Phosphating Basics as a Pretreatment

A Guide to Phosphatizing Processes

Industry has long relied upon phosphatizing as an integral part of a variety of finishing applications. Most commonly, a phosphate surface provides a base for subsequent coating (such as paint, oil, soap or wax), that further enhances adhesion, corrosion protection, or lubricity on the finished part. Preconditioning a part with a BroCo phosphate is an important step in obtaining quality performance for a finished surface.
The following discussion provides a brief overview of the marketplace for phosphate conversion coatings.

Steel surfaces have microscopic peaks and valleys that are electrically conductive and susceptible to corrosion. The phosphatizing solution reacts with the part, converting the metal surface into a layer of crystalline or amorphous metal phosphate. The resultant phosphate layer is tightly bound and inhibits oxygen attack. The phosphate covers the peaks and valleys making the surface uniform and non-conductive. The most common phosphates are iron, zinc, and manganese. The parts’ end use function, aesthetic demands, and cost, dictate the appropriate phosphate finish. Iron phosphates are almost exclusively used as pretreatment for painting. Painted items requiring more corrosion protection can be coated in either fine grain or microcrystalline zinc phosphate. Heavier zinc phosphate coatings are more commonly used as bases for oils, paints, and lubricants to increase the film retained on the surface, and thereby the subsequent performance of the product. Manganese phosphates find similar application to zinc phosphates, but are commonly used where parts must rely on the phosphate (along with a subsequent coating) for surface lubrication during the initial wear period.

All BroCo phosphatizing products are marketed as powerful concentrates. Surface activators, accelerators and stabilizing agents are blended with metal salts to provide the highest quality formulations. BroCo phosphates have achieved recognition for ease of startup and ongoing bath operation, with minimal operator maintenance.

The following provides a quick overview of the difference between iron, zinc, and manganese phosphates:



Generally iron phosphates will produce 25 to 100 mg/ft2 coating weight. The surface develops a visible color on the metal that may range from an iridescent blue/gold to a bluish brown, or uniform gray. Measuring coating weights, salt spray results (accelerated corrosion tests), and crosshatch tests on finished parts, are a few of the most commonly used quality performance criteria. Iron phosphates are excellent for paint and powder coat adhesion. Commonly, iron phosphates combine cleaning and phosphating, but it is always best to preclean with an alkaline cleaner.

Iron phosphates are generally used on products where paint attractiveness, adhesion, and corrosion protection, at the lowest possible cost, are the primary considerations. Examples of surfaces commonly pretreated with iron phosphate coatings are: office furniture, metal wall partitions, and file cabinets. When compared with other phosphate coatings or spray “priming” surfaces, the cost of iron phosphate chemistry, equipment, and operational maintenance is very low.

When paints and powder coatings are to be applied, a pretreatment is necessary for consistent quality performance. While paint and powder can be applied directly to metal surfaces, the resultant adhesion, corrosion protection, and future rust confinement are not acceptable. These coatings adhere better and retain the desired appearance for a longer period of time when a phosphate pretreatment is applied. If you have ever noticed a scratch on a car that goes through to bare metal, you will see the rust formation is confined to a limited area around the scratch. This damage containment is what the phosphate does for the painted surface to prevent the creeping underlayer rust.

In many instances, BroCo iron phosphates can be used to clean and phosphate at the same time. The iron phosphate coatings may be applied in 3 or 5 stage (clean/iron phosphate-rinse-seal or clean-rinse-iron phosphate-rinse-seal), spray or soak systems. Process equipment may be of carbon steel, or other materials suited for the operating temperature of 120 to 130oF. In some instances, the operating temperature may be as low as 90oF. To maintain consistent quality finishes, control of the acid concentration and pH is all that is necessary.

BroCo iron phosphates are available for use on steel, iron, zinc, zinc plated, galvanized steel, aluminum, and cadmium plated surfaces. Waste treatment is usually a simple neutralization process; however, it is best to discuss this with local authorities to be sure your disposal process is in compliance with associated restrictions. As noted above, the final stage after phosphating is often referred to as a seal. The seal is actually a final rinse with additives to further improve surface treatment. These seal additives are discussed in the “BroCo Post Phosphate Seals” section in the
following pages.


Parts to be finished are often too large to move around or to be processed in an immersion tank. In these instances, preparation of the part is best done with equipment employing a steam, or high pressure heated phosphate solution. Most often this “manual spray” processing is completed in a single step. Spray concentrations of between 1/2% to 3% solution at the surface should be sufficient to clean and iron phosphate. BroCo 206M, BroCo 207M and BroCo 207M-II are three of the most frequently used materials. One must keep in mind that the quality of phosphate is as good as the operator, since he controls the amount of time the spray is in contact with the part. The coating weight of the phosphate can vary significantly, but the goal is to produce a clean part that has a consistent coating of iron phosphate. This process is inexpensive to setup but may be labor intensive — it takes a good, conscientious operator to do a quality job with this method.



These phosphates produce a grey, fine grained compact crystalline structure and are typically referred to as light or microcrystalline zinc phosphates. Generally, the coating weights vary from 100 to 600 mg/ft2. These coatings provide excellent surface preparation for maximum corrosion resistance and adhesion under paint or powder. With these coating weights you can easily scratch the surface with your fingernail to
identify the existence of a metal phosphate layer.

