cleaning, pretreatment & surface preparation

NON-PHOSPHATE TRANSITION METAL COATINGS

BY BRUCE DUNHAM AND DR. DAVID CHALK, DUBOIS CHEMICALS,

SHARONVILLE, OHIO

INTRODUCTION

Traditional iron phosphate and zinc phosphate conversion coatings havebeen used for more than a century as pretreatments for painting over a varietyof metals. These “legacy” phosphate pretreatments have served well; however,environmental regulations restricting phosphate discharge, increasedphosphate and zinc costs, and higher corrosion-resistance requirements have provided impetus for the development of non-phosphate alternatives. Duringthe evaluations of the various technologies, it was discovered that these newnon-phosphate pretreatment conversion coatings conferred significant costsavings and operational benefits along with their promised decreased environmentalimpact.Considered new and experimental in the New Millennium (Y2K), these nonphosphate

conversion coatings have gained significant traction in the pretreatmentmarket and are rapidly becoming the technology of choice for paint andpowder coating pretreatment. The purposes of this article are to provide backgroundinformation for those new to non-phosphate pretreatments, and to answersome frequently asked questions about the non-phosphate conversion coatings.

WHAT ARE TRANSITION METAL COATINGS?

If iron phosphate and zinc phosphate can be referenced as “Traditional MetalPhosphates”, the new non-phosphorus pretreatments can rightly be called“Transition Metal Coatings” (and will be referenced as “TMC” coatings in the

remainder of this paper).The term “transition metal” refers to a metal’s position in the Periodic Tableof the Elements, and is a term chemists use to describe the location of a groupon the Table.Zirconium (Zr) is at the center of a group of elements in the Periodic Table thatare considered relatively environmentally friendly. (See Figure 1) Oxides of zirconium,titanium, and/or vanadium are the most commonly used transition metalcoatings, with zirconium as the most frequently encountered transition metal.

Note the location of these metals relative to chromium. The closer two given elementsare to each other on the Periodic Table, the more similar their properties.The first recorded application of zirconium oxide on steel was in 1996, whenthe first non-chrome seal rinse based on zirconium was introduced. Applied overa traditional metal phosphate conversion coating, the sealer conferred corrosion

resistance that was close to that offered by the chromium seal rinse that hadtraditionally been used.

The chemistry was then modified in 1998 to serve as a chromium replacementfor conversion coating on aluminum. The first applications for steel arrived in2002.

WHAT IS THE APPEARANCE OF A TRANSITION METAL COATING

PRETREATMENT?

Zirconium oxide is a very versatile material, taking on such varied forms as

ceramic bake ware, or when fused as jewelry, cubic zirconia. Imagine claddinga reactive metal in an inert substance like cubic zirconia, then applying a corrosion-resistant organic coating. This is the promise of the modern transitionmetal coatings, once referenced as nano-ceramic.Figure 2 shows the relative thicknesses of the pretreatments. When gaugingrelative thickness of applied pretreatments, zinc phosphates are by far the heaviestand thickest pretreatments, depositing a mineral layer of some 1000 to 5000nanometers (nm) in thickness. (Footnote 1) Iron phosphate applies typically a 250to 500 nm thick coating. TMC pretreatments are approximately 50 nm, with someapproaching 200 nm in thickness. They are the smallest, thinnest of the pretreatments,and are much thinner than the traditional metal phosphates they replac

Figure 2a. Pretreatment comparison.

TMC* Iron Phosphate Zinc Phosphate

Coating

Structure

Amorphous Amorphous Crystalline

Typical Coating

Thickness

~50 nm ~250 nm 1000 nm

Typical Coating

Weight

50-150 mg2

5-15 mg/ft2

300-700 mg/m2 25-

65 mg/ft2

2-3 g/m2

180-300 mg/ft2

*TMC

Much like traditional phosphates, TMC pretreatments can exhibit an arrayof colors, from nearly colorless, to tan, gold, and iridescent blue. Investigationhas revealed that the appearance of pretreated metal is related to the thicknessof applied coating. When the substrate is mild steel, the coating color goes fromthe original appearance of the substrate to light gold or tan, to a deep gold, tolight blue and gold, to blue, to deep iridescent blue as the coating becomes morecomplete and increases in coating weight or thickness. As with traditional phosphates,the coating will become higher with increases in one or more of the followingvariables: chemical concentration, contact time, pressure (spray) or agitation(immersion), or temperature.Another significant variable that impacts appearance of the coating is the type

of steel a finished good is made from and the fabrication steps required to produceit. Heat treatment, welding, grinding, bending, blasting and other commonmanufacturing processes impact the amount of carbon (or scale) and iron at thesurface of the part. The more carbon at the surface, the less reactive the surface isto the pretreatment solution. The more iron at the surface, the more reactive thesurface is to the pretreatment solution. The photos below show a mild steel andhot rolled steel panel. Both panels were processed through a five-stage pretreatmentprocess (Clean, Rinse, TMC, Rinse, Final Seal). The cold rolled steel is aneven deep blue and the hot rolled steel is an even grey color because of the highcarbon content at its surface.

