Comparison of the efficiency of inorganic nonmetal pigments with zinc powder in anticorrosion paints
Introduction
Protection of metals from ever progressing corrosion presents one of the topical issues of this century. The increasing industrialization of our life is accompanied with the ever-growing number of metals that corrode and become devalued. Corrosion is a natural event during which a metal, usually obtained from salts, transfers to a more energetically convenient state; i.e. it transforms into the form of an oxide. The speed and course of metal corrosion are affected by a large number of factors such as pollution with corrosion enhancing (stimulating) substances.
Preservation of metals in their pure form can be achieved only through their proper protection. One of the possible methods is protection by means of an organic coating [1] that ensures both a chemical and electrochemical reaction between an anticorrosion pigment and the metal or a corrosion environment penetrating the coating [2]. In this case we deal with prime-anticorrosion coatings.
Various types of pigments with diverse anticorrosion properties were used during the years in anticorrosion paints formulation. Some of the pigments – such as chromated or lead pigments – were used quite often thanks to their excellent anticorrosion resistance [3]. At present, however, environmental aspects require that these pigments be eliminated in paints. The end of the 1980s witnessed the ongoing efforts to continue the application of chromated and lead pigments, yet a new form of less toxic core pigments started to become promoted. The 1990s thoroughly supported the full replacement of these pigments in paints [4]. When searching for the technical and application properties of pigments, it is necessary to consider the price of raw materials needed for paint formulation. New pigments and fillers are usually more expensive than classical pigments. Despite these problems, the application of organic coatings can still be labeled as one of the most economical methods of corrosion protection, which is due to its relatively low costs of acquisition compared to the other methods of protection such as chromate coating, zinc coating, enamel coating or the use of finer materials. The true advantage of the use of paints consists in their all-purpose and easy application onto the surface of various materials.
At present, zinc phosphate is the anticorrosion pigment most frequently applied onto paints [5], [6], [7], [8]. The massive use of nontoxic and environmentally acceptable zinc phosphates developed an immense pressure on regulations prohibiting the use of lead and chromated anticorrosion pigments. The most significant applications of zinc phosphate are in water–borne dispersion or emulsion paints [9].
For many years, metal zinc particles have been used as an anticorrosion pigment in paints designed for the heavy-duty corrosion protection of metals. The mechanism of zinc action used to be explained as cathodic protection based on the electrochemical theory [10]. Professional literature introduces opinions claiming that this pigment is efficient at concentrations ranging from 94% to 96% in paints [11]. However, this requirement is confronted with certain problems. With a very small volume amount of a binder, this system is very difficult to produce. Another pitfall consists in very poor physico-chemical properties of such paints. Cathodic contact protection was considered efficient in the first stage of corrosion stress. The action of the electrochemical reaction, however, leads to the release of zinc oxidation products that seal the pores between zinc particles until reaching a stage when the system becomes an electric nonconductor and protection is secured by a mere barrier effect. The paint coating then consists of these three components: metal zinc particles, a very small amount of the binder and zinc oxide that seals the highly porous paint coating [12]. The drawback of this system is the fact that zinc oxide is easily soluble in a diffusive corrosion environment. In the past, the efficiency of zinc-pigmented coatings was identified especially by means of electrochemical methods [13].
Section snippets
Pigments
The anticorrosion pigments used in this study are summarized in Table 1. The choice of the anticorrosion pigments is considered to represent all the types of these substances that are currently industrially used. Phosphates, modified phosphates, phosphosilicates, polyphosphates, borates, ferrite, and – for the purpose of comparison – also chromates were subjected to tests. Table 1 features the characteristics of applied anticorrosion pigments.
Zinc dust was used as a reference pigment to which
Zinc-pigmented paints
Metal zinc particles contained in paints provide the steel base with cathodic protection. This protection requires a relatively high concentration of these particles in an organic binder. As listed in previous publications, the anticorrosion effect of metal zinc is not only of electrochemical nature but it also has neutralizing effects. Perceived from the point of steel anticorrosion protection, the most efficient paints are those that contain zinc whose concentration falls within its CPVC
Conclusion
The results of corrosion tests performed in a SO2 condenser chamber, salt spray cabinet and the results of chemical resistance were sequenced according to the efficiency of individual anticorrosion pigments. The final order applicable to all types of corrosion tests was obtained by calculating the difference factor ΔU.
The following order of results applies to the concentration of anticorrosion pigment PVC = 10% at the PVC/CPVC ratio = 0.65:
- •
zinc phosphomolybdate > calcium hydrogen phosphate > zinc
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