An Effective Non-Toxic Anti-Corrosive Pigment

 

Zinc Phosphate

Research has intensified on zinc phosphate as a replacement for lead and chromium pigments. Today, zinc phosphate is one of the most widely used phosphate-containing anticorrosive pigments. Due to its much lower solubility compared to chromates and other pigments, and consequently lower reactivity, it has gained economic importance in the market by enabling various formulations. Zinc phosphate can be used in a wide variety of resin systems. According to ISO 6745 standard 'Zinc phosphate pigments in paints' 'Specifications and test methods', a zinc phosphate pigment is defined as a corrosion-preventive white pigment that consists predominantly of zinc phosphate dihydrate (Zn3 (PO4)2 · 2H2O) or a mixture of zinc phosphate dihydrate and zinc phosphate tetrahydrate (Zn3 (PO4)2 · 4H2O) or primarily contains zinc phosphate tetrahydrate. With respect to the effect on protection against corrosion, ISO 6754 states that various zinc phosphate pigments may have different corrosion-prevention properties. The fundamental requirements for product properties of zinc phosphate pigments are summarized in Table 1.

Table 1: Quality requirements for zinc phosphate.

Zinc phosphate is normally produced on an industrial scale by a wet-chemical reaction containing zinc oxide (ZnO) and orthophosphoric acid (H3PO4). This is followed by filtration, washing, drying and grinding operations.

The performance properties of zinc phosphate pigments depend on their chemical effectiveness and their ability to form inhibitor complexes and adhere to the applied surface. In addition, in the case of zinc phosphate, attention must be paid to electrochemical effectiveness, particularly in anodic areas, since small amounts of zinc phosphate will hydrolyze under humid conditions. As a result of this reaction, the formation of zinc hydroxide and the generation of secondary phosphate ions capable of forming protective layers on metal surfaces in anodic areas are proposed. Furthermore, under humid conditions, it is discussed in the literature that basic complexes can be formed by reacting zinc phosphate with inorganic ions or carboxylic groups of the metal resin used, and that the reaction with metal ions results in adhesion to the surface, cross-linking and the formation of inhibitor complexes. Based on the assumption that the hydrolysis process is a prerequisite for the effectiveness of zinc phosphates, this means that these pigments require a certain period of time before becoming active. This leads to the conclusion that zinc phosphates do not possess the well-known electrochemical effectiveness of chromate pigments. In the case of zinc phosphate, the formation of hydrolysis products depends on the permeability of the protective paint. The permeability of the protective paint itself is affected by the type of resin used and in particular by PVC (Pigment Volume Concentration). This means that the selection of resin, pigment and filler, and therefore the entire formulation, has a significant effect on the corrosion-protection function of protective paints containing zinc phosphate. Perhaps the proposed mechanism of action of zinc phosphate is more theoretical than fully proven knowledge. However, practical experience has shown that zinc phosphate is an active anticorrosive pigment, but the protective effect of lead and chromate pigments can only be achieved in some systems.

Modified Orthophosphates

As stated, although zinc phosphate has the desired properties as an anticorrosive pigment, it does not provide the degree of corrosion protection of lead and chromate pigments. For this reason, the pigment industry has focused on the development of phosphate-based pigments with enhanced properties. Through controlled chemical modifications, considering different perspectives with appropriate elements and compounds related to the optimization of production processes, it has been possible to increase zinc phosphate effectiveness in many applications. A general overview of modified orthophosphate pigments that have gained economic importance is given in Table 2. Looking at Table 2, it is evident that the pigment industry has developed various zinc phosphate variations to improve performance properties. These developments have been made possible by considering the effects of synergistic values. For example, the development of basic zinc phosphates is based on knowledge that as hydroxyl ion concentration increases, the local cathodic reaction balance will be stabilized and this will lead to the prevention or inhibition of electron emission. Additionally, a pH-balancing effect in the paint is also discussed depending on the presence of basic compounds in the pigment. The purpose of phosphate-borate combinations was to accelerate hydrolysis readiness because, as discussed, hydrolysis is a precondition for the effectiveness of zinc phosphates, but a certain time is required for the activation of these pigments.

The improved anticorrosive activity of zinc phosphate molybdates is attributed to the inhibiting effect of water-soluble molybdate ions.

