Anticorrosive pigment

As the main part of this section deals with anticorrosive pigments as such, we shall not go into more detail on the discussion of corrosion processes in this section. However, it should be mentioned that one key feature of the electrochemical process of corrosion is the presence of an electrolyte. This electrolyte is made up of rain, snow and dew, leading to ionogenic surface contamination. The electrolyte contains material coming from airborne pollution, such as dirt, water-soluble chloride and sulfate salts. Chlorides and sulfates are well known as so-called corrosion stimulators, which have a distinct influence on the course of the corrosion reactions [5.51, 5.54]. 

Anticorrosive pigments can affect the performance of protective coatings in several ways, such as 

Anticorrosive pigments may be classified by their mode of action, for example chemical and/or electrochemical (Active Pigments), physical (Barrier Pigments), and electrochemical/ physical (Sacrificial Pigments) [5.54—5.56]. 

Active pigments are anticorrosives with a chemical and/or electrochemical action. They are also described in the literature as inhibitive pigments. These pigments interact chemically, either directly or via intermediates with the metal substrate to reduce the rate of corrosion. Such intermediates can, for example, be formed by reaction with the resin system. 

Another mechanism of active pigments is neutralization of corrosive substances such as sulfates, acids and chlorides resulting in maintaining a constant pH value in the coating. 

Apart from optical properties such as color and hiding, certain pigments are used in protective coatings to increase their corrosion resistance properties. The anticorrosive pigments used in conjunction with suitable binder in protective coatings systems play a major role in protection of metals from corrosion. 

Based on their mode of action to prevent corrosion, anticorrosive pigments may be classified in the following types  

Active pigments (inhibitive pigments) These are anticorrosive pigments with chemical and/or electrochemical action. When water penetrates the coating film, these pigments release soluble passivating ions that help in corrosion protection by promoting the 

Coating with lamallar pigment Corrosive species 

Some commercially important barrier pigments are micaceous iron oxide (Section 3.3.3.1) and aluminum flakes (Section 3.3.5.1). The only 

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A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

The pigment is the principal agent in the electrochemical control of corrosion by primers (see Section 14.3). Probably the best known anticorrosive pigment is red lead. When used in conjunction with linseed oil as the binder it gives very good primers which will perform well over relatively poorly prepared (manually abraded) steel surfaces. Present-day use of red lead (and lead pigments, generally) in paints has been drastically curtailed as a result of understandable pressure from the environmentalists. 

The search for new, effective anticorrosive pigments with low toxicity to replace red lead and chromates in paints has occupied the attention of many paint-making companies recently. Barium metaborate, calcium molybdate and zinc molybdate have been identified as possible compounds but they have not found general acceptance in the United Kingdom and western Europe, most probably because of their lower cost effectiveness. 

Anticoccidial activity, of polyether antibiotics, 20 135-136 Anticoccidials, 20 135—136 Anticorrosion agents, molybdenum compounds in, 17 39 Anticorrosion coatings, organic titanium compounds in, 25 134 Anti-corrosion metallic coatings, 1 713-714 Anticorrosion pigments, 19 411 Antidegradants... 

Unfortunately some corrosion stimulants, like Cl, SO or NO,, strongly oppose inhibition by anticorrosive pigments and inhibitors ( ). Steel corrodes in saturated aqueous solutions of an anticorrosive pigment in presence of small amounts of these stimulants, e.g. 1% w/w NaCl is sufficient to make a saturated aqueous extract of zinc chronate corrosive (7). Therefore, irrespective of environmental requirements, the usefulness of active anticorrosive pigments and inhibitors as well has beoome questionable. 

The anticorrosive pigments zinc chromate, zinc tetraoxychromate, and strontium chromate are described in Section 5.2.6... 

This chapter deals with pigments that produce special effects magnetic pigments (Section 5.1), anticorrosive pigments (Section 5.2), luster pigments (Section 5.3), transparent pigments (Section 5.4), and luminescent pigments (Section 5.5). 

This chapter deals with the protection of metal surfaces against corrosion by means of coatings that contain anticorrosive pigments. The degree of corrosion protection depends not only on the pigment, but also on the binder, and these must complement each other chemically. The uses of anticorrosive pigments in paint binders are summarized in Table 39. For standards, see Table 1 ( Corrosion testing )-... 

Table 39. Possible uses of anticorrosive pigments in various primers... 

Anticorrosive pigment Air- drying alkyd primers Alkyd- melamine primers 2-Compo- nent epoxy primers 2-Compo-nent acrylic isocyanate primers Chlor- inated rubber primer Aqueous polymer dispersions Poly(vinyl- butyral) primers Electro- deposition coatings Mirror coatings References... 

Anticorrosive pigments may be divided into three types [5.53] ... 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

Table 41. Composition and properties of aluminum phosphate anticorrosive pigments...

Chromium phosphate has a low solubility. It is therefore nearly always used in combination with other anticorrosive pigments. It is an extremely good long-term inhibitor, but is less effective during the initial phase of corrosion protection. 

The compositions and properties of some commercial multiphase anticorrosive pigments are listed in Table 43. Their anticorrosive properties in various binders are shown in Table 39. 

Iron Phosphide. Commercial iron phosphide anticorrosive pigments usually consist of Fe2P [12751-22-3], Mr 142.7, with traces of FeP and Si02. The pigment is a powder with a metallic gray color and contains 70 % Fe, 24 % P, 2.5 % Si, and 3.0 % Mn. The density is 6.53 g/cm3 and the mean particle size is ca. 3 5 pm. 

Iron phosphide anticorrosive pigments are recommended by manufacturers as replacement materials for zinc dust to reduce the price of zinc-rich paints [5.103]. For toxicological data, see Section 5.2.14. 

Molybdenum-based anticorrosive pigments offer a nontoxic alternative to the zinc chromate pigments [5.120], They all have a neutral color (white) but the pure compounds are very expensive. To produce economically competitive pigments molybdate and phosphate pigments are combined, or molybdate compounds are applied to inorganic fillers (e.g., zinc oxide, alkaline-earth carbonates, or talc) [5.75], [5.121]—[5.123]. 

The cyanamides are active anticorrosive pigments which have a passivating action under alkaline conditions. The action of lead in anticorrosive pigments is discussed in Section 5.2.10.1. Heavy metal oxides, especially of iron and manganese, catalyze the conversion of cyanamide to urea even below 20 °C [5.126] ... 

Ion-exchange pigments should be classified as active anticorrosive pigments. They act by exchanging their calcium ions for hydrogen ions in the paint film. In this way... 

Zinc oxide [1314-13-2], ZnO, Mr 81.37, is a white powder that is usually used in combination with active anticorrosive pigments. For a detailed description, see Section 2.3. It has the following physical properties ... 

Other Organic Anticorrosive Pigments. The following organic anticorrosive pigments are either commercial products or have been reported in the literature ... 

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