Pretreatments

Pretreatments Used with Coatings

To view recommended Pretreatment Process Flows for most dry film lubricant systems, based on substrate, please select from the charts below:

  • Steel-Chromate (shows processes for steel, stainless steel, aluminum, nickel & chrome plate)
  • Cadmium-Titanium (shows processes for cadmium or zinc plate, copper or zinc plate, magnesium & titanium)

The marriage of pre-treatments and dry film lubrication is a bit like the Chinese concept of the forms Ying and Yang. Each can perform independent of the other, but a single half lacks the symmetry of the whole, and therefore, lacks optimum function. Combined together they form the two halves of the perfect circle. The corollary in dry films are the functions of adhesion, cohesion, wear, corrosion resistance, and coefficient of friction for any dry film lubricant can all be markedly improved by a proper pretreatment. The optimum pretreatment can make a dramatic difference. The level of difference can be quantified by test for any specific application, but is nearly impossible to predict precisely without testing a specific application. Any generalized prediction is subject to many factors such as specific alloy, temper, initial surface condition, specific conditions of use, etc.

Most dry film coating manufacturers such as Acheson Colloids, DuPont Corporation, Everlube Products, Whitford Corporation, Sandstrom Corporation, and others will have a chart with information very similar to the one available below, stipulating the recommended pre-treatments for military specification coatings. These Mil. Spec. suggestions, proven over many years, are generally applicable to any resin bonded dry film lubricant. Be careful as cure temperatures above 600° F precludes some specific plating or conversion processes from being used as a pre-treatment. Click to view the “Recommended Process Flow for Most Dry Film Lubricant Systems” charts.

The information presented here is not intended to supplant the expert knowledge and recommendations of your plating, or conversion coating sources for ordinary uses. The information here is structured to provide a generalized view of various platings, electro-chemical conversions, and straight chemical conversion coatings specifically deemed useful in improving dry film lubricant, coating, and painting applications. Much of the information is specifically directed to special needs encouraging polymeric adhesion to various base materials with consideration of unique time/temperature/humidity, etc., involved in thermoset coatings. Discussions with our application engineers prior to final print/specification formulation are strongly recommended.

The general “rules of thumb” for dry film lubricant films, platings and conversion coatings apply. Of course nothing replaces actual test data. The general rules are:

  • Dry lubricant films give their best friction and wear results when applied onto the hardest substrates. Very little is gained by applying dry films thicker than their optimum cohesive limits. Most layer lattice lamella dry film lubricants and binder systems perform best in the .0003″ to .0007″ range. Occasionally, if being used for ancillary reasons such as corrosion resistance, a slight loss of wear/friction qualities of the thicker films is tolerated for that end. DuPont Teflon-S® coatings usually are in that range. Pure DuPont Teflon® finishes used for release or wear usually run between .001″ to .002″, although films for chemical resistance can be much thicker.
  • Hard facing pretreatments, especially thin ones, on soft substrates are subject to problems of fracture/spalling under high point loads. Some pre-treatments, such as aluminum anodic conversions, if they particulate under stress, can be more damaging than the original potential failure. Aluminum oxide is 9 on the moh’s scale of hardness. That is just below the diamond hardness measure of 10. It would be impractical to itemize all the potentials, good and bad, inherent in various pretreatment potentials in a format as brief as this.

For the preponderance of parts the following pre-treatments are the norm:

STEEL PARTS:
There are four basic pre-treatments utilized. The processes are iron phosphates, heavy zinc phosphate, fine grain phosphate (Bonderize type), and grit blast. Below they are listed in order of their typical overall efficacy with the lowest performer listed first.

  1. Iron phosphate. This coating is an amorphous structure. It provides a fairly low degree of adhesion. Little to no corrosion resistance is imparted to the steel independent of the coating.1 Iron phosphate applied costs tends to be low.
  2. Grit blast imparts a fair degree of adhesive character for most coating systems. For a few of the high temperature cure materials such as PTFE (aka Teflon®) processes, grit blast is the only recommended pre-treatment. After grit blast you are dealing with a highly active surface which offers increased potential for atmospheric, or handling induced corrosion. It is essential to apply coatings as expeditiously as possible.2 Grit blasting is adequate for many applications. Hand grit blast has a high labor content. Barrel blasting, where possible, tends to be at a bulk application cost.
  3. Heavy zinc phosphate (dry with no oil) provides a good base for coatings and enhances corrosion resistance. All the phosphate crystals must be fully covered by the applied coating with no leak path to the phosphate crystal structure, or moisture can wick to the steel base leading to early corrosion potential. This process tends to increase coating total thickness. Oil is often used as a leak path seal to enhance corrosion protection, but this is applied only after the coating is final cured. Heavy phosphate requires significantly more coating by weight and volume to cover the crystal structure than with a lighter weight fine grain system. Final cost will be higher than with thin phosphates. Heavy phosphate crystal structures typically do not reinforce cohesion/adhesion as well as the fine grain zinc phosphate crystals.1 This is a bulk cost process.
  4. Fine grain, moderate weight (approx. 350 to 800 mg/sq. meter range) phosphate structures (dry with no oil), such as “bonderize” offer excellent adhesion, and corrosion enhancement using less total coating by volume/weight than with the heavy phosphates. Typically due to the finer, denser crystal structure, the fine grain phosphate provides superior wear resistant properties, and improved corrosion resistance.1
  5. Other phosphates, such as iron manganese are occasionally utilized. Iron manganese phosphate has friable crystals. For this reason the manganese crystal has a lower frictional characteristic than the non-friable zinc crystal, but typically would not be as good at reinforcing cohesion of a specific film. Manganese phosphates are typically used with oil for a degree of lubricity. We know of no quantitative tests defining performance differences between manganese and zinc phosphate under post-applied dry lubrication films. There is no end of subjective opinion. We recommend fine grain zinc phosphate for maximum adhesion on steel.
  6. Another lubricating phosphate is Endurion®. This is a zinc crystal formation seldom used today, which infuses metallic lead into the crystal structure by use of lead plates, or lead tanks for the chemical conversion process. We know of no quantitative data on improvement of post-applied dry lubrication film performance for specific topcoat materials. Here too is much subjective opinion. Again our preference for optimum wear life is the fine grain zinc phosphate.