WHAT ARE THE BENEFITS OF REPLACING PHOSPHATE

CONVERSION PRETREATMENTS?

The primary benefit of replacing traditional metal phosphate pretreatments issignificant and measurable cost savings in the operation/application of TMCpretreatments.A significant benefit of replacing traditional metal phosphate with TMC pretreatmentsis the elimination of phosphorus from the waste stream. Phosphorusis becoming increasingly regulated, especially in areas near large bodies of freshwater such as the Great Lakes region; watersheds such as the Chesapeake Bay;and other areas where municipalities are trying to reduce phosphates in theFootnote 1 - Note that a nanometer is on one billionth of a meter so the “smallness” of the

concept is difficult to grasp. Put more succinctly, a nanometer is to a meter, what a marblis to the eartwater they discharge back into the environment.Minimizing phosphates in water is astrategy aimed at reducing eutrophication(Footnote 2). Adopters of TMC pretreatmentsoften claim a green pretreatment strategy; thedisposal procedures are generally inexpensiveand uncomplicated.TMC pretreatments are very reactive so heatis not needed to drive the reaction of the zirconiumwith the metal at the surface of the part.Thus, TMC pretreatments can run at ambienttemperature, whereas the traditional metalphosphates require significant heat to drivethe deposition reaction. This saves significantenergy cost.Most TMC pretreatments operate between90 and 105¡F. The heat carried in by the partscoming from the heated cleaner stage andthe energy generated by the pump in a spraysystem are typically enough to maintain thistemperature range.Early adopters of TMC pretreatment technologyenjoyed a minimum of 15%, to as muchas 40% lower costs when converting from traditionalmetal phosphate pretreatments. Thesekinds of savings persist with the modern renditionsof TMC pretreatments.Another key benefit of TMC pretreatmentsis much better corrosion performance in service,as well as in accelerated testing, whencompared to the legacy metal phosphates(10% to 30% longer salt spray hours and moreintervals of cyclic corrosion testing have beenobserved with the first versions of this newclass of chemistry). Several suppliers of pretreatmentchemistry have developed TMCpretreatments that are approaching the performanceof zinc phosphate. Because of highoperational and disposal costs associated withrunning a successful zinc phosphate process, OEM’s are investigating substitutingTMC for zinc phosphate pretreatment, and several organizations havesuccessfully made the transition.There are several reasons why TMC pretreatments provide excellent corrosionprotection. As previouslynoted, TMC contain elements that are near chromiumon the period table; the oxides of these elements are relatively chemically inertso they do not dissolve as easily as phosphate metal coatings. Zirconium oxidesare so stable that hydrofluoric acid, which is extremely corrosive and aggressive,is needed to dissolve them. Secondly, TMC are made of much smaller particlesthan amorphous iron phosphate coatings or zinc phosphate crystals. BecauseFootnote 2: Eutrophication: or more precisely hypertrophication, is the ecosystem responseto the addition of artificial or natural substances, such as nitrates and phosphates, throughfertilizers or wastewater, to an aquatic system.

Figure 3 Top: Heavy zirconium oxide

coating on hot rolled steel. Bottom:

Heavy zirconium oxide coating on

cold rolled steel.

the particles are so small, they are able to pack closer together. This results inless void space within the matrix of the TMC when compared to conventionalphosphate metal coatings, so there is less room for air, moisture, and salts totravel to the substrate and cause corrosion. These coatings also inhibit galvaniccorrosion because the transition metal has electrons that would be sacrificed

prior to the electrons of the iron in the base metal.Paint/powder coating adhesion and corrosion resistance also benefit because

of the efficiency of the reaction. As stated, the efficient reaction results in verylittle sludge formation, so there is much less suspended solids in the pretreatmentsolution. As the pretreatment bath ages and the level of insoluble suspendedsolids increases, they can become incorporated in the phosphate coatingsand/or dry down on top of them despite rinsing. The result is a powdery

appearance on the parts that provides an inferior surface for adequate paintor powder coating adhesion. If you have managed an iron or zinc phosphatepretreatment process, you have likely made the decision to dump the bath at theend of its useful life due to powdery part appearance in your past.The reader may be thinking, “If it saves costs, increases environmental compliance,and gives better performance, what’s not to like?” The market agrees,and adoption of TMC pretreatments is therefore rapidly increasing in the marketplace.

HOW ARE TRANSITION METAL COATINGS DIFFERENT

(FROM TRADITIONAL PHOSPHATE)?

