If phosphating and its advantages in various prepaint applications have been something of a mystery, here are the whys and wherefores.

There has been a great deal written about the increased demands placed on prepaint applications in todays market place. The advent of powders, high solids and water-reducible paints has increased the need for uniformly cleaner surfaces and heavier, more uniform coating weights.

At the same time, there are coatings which now cannot be used because of existing and projected EPA regulations. For most plants, chromated seals are out, as are zinc phosphates and chromate conversion coatings. That leaves iron phosphate as the primary tool for developing high-performance prepaint coatings.

Ideally, new equipment with increased capabilities could be purchased. Realistically, however, there is one alternative: make the chemicals and equipment available operate at peak efficiency. To do this, one must have a basic understanding of how iron phosphate coatings are formed and how they function.

Iron Phosphate Function

An iron phosphate is actually a crystalline conversion coating that is chemically bonded to the metal surface. Picture sandpaper with microscopic sand grains firmly glued to the surface. The grains (phosphate crystals) are stacked vertically and closely packed horizontally. If stacked in sufficient quantity and packed tightly enough, they form thousands of tiny canals, or capillaries. Paint flows into the capillaries and, when cured, is mechanically locked into the crystal structure. The mechanical lock provides excellent paint adhesion.

The crystals are also nonconductive. Corrosion, on the other hand, is an electrochemical process. To form rust, a flow of electrons must occur between oppositely charged sites on the metal surface. The nonconductive phosphate crystals act as barriers to the flow of electrons, but only those areas actually covered by crystals are protected. If packed tightly enough, the coating will provide very good under-paint corrosion resistance.

The process of producing a highly functional coating now becomes three-dimensional. To function properly, the coating must be present in sufficient quantity, which is called coating weight. Crystals must be tightly packed across the metal surface, the measure of which is called crystal density. Finally, the coating must be uniform over the entire metal surface.

Prerequisites to Quality

An iron phosphate coating is the result of a chemical reaction between a metal surface and chemical ingredients present in the iron phosphate bath. The quality of the coating is, therefore, dependent upon the nature of the metal surface as it encounters the phosphate bath, the composition of the bath, and the environment in which the bath operates.

To develop highly uniform coatings with dense crystal structures requires (1 ) that the metal surface be uniformly clean and reactive; (2) the composition of the bath to be maintained within strict limits; and (3) the environment inside the application equipment must match an optimum set of conditions dictated by the chemical-to-part match.

Clean Metal Surfaces

In order to accept a coating, the metal must be free of any soils or conditions which may insulate it from the chemical bath. Oils, greases, forming lubricants and similar soils must be removed. In five-stage systems, the key is to remove them before they enter the phosphating stage. A five stage system is arranged as follows:

1. Alkaline cleaner 4%, 90 sec.,140 F.

2. Rinse, 30 sec., ambient

3. Cleaner/coater (variables as indicated)

4. Rinse, 30 sec., ambient

5. Acidic seal, 0.1%,120 F. 30 sec. (except as noted)

In a three-stage cleaner/phosphate system, the key is to remove soils as quickly as possible. The arrangement is:

1. Cleaner/coater (variables as indicated)

2. Rinse, 30 sec., ambient

3. Acidic seal, 0.1%, 120 F. 30 sec.

There are two options in attempting to provide a clean surface: alter the cleaning process or change the soils. Either is acceptable, but neither may be feasible. In many plants, the limitations of existing equipment or production demands rule out changes. Chemical additions to cleaning solutions may become a third alternative. Atypical alkaline cleaner is formulated as follows:

Caustic 8 to 10%

Phosphate 4 to 5%

Sequestering agents 0.5 to 1%

Buffer (soda ash) 3 to 5%

Detergents 8 to 10% (anionic/ nonionic)

Uniform Surface Reactivity

Variations in surface reactivity of the metal will produce variations in the amount and quality of the coating applied. Specifically, passive surfaces react more slowly than highly active ones. Crystal structures produced on such surfaces are lower in coating weight and lack the crystal density found on more reactive surfaces. They perform accordingly in QC programs. Two common causes of poor surface reactivity are variations in alloying elements and work hardening. For steels, alloying elements such as nickel, chromium, lead and silicon tend to passivate surfaces if present in significant amounts. Copper tends to make the surface more receptive to coating formation.

Aluminum alloys containing large amounts of nickel and chromium are also more passive. High-silicon aluminum alloys are overly sensitive, and may create a different set of problems. When selecting metals for a product, consider more than the machining and forming characteristics if they must be painted.

Work hardening occurs during metal forming operations. Severe stress and heat change the crystal structure of the metal, which affects surface reactivity. Heat treating, annealing, etc. may have similar effects. Typically, punched "ears," drawn parts and highly machined surfaces exhibit this type of behavior in finishing operations. In most cases, the cause cannot be eliminated, the prepaint process must be altered to include additional steps appropriate to the need.

The Phosphate Bath

Understanding the operation of an iron phosphate bath requires some fundamental knowledge of the mechanics of coating formation and the function performed by individual ingredients in the bath. This forms the basis for changing control parameters and, ultimately, the quality and performance of the coating produced.

An iron phosphate formulation consists, essentially, of primary phosphate salts and accelerators dissolved in a phosphoric acid solution. A typical formulation is:

Phosphate salts 12 to 15%

Phosphoric acid 3 to 4%

Molybdate accelerator 1/4 to 1/2%

Detergents 8 to 10% (anionic/ nonionic)

The acid begins the reaction by attacking the metal surface. As attack progresses, acid is consumed and solution pH at the metal surface rises slightly. The change in pH causes the primary phosphate salts to drop out of solution and react with the metal surface, forming the desired crystalline coating.

During the process, accelerators act like parking attendants: they speed coating formation by creating additional sites on which crystals can form. The type of accelerator used must match the metal being coated and it must not be so aggressive that it detracts from coating quality . A variety of accelerators is available for ferrous, non-ferrous and multi-metal applications. Results may vary significantly if the wrong one is chosen. Consult your supplier for details.

Key elements in controlling operation of the chemical bath are: (1 ) having sufficient volume of chemical available to initiate the acid attack phase and sustain the coating reaction, and (2) ensuring that the correct pH change occurs.

Chemical Concentration

Experience has shown that higher concentrations do produce heavier coating weights; there are simply more ingredients available to participate in and sustain the coating reaction. Table 1 illustrates the effect of increasing concentration in an iron phosphate bath, while holding all other factors constant. There are limits, however. Excessively high concentrations produce loose, dusty coatings that normally yield poor paint adhesion and corrosion resistance. If operating on the low side of the recommended concentration range, an increase may be much less expensive than reprocessing rejects. If concentration is already at the recommended maximum, try another approach.

Solution pH

Solution pH should be viewed as a measure of reactivity in the phosphate bath. If too reactive, the solution produces only the acid attack phase. If not reactive enough, the solution will produce only part (possibly none) of the acid attack phase. In either case, coating formation, crystal density and uniformity are retarded because the reaction cannot go to completion.

Iron phosphates form between pH values of 3.5 and 5.0. However, the reaction between different metals, alloys, water conditions, and the specific chemical formulation used may dictate that optimum pH ranges be held much tighter. Optimum for most operations will be a spread of no more than 0.4 to 0.5 pH points. As pH drifts out of the optimum range, crystal density and coating uniformity begin to decrease.

Figure 1 (figures and tables not available) shows the results of a laboratory study with pH as the only variable in a five-stage iron phosphating operation. Optimum pH values were established between 4.5 and 4.8 on cold rolled steel Q panels. In addition, the panels were subjected to 48 hours of salt spray, using 1.5 mils of an acrylic melamine paint rated at exactly 48 hours. The results show creepage of rust from the score, expressed in fractions of an inch. Coating weight and performance can be seen to vary directly with pH.

Determination of optimum pH is a trial and error process. Begin with values recommended by the supplier, then move pH values up and down in small increments. At each "set point," run test panels of the metal and ship them to a qualified lab for analysis.

Note, however, that optimum ranges will change when significant changes are made in other operating parameters. Changes in concentration, temperature, contact time, etc., may require that the evaluation process be repeated.

Measuring pH

Because of the narrow ranges for "optimum" solution pH, the instrument used to measure it is critical. A well maintained pH meter is the preferred method. Use of pH papers is usually confined to operations in which paint adhesion is the only concern. They are seldom accurate to better than 1/2 pH point and lose accuracy quickly in the humid environment surrounding phosphating operations.

Measuring "free and consumed acid" content is a second, very useful, means of controlling pH. This is a titration, using bromcresol green as an indicator. It is useful because it picks up minor changes in acid content as well as solution pH changes. In some cases, these two items do not parallel each other. The graph of Fig. 2 shows a plot of acid content vs. solution pH for one specific operation.

For difficult-to-coat metals or difficult-to-handle water conditions, using free and consumed acid titrations instead of a pH meter often can be extremely helpful. The same is true where QC standards are very tight and equipment capabilities are somewhat marginal.

