What is the desired result of heavy zinc phosphating?

To produce a coating of 1000 to 4000 mg/ft2 capable of absorbing close to 1 gal of water-soluble, oil-type rust inhibitor per 1000 ft2 of metal processed. At this coating weight, and with the recommended concentration of soluble oil in water, treated parts should be capable of withstanding more than 96 hr in a salt-spray test cabinet.

The rust inhibition comes almost entirely from the oil or whatever other protective compound is applied to the phosphated surface. By itself, the zinc phosphate coating provides very little in the way of protection against rusting. Its main purpose is to soak up enough oil (at the recommended concentration) to do the job of protecting the metal against corrosion. Too thin a phosphate coating does not hold enough oil to provide the necessary protection. The lighter phosphate coatings plus oil are used on hinges and other moving parts where non-binding movement is required.

Parts processed in a zinc phosphate system consisting of a soak cleaner, rinses, acid dip, chromic acid passivator, and oil dip are usually small, and are carried through by means of either a slowly rotating barrel (similar to those used on plating lines but made of stainless steel instead of plastic) or free-rinsing, expanded metal baskets. Larger parts that do not lend themselves to bulk processing may be processed using specially designed racks of steel construction.

Typical Line

A good percentage of parts that require heavy zinc phosphate coatings have surface defects that may consist of rust or heat-treat scale. Because of these defects, a typical zinc phosphate Wine requires a pickling tank to remove the rust or scale. A representative process line incorporates the following steps:

1. Alkaline soak clean at a cleaner concentration of 8 to 10 oz/gal and 170 to 190 F. In most cases, about 5 min of immersion should be enough to remove soils, oils, and drawing compounds.

2. Double rinse (cascade type) for 30 sec in each tank for maximum effectiveness.

3. Pickle in 25 to 50 percent by vol hydrochloric acid at ambient temperature for 5 min or whatever time it takes to completely remove rust or scale.

4. Double rinse as in Step 2.

5. Zinc phosphate using 12 to 18 g/L (30 to 45 points) total acid, according to titration with 0.1 N NaOH and phenol phthalein indicator. There should be a 6:1 ratio of total acid to free acid. For racks and baskets, the recommended immersion time is 15 to 20 min at 180 to 200 F; for barrels with a 0.3-0.5 percent iron content,30 min.

6. Double rinse (cascade type) for 1 min.

7. Immerse in chromic/phosphoric acid passivating rinse at 120 to 160 F. The total acid concentration should be 0.48 to 0.8 g/L (3 to 5 points), according to titration with 0.1N NaOH and phenol phthalein indicator. The free acid should range from 0.03 to 0.08 g/L (0.2 to 0.5 points) titrated against brom cresol green.

8. Double cascade rinse with in/out immersion.

9. Oil dip in water-soluble material at 15 to 30 percent by vol and 120 to 150 F. Some customers may demand that all excess oil be removed. This is usually accomplished by spinning parts in a centrifugal dryer.

Most metals require activation by pre-pickling because they are not reactive enough to produce a good, even phosphate coating. If the metal is relatively clean (meaning free of rust or scale) and it is found that it does not phosphate properly when the pickling stage is omitted, then the operator must include pickling in production, otherwise, this step can be left out. It is often the case that smooth, shiny surface metals are the most difficult to phosphate, whereas the hot-rolled, pickled steels are the easiest.

One point worthy of mention: The oil-finish material should be capable of tolerating chromic acid residues dragged into it by production loads. Some oil emulsions will "break out" of dispersion with water in the presence of the acidic residues, rising to the surface as a fully separated oil or as a semi-separated curd that covers the parts with an unsightly and messy scum. This condition also presents a problem in testing for the strength of the oil. The bath is either too low in pa (below 8) or has lost a portion of its emulsifier due to lengthy overheating. When only the former is the case, a simple adjustment of pH by adding small volumes of concentrated ammonium hydroxide (200-mL increments) usually will rectify this situation. However, if the bath does not respond to this treatment, an addition (gallon at a time) of a suitable, non-ionic, low-foaming wetting agent will re-emulsify the unsightly separated oil.

