitive species in this pigment is the molybdate anion, which is thought to repassivate the corrosion pits in steel [25].

The present work will concentrate on the evaluation of epoxy coatings over steel formulated with different contents of standard zinc phosphate and organophilized zinc molybdenum phosphate by means of neutral salt spray test (NSS) and accelerated cyclic electrochemical test (ACET) [26], [27], [28], [29], [30], [31], [32], [33], [34] and [35]. Moreover, these techniques will provide vital information related to the anticorrosive properties of the tested epoxy coatings on steel and according to the results of salt spray test. The use of salt spray test and ACET technique will allow us to determine the best type of pigment and its optimum amount for the tested epoxy systems”.

2. Experimental

2.1. Substrates

Standardized cold rolled steel panels (S-46 from Q-Panel) were used in this study. All test panels were degreased with acetone and a single layer of paint was applied on them at 62 ± 7 μm dry film thickness, using application bars of 150 μm. The systems were dried under ambient air conditions for three weeks at room temperature prior to testing.

2.2. Pigments

Anticorrosive pigments were used in order to protect metallic surfaces with the idea to slow down and minimize the negative impact of corrosion. Pigments used were a standard zinc phosphate (Nubirox N2^®) and a unique zinc phosphate (Nubirox N106^®) whose molybdenum content and organophilic treatment enhances its compatibility and performance both in solvent based and water based systems.

2.3. Paints

Seven paints (Table 1) were formulated containing each of them, anticorrosive pigment at three different dosages: 6%, 8% and 10% (expressed as anticorrosive pigment volume concentration in dry film volume) and the reference that does not contain any anticorrosive pigment. The paint formulas were calculated keeping the same solids volume percentage (40%) and the same PVC/CPVC ratio (0.75), being CPVC the critical pigment volume concentration, in order to compare the pigment effect at the same free binder volume level.