This type of phosphate is applied when extended corrosion protection and adhesion are required because of outdoor exposure conditions, or when the atmosphere the part is to be exposed is such that additional corrosion protection is demanded. Examples whereas fine grained zinc phosphate may be used are car bodies, lawn mowers, outdoor furniture, and washing and drying machines.

Operation of a zinc phosphate bath is typically more complex than that of an iron phosphate. Total and free acid (and sometimes iron levels) are analyzed to assure proper ratios are maintained. In some instances (particularly spray applications), several different chemicals are used to maintain the bath. When compared to iron phosphates, zinc phosphates produce considerably more phosphate coating weight on the surface, and therefore much more sludge. Consequently, equipment maintenance becomes critical. Desludging of the tank, as well as cleaning nozzles and heating equipment, are regular maintenance items.

This coating produced is much more complex and heavier than an iron phosphate. For purposes of prepaint treatment on finished product, coating weights in excess of 250mg/ft2 are not uncommon. Typically, spray systems produce 150-250 mg/ft2, and immersion systems produce 300-500 mg/ft2. As a result of this, zinc phosphates absorb more paint and help the paint firmly adhere to the surface. Zinc phosphate should not be done in less that five stages (clean-rinse-zinc phosphate rinse-seal). Systems may be designed as soak or spray operations where the phosphate and the subsequent rinse stage are of stainless steel construction. Phosphates used in these applications are most often operated at temperatures of 100to 150oF.

In comparison to iron phosphating, zinc phosphating is more expensive to operate, demands more attention, and requires more frequent equipment maintenance. The trade off is the improved corrosion resistance that is required for the corrosion demands of the parts being finished.

In paint applications, the final rinse after zinc phosphating often contains seal additives. These additives are discussed in the “BroCo Post Phosphate Seals” section in following pages.


This type of zinc phosphate is used where maximum corrosion protection is required. Coating weights are 1000 to 2500 mg/ft2. Common examples of parts that are specified for heavy zinc phosphate finish are fasteners that are either phosphate and oiled, or phosphate and spin-dip painted. We see this most frequently in high volume processes on parts such as clips, nuts, bolts, small under-hood automotive parts. The heavy crystalline structure easily absorbs the oil or paint.

These coatings require the minimum 5 stage cycle and are usually processed in barrels or baskets. Phosphates used in these applications are operated at temperatures of 170 to 190oF. Equipment and operational demands for heavy zinc phosphates are similar in scope to that discussed earlier for the “BroCo Prepaint Zinc Phosphates”.


Heavy zinc phosphates have long been used in cold forming and extruding operations to obtain improved parts and longer die life. The work is phosphated and immersed in a soap material before fabrication. The application of the dense phosphate coating provides for greater absorption of the soap lubricant. The resultant increased lubricity protects the manufacturing dies against galling, and the work from undue distortion stress. Operating parameters are similar to those found in “BroCo Heavy Zinc Phosphate” discussed earlier.


This is an immersion process that produces very high coating weights (2000-4000 mg/ft2). Manganese phosphate is used as a “break in” surface on gears and other parts where premature wear at startup is of concern. The manganese phosphate prevents minor steel fragments left on the part surface from machining operations, from welding to each other under pressure, and tearing the metal surface. Due to the high coating weights normally specified for manganese phosphate coatings, a considerable amount of sludge is produced in these processes. Similar in application to heavy zinc phosphatizing, manganese phosphate coatings are applied in 5 stage, soak operations where phosphate temperatures are typically maintained at 190 to 200oF.

Manganese phosphate coatings are generally more expensive to apply than zinc phosphates. This due to higher operating temperatures, longer immersion times, greater chemical consumption, and the increased maintenance requirements. Parts that requiring this type coating have usually been well researched to provide substantiation for need of specifying manganese phosphate.


Seal compounds are concentrates used in post phosphate rinses at very small concentrations to improve corrosion protection and assist rinsing. When properly applied, these solutions will fill phosphate voids and further passivate the phosphated surface. The improved rinsing reduces the likelihood of residual salts causing oxidation, and therefore surfaces exhibit better short term corrosion protection. Most commonly, the seals are applied as pre-paint treatment, but may be used as inhibitors over heavier coatings of either zinc or manganese phosphated parts. It should be noted that the paint adhesion improvements created by the phosphate coating, may be more fully realized when a seal material is properly selected and applied.

There are many types of post phosphate seal materials, but industry generally subdivides the two basic categories by whether or not the seal contains a chromium constituent. The performance of chrome based seals has historically been accepted as the benchmark for corrosion protection of the finished surface and application ease. Chromium has long been used as a corrosion inhibitor and, provided as an oxidizer, these solutions do an excellent job of removing residual phosphate salts. Yet the environmental and health risks associated with chrome processes has pushed modern developments in chrome-free materials.

Non-chrome bearing seals vary considerably and choosing the correct material depends on a variety of application parameters. Although typically acidic, some seals may operate at neutral or even alkaline pH. Both organic and inorganic materials are used, sometimes in conjunction with reactive metal salts. When compared to chromium seals, the newer generation of materials require more frequent makeup and must be diligently maintained to provide consistent quality results.

For a more complete discussion relating to phosphatizing chemistries and equipment, please contact your local BroCo Products Representative or call BroCo Products, Inc. direct at (216) 531-0880 OR email us at [email protected].