Hydrochloric acid is recommended for pickling because of its superior capability to remove heat scale and rust with the least amount of surface disruption to the metal. No inhibitor is recommended for the acid, as there is a strong possibility that drag-in of the inhibitor to the phosphate bath would result in poor or no formation of phosphate crystals on the metal surface.

Phosphoric acid, which would seem to be the logical choice for pickling because it is used in the manufacture of the zinc phosphate chemical and therefore its drag-in would not harm the bath, really is not recommended because it tends to passivate the metal. This passivation prevents the necessary reaction of the phosphate solution with the workpiece surface, resulting in either a slowdown or cessation of crystal formation.

Phosphate Stage

The seemingly long immersion time in the zinc phosphate solution as compared to spray phosphating is necessary to achieve complete coverage and add coating weight, though the latter reaches close to requirement the first 5 to 10 min. The main reason for the longer immersion time is for "filling in" voids or bare spots, tiny as they may be, which are not always covered in the first few minutes.

With the likelihood that some steels have microscopic areas of passive metal, occluded foreign materials, or localized concentrations of carbon

from the alloying elements, all of which do not promote phosphate crystal formation, it is imperative to adhere to the immersion time of 20 to 30 min. If this is not done, rust can begin to develop in voids or thinly coated areas and spread rapidly. A coating weight of 2000 to 2500 mg/ft2 is generally necessary to adequately cover the surface and minimize the detrimental effects of voids.

At one time, not too long ago, it was generally believed that larger phosphate crystals were superior and would absorb more oil. It has since been found that this is not the case. The larger crystals have fewer spaces or pockets in which to hold oil, whereas the smaller crystals have a great many more spaces to soak it up. Though crystal size has a pronounced influence on oil absorption, it is coating weight that must be met to ensure that a given part withstands the corrosion test requirements.

Table 1 compares oil absorption capacity as a function of coating type and crystal size. The confusing factor here is the fact that si nce smal I crystals pack closer together in what is a tighter formation, less phosphate coating is necessary to effect complete coverage. So, it would seem that a tighter coating would have fewer spaces or openings in which to soak up oil. In common usage, when a tighter coating is involved, less coating weight may be produced if proper immersion time is not adhered to; consequently, there would be less oil absorption.

Troublesome Iron

One of the most troublesome features connected with heavy-duty zinc phosphating is controlling the iron content of the bath. Too little iron can result in sparse, loosely adherent, coarse crystals, while too much can cause a powdery film or a coating with many obvious voids (non-coated areas), both of which contribute to poor corrosion resistance. Generally, an iron content of 0.3 to 0.5 percent is desirable. These figures, however, are based on the relative zinc concentration. For best results, the iron and zinc concentrations should be equal, with 0.5 percent of each being optimum.

With a new bath, it is advisable to run several loads of scrap parts through the phosphate bath to introduce sufficient iron before production begins. Another method is to soak several packages of steel wool for a few hours. Still a third method of getting enough iron in the new bath is to save a small portion (about2 percent by vol) of the old one.

As production proceeds, the iron concentration begins to climb and steps must be taken to prevent rapid and excessive buildup. Why the buildup? To produce heavy coatings (2000 mg/ft2 or more), the acid attack on the steel must be accelerated considerably by heating the solution to at least 190 F. with the total acid concentration being kept at 12 to 18 g/L (30 to 45 points). When more iron is removed than is needed to help form the mixed zinc/iron phosphate crystals, the excess remains in solution or sludges out and falls to the bottom of the tank.

This condition arises from three major causes:

1. High Loading: Loads too great for the solution volume are one cause. Generally, it is possible for the iron concentration to remain fairly constant or stable, rising slowly through the life of the bath. As long as the bath is in full production, the iron level will rise.

A rule of thumb states that about 11b or 1 ft2 Of parts per gallon of solution is the ideal load in phosphating. More than this will necessitate early dumping due to abnormally high iron buildup, which will require treatment to remove the excess iron. With more surface area of iron exposed to the acidic solution, it is only natural that more acid will be consumed, requiring replenishment to bring it back to the operating range.

2. Insufficient Temperature: The lower the temperature under the recommended range, the greater will be the rise in the iron concentration. At proper temperature, the zinc phosphate coating will form rapidly on the steel, in effect passivating the surface so that little or no further attack takes place.

