cleaning, pretreatment & surface preparation

PREPARATION OF NONFERROUS

METALS FOR PAINTING

BY EARL GROSHART

CONSULTANT, SUMNER, WASH.

Metals to be painted should be cleaned to remove oily soils and loose materials.It may not be necessary to remove the natural oxides if they are solid,such as the blue scale on heated steel; however, for many active metals it maybe advantageous to remove the natural oxides and replace them with artificiallyformed oxides by “anodizing.” For the preparation of aluminum, seeboth “Paint Pretreatments for Aluminum” and “Anodizing as a Pretreatmentfor Aluminum,” in this section of the Guidebook. The preparation of cadmium,chromium, cobalt, copper, lead, magnesium, molybdenum, nickel, silver, stainlesssteels, titanium, zinc, and alloys of zinc-nickel and tin-zinc are covered here.Each metal requires its own processing, but a few general steps are the samefor all metals.

GREASE REMOVAL

“Degreasing,” i. e. , the removal of oils, greases, waxes, and corrosion-inhibitingcompounds, which have in the past been removed in vapor phase degreasers withchlorinated solvents, should now be removed in an environmentally approvedmanner. These include closed vapor degreasers, solvent (vat) washing, parts washerswith cleaning agents in water, and emulsion degreasers. There are a numberof closed-loop vapor degreasers that can safely be used with chlorinated solvents.These satisfy and offer the least disruption to production that in the past hasused vapor degreasers. Solvent washing with a hydrocarbon solvent, usually ablend of several solvents and a combined low-vapor pressure (below 45 mm Hgat 20°C for the aerospace industry), has replaced some degreasers. If properlycovered, these will meet environmental requirements. The parts washers and theemulsion cleaners can be selected so that they leave the work with a water-breakfreesurface and, thus, would not require further cleaning. The solvent cleaningmethods will require alkaline cleaning to provide this water-break-free surface ifother processing, such as conversion coating, is to be applied. If not further wetprocessed, the emulsion cleaning and washer cleaning will require an extra dryingstep prior to painting.

HAND SOLVENT WIPING

As painting is generally done after the cleaned parts have been exposed, i. e.,after assembly or storage, most parts will require a hand solvent wipe justprior to painting. This is especially true prior to painting with the new environmentallydesigned high-solids and water-reducible paints. Because theyhave little or no organic solvent, they cannot tolerate even small amountsof organic contamination on the surface of the substrate. To exacerbate theproblem further, as the Clean Air Act of 1990 becomes more defined for variousindustries, fewer solvents are being allowed for cleaning and their vaporpressures are going down, making them more difficult to remove. Solventsand wipe cloths should be clean. A level of 100-ppm dissolved contaminationin the solvent with clean, lint-free wipes should be maintained. Before thesolvent dries, it should be wiped off with a clean dry wiper. This way the soilis removed, not just spread around. Because these solvents find their way intothe atmosphere, the wipers should be placed into closed containers until sentto reclamation or to hazardous waste disposal. Sealed plastic bags will work.“Old work,” which is defined as having previously been painted or primed,

generally requires different handling, in general. If the work is to be stripped ofthe coating and started over, one should proceed with the stripping and, whencompleted, remove or arrest the corrosion and start as if it were new work.If it is not necessary to remove the old paint, the surfaces can be preparedby mechanically removing any loose material. Wire brushing by hand or with amotor or a light brush-off blast are all methods that work well. The wire brushesshould be devoted to a substrate. Although stainless steel can work on all substrates,if it has been used on iron or copper, it should not be used on aluminumor magnesium unless thoroughly cleaned including an acid pickle to remove allforeign metal. Iron wire and copper or brass brushes should only be used on likesubstrates.Finally, hand sanding, using sandpapers or mat abrasives, or “dust blasting”should be used on the surface of good materials to roughen the surface and toremove the surface oxidized layers of materials. This will promote better adhesionbetween the old and the new paint.After any of the above mechanical treatments, the work should be solvenwashed or wiped to remove all of the loose materials, and it should then be finalwiped as described above just prior to painting.

WASH PRIMER

This paint preparation goes by a number of names. In addition to wash primer,it is known as pretreatment coat, resin-acid coat, and acid-etch primer, andit has a number of military and commercial specifications that describe thematerial. The coating is a two-part mixture. The first part is composed of a(polyvinyl) butyl resin dissolved in ethyl and butyl alcohol with small amountsof zinc chromate and magnesium silicate added. The second part, which makesup 20% of the volume, is composed of phosphoric acid (85% ortho), ethyl alcohol,and water. The two parts are mixed just prior to application. The purposeof the phosphoric acid is to react with the metal surface, forming an in situ

metal phosphate. As the water and alcohol evaporate, the very thin resin formsa stabilizing coating over the phosphate and promotes adhesion of subsequentcoats. This system is only good when the metal substrate will react with thephosphoric acid to form the phosphate. If the acid is not essentially neutralizedby the reaction, the coating may appear satisfactory upon drying; but onaging, as water vapor penetrates through the coating, the acid will be reformedand will cause failure of the coating in service. The coating, nevertheless, isuseful as an adhesion promoter on some metals and will be recommended foruse when appropriate. There are some low-solvent wash primers on the market;these should be investigated if the material usually used will not meet the localair quality district’s requirements

CADMIUM

Cadmium surfaces should be stabilized with a conversion coating prior topainting. A phosphate coating is the usual way of doing this, but chromatecoatings, though usually used for corrosion protection, can be painted. Becausechromium (hexavalent) poses a health hazard, however, its use is discouraged.Phosphate coatings are usually applied from proprietary baths, all of whichshould be satisfactory as a paint base. Immersion for 2 to 4 minutes in thefollowing solution will leave the cadmium surface stable enough for painting:Phosphoric acid (85% ortho), 10 oz/galZinc phosphate, 3 oz/galRoom temperature

CHROMIUM

A fine sandblasting with one of the hard abrasives — aluminum oxide, siliconcarbide, etc. — to provide a matte but smooth finish, followed by solvent wipingto remove dust and a thin coat of primer, will develop satisfactory adhesion.Keep the time between blasting and priming as short as possible.

COBALT

Light sand blasting with aluminum oxide or silicon carbide to provide a matte finish,followed by solvent wiping to remove dust, will be adequate for priming. Washprimer may add additional topcoat adhesion if a traditional primer is not used.

COPPER

Conversion coatings on copper are usually not necessary; however, the blackoxide treatments used for adhesive bonding of copper are also paintable.Removal of oily soils, followed by wash primer, will give sufficient topcoatadhesion.

LEAD

Cleaning to remove all oily soils will be satisfactory for lead. The dull natural surfaceresulting from atmospheric exposure should not be removed. Wash primeror a very thin coat of traditional primer is recommended but not required,provided the first topcoat is applied thin.

MAGNESIUM

Magnesium must be conversion coated or anodized before painting. Productsused in mild environments and not subject to mechanical abuse can be conversioncoated. The following treatment can be used:Immerse parts for 1 1/2–2 minutes with agitation in the followingsolution:Ammonium phosphate monobasic( NH4H2PO4), 16 oz/galAmmonium sulfite [(NH4)2SO3.H2O)], 4 oz/galAmmonia (30%), 2 fl oz/galRoom temperatureAfter rinsing in cold water, immerse for 1 to 2 minutes in the following solution:Sodium hydroxide, 16 oz/galTemperature, 160–190°FMild agitationHot water rinse and dry and the part is ready for priming. This treatmentprovides less corrosion protection than the chromates used in the past, but whenused with a corrosion-protective paint system, it is satisfactory.Anodizing should be done on products used in a severe environment or sub-ject to mechanical abuse that may damage the paint. The following solution is

recommended:Potassium hydroxide, 22 oz/galAluminum hydroxide (see note 1), 4.5 oz/galPotassium fluoride (anhydrous), 4.5 oz/gal

Trisodium phosphate (Na2PO4. 12H2O), 4.5 oz/galPotassium manganate [or permanganate (see note 2)], 2.5 oz/galRoom temperature

Current density, 18–20 A/ft2Voltage, 0–60 VACTime, ~8 minNote 1: Aluminum (1100 alloy to prevent impurities in the bath) can bedissolved into part of the potassium hydroxide in a separate tank and thendecanted (or filtered) into the tank.Note 2: Dissolve permanganate in water prior to adding to the tank.Make the parts one electrode and the tank the other electrode, or (in a plastictank) divide the load into approximately equal areas and make each half aseparate electrode.

MOLYBDENUM

After cleaning, to provide a water-break-free surface, the paint coatings can beapplied. Wash primer can be used but is not necessary.

NICKEL

Nickel will respond to the wash primer pretreatment before applying the topcoat.A light sandblasting to produce a matte surface, which can be primed withthe wash primer or a regular primer, will produce satisfactory topcoat adhesion.

SILVER

The cleaned surface can be painted without further processing. Silver that hasbeen chromate conversion coated will also paint satisfactorily. The conversioncoating stabilizes the silver from sulfide tarnishing. Surfaces with sulfide tarnishwill accept paint so long as the tarnish is not powdery.

STAINLESS STEEL

Stainless steel should be cleaned and can then be given one of the passivationtreatments followed by wash primer, or it can be sandblasted and followed assoon as possible with either a wash primer or a regular primer.Heat-treated materials that have been put through an alkaline scale conditionerand have then either been acid cleaned or hydroblasted to remove scaleresidues can be painted without further processing.

TITANIUM

Untreated titanium exists in the atmosphere as a passive material, and as such,it is paintable; however, adhesion is only moderate. A conversion coating or ananodize coating is recommended. A phosphate–fluoride conversion coating isapplied by immersion for 2 to 3 minutes in the following solution:Sodium phosphate (Na2PO4), 6–7 oz/galPotassium fluoride (KF.2H2O), 2–3 oz/galHydrofluoric acid, 2–3 fl oz/galRoom temperatureAnodizing can be accomplished by making the part anodic at 10 VAC for 20minutes in a 70 to 80 oz/gal solution of potassium hydroxide. Steel tanks canbe used as cathodes and to hold the solution. Titanium will heat and the solutionwill produce a caustic spray, so it should be covered and handled carefully.Both of the treatments will provide satisfactory paint adhesion.

ZINC

A phosphate coating applied after electroplating will render zinc-plated partspaintable. A solution containing 10% phosphoric acid (85% ortho) and zincphosphate or dissolved zinc (0.5–2.0 oz/gal) will provide a coating satisfactoryfor painting; however, the proprietary processes intended for heavy phosphatecoating are also satisfactory.New galvanize should be thoroughly washed and given a phosphate conversion

coating prior to painting. An alkaline resisting primer will give best resultsfor outdoor exposures. Assembled and weathered galvanize can be washed witha solution of 25% phosphoric acid (85% ortho), 25% isopropyl alcohol, 0.25%wetting agent, and balance water. Allow the solution to stay wet on the partfor a few minutes, then wash off by spraying with water (hose or spray bottle,depending on size), and then dry and paint.Wash primer can also be effective on weathered galvanize, but it may notwork well on new work.

ALLOYS

The zinc-nickel and tin-zinc alloys that are emerging as cadmium replacementswill respond to both chromate and phosphate conversion coats. Owing to environmentalpressures the chromates are not recommended; hence, the phosphatecoatings are excellent. Wash primer without conversion coating also works well

for a pretreatment.