By David Gecic
An iron phosphate is a chemical coating on the surface of a metal which increases the paint adhesion and the corrosion protection of paint and other coatings.
Coatings of this type are conversion coatings since part of the metal surface changes chemically to a different material. Unlike paint which covers a surface, a phosphate coating is part of the surface itself.
It leaves molecular roughness on the surface. A coating, such as paint or powder, adheres to the surface better. The change in surface profile gives the molecules of paint something to intertwine into. In addition, the chemical nature of the surface changes to give more corrosion resistance.
Use of phosphate coatings
There are different types of phosphate coatings currently in use. Iron, zinc, and manganese phosphates each have different applications. The most common type is iron phosphate. It is the most common pretreatment for painting steel. In a high production painting line it is very common.
Metal furniture, automotive parts, motor shells, any type of metal part may be treated with a phosphate coating prior to painting. Although used mostly on steel, there are also specialty products which will deposit a coating on aluminum, magnesium, zinc, and other metals.
Except in extremely special cases they are diluted with water. They are applied in a single-stage to multiple stage operations. They are applied by spray, immersion, or even hand wiping. Most large industrial operations use a multistage, often with an overhead conveyor that moves the parts from stage to stage. Large parts such as truck or tractor bodies are normally processed using a high-pressure hand held spray in a one or two stage operation.
Iron phosphate formations
Chemical solutions that deposit iron phosphate coatings are acidic. They produce coating through a multi-step process. First, the acid dissolves a small amount of metal from the surface. Second, the dissolved metal reacts with phosphate ions in the solution. Third, the acidity level drops because the acid has dissolved some of the metal, the iron and phosphate combination is no longer soluble. Fourth, the now insoluble iron phosphate deposits on the surface of the metal.
These reactions happen in a fraction of a second and within a few nanometers of the surface.
There is no detectable dimensional change to the surface, because the metal removed deposits back on the surface. The coating is thin.
Phosphate coating weight and thickness
An iron phosphate coating is usually measured in milligrams per square foot or grams per square decimeter. It usually deposits a coating which has a weight between 15 and 80 milligrams per square foot. Typically, about 30 milligrams per square foot is an effective amount of coating for good paint adhesion and corrosion protection. The coating is very thin. In order to have a pound of coating a piece of metal would be about 125 x 125 square feet.
Color
The color of the phosphate will vary depending on the type of accelerator used, the acidity level of the bath, and other factors. Some phosphate coatings are gray or pale blue at any coating thickness. Most common iron phosphates change from brown to blue to gold to a multicolored “rainbow” as the coating weight increases. The brown and gold color on the surface of the metal can sometimes look like rust but it is usually reflective instead of dull as rust would be. Generally a brown or blue color is sufficient for good paint adhesion.
Some of the higher quality coatings are grey colored. Traditionally these are used in multistage operations. Some of these products are strong oxidizing agents. Many these products do not have accelerators that are stable with organic materials. Because of this they cannot be used in a combination cleaner-phosphate product.
Single-stage operations
Coating larger objects works well in one-stage operations. These may be called pressure-wash phosphates, high-pressure phosphates, or hydromation. Hydromation refers to the movement of water. These are generally higher temperature and higher pressure processes, although there are many exceptions.
Normally parts are cleaning and coated in a single pass. The coatings produced provide excellent paint adhesion and sometimes good corrosion protection when done properly. One of the biggest problems in a stage phosphate operation is the management of flash rusting. Often improper concentrations or improper product selection leads to flash rusting. More information on Custom Designed Chemicals’ single stage phosphates.
Pressure washers are used for these operations. They have automatic injectors to mix the phosphate chemicals into the incoming water. They may be gas heated or electric heated. Some specialty operations may use room temperature water.
Using a rinse after a one stage operation is sometimes needed. They are really two stage operations. Only certain products can be used without a rinse afterwards. Very heavy soils may be cleaned in pressure wash operation, however they also need to be removed from the surface.
Three-stage operations
A three-stage operation generally has a first stage which is a cleaning and treatment stage. The second stage is a water rinse. The third stage is a seal rinse. A seal rinse is a chemical that gives extra corrosion protection or flash rust protection. It removes unreacted salts from the surface of the metal.
Three-stage operations are very common. They give very good paint adhesion. Most soils are effectively removed.
Five-stage Operations
A five stage operation separates the cleaning and coating stage. Therefore an alkaline cleaner may be used and a wider variety of soils may be cleaned. Heavier soiled parts can be cleaned quicker. The first stage is a cleaning stage. The second a rinse. The third a phosphate coating stage. The forth a rinse. The final stage is a seal rinse.
A separate cleaning stage gives a longer bath life. Therefor less maintenance is required. Chemical costs are sometimes lower. However they increase because a separate coating chemical is needed. You usually obtain better results in a five-stage than a three-stage.
Other Stages
There are many variations on these three models. For instance, a separate descaling stage may be used. A final deionized water rinse may be needed. In other words, these three models are just a guide.
The number of stages depends on several factors. Firstly, does it require a separate cleaner and coating stage? Secondly, are additional special cleaning stages needed? For example, is descaling or derusting required? Thirdly, is a rinse required? In addition are rinses required between these stages.
Of course, the bigger the washer is and the more stages it has the more expensive it is. Generally a separate cleaning and coating stage in more efficient than a combination stage.
Phosphate Selection
There are several factors that affect the choice of a phosphate. Certain metals such as aluminum and zinc do not develop a traditional phosphate coating, The coatings however do give better paint adhesion and corrosion protection over the bare metal.
The amount of cleaning ability of a phosphate product can vary from 0 to 25% by volume. A one-stage or three-stage phosphate cleans and coats at the same time. The amount of detergent will also vary with the amount of cleaning required. A five-stage phosphate does not require detergent. However, some five-stage products may still contain some detergent to wet the surface of the metal better.
Chemically, different phosphate coatings have different compositions. The ratios of the amount of iron oxide to iron phosphate varies. The amount of water that is bound into the molecule varies. There is chemically two types of iron, ferric and ferrous. The amount of these two ingredients must be managed in the phosphate bath. The accelerators used in phosphate formulations are the primary reason for composition differences between products. Therefore it is very important to get the proper type of phosphate for your application.