Improving Equipment Performance

The basic axiom in prepaint applications is: "No chemical is better than the equipment used to apply it." In actual practice, the equipment provides impact on and contact between the phosphate bath and the metal surface. The reaction which forms iron phosphate coatings thrives on high levels of bothóit takes time and physical impingement to develop high-quality iron phosphate coatings. Determination of what is acceptable is relative to requirements. Less than 60 seconds of actual contact time in a washer, or three minutes in an immersion system is usually inadequate for development of a good coating. From experience it can be said that a minimum of 90 seconds in the first stage of a three-stage washer system is required. If soil conditions are difficult or amounts heavy, as much as 120 seconds, or addition of an alkaline precleaner should be considered. In immersion systems, at /east five minutes should be allowed.

Figure 3 (data in Table 3) illustrates the increased performance possible from using time as the only variable in a five-stage system. Again, the paint used was 1.5 mils of an acrylic melamine. All panels were pulled after 48 hours, and creepage of rust from the score recorded in fractions of an inch.

In addition to minimum time requirements, phosphates like to be "banged" onto a surface. In immersion systems, "brisk" agitation increases coating weights by as much as 25 percent per unit time. In spray-washers, similar gains are realized, provided parts remain inside the washer silhouette. Above 25 psi, parts become susceptible to "blow-by," and coating weight starts to decrease.

Table 4 shows the effect of nozzle pressure on coating formation. Note the "blow-by" effect caused by excessive pressure. Coating weight and performance can be increased by adding to the number of spray nozzles or by changing to full-cone-shaped nozzles. Unlike whirl jets, full cones provide impact similar to that of V-jets, but can cover more than three times the surface area.

Large sums of money are not required to increase the number of nozzles or pressure, nor to provide brisk mechanical agitation in an immersion system. Doing this, however, can produce significant benefits even in systems with slightly marginal contact times.

Temperature

Temperature definitely affects coating formation and performance. More energy accelerates the reactions and reduces the time required to produce acceptable iron phosphate coatings. This is especially true for three-stage systems in which cleaning and phosphating occur in the same stage (or tank). In four- and five-stage systems, gain in coating weight, crystal density, and uniformity may be somewhat lower, but are still worth considering as long as losses from poor product quality exceed the additional energy costs.

Table 5 illustrates the results obtainable by varying only the temperature. The same acrylic coating as before was used and the panels pulled after 48 hours exposure. Note particularly the result with Panel Set 14. The relation between temperature and coating weight is clear, but so is the effect of going beyond the generally accepted coating weight range: 30 to 60 mg/ft. Deterioration of coating performance is not peculiar to temperature variation, however; changes in any control parameter which cause excessive coating weights are unacceptable.

Make up Water Quality

Variations in water quality also tend to have a detrimental effect on coating performance. The most common problems are caused by high levels of hardness, alkalinity, iron, chlorides, sulfates, and dissolved solids. Absolute maximum limits are shown in Table 6; lower operating levels are highly recommended.

Iron, chlorides, and sulfates represent impurities which, if left on a phosphated surface even in small quantities, are highly corrosive. Most suppliers have established maximums for any one, or combination, of these contaminants. If the available water supply exceeds the stated maximum, it may be necessary to substitute deionized water in the final seal or rinse stage, as a minimum. Softening, to lower high iron content, is usually not a remedy. Common softeners merely substitute sodium ions for iron. Unfortunately, sodium is also corrosive when left on phosphated surfaces.

Hardness and high alkalinity tie up or precipitate ingredients of the phosphate bath. Primary phosphate salts in the phosphate formulation will act as sequestering agents and tie themselves to the calcium and magnesium ions in the water. High levels of natural alkalinity neutralize free acid content in the phosphate bath.

In even moderately severe cases, pH control problems and high chemical consumption are common because of the constant influx of high hardness and high alkalinity make-up water. Heavy scaling of heater tubes, risers and nozzles will occur.

If poor quality make-up water is unavoidable, some difficult decisions will be necessary. First, phosphate formulations that specifically meet the make-up water conditions must be used. Second, use of deionized water may be necessary in a final seal or rinse. Third, consultation with water treatment experts may be necessary, as well as consideration of a treatment program for all water used in the prepaint process.

Rinsewater Cleanliness

Rinse stages do have an effect on the performance of chemical baths, and they do affect the quality of the finished product. To ignore this is to ask for trouble.

The first point to keep in mind is that in many installations, rinses do not remove anything; they only dilute, since the rinsewater is recirculated. Typically, alkaline cleaners build up in the succeeding rinse stage and are eventually carried into the acidic iron phosphate. The alkali neutralizes the acid, chemical consumption goes up in the phosphate bath, and proper pH is nearly impossible to maintain. So is product quality.