New users observe several differences when converting from the legacy phosphatepretreatments.

• TMC are best applied at cool temperatures, not warm-to-hot like phosphate.

• TMC are MUCH more reactive than phosphates during application, yetthey sludge much less. They benefit from a continuous filtering regimen toremove iron solids.

• TMC can be (and are) used in mild steel washers, but are best applied fromstainless equipment.

• TMC are equally as well applied via spray, immersion, and pressure wand.

• TMC require excellent rinsing and low-salt content applications along with avery clean surface.

HOW ARE TRANSITION METAL COATINGS THE SAME (AS A

PHOSPHATE)?

New users of TMC pretreatments are delighted to find that there are manysimilarities with the traditional metal phosphates.

• TMC pretreatments are usually applied from a washer and generally willchange the color of the metal substrate (if it’s steel). The color change cangive a good visual indication of a properly running process.

• The application mechanism of TMC pretreatments is somewhat similar tophosphate, with pickling of metal and depositing of coating. There is a bitof a difference in that the substrate metal is not generally believed to be aparticipant in the deposition reaction mechanism.

• The application requires some measure of control and attention to the process. Typical measurements are for pH, acidity, and perhaps a

CONVERTING FROM PHOSPHATE TO TMC

There are generally two types of conversions: the washer is an existing traditionalmetal phosphate application, or the washer is newly constructed for applicationof TMC pretreatments. The two applications require different approaches.A legacy metal phosphate washer will typically feature a cleaning stage, onerinse, a pretreatment application stage, one or more rinses and perhaps a seal

rinse.The best practice for TMC is stainless steel for the TMC stage, good cleaning,very good rinsing, and perhaps provision for a seal rinse. At first glance it wouldbe considered fairly straightforward to simply replace the traditional metalphosphate stage with a TMC stage. Unfortunately, this overlooks the need forvery thorough rinsing ahead of the TMC stage. More frequently, a conversionof the legacy metal phosphate stage to a rinse, followed by conversion of a legacyrinse to a TMC stage is more successful. New washer construction takes intoaccount the requirement of sufficient rinsing ahead of pretreatment by insertingan extra rinse after cleaning.

Three-stage washer configurations present a particular challenge becausethe process must both clean the metal and provide a conversion coating.Surfactants cannot be incorporated into zinc phosphate baths so the cleaning

and pretreatment must be in separate stages, separated by a minimum of 1 rinse.Zinc phosphate requires a minimum of 5 stages and more typically 7 stages.Iron phosphate is commonly applied in a three-stage process, where the cleaningand conversion coating is accomplished in the first stage. The cleaning isachieved the addition of both the surfactant and/ or solvent into the formula.

When parts are heavily soiled with mill oils and other metalworking fluids, it isnot uncommon to use a tank side additive to improve cleaning and extend theuseful life of the bath.As previously stated, TMC do not contain phosphate. While this is an environmental

benefit, it diminishes the cleaning capability of the chemistry becausephosphates are a detergent and help cleaning. TMC must rely solely on surfactantsand solvents in the formula to degrease the metal. cleaner-coaterTMC products are available and are best used when the soil load is light andconsistent.The most common way of using TMC in three-stage pretreatment processes

is to apply them in the final stage. The clean, rinse, coat configuration is idealfor manufacturers that have tough to remove soils and inconsistent sources ofsteel. A custom coater with 3-stage washer is a good example of an organizationthat could benefit.Users of these technologies report varying degrees of satisfaction, but withcreative use of vestibule space for misting risers to provide better rinsing, andgood chemical product selection, a 3-stage washer can give satisfactory service.The 5-stage washer configuration will give much more flexibility in the application of TMC pretreatments Of particular interest are conversions of legacy zinc phosphate systems toTMC pretreatments. With performance of TMC pretreatments approachingthat of zinc phosphate, without the detriments of heavy sludge, high processcost, and need for onsite waste water treatment, these old washers are beingconverted with increasing frequency. The most important consideration is theremoval by acidic descaling of the old sludge and activator products from thewasher surfaces. Using a hot recirculating solution of muriatic acid will dissolveold zinc phosphate scale, corrosion products, titanium salts, and other deposits,leaving the washer ready to accept the new TMC pretreatment. Failure todescale the washer will cause contamination and compromise performance ofthe new pretreatment.

CONCLUSION

TMC are now widely used across the globe by hundreds of users in a broad rangeof industries. This technology is the fastest growing powder coating and paintpretreatment and is firmly established in the finishing market. It is no longerconsidered “new”. The technical support to convert existing metal phosphatepretreatment systems is well established. The newest TMC products are easyto run, so there are no barriers to enjoying the benefits to your process offeredby TMC. If you are interested in improving the corrosion resistance of your

product and the environmental profile of your pretreatment program, you may consider TMC.