The practical application of phosphate pretreatment processes

By Brian Carroll, BC Metal Finishing Services Ltd

Many articles have been written on the chemistry of phosphate conversion pretreatment processes. The purpose of this article is to illustrate the practical application of such processes and how they are used in industrial applications prior to powder coating and wet paint.

For over one hundred years phosphate conversion coatings have been used in industrial applications. These date back to the humble beginnings using phosphoric acid to the sophisticated products in present day use.

The application of phosphate pretreatment covers a diverse range, however, to concentrate on the practical application this article will focus primarily on the use of lightweight iron phosphates and zinc phosphates as a pretreatment for steel alone or steel, zinc and aluminium in mixed metal production prior to powder coating and wet paint.

The phosphate process
The use of phosphate pretreatment processes improves the adhesion and enhances the corrosion resistance when compared to solvent degreased substrates. The phosphate conversion coating is produced by the chemical reaction of the process solution with the metal surface. This results in the formation of an insoluble amorphous or crystalline phosphate coating on the metal substrate. When finished with an organic finish, the phosphate coating upgrades the overall system by:

  • Increasing the surface area, thereby promoting the adhesion of the applied organic film.
  • Providing a barrier to prevent possible chemical reaction between organic films and the metal substrate.
  • Insulating the metal surface, providing corrosion protection against physical, chemical and electrical attack.
  • Providing an ideal surface key.

Process plant
When installing pretreatment plant the choice is between immersion or spray application.

The main advantage of spray application techniques are faster process times owing to on line fully conveyorised systems.

Process requirements
These are as follows:

  • Degreasing
  • Pickling (if required)
  • Phosphating
  • Passivation
  • Dry off

Immersion plants
These are a series of tanks which can be serviced by baskets and a simple hoist, or by a fully programmed automated system. The basic requirements are:

Immersion phosphatic system to remove grease only

  1. Degrease
  2. Water Rinse
  3. Water Rinse
  4. Phosphate Treatment
  5. Water Rinse
  6. Passivation Seal
  7. Dry Off

Corrosion products are not removed in the above system and if present an acid pickle has to be introduced into the system.

Immersion phosphate system to remove grease and corrosion products

  1. Degrease
  2. Water Rinse
  3. Water Rinse
  4. Acid Pickle
  5. Water Rinse
  6. Water Rinse
  7. Phosphate Treatment
  8. Water Rinse
  9. Passivation Seal
  10. Dry Off Oven

When the amount of soil is minimal and the main consideration is the removal of corrosion products, the degreasing and pickling can be combined into one stage.

Combined immersion system to remove grease and corrosion products

  1. Combined Degrease Pickle
  2. Water Rinse
  3. Water Rinse
  4. Phosphate Treatment
  5. Water Rinse
  6. Passivation Seal
  7. Dry Off Oven

Spray plants
The original convention spray plant was known as a six stage plant with separate cleaner and phosphate stages which is shown below.

Six stage spray pretreatment plant

  1. Degrease
  2. Water Rinse
  3. Water Rinse
  4. Phosphate Treatment
  5. Water Rinse
  6. Passivation Seal
  7. Dry Off Oven

This plant will cater for both iron and zinc phosphates and must be used when zinc phosphating is required.

A process sequence developed from the above shows that the addition of a surfactant system to a light weight iron phosphate concentrate can effect both degreasing and phosphating in one operation by spray application.

This system although a compromise has the advantage of smaller pretreatment plant, which has proved popular in practice. It is not suitable for zinc phosphate processes.

Four stage spray pretreatment plant

  1. Degrease/Lightweight Iron Phosphate
  2. Degrease/Lightweight Iron Phosphate
  3. Water Rinse
  4. Passivation Seal
  5. Dry Off Oven

The general principle of this plant is that stage one does the bulk of the cleaning and stage two the phosphating. This plant can be further reduced by combining stages one and two as shown.

Three stage spray pretreatment plant

  1. Degrease/Lightweight Iron Phosphate
  2. Water Rinse
  3. Passivation Seal
  4. Dry Off Oven

The above plants are conventional and do not remove corrosion products. When heavily corroded products are to be phosphated these must be removed off line by either pickling or shot blasting. When light rust or oxide films are present they may be removed in the following system.

Spray pretreatment plant for removal of grease and corrosion products

  1. Degrease
  2. Water Rinse
  3. Water Rinse
  4. Derust
  5. Water Rinse
  6. Water Rinse
  7. Phosphate Treatment
  8. Water Rinse
  9. Passivation Seal
  10. Dry Off

If the amount of soils are light the degreasing and derusting can be combined to form one degrease/derust stage.

Summary of plant and process requirements
On both immersion and spray plants compromises can be made on the process sequences to reduce the number of stages. The best results are always obtained using separate cleaner and phosphate stages as this offers more flexibility.

Process chemicals


Emulsifiable cleaning
Used by immersion the components are soaked in the emulsifiable cleaner for up to five minutes. After soaking, the components are transferred to the two water rinse stages both of which are agitated and overflowing. This agitation rinsing action completes the degreasing cycle by emulsification of the oils.

The advantages of this system are that it is used at ambient temperatures, can be used on mixed metals and is easy to control. The disadvantages are the flammable nature and the high VOCs.

Alkali cleaning
This is the most popular and best accepted method of degreasing on pretreatment plants.

When used by immersion, ferrous components are immersed from times varying from five to fifteen minutes up to temperatures of 90°C over a wide range of concentrations. When used by spray the concentrations are lower and the combination of the impingement and choice of surfactant system allow the cleaner to operate at temperatures down to around 25°C on certain applications.

The choice of cleaner is critical when steel; zinc and aluminium are being processed in mixed metal production.

The term pickling is used to describe acid treatments for the removal of rust formed by atmospheric corrosion and scale formed by heat treatment processes. Essentially three acids or a combination are used to remove corrosion products which are phosphoric, hydrochloric and sulphuric acids all of which will remove rust but only sulphuric and hydrochloric will remove scale. They operate under the following conditions.

Phosphoric acid
Normally used by spray or immersion at concentrations from ten to fifty percent by dilution with water at temperatures from ambient to 70°C.

Hydrochloric acid
Normally used by immersion at fifty percent by volume with water at ambient temperature.

Sulphuric acid
Used by immersion at around ten percent dilution with water at temperatures around 60°C

Products are available that will degrease and pickle in one operation and consist of the above acids alone or in combination blended with solvents and surfactants.

The phosphates used prior to painting are usually lightweight iron phosphates conforming to BS 3189:1973 Type 4 or DEF STAN 03-11/1 Class IV or zinc phosphates conforming to BS 3189:1973 Type 3 or DEF STAN 03-11/1 Class III. The heavier zinc and manganese phosphates can and are over painted but it would be more conventional to finish these with an oil type finish.

The choice of phosphate mainly depends on the conditions of service required by the end product. If the application is for internal use or low to moderate exposure requirements, iron phosphates would normally be used. If the application is for external use or a hostile environment, zinc phosphate is the preferred process.

Lightweight iron phosphates
These processes can be applied to steel, zinc and aluminium in mixed metal production. Although no iron phosphate is produced on zinc or aluminium, the generic name is used to describe the process on all three metals.

The coatings produced from iron phosphate solution are amorphous in nature giving an iridescent appearance ranging from yellow through to blue. The conversion coating produced is very light, giving coating weights in the range of 0·2 to 0.8g per square metre, dependent on the type of process used.

The iron phosphate processes are very economical in use, easy to control, operate at low temperatures and do not present a major effluent problem.

Zinc phosphates
These processes are predominantly applied to steel but will also treat steel, zinc and aluminium in mixed metal production.

The coatings produced on steel are crystalline, grey in colour and extremely adherent. The conversion coating produced is light to medium, giving coating weight usually in the range 1·5 to 6·0 gms per square metre.

Some zinc phosphate processes are susceptible to harsh cleaning methods which can be overcome by using a conditioning agent in the rinse prior to phosphating. An alternative would be to use a calcium modified zinc phosphate which produces fine uniform coatings even after harsh cleaning.

In comparison with lightweight iron phosphates, zinc phosphates are slightly more expensive to operate, requiring closer control, form more sludge as a by product and may give rise to effluent problems.

The choice of phosphate pretreatment is usually clearly defined by the end use of the product. Whether lightweight iron or zinc phosphate are used, both are simple to operate, very economical in use and considerably upgrade the overall coating performance particularly when zinc phosphates are used.

After phosphating the components are rinsed in two stages of water the first cold and overflowing to prevent excessive contamination, the second which may be hot containing a passivation treatment. The use of a final demineralised water rinse is advisable, particularly in hard water areas. If the work is to be electrophoretically painted a final demineralised rinse is essential.

Passivation treatments
These post treatments which is an integral part of the process are applied in the final rinse stage. The passivation processes are used very dilute at temperatures from ambient to 70°C with process times between one and two minutes.

These treatments have a pronounced affect on the performance of the overall system. On accelerated corrosion tests, these treatments can enhance the performance of lightweight iron phosphates by up to one hundred percent and zinc phosphates by around 20%.

This final treatment on the pretreatment line is once again an integral part of the process and should not be neglected. Dependant on the nature of the work components are oven dried at temperatures from 100 to 150°C for five to ten minutes. The drying process can actually increase the performance of zinc phosphates under certain paint and powder finishes.

Plant construction
The ideal plant would be that manufactured essentially from stainless steel (Grade 316 S16). Due to economics this is usually the exception rather than the rule, hybrid construction being more favoured.

On pretreatment plants, with the exception of the pickling stage, zinc phosphate and spray nozzles, the construction may be mild steel throughout. The system can be upgraded by rubber lining the mild steel rinse tanks and using stainless steel for the phosphate and final rinse stages. The spray tunnel must be of stainless steel or coated mild steel construction.

The acid pickle stage must be manufactured from acid resistant materials. This is usually stainless steel for the phosphoric acid solutions and rubber lined or other suitable materials for hydrochloric acid and sulphuric acid solutions. Special consideration must be given to the materials used to heat the tanks. On spray plants the nozzles must be constructed from stainless steel or polypropylene.

The tanks may be heated by gas, electric or steam. The sighting of burners on immersion tanks is of paramount importance as process tanks must not be heated from the bottom. This disturbs the sludge which then deposits on the workpiece.

Present and future trends
Although the purpose of this article was to concentrate on practical application of traditional and well established coatings it would be remiss not to mention the present state of the industry.

In the well established products, zinc is a major constituent in the corrosion protection on steel and chrome on aluminium. Both these are heavy metals with their associated health and environmental problems. The aim for many years has been to replace these with safer and more environmentally friendly processes.

A recent break through in the replacement of zinc phosphates is the introduction of the organo silane products as a drop in replacement. These products form nanoscale ultra thin coatings which operate at ambient temperature and form little or no sludge as a by product. They can be used in the traditional plant, however, require demineralised water for make up and for rinsing prior and after treatment.

The replacement of chrome has already taken place in the products for use in the final rinse passivation treatments on the convention pretreatment lines. Chrome free products are also available to replace the traditional chromate conversion on aluminium and are now being actively promoted.

Phosphate pretreatment is used extensively throughout the metal finishing industry and it is hoped this article provides a clear concise guide as to what is required for the practical application of phosphate pretreatment processes.

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