Phosphate conversion coating
Encyclopedia
Phosphate coatings are used on steel
parts for corrosion
resistance, lubricity
, or as a foundation for subsequent coatings or painting. It serves as a conversion coating
in which a dilute solution of phosphoric acid and phosphate salts is applied via spraying or immersion and chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates.
Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin.
The main types of phosphate coatings are manganese
, iron
and zinc
. Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for rust proofing (P&O), a lubricant base layer, and as a paint/coating base and can also be applied by immersion or spraying.
and takes advantage of the low solubility of phosphates in medium or high pH solutions. Iron, zinc or manganese phosphate salts are dissolved in a solution of phosphoric acid. When steel or iron parts are placed in the phosphoric acid, a classic acid
and metal
reaction takes place which locally depletes the hydroxonium (H3O+) ions, raising the pH, and causing the dissolved salt to fall out of solution and be precipitated on the surface. The acid and metal reaction also creates iron phosphate locally which may also be deposited. In the case of depositing zinc phosphate
or manganese phosphate the additional iron phosphate is frequently an undesirable addition to the coating.
The acid and metal reaction also generates hydrogen
gas in the form of tiny bubbles that adhere to the surface of the metal. These prevent the acid from reaching the metal surface and slows down the reaction. To overcome this sodium nitrite
is frequently added to act as an oxidizing agent
that reacts with the hydrogen to form water. This chemistry is known as a nitrate accelerated solution. Hydrogen is prevented from forming a passivating layer on the surface by the oxidant additive.
The following is a typical phosphating procedure:
The performance of the phosphate coating is significantly dependent on the crystal structure
as well as the weight. For example, a microcrystalline
structure is usually optimal for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil, however, may be the most desirable for wear resistance. These factors are controlled by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, concentration, and phosphating time. A widely used additive is to seed the metal surface with tiny particles of titanium salts by adding these to the rinse bath preceding the phosphating. This is known as activation.
.
Most phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation. The porosity allows the additional materials to seep into the phosphate coating and become mechanically interlocked after drying. The dielectric nature will electrically isolate anodic and cathodic areas on the surface of the part, minimizing underfilm corrosion that sometimes occurs at the interface of the paint/coating and the substrate.
Zinc phosphate coatings are frequently used in conjunction with sodium stearate
(soap
) to form a lubrication layer in cold and hot forging. The sodium stearate reacts with the phosphate crystal which in turn are strongly bonded to the metal surface. The reacted soap layer then forms a base for additional unreacted soap to be deposited on top so that a thick three part coating of zinc phosphate, reacted soap and unreacted soap is built up. The resulting coating remains adhered to the metal surface even under extreme deformation. The zinc phosphate is in fact abrasive and it is the soap which performs the actual lubrication. The soap layer must be thick enough to prevent substantial contact between the metal forming dies and phosphate crystal.
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
parts for corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
resistance, lubricity
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...
, or as a foundation for subsequent coatings or painting. It serves as a conversion coating
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
in which a dilute solution of phosphoric acid and phosphate salts is applied via spraying or immersion and chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates.
Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin.
The main types of phosphate coatings are manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for rust proofing (P&O), a lubricant base layer, and as a paint/coating base and can also be applied by immersion or spraying.
Process
The application of phosphate coatings makes use of phosphoric acidPhosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
and takes advantage of the low solubility of phosphates in medium or high pH solutions. Iron, zinc or manganese phosphate salts are dissolved in a solution of phosphoric acid. When steel or iron parts are placed in the phosphoric acid, a classic acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
reaction takes place which locally depletes the hydroxonium (H3O+) ions, raising the pH, and causing the dissolved salt to fall out of solution and be precipitated on the surface. The acid and metal reaction also creates iron phosphate locally which may also be deposited. In the case of depositing zinc phosphate
Zinc phosphate
Zinc phosphate is an inorganic chemical compound used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment . Zinc phosphate coats better on a crystalline structure than bare metal, so a seeding agent is often used as a...
or manganese phosphate the additional iron phosphate is frequently an undesirable addition to the coating.
The acid and metal reaction also generates hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas in the form of tiny bubbles that adhere to the surface of the metal. These prevent the acid from reaching the metal surface and slows down the reaction. To overcome this sodium nitrite
Sodium nitrite
Sodium nitrite is the inorganic compound with the chemical formula NaNO2. It is a white to slight yellowish crystalline powder that is very soluble in water and is hygroscopic...
is frequently added to act as an oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
that reacts with the hydrogen to form water. This chemistry is known as a nitrate accelerated solution. Hydrogen is prevented from forming a passivating layer on the surface by the oxidant additive.
The following is a typical phosphating procedure:
- cleaning the surface
- rinsing
- surface activation (in some cases)
- phosphating
- rinsing
- neutralizing rinse (optional)
- drying
- application of supplemental coatings: lubricants, sealers, oil, etc.
The performance of the phosphate coating is significantly dependent on the crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
as well as the weight. For example, a microcrystalline
Microcrystalline
A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination.-See also:* Macrocrystalline* Microcrystalline silicon* Protocrystalline* Rock microstructure...
structure is usually optimal for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil, however, may be the most desirable for wear resistance. These factors are controlled by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, concentration, and phosphating time. A widely used additive is to seed the metal surface with tiny particles of titanium salts by adding these to the rinse bath preceding the phosphating. This is known as activation.
Uses
Phosphate coatings are often used to provide corrosion resistance, however, phosphate coatings on their own do not provide this because the coating is porous. Therefore, oil or other sealers are used to achieve corrosion resistance. This coating is called a phosphate and oil (P&O) coating. Zinc and manganese coatings are used to help break in components subject to wear and help prevent gallingGalling
Galling usually refers to the adhesive wear and transfer of material between metallic surfaces in relative converging contact during sheet metal forming and other industrial operations....
.
Most phosphate coatings serve as a surface preparation for further coating and/or painting, a function it performs effectively with excellent adhesion and electric isolation. The porosity allows the additional materials to seep into the phosphate coating and become mechanically interlocked after drying. The dielectric nature will electrically isolate anodic and cathodic areas on the surface of the part, minimizing underfilm corrosion that sometimes occurs at the interface of the paint/coating and the substrate.
Zinc phosphate coatings are frequently used in conjunction with sodium stearate
Sodium stearate
Sodium stearate, or sodium octadecanoate, is the sodium salt of stearic acid. It is the major component of some types of soap, especially those made from animal fat. It is found in many types of solid deodorants, rubbers, latex paints, and inks. It is also a component of some food additives and...
(soap
Soap
In chemistry, soap is a salt of a fatty acid.IUPAC. "" Compendium of Chemical Terminology, 2nd ed. . Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford . XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN...
) to form a lubrication layer in cold and hot forging. The sodium stearate reacts with the phosphate crystal which in turn are strongly bonded to the metal surface. The reacted soap layer then forms a base for additional unreacted soap to be deposited on top so that a thick three part coating of zinc phosphate, reacted soap and unreacted soap is built up. The resulting coating remains adhered to the metal surface even under extreme deformation. The zinc phosphate is in fact abrasive and it is the soap which performs the actual lubrication. The soap layer must be thick enough to prevent substantial contact between the metal forming dies and phosphate crystal.