Anticorrosive Pigments in the Protection of Metal Surfaces

  Anticorrosive Pigments: The anticorrosive effect of chromate pigments is based on both chemical and electrochemical reactions. Electrochemical passivation and chemical reaction are shown below. Passivation is based on electrochemical processes in the cathodic region. Additionally, a protective film is formed by reacting chromate ions with metal ions on the substrate surface to form metal oxide hydrates. The anticorrosive properties of this pigment class depend on the following factors: • Content of water-soluble chromate ions, • Ratio of water-soluble chromate ions to water-soluble corrosion-promoting ions (chloride and sulfate ions), • Activity of the pigment surface such as particle size distribution in the paint and dispersion properties. Chromate-containing pigments are classified as toxic; therefore their use is highly restricted and they must be labeled appropriately. Zinc-Containing Anticorrosive Chromate Pigments Zinc Chromate: Zinc chromate is produced by reacting dissolved chromate ions with an aqueous zinc oxide or hydroxide suspension. It is then neutralized or precipitated by dissolving zinc salt with dissolved chromate salts. Zinc tetraoxychromate is produced from zinc oxide and chromic acid in an aqueous medium. Basic zinc potassium chromate is obtained by reacting an aqueous zinc oxide suspension with potassium dichromate and sulfuric acid. The pigments are washed, filtered, dried and ground. Strontium Chromate: Strontium chromate is produced by precipitating from solutions of sodium dichromate and strontium chloride. It is then filtered, washed, dried and ground. Lead Silicochromate: Lead silicochromate is an orange powder. This pigment is a core pigment with the active pigment compound (PbCrO4) precipitated onto an inert core (SiO2). Properties of lead silicochromate

Anticorrosive Molybdate Pigments

Molybdenum-based anticorrosive pigments are presented as a non-toxic alternative to zinc chromate pigments. All are white in color but pure compounds are very expensive. To produce economically competitive pigments, molybdate pigments are combined with phosphate pigments or inorganic fillers such as zinc oxide, alkaline-earth carbonates or talc can be added to the molybdate compounds. Phosphate-containing molybdate pigments are particularly suitable for water-reducible or latex-based binders because they improve adhesion to iron surfaces. Other molybdate pigments are mainly used in solvent-based binder systems. Unlike chromate ions in chromate pigments, MoO2−4 ions in molybdate pigments are not chemically reduced in most paints. Therefore they are ineffective for cathodic protection. It is assumed that their protective effect originates from their activity in the anodic region, similar to phosphate ions. Like protective phosphate films, molybdate films are highly resistant to chloride and sulfate. Maximum activity duration depends on the metal ions used in the pigment and is likely due to differences in solubility.

Anticorrosive Lead and Zinc Cyanamide

Lead cyanamide is a lemon-yellow powder. Zinc cyanamide is a powder ranging from beige to white. Cyanamides are active anticorrosive pigments with a passivating effect under alkaline conditions. Properties of lead and zinc cyanamide The main application areas for this type of anticorrosive pigment are mirror coatings, electrodeposition topcoats and primer coatings. Cyanamide pigments are first produced from industrial-grade calcium cyanamide which dissolves. Sulfite and phosphorus impurities are precipitated as iron or lead salts or oxidized and separated along with graphite impurities. Pure calcium cyanamide is reacted in an aqueous medium with soluble lead or zinc salts or lead oxide or zinc oxide suspension. The pigments are filtered, washed, dried and ground. Zinc cyanamide and pure lead cyanamide are not explosive. If an explosion occurs during lead cyanamide production, it can be caused by contamination with small amounts of acid or nitrate. Zinc cyanamide is not toxic, but the toxicological classification of lead cyanamide should be evaluated taking into account its lead content. Ion Exchanger Pigments: Ion exchanger pigments were developed as non-toxic alternatives to chromate pigments. They consist of a silicate carrier such as zeolite or amorphous silica gel with bound calcium ions. Commercial ion exchanger pigments have the following properties. Ion exchanger pigments should be classified as active anticorrosive pigments. They work by exchanging calcium ions for hydrogen ions in the paint film. This neutralizes acidic chemical substances. Calcium ions then bind to the metal oxide surface. These pigments have good anticorrosive properties due to their high pH values, but depending on the binder, there is a risk of pinholing on the surface. Anticorrosive Metal Oxide Pigments Red Lead: Red lead is a red powder that crystallizes tetragonally. It decomposes at approximately 500°C and atmospheric pressure. Red lead should be considered as the lead salt of orthoplumbic acid H4PbO4, that is lead (II) orthoplumbate Pb2PbO4 in which Pb (II) ions are bonded to PbO6 octahedra. Red lead is produced industrially by passing lead monoxide (PbO) through a stream of air at approximately 460-480°C for 15-24 hours. Most red lead is used in the glass, ceramic and accumulator industries where a bulk density of less than 2 g/mL is sufficient. However, for the paint industry, normally finely ground red lead is required, with a residue on 0.063 mm mesh sieve of less than 0.1% and bulk density of 1.3-2.0 g/mL. The electrochemical effect of red lead stems from lead having valence states 2 and 4 in orthoplumbate lead: Pb (IV) compounds are reduced to Pb (II) in the cathodic region. The chemical anticorrosive effect of this pigment results from lead soaps formed by the reaction of fatty acids in the binder with red lead. Lead soaps spread as thin layers in the paint film and provide good mechanical strength, water impermeability and adhesion to steel surfaces. Additionally, corrosion-producing chloride and sulfate ions are precipitated with lead (II) ions. Red lead is still used in corrosion prevention applications under severe environmental conditions, particularly for surfaces bearing residual rust traces. In water-based paints, red lead is not more effective than zinc phosphate. Calcium Plumbate: Calcium plumbate, Ca2PbO4, is a beige powder with a density of 5.7 g/cm3, formed by passing a mixture of lead monoxide and calcium oxide through a stream of air at approximately 750°C. The anticorrosive properties of calcium plumbate are lower than those of red lead. When water enters a primer containing calcium plumbate, calcium hydroxide, a hydrolysis product, is released. The pH value on the metal surface then rises to approximately 11-12. This helps prevent corrosion. The most important use of calcium plumbate is in primers used on zinc-coated surfaces. The pH change that occurs during hydrolysis of calcium plumbate, particularly on hot-dip galvanized steel surfaces, abrades the zinc surface, improving primer adhesion. Anticorrosive Zinc and Calcium Ferrites: Iron oxides are used as pigments or fillers in many paint formulations. They act only to a small extent as physically protective anticorrosive pigments. To obtain a chemically protective anticorrosive pigment with an active ingredient, iron oxide is heated with oxides or carbonates of alkaline-earth compounds (CaO, CaCO3) or with zinc (ZnO) to form ferrite-type pigments. In the paint, these pigments are hydrolyzed with water to form alkaline-earth hydroxides or zinc hydroxide, which increase pH and prevent corrosion. Alkaline-earth soaps are also formed in some binder resins. However, high pigment volume concentration is necessary for good results. Only zinc ferrite pigment has gained economic importance among these. Zinc Oxide: Zinc oxide is a white powder generally used together with active anticorrosive pigments. The inhibiting effect of zinc oxide is based on its ability to react with corrosive compounds and maintain alkaline pH in the paint. It also reacts with acidic compounds in the binder resin to form soaps and absorbs UV light. The lead content of commercial zinc oxide depends on its production and ranges from 0.002-1.5%. For zinc oxide to be considered lead-free, the lead content of the paint containing zinc oxide must be less than 1.5%.

Anticorrosive Powder Metal Pigments

Zinc Dust: Zinc dust is a free-flowing blue-gray powder consisting of spheroid-like particles. Metallic zinc is produced by melting in a pot, vaporized at approximately 900-950°C, then the product is condensed and screened. Alternatively, molten zinc is atomized by spraying from a nozzle to powder form and then screened. The effect of zinc dust in primers prepared with organic binders is based on sealing effects and electrochemical processes. Zinc reacts with atmospheric oxygen and water that has entered the binder to form zinc hydroxide, which is then neutralized by sulfuric acid from SO2 in the air and hydrochloric acid from chlorine-containing materials such as NH4Cl in the air. This causes an increase in volume and reduces permeability. Zinc corrosion products also have a rust-preventing effect. Cathodic protection occurs when zinc and iron come into contact; for this, the zinc content in the primer must be at least 94-96%. Zinc dust paints are used in large quantities in structural steels, particularly in underwater steel construction and shipbuilding. Zinc dust is also used in two-component inorganic binder systems such as alkaline silicates or alkyl silicates.

Lead Dust:

Lead dust is a dark gray powder containing approximately 99% metallic lead and 0.5% lead (II) oxide. It is produced by spraying molten lead and then screening. Due to the high surface area of particles sized 1-15 μm, it is prone to oxidation and is therefore supplied in airtight packages or as a paste. Lead dust can be used with many binders. This does not affect paint stability or viscosity. However, it is suitable for use with binders that absorb very little water, such as epoxy resins and chlorinated rubber. When formulating paints containing lead dust, care must be taken not to mix more than 5% by volume with other pigments and fillers in the formula. Lead dust paints are mainly used to provide protection against aggressive chemicals. They have high UV reflectivity and are extremely flexible. Lead dust pigments and pastes are also used in radiological protection.

Platelet-Shaped Anticorrosive Pigments:

They increase the barrier resistance of the paint against water and aggressive gases by increasing the length of the diffusion path into the paint. The interaction between pigment and binder should be as water-resistant as possible to prevent water from diffusing from the paint surface into the paint. In principle, all platelet-shaped lamellar minerals such as mica iron oxide, mica, wollastonite and talc can be used as barrier pigments. However, untreated mica and talc are not very suitable because they are quite permeable to water. The surface can be modified, for example with silanes or titanates, to reduce water permeability and improve adhesion. If the chemistry of the treated pigment surface is adapted to the functional groups of the binder resin, the pigment can also be used in water-based anticorrosive dispersions. Flake aluminum pigments with variable platelet thicknesses and shapes are used for corrosion protection. Their surfaces are coated with a water-repellent, oily film and are therefore particularly suitable for conventional solvent-based paint systems. They are resistant to an extraordinary degree to external environmental conditions. Flake-shaped zinc pigments provide a barrier effect and also act through a cathodic anticorrosive mechanism. Compared to zinc dust paints, flake-shaped zinc pigments are formulated with low pigment volume concentrations.

Anticorrosive Organic Pigments:

Unlike soluble anticorrosive inhibitors, organic anticorrosive pigments are sparingly soluble organic compounds or metallic salts of organic acids. They are used with binders instead of or in addition to inorganic anticorrosive pigments. These inhibitors are not suitable to replace the active anticorrosive pigments in paints. Organic anticorrosive pigments were developed to replace toxic, chromate-based anticorrosive pigments.

Zinc Salt of 5-Nitroisophthalic Acid:

It is produced as a pigment that can be easily dispersed through a wet chemical process from 5-nitroisophthalic acid and zinc oxide, with properties given below. A more cost-effective replacement for zinc chromate can be obtained by combining zinc salt of 5-nitroisophthalic acid pigment with zinc phosphate pigment. It is recommended to use at a rate of 0.5-2.0% based on total paint. The electrochemical anticorrosive effect of zinc salt of 5-nitroisophthalic acid is similar to that of zinc potassium chromate. Furthermore, this pigment is not toxic.

Other Organic Anticorrosive Pigments:

The following organic anticorrosive pigments are mentioned in the literature as well as being commercial products: 1) (2-Benzothiazolylthio) succinic acid, 2) Zinc mercaptobenzothiazole, 3) Basic zinc salt of A-benzenesulfonananilic acid These organic anticorrosive pigments are used in 0.5-2.0% ratio combined with zinc phosphate or other suitable anticorrosive pigments in the binder resin for better results. .