Table 2: Orthophosphate-based anticorrosive pigments

An aluminum zinc phosphate is produced by precipitating primary aluminum phosphate (Al (H2PO4) 3) together with zinc oxide. These types of pigments have a higher phosphate content than standard zinc phosphate. The improved performance properties are attributed to this higher phosphate content. In organically treated modified phosphate pigments, it is discussed that there is a strong bond between the pigment and binder, as well as a strong bond between the paint and the surface. Another example is phosphate silicates, commonly referred to as mixed phase or core pigments. By fixing active components to the surface of wollastonite (calcium silicate core) and adjusting the pH value to near neutral, these pigments can be used relatively universally. Modified Polyphosphates During research for anticorrosive pigments with improved performance properties compared to zinc phosphate, the development of modified polyphosphate pigments was another focus point. Orthophosphates are produced using the reaction of orthophosphoric acid with basic and/or amphoteric substances. Polyphosphates are obtained through condensation of acidic orthophosphates at higher temperatures: • Orthophosphates e.g. 3 ZnO + 2H3PO4 → Zn3 (PO4) 2+ 3H20 • Polyphosphates e.g. Al (H2PO4) 3 → AlH2P3010. 2H2O Today, modified polyphosphate pigments of practical importance are predominantly reaction products of acidic aluminum tripolyphosphate with zinc, strontium, calcium and magnesium-based compounds (Table 3). Development activities together with polyphosphate pigments have focused on the high chelate-forming potential of acidic aluminum tripolyphosphate with metal ions.

Table 3: Polyphosphate-based anticorrosive pigments.

However, attempts have been made to enhance specific properties of certain products in one product (synergistic effects). Although the use of synergistic effects is not new to the pigment and paint industry, it has yielded interesting results in the field of anticorrosive pigments.

Recent research studies have reported that special synergistic effects can be observed when a phosphate-based pigment is used in combination with an organic corrosion inhibitor. Due to the use of such a combination, it is possible to see improved substrate protection during the initial stage of exposure to external conditions, which as a result means that improved long-term protection has been developed in special applications. As discussed earlier, the development of modified orthophosphates and polyphosphates is possible through the effects of synergies. The most important characteristic of best-modified phosphate-based pigments is their compatibility with a wide variety of resins. Although modified anticorrosive phosphates show broad compatibility, it is recommended to select pigments recommended by the pigment manufacturer for any given application. This is one of the reasons why the pigment industry is working on the development of phosphate-based pigments for universal applications. Since 2003, typical properties of two anticorrosive pigments derived from modified orthophosphates found on the market for universal applications are summarized in Table 4.

Table 4: Typical properties of two modified orthophosphate pigments for universal use

Other Phosphates

In recent literature, references have been made to other phosphate-based pigments such as barium phosphate, manganese phosphate, lead phosphate and chromium phosphate. The use of such products is limited due to economic considerations.

Other Phosphorus-Containing Pigments Zinc Hydroxyphosphite

A zinc hydroxyphosphite anticorrosive pigment is defined by the theoretical formula [2 Zn(OH) 2 · ZnHPO3] · xZnO, where x = 0-17. This pigment is produced by reacting zinc oxide slurry with phosphoric acid in the presence of a complex regulator zinc hydroxyphosphite. It is a white pigment with a basic structure. The effectiveness of this anticorrosive agent is attributed to the ability of phosphite ions to prevent anodic corrosion reactions by forming iron phosphite and iron phosphates. In addition, the ability to form zinc soap in oleoresinous resins that can also prevent corrosion is also relevant.

Iron Phosphite

Commercial iron phosphite anticorrosive pigments consist of Fe2P containing small amounts of FeP and SiO2. These pigments are metallic gray powders. They are defined as conductivity-enhancing pigments designed to partially replace zinc dust in zinc-rich organic and inorganic paints. When replacing up to 50% of zinc dust in a paint, weldability will be improved. The use of a certain amount of iron phosphite will lead to a reduction in price. However, when compared with the use of zinc dust and layered zinc flakes, iron phosphites are not seen to have as much economic importance in the market as might be expected. M.Namık Kayaalp - Chemical Engineer - Ecelak Boya Kimya Ltd. Şti.

References 1. Industrial Inorganic Pigments, Dr. Gunter Buxbaum, 47812 Krefeld Germany, Dr. Gerhard Pfaff, 64271 Darmstadt Germany, Revised Edition 2005 2. Smith, A,Inorganic Primer Pigments, Federation Series on Coating Technology, Philadelphia, PA 1988, 3. Austin, M. J.,Inorganic Anti-Corrosive Pigments, ASTM Manual 17, American Society for Testing and Materials, Philadelphia, USA, 1995, 4. Krieg, S.,European Coating Conference Proceedings, Anticorrosive Pigments, Berlin, 2000, 5. Adrian, G., Bittner, A., Grawol, M., Farbe + Lack 87 (1981) 6. Dr. Hans Heubach GmbH & Co. KG, Heucophos Program Overview, company information, Langelsheim, Germany, 2001 7. Halox, Product Application Guide, company information, Hammond, USA, 2004