Similar problems arise when acidic iron phosphates are allowed to build up in the final rinse stage. The residual phosphate acts like any other "salt" when allowed to dry on a phosphated surface.

For these reasons, rinses must be titrated and controlled like any chemical bath. Levels of chemical carryover, dissolved solids content, and pH values must be monitored and adjusted to prevent contamination of succeeding tanks and phosphated surfaces. Limits must be established and rigorously maintained for each control parameter. If a rinse exceeds its limits, it should be dumped or diluted with make-up water.

Rinsing Efficiency

Efficiency and cleanliness do not always go hand in hand in rinse stages. A clean rinse with insufficient physical impact or coverage of parts does not dilute very effectively. Cleaner or soil films may cling tenaciously in sufficient quantity to contaminate other baths and the finished parts.

Like phosphate solutions, rinse solutions need physical impact, contact and time. Minimum spray pressures of 12 to 14 psi should be used, provided parts remain within the silhouette created by the nozzles. Mandatory contact time is at least 30 seconds. And the more nozzles the better.

If possible, two separate rinse stages, back-to-back, are best. If not possible, add a fresh water rinse arch in the vestibule as parts exit the rinse stage. The action of the second rinse can improve rinsing efficiency by as much as 50 percent or more, if properly set up. If designed as part of a counterflow rinse system, it can reduce water consumption and waste treatment cost by a rather significant amount.

Final Seals

It was stated earlier that small interstitial spaces exist between phosphate crystals. In addition, there will be some unreacted phosphate salts clinging to the metal as it enters the final rinse stage. These spaces represent unprotected metal and the phosphate salts are corrosive if left on the surface.

A final chemical seal is used to eliminate both potential problems. Seals dissolve the salts and lay a protective barrier over unprotected metal areas. It is generally accepted that chromic seals give better results than non chromic ones, but non chromic seals work well if matched to the metal surface and to the paint being applied. This is illustrated in Table 7.

Hot-rolled steel panels were tested in a three-stage washer, where the only variable was the type of final seal used. The paint was a 1.8-mil coating of a water-reducible epoxy. All panels were run to failure using existing plant standards.

Because there are hundreds of non chromic formulations on the market, only general guidelines can be offered: Nonferrous metals seem to prefer acidic seals. Experience has shown that those containing amine derivatives perform better than seals containing only inorganic acids, which are not normally considered for high performance, high-quality prepaint applications.

Porous ferrous metals, including hot-rolled steel, react very favorably with the newer alkaline seals. Nonferrous metals do not. In addition to their normal functions, the newer seals are believed to displace water from metal pores. This can make a big difference in under paint corrosion resistance. However, this type of seal should not be used with paints that are acid catalyzed.

Cold-rolled steels can be treated with either of the two recommended types. Their performance is roughly equivalent with either acidic/amine or alkaline/amine formulations.

Controlling Final Seals

Solution pH, concentration, and dissolved solids content of the bath are critical factors affecting seal performance. Almost all final seals are pH dependent, working properly only if maintained within a given pH range.

Since many of the newer seals deposit a physical coating, it is essential to keep concentrations at or above minimum levels. However, concentrations in excess of those recommended may produce coatings that interfere with paint adhesion. Consult suppliers for "exact" concentration ranges and be sure that they are maintained.

Finally, monitor dissolved solids content of the final seal. Left on phosphated surfaces, they are corrosive. The rule of thumb is to limit dissolved solids content to not more than 1.5 times the combined level of seal plus water, when first made up. In water with high dissolved solids content, an even lower limit might be considered. z

References

1. Training Manual, "Phosphate Coatings," The Chemical-ways Corporation, Lake Bluff, I L 60044-0215. 2. Data developed from field experience by the author.

3. Data in tables developed by the Customer Service Laboratory, Chemical-ways Corporation, Lake Bluff, l L 60044-0215.

About the Author

Richard W. (Dick) Phillips is regional manager at Chemical-Ways Corporation, 901 Sherwood Drive, Lake Bluff, IL 60044-0215. He has 18 years experience in the preparation of metals for painting, and has been with Chemical-Ways for almost seven years. Mr. Phillips has an extensive background in the technical training of new personnel, and acts as a consultant for a number of firms installing new paint systems . I n 1965, he received his BA in biology and minors in both chemistry and physics from the Virginia Military Institute, Lexington, EPANSWER:

Mr. Phillips is a member of the AESF.