At low temperatures (e.g., 160 F), phosphate formation is much slower, thus allowing the acid to attack the partially passivated steel. The lower temperatures allow for too much pickling and promote too little coating.

3. Tank Deterioration: Even though the life of a mild steel tank is short, some plants, due to the high initial cost of stainless steel, use mild steel tanks instead, choosing to replace them when they deteriorate beyond use. All they are doing is postponing the expenditure of money, which they will inevitably spend more of in the long run.

Aside from the economic consideration, mild steel tanks are not recommended because too much iron will be removed from the steel of the tank walls and bottoms by the acid in the phosphating solution, thereby increasing the iron concentration. Despite the coating of the tank with a phosphate crystal formation, there will always be some attack on the metal through the coating.

There are two major methods of removing iron from the bath: decanting and treatment with hydrogen peroxide. Decanting is probably the best answer and can be done in two ways.

When in the middle of production, the operator simply siphons off a portion, the volume depending on how excessive the iron concentration is, then tops off with heated water and sufficient phosphate concentrate to replenish that which is lost through decanting. The other way is to pump out the top two-thirds to a storage tank, clean out the sludge and bottom third of the solution, then pump the usable phosphate solution back to the process tank, topping up with heated water and adding phosphate concentrate to bring the bath back to operating strength. As much as 0.2 to 0.3 percent of the iron concentration can often be removed in this manner.

Removal of excess iron by hydrogen peroxide treatment has been performed for years but has lost popularity due to the major disadvantage that it greatly accelerates sludge formation. The other drawback is that, along with iron it precipitates some zinc as well. Therefore, when hydrogen peroxide is used, the loss of zinc must be replaced by adding zinc carbonate.

Equipment Recommendations

There are six key aspects associated with equipment and its usage for heavy zinc phosphating: heating capacity, heating coils, pickling tanks, barrels, phosphating tanks, and transfer time.

ï Heating Capacity: Phosphate solutions require at least 50 percent more heating capacity than cleaners do. This is due to the insulating qualities of phosphate scale on coils.

ï Heating Coils: These should be stainless steel, type 316, plate coils attached in such a way as to be at least 6 in. from the bottom of the tank to avoid being immersed in the sludge. Several spare, clean coils should be available at all times to replace those that become heavily scaled. At least one and possibly two coils should be replaced each working day. This minimizes excessive scale buildup on the coils, a film which not only drastically lowers heat transfer, but takes much longer to remove in descaling solutions.

ï Pickling Tanks: These should be plastic or rubber-lined. Stainless steel is not recommended when hydrochloric acid is used because of the corrosive action involved, even though it might be slow.

ï Barrels: If the parts are small (e.g., nuts, bolts, washers or small stampings), they are processed in barrels usually made of stainless steel. Some plants may employ regular plastic plating barrels but these tend to warp under the high loads and high-heat cleaners and phosphate solutions.

ï It seems a contradiction to state that stainless steel tanks should not be used for pickling and then recommend stainless barrels for parts processing. But even though the barrels pass through the hydrochloric acid pickling tank, the degree of attack on the barrels is so negligible as to be no problem whatsoever.

Rotation speed of the barrel should be on the order of one revolution every 11/2 to 2 min. This minimum rotation rate is necessary to ensure that a!! areas of every part contact the phosphate solution. The rotation speed in the cleaner and in the acid may be considerably faster. Some parts, due to their shape (flat or formed in such a way as to nest into stacked bundles), require lighter loading of the barrels to allow more space for the workpieces to fall apart for solution contact. These types of parts also require longer processing time.

Too fast a rotation rate in the phosphate stage leads to abrasion of the coating, causing it to be rubbed off completely or else marring and scratching to the point of unacceptability. Compared to rack or basket processing, barrel-phosphated work must remain longer in the phosphating stage to allow sufficient time for all parts to be exposed to the solution for complete coverage and proper buildup of coating weight.

It is recommended (though not absolutely necessary) that after parts emerge from the chromic acid rinse, they be transferred to another barrel utilized solely for immersing parts in the final-stage oil dip. The reason for this being that oil would not be carried over to the first-stage cleaner, thereby further contaminating it.

Phosphate Tank: This should be constructed of Type 316 stainless steel (remainder of article missing--contact AESF for copy)