1Employing grit blast prior to a phosphate substantially improves the performance of the lubricated coating. The grit blast induces more points of initiation for the phosphate conversion providing increased, and improved crystal structure plus a profile below the metal surface for additional reservoir of lubricated coating. The total corrosion and wear improvement can be measured in at least a full magnitude or more. DOD-P-16232 for zinc and manganese phosphates requires a grit blast prior to the phosphate immersion process.

2An iron or zinc phosphate passivates the metal surface offering less potential for atmospheric or handling corrosion prior to the coating operation.

The most effective sealers for phosphates are those incorporating chromate. If not specifically instructed to eliminate this process, many phosphate sources use chrome sealers as a first choice. Be sure to be fully aware of your end customers’ hazardous materials purchasing policies, and specifications. Most automotive customers have established, or are promulgating requirements to eliminate usage of chrome additives in coatings and coating pre-treatments.

In order to understand the benefit of a phosphate crystal used with a wear coating, visualize the benefit of a steel belted tire vs. a non-belted tire. With steel belted tires, due to improved cohesion, you get reduced squirm of the rubber against the road. This is especially true at high speed / heat. The matrix formed by the coating and phosphate crystal operate somewhat analogous to that vision.

Be aware that cure temperatures can affect phosphate quality. Thermogravimetric, and differential thermal analysis have shown that zinc phosphate crystals lose the first two molecules of water at 180° C (350° F). This transforms the zinc phosphate crystal from a tetrahydrate crystal to a more firmly attached, smaller dihydrate crystal, thus markedly improving both adhesion and corrosion resistance. Typically this works best with the bake done prior to applying a coating, however; a curing bake at 180° C to 200° C (350° F to 390° F) after coating will provide positive benefit of corrosion resistance in addition to releasing most molecular hydrogen at the part surfaces. Do not cure phosphated parts at temperature levels above 320° C (600° F) as the second two molecules of water are lost, and phosphate quality decreases markedly due to severe crystal shrinkage. Grit blast only is the recommended pretreatment for cure requirements above 600° F.

CHROMATE CONVERSION PROCESSES:
These processes are commonly used for improving the corrosion resistance of some metals, and to promote adhesion of coatings, and paints. These chrome-based processes are a science unto themselves. This precludes any depth of coverage here. We are only going to provide a few rudimentary precautions in their use with dry film lubricants.

  1. Chromate conversions, by design, seal with age. Extensive work done by 3M Corporation, and Everlube Products of a coating process named Sumar® (no longer available), incorporated extensive studies regarding the affects of chromate aging on adhesion of bonded coating/paint films. These studies showed that atmospheric aging over 72 hours produced a level of chromate film sealing precluding effective long term adhesion of applied coatings/paints. Removal of the aged chromate, and a fresh chromate application process was required for proper adhesion.
  2. Thermoset films and other post coating needs, curing above 325° F can cause breakdown of the bond of the chromate film to the base metal. In addition, temperatures above 375° F can affect adhesion of zinc or cadmium platings to the base metal. Note: High temperatures may induce eruptions of trapped gas in various die-castings (or other porous substrates) especially those of zinc and aluminum.
  3. In general, chromate processes which produce thicker / duller appearance films, promotes adhesion of coatings superior to chromate’s that are shiny and thin. Discuss this feature with your coating application engineer.

STAINLESS STEEL:

  1. The basic process used for stainless steel grades is the aluminum oxide grit blast. This can be either a hand blast operation or a barrel blast operation dependent on part configuration. It is recommended that chemical passivation, if needed, be employed after the grit blast, not before.
  2. A ferric chloride etch provides a minimum base of adhesion at a bulk process cost.3
  3. A black oxide (with NO oil or wax) will also provide a minimum base for a dry film coating. This can be especially useful for dip/spin applied coatings with a dark or black color. Grit blasting prior to the black oxide will dramatically improve adhesion.3

3Passivation for stainless steels should be an integral aspect of the above treatments. This will add to total cost.

ALUMINUM:
In this we are only going to discuss aluminum anodizing processes which are the most broadly utilized. We will refer to those processes covered by Mil-A-8625, a standard reference. A great deal of literature is available.

Three basic types of anodic conversion are commonly used. These are the chromic acid processes, sulfuric acid at approximately ambient temperature, and sulfuric acid at lower temperatures, which is called ‘Hardcoat’. The essential difference in the anodic film character, as induced by the acid and applied power variables, is the anodic film density. All three films produced are amorphous alumina (trioxide of aluminum, or Al2O3). Their performance differences result from variations in density, thickness caused by process environment, pre & post treatments, and the voltage or current density applied.

Chromic acid and standard sulfuric acid films are formed by constant voltage. ‘Hard Anodize’ (Hardcoat) is formed with constant current density, sometimes with a superimposed AC current over the DC current to prevent process initiation “hot spots”. This is especially useful with high copper content alloys. When these processes are used with dry film lubrication coatings they should either be left unsealed, or only hydrated with hot water. Salts precipitation from seal operations such as nickel acetate inhibits adhesion of applied coatings.

Dimensional changes resulting from the anodize processes and subsequently applied dry film coatings must allow for all OD, ID and thread pitch diameters. The typical film thickness of the various anodize film’s are .00004″ to .00008″ for chromic acid coatings, .0004″ to .0008″ for sulfuric anodize, and Hard Anodize is typically formed between .0005″ to .004″ thickness. With chromic and standard sulfuric anodize about 70% of the film is buildup, and with hard anodize only one half the total coating thickness is in dimensional buildup. The other portion of the coating thickness results from reduction of the original basis material consumed in the conversion process. The thicker the film that is formed, or the higher the heat of the acid bath, the more porous the coating will be at the outer surface due to increased exposure to the acid, which causes degradation of the film. The film at the interface between the metal alloy and anodic film interface is always the highest density (hardest). Often a burnish is used to get to this layer when the anodic film is used as the single treatment for wear, or friction reduction. Of course, a burnish would reduce the adhesion of any applied lubricated coating by damaging the surface porosity. We highly recommend discussing any pretreatment processes being considered with your dry lubricant applications engineer.

Aluminum & Anodize References used:

  • Products Finishing magazine, “A Hard Face for Aluminum” (January 1965),
  • Metal Progress Magazine (Feb. 1965),
  • Michigan-Dynamics Engineering Bulletin #14, “Aluminum Natural Phenomena and Anodic ‘Hardcoat’ Data” (© 1964), all authored by Colonel R. Kliemann

ZINC AND CADMIUM PLATINGS:
The use of zinc or cadmium platings over steel, and under organic coatings can improve corrosion resistance by several magnitudes over either’s individual capacity to inhibit corrosion. For example, a commercial zinc plate (.00015″ thick) with a chromate conversion and a thermoset organic film topcoat (.0004″) can yield 240 hr’s resistance to red rust. Minor amounts of white corrosion may appear earlier. This is in accordance to electromotive scales where zinc sacrifices to steel. This can be significantly enhanced by the use of an aluminum-filled organic topcoat in addition to the above process. In this scenario, the aluminum sacrifices to the zinc, and the zinc sacrifices to the steel. In fact, more often than not, hardened substrates >40 Rockwell C, a zinc-filled organic coating is used in place of zinc plating. By controlling coating thickness, types, and volume of organic fill, 5% neutral salt spray results of 1,000 hr’s or more can be shown.

Cadmium performs equal to or superior to zinc, and exhibits slightly better friction characteristics. However, many countries legally preclude use, or import of cadmium as it is a heavy metal. We understand US multi-national corporations, such as the US headquartered automobile companies, will not allow use of cadmium after January 2003. This precludes the use of cadmium in large measure. Cadmium has been off & on the US’s banned list throughout the years. It appears cadmium may not be used after January 2003.

NICKEL AND CHROME PLATINGS:
A grit blast is the recommended process for these platings. They must be adherent enough, and thick enough to tolerate the resultant profile. The profile will vary dependent on the mesh selected, the blast pressure, nozzle angle, and nozzle distance from the work piece.

COPPER OR COPPER ALLOYS:
Adhesion to copper (or copper alloys) is difficult. Grit blast helps in providing increased adhesion through increased profile; however, a copper surface should then be made passive after the blasting operation. A chromate works well with copper. Bronze should be coated without delay after the grit blast, so ambient oxidation doesn’t occur. Ambient oxidation will decrease adhesion markedly.

TITANIUM:
Typically a grit blast is employed, however, a titanium anodize process can be useful.

MAGNESIUM:
The process flow referenced above suggests grit blast, dichromate, or anodizing. Special chemicals for these processes are available from major manufacturers. Be careful when working with magnesium, it is highly flammable!

SPECIAL COMMENTS:
The information contained here is general in character. Any use of pre-treatments in conjunction with dry films should be discussed with your dry film lubricant application engineer, and thoroughly evaluated and tested prior to final use.