مقالات آماده سازی قطعات

cleaning, pretreatment & surface preparation

PAINT PRETREATMENTS FOR

ALUMINUM

BY ANTHONY O. ITA

HOUGHTON METAL FINISHING CO., ALPHARETTA, GA.

Chemical conversion coatings are the most widely used prepaint treatment processesfor metal substrates. Processes specifically designed for aluminum are ofrecent origin. These include accelerated chromate phosphates, chromate oxides,anodizing and, very recently,nonchromate formulations. Historically, however,phosphoric acid cleaners, wash primers, and iron and zinc phosphates have allbeen utilized as paint pretreatments with satisfactory results. Among these,thechromate types provide the most reliable under film corrosion protection and

paint adhesion. Anodized coatings, especially unsealed sulfuric and chromic acidtypes, are also comparable in performance, but fall far short of the productivityand cost effectiveness of the chromate processes.A careful evaluation of critical product requirements is essential in selectingthe right conversion coating for anticipated field performance. Paint pretreatmentsmust assure these corrosion protection functions: passivation of basealuminum, action as a barrier against moisture, oxygen, and other corrosiveagents, electrochemical insulation, and protection against mechanical erosion.In addition,conversion coatings must provide other essential interfacial propertiescomplementary to the paint top coat, including an effective and continuousbonding site; chemical stability during the service life of painted products;remaining insoluble, impervious, and flexible; providing a wettable subsurfacefor paint application; and maintaining adhesive integrity between the base metaland painted film. See Table I for typical performance data.

CRYSTALLINE PHOSPHATES

These are primarily accelerated iron and zinc phosphate processes adapted fromiron and steel pretreatment. Phosphating solutions typically contain metalhydrogen phosphate salts with limited free acidity. The metal phosphates aresoluble in strong acids but crystallize out when the acidity is reduced. Thismechanism occurs as the acid ions react with the aluminum surface, become neutralized, and produce an integral crystal growth on the metal surface. Thealuminum surface is therefore converted to a finely crystalline phosphate filmwith acceptable texture for paint bonding. Crystalline phosphate films may beiridescent to gray. Coating weights range from 10 to 50 mg/ft2 for iron phosphatesand 100 to 300 mg/ft2 for zinc phosphates.Properly applied, this group of phosphates provides good corrosion protection.

Iron and zinc phosphates find widespread use in mixed steel and aluminumproduct lines. They are popular because of low operational costs and mildenvironmental toxicity. Bath life is, however, very limited due to low tolerancefor aluminum ion accumulation. Application is by immersion at 125 to 140¡ for1 to 4 minutes, or spraying at 125 to 160¡F for 30 seconds to 2 minutes. Productselection should be restricted to moderate field service environments.

CHROMATE PHOSPHATES

Chromate-phosphate coatings enjoy a privileged position in aluminum prepainttreatment. They have a historic significance as being the first pretreatmentspecifically developed for aluminum in 1945. Since then, these productshave performed remarkably well for the architectural metal and beverage canindustries. Demand for cans,however, is on the decline. Recently introducedhigh-performance topcoats are more forgiving toward nonchromate prepainttreatments.Chromate-phosphate coatings are applied by spray or immersion.Immersiontimes range from 30 seconds to 3 minutes at 110 to 130¡F, whereas sprayingis done at 15 to 45 seconds at 95 to 130¡F. These baths produce crystalline oramorphous coatings of 15 to 1,000 mg/ft2. The film is iridescent to grayishgreen. Thickness can be as high as 0.1 to 0.4 mil. A typical air-dried coating isgiven as 50% to 55% chromic phosphate, 17% to 23% aluminum phosphate, 22%to 23% water, and a trace of fluorides.Performance properties of chromate-phosphate films are generally very closeto chromic acid anodizing films and those of chromate-oxide films to be discussedlater. Adhesion and corrosion protection increase with coating weight upto a point, then fall off. The best range is between100 and 200 mg/ft2.Because of its excellent qualities, the American Architectural ManufacturersAssociation (AAMA) has designated the chromate-phosphate process as a standardprepaint treatment. It also meets U.S. military specifications. Generally, thisprocess is recommended for severe and long-term service conditions.

CHROMATE-OXIDE COATINGS

Chromate-oxide films are more versatile and widely applied than the chromatephosphatetreatments. They comprise the bulk of treatments for the coil stockand transportation industries. In applications where anodizing is not feasible,for example, where parts are too long or assembled with dissimilar metals, chromatetreatments of this type have been used in place of anodizing.Typically, a chromate-oxide bath consists of three principal constituents: acidchromates, etchants and accelerators or complexing agents.

Application may be by spray, immersion, or brush at 70 to 110¡F for 15 to45 seconds. The aluminum surface is converted to an iridescent golden yellowcolor. The film is tightly adherent, amorphous, and mixed with metallicoxide products. Film thicknesses range between 0.005 and 0.04 mil. Coatingweights are from 15 to 100 mg/ft2. A freshly formed film can be leached to anearly colorless appearance.Chromate-oxide films have superior performance ratings compared to chromate-phosphate coatings. Unpainted films have almost twice the salt sprayresistance of a chromate-phosphate coating. Chromate parts are known to havepassed 2,000 hours of salt spray. Such performances are partly explained bythe fact that these films retain hexavalent chromate ions in their structures. Incorrosive environments, they trigger a repair mechanism into action. Chromatefilms are capable of withstanding very severe service conditions. They complywith military specification MIL-C-5541 and AAMA 605.2.

ALKALINE CHROMATES

These are among the oldest coating processes analogous to the phosphatetreatments described earlier. One ingredient serves to attack the aluminumsubstrate to a limited degree, while a second functions to form an oxide coatingon the reactive substrate. Alkaline chromates are primarily solutions of 2% to3% sodium carbonate and 0.5% potassium dichromate. Immersion times rangefrom 10 to 20 minutes at 180 to 190¡F. Thin (0.04 to 0.1 mil) gray and porousfilms made up of aluminum oxide and dispersed chromate oxides are produced.Maximum corrosion resistance is achieved by sealing in hot 5% potassiumdichromate. Consistent coating action depends on the correct ratio of carbonateto dichromate in the treatment solution.

MISCELLANEOUS TREATMENTS

No survey of prepaint treatment is complete without mention of numerousother products being successfully used in diverse applications. Some of the oldersuccessful ones include wash primers, chemical oxide films, and mechanicaltreatments such as wire brushing and sandblasting. Wash primers are primarilypigmented polymeric organic chromate compounds similar to paint. They haveexcellent corrosion protection and adhesion properties. Humidity resistance ispoor.Environmental restrictions of the last decade have generated exciting interestin chromium-free products. A substantial number of these have met acceptablerequirements for the beverage can industry. Testing is in progress in coil coating

and architectural segments. There is a strong indication that chromium-freeproducts will eventually meet the stringent requirements of the architecturaland transportation industries. Some of the shortcomings of current productsare being gradually eliminated. New high performance top coat paints such assilicones, fluoropolymers, and powder coats can minimize performance requirementsfor these prepaint treatments. Advances in application technology ofelectrocoat systems are also proving beneficial.

PROCESS GUIDELINES

Conversion coating processes are essentially multistep operations.Preconditioning stages, such as cleaning, rinsing, and postrinse treatments aresignificant factors in assuring critical final results. Spray and immersion systemsusually show the same order of operation. Stages consist of the following:

1. Clean

2. Rinse

3. Acid deoxidize (optional)

4. Rinse

5. Conversion coat

6. Rinse

7. DI/Acidulated/Seal rinse (optional)

8. Dry.

In the basic five-stage operation, the deoxidize and seal rinse stages are oftenomitted. The chromate bath initially functions as a deoxidizer before reactingto produce the desired conversion coating. Many installations easily meet thisdesign criteria, but others, utilizing alkaline cleaners loaded with dissolved aluminum,generally produce poor quality and powdery films.Cleaning, on the other hand, is very critical. General dirt, surfaces oils, grease,debris, and natural oxides must be removed in the cleaning process to achieveuniform coating and paint adhesion. Nonetch and etch-type cleaners may beapplied with satisfactory results. Slight etching is desirable. Timely rinsingprevents soil dry-on and redeposition.Rinse stages require fresh water input to neutralize, dilute and prevent crosscontaminationbetween the stages. Water quality requirements for the chromateand final rinse stages are even more critical. Excessive hard water salts andsoluble contaminants are detrimental to coating performance. Very often, thesecontaminants have been known to precipitate on the chromated film and causepaint blistering or corrosion problems when moisture penetrates the paint filmin the field. The water supply should not exceed the following impurity limits.Total dissolved solids should not exceed 150 ppm, chlorides 15 ppm, sulfates(as SO4) 25 ppm, and total hardness (as CaCO3) 200 ppm. Deionized water isrecommended for the chromate and final rinse stages if the incoming waterquality falls below these limits.

TESTING AND QUALITY CONTROL

Testing and quality control are important parts of any manufacturing process.A large majority of the prepaint processes are proprietary. Supplier recommendationson equipment and bath maintenance must be observed to produceacceptable results consistently. Frequent bath analysis or on-line process controlgreatly reduces rejects and lowers cost.

COATING WEIGHT PROCEDURE

Coating weight and occasional salt spray determinations on the unpainted partsare important tools in evaluating work quality. The coating weight procedureis as follows:

1. Solvent degrease and blow dry a 3- x 3-inch panel. Weighaccurately to ± 0.2 mg.

2. Immerse for 2 to 5 minutes in a molten salt bath (temperature 620to 670¡F) consisting of reagent grade sodium nitrite (NaNO2). Agedpanels may require up to 15 minutes for stripping.

3. Remove from the salt bath and carefully rinse in cold water.

4. Dip for 30 seconds in equal parts by volume of concentrated nitric acid and water at room temperature.

5. Rinse thoroughly in cold water and blow dry.

6. Reweigh.

7. Repeat steps 2 through 6 until the weight loss is less than 0.6mg.

8. Calculate the costing weight as follows:Coating weight = total weight loss/total areaVerify that the observed coating weight meets the required specifications.

Manufacturing specifications for coating weights differ for various applications.Guidelines for quality manufacturing practices are covered by specificationsfrom agencies such as American Society for Testing and Materials (ASTM),American Architectural Manