Anticorrosive Pigments in the Protection of Metal Surfaces

An organic paint system protects metal surfaces from corrosion through one or more mechanisms, which can be explained as follows:

• Protection against corrosion by reducing the rates of anodic (oxidation) and/or cathodic (reduction) partial corrosion reactions occurring at the metal/paint interface.

• Protection against corrosion by introducing high electrical resistance into the metal/electrolyte corrosion cell circuit caused by the non-conductive organic film applied to the metal surface.

• Protection against corrosion by acting as an effective physical barrier against the transport of water, such as the basis of electrolyte/moisture conditions, oxygen as an oxidizing agent, and aggressive species such as Cl–, SO4–2 ions that cause various types of corrosion to the metal surface.

Generally, the corrosion-inhibiting effectiveness of an organic primer paint film is evaluated as a function of four factors:

1. The structure of the metal surface, namely its susceptibility to oxidative corrosion. 2. The condition of the metal/primer interface. For example, properties such as electrical resistance and capacitance, and the nature of ions or molecules present in this region. 3. The composition of the primer paint and the effectiveness of corrosion-inhibiting pigments and other inhibitors. 4. The corrosion resistance of the environment and the characteristic nature of the environment to which the painted metal is exposed.

Metal corrosion and related electrochemical theory can be addressed as a separate topic. This section pertains to the protection of metal surfaces from corrosion through paints containing corrosion-inhibiting pigments. The degree of corrosion protection depends not only on the pigment, but also on the binder, and they must complement each other chemically. Anticorosive pigments can be divided into three types:

1) Pigments with Physical Protective Effect (Passive Anticorosive Pigments):

These are chemically inert and are called inactive or passive protectants. Micaceous iron oxide can be given as an example. These lamellar pigments are present as platelets in the paint. They prevent ions from the surface from spreading inward through the film by lengthening their paths. They improve the adhesion between the substrate and paint, absorb UV radiation and thereby protect the underlying binder.

2) Pigments with Chemical Protective Effect (Active Anticorosive Pigments):

This type of pigment contains soluble components and can maintain a constant pH value in the paint. They are called active. Their effects depend on reactions at the interface between the pigment and substrate, between the pigment and binder, or between the pigment and ions dispersed in the paint. To give an example, red lead can be cited. Redox reactions may occur to form protective compounds such as oxides or oxide hydrates that can contain pigment cations. Saponification of the binder or neutralization of acidic decomposition products ensures the maintenance of a certain pH value in the paint.

3) Pigments with Electrochemical Protective Effect

This type of pigment passivates the metallic surface. Like phosphate pigments, they prevent iron corrosion by forming a protective coating. These are considered active in the anodic region of the metal surface (anodic protection). It is assumed that pigments like chromates, which prevent rust formation due to their high oxidation potentials, are active in the cathodic region (cathodic protection). Iron corrosion is explained by the position of iron in the electrochemical series of elements (Fe / Fe2+:—0.44 V). In steel, for example, due to the presence of phases containing carbon, carbides and oxides, there are local anode and cathode areas. These hidden local cells are activated by moisture, oxygen and current-carrying electrolytes, and the following reactions occur between anode areas consisting of iron and cathode areas containing carbides or oxides. Rust can cause further corrosion. The OH– ions formed at the cathode produce high local alkalinity and cause hydrolysis of esterified binders resulting in delamination of the primer from the substrate. Active anticorosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and primer. Water dispersed in the binder dissolves soluble anticorosive components such as phosphates, borates or organic anions formed from pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on iron is thereby strengthened and sometimes chemically modified. Damaged areas are repaired with the aid of active substances. The formation of a protective film with inhibition is the most important mode of action of common anticorosive pigments. Phosphate Pigments: The most important phosphate-containing pigments are listed below.

Zinc phosphate,	Barium phosphate,
Basic zinc phosphate,	Aluminum zinc phosphate;
Chromium phosphate,	Zinc / iron phosphate.
Aluminum triphosphate,

  The phosphate ions here form protective coatings of basic iron (III) phosphate on the iron surface. The reactions of phosphate pigments in binders and on metal surfaces have been described. It is assumed that basic complexes consist of phosphate pigments containing crystallization water and react with inorganic ions or carboxyl groups of the binder. The reaction of the pigment with the binder results in physical changes such as loss of adhesion or gelation. In paints containing hydroxy and carboxy functional binders, gelation during storage can be prevented by using excessive amounts of alcohol as solvent.

Zinc Phosphate:

The most important phosphate-containing pigment is zinc phosphate. It is defined by the formula Zn3(PO4)2·4H2O. It can be used with many binders and has a very wide range of applications. Zinc phosphate is typically produced on an industrial scale from zinc oxide and phosphoric acid or from zinc salts and phosphates. The mechanism of action of zinc phosphate is shown below. The zinc phosphate dihydrate pigment hydrates to tetrahydrate in an alkyd resin binder. The tetrahydrate then hydrolyzes to form zinc hydroxide and secondary phosphate ions that form a protective basic iron (III) phosphate film on the iron surface. The anticorosive effect of zinc phosphate depends on particle size. Micronization improves anticorosive properties.

Aluminum Phosphate:

Commercial aluminum phosphate anticorosive pigments consist of aluminum zinc phosphate hydrates or zinc-containing aluminum triphosphate. Their composition and properties can be seen in the table. Aluminum zinc phosphate hydrate pigments are produced by reacting acidic solutions of aluminum hydrogen phosphate with zinc oxide and alkali aluminate. The precipitated pigment is filtered from the mother liquor, washed, dried and ground. Commercial aluminum triphosphate pigments contain trimeric phosphoric acid ions that form stable aluminum-containing iron phosphate complexes. Aluminum phosphate pigments ensure good adhesion of the paint film to the metallic surface.

Property	Aluminum Triphosphate-1	Aluminum Triphosphate-1	Hydrated Aluminum Zinc Phosphate
Al Content, %	5.5-7.7	4.7-6.9	4-5
Zn Content, %	11.6-14.9	21.3-24.5	35-39
P2O5 Content, %	42.0-46.0	36.0 - 40.0	30/37
SiO2 Content, %	13.0-17.0	11.0-15.0
Ignition Loss (600°C)	Approx. 10%	Approx. 8%	10-16%
Density, g/cm3	3.0	3.1	3.2
Oil Absorption, g/100 g	35	30	30/35
Particle Size, µm	0.5-10	0.5-10	0.5-10
pH Value	6-7	6-7	6.5
Water-Soluble Content	max. 1.0%	max. 1.0%	0.1%
Color	White	White	White

 

Composition and properties of anticorosive aluminum phosphate pigments

Chromium Phosphate: Chromium phosphate CrPO4·3H2O is produced from chromium (III) salts and alkali phosphates. Physical and chemical properties are listed in the table below. Chromium phosphate has low solubility. Therefore, it is almost always used in combination with other anticorosive pigments. It is an effective long-term good inhibitor, but is less effective in the initial stage of corrosion protection.

New Anticorosive Pigments Based on Metal Phosphates:

Developments in the field of phosphate pigments aim to address deficiencies such as moderate corrosion protection of phosphate during the initial stage of corrosion protection against accelerated air conditions in salt spray testing. Improvements include the substitution of zinc cations by calcium to form new structures, modification of anions such as phosphovanadates and phosphocarbonates, optimization of particle size distribution, use of zinc ions and phosphate ions in separate compounds, and the use of synergistic effects.

Multiphase Phosphate Pigments:

Multiphase anticorosive phosphate pigments are prepared as suitable combinations of inorganic phosphates. They are low-solubility and electrochemically active organic corrosion inhibitors. Multiphase anticorosive pigments are produced in a combined synthesis. A precipitated zinc phosphate slurry is intensively mixed with zinc or calcium salt of 5-nitroisophthalic acid, filtered and washed and ground until the specified conductivity is reached. These are more effective than physical mixtures of this type of pigment produced by separate processes. When the anticorosive effect of pure zinc phosphate is insufficient in the initial stage of external durability such as salt spray testing, multiphase phosphate pigments are used instead of zinc chromate. The anti-corrosive effect of multiphase pigments is very good at a level comparable to zinc chromate in air-drying alkyd resin paints or alkyd-melamine oven enamels, even in open air conditions. Commercial multiphase anticorosive pigments differ primarily in the solubility of the organic corrosion inhibitor. Inhibitor systems with higher solubility are used with binders having low swelling capacity, such as epoxy resins or chlorinated rubber.

Other Phosphorus-Containing Pigments

Iron Phosphite: Commercial iron phosphite anticorosive pigments generally consist of Fe2P with small amounts of SiO2 and FeP. Iron phosphite anticorosive pigments are recommended by manufacturers as a replacement for zinc dust to reduce the cost of zinc-rich paints. Epoxy zinc-rich primer paint or topcoat paint, inorganic zinc silicate primer or topcoat paint, or inorganic zinc silicate coating, are used to make corrosion-resistant and in the production of conductive paints. The trade name for iron phosphite is Ferrophos.

Properties of iron phosphite

Zinc Hydroxyphosphite: Commercial zinc hydroxyphosphite is a white, non-toxic pigment. As an anticorosive pigment, zinc hydroxyphosphite is mostly used in air-drying alkyd-based primers, 2-component epoxy primers, chlorinated rubber primers and water-based polymer dispersion paints. The properties of the pigment are given below.

Properties of zinc hydroxyphosphite pigment

Borosilicate Pigments: Borosilicate pigments are pigments generally containing calcium or zinc ions in a silicon dioxide and boron trioxide matrix as x(Ca, Zn)·ySiO4·zBO3. The aqueous slurries of this white pigment are alkaline. Borosilicate pigments are proposed as non-toxic alternatives to basic lead silicate. The main application area is water-based binders and electrodeposition paints. Commercial product calcium borosilicate properties are as follows.

Properties of calcium borosilicate pigment

Borate Pigments: Barium metaborate and zinc borophosphate are colorless pigments. Both are relatively highly soluble in water. Barium metaborate is coated with silica to reduce solubility. The anticorosive effects of these pigments depend primarily on their ability to maintain high pH values in the paint. They are the most effective anticorosive pigments in the initial stage of corrosion protection. Borate ions neutralize acidic foreign ions and binder decomposition products. However, it is also assumed that they act as an anodic passivator forming a protective film. The effectiveness of metal borate pigments has also been established in water-based air-drying anticorosive paints. Metal borate pigments are classified as low-toxicity pigments. Barium metaborate as an anticorosive pigment is mostly used in air-drying alkyd-based primers, alkyd-melamine oven primers and aqueous polymer dispersion paints. Zinc borophosphate as an anticorosive pigment is used in air-drying alkyd-based primers, 2-component epoxy primers, aqueous polymer dispersion paints and PVB primers.

Properties of metal borate pigments

M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.    

References 1)ICL Advanced Additives, a division of ICL Specialty Products Inc. 2018 2)Industrial Inorganic Pigments, Dr. Gunter Buxbaum, 47812 Krefeld Germany, Dr. Gerhard Pfaff, 64271 Darmstadt Germany, Revised Edition 2005 3)Smith, A. Inorganic Primer Pigments, Federation Series on Coating Technology, PA 1988, 4)Paint Formulation, J.Boxall, G.Godwin Ltd.-London 1980 5)Organic Coating Technology-II, H.F.Payne, J.Wiley and Sons Inc.-Newyork 1960 6)Oil, Colours Chemists'Association-London 1966 7)Ullmann's Encyclopedia of industrial Chemistry, H. Wienand, Vol. A20, Sec. 3.3, VCH Inc. 8)Paint and Coating Testing Manual 15. Edition of the Gardner-Sward Handbook, Joseph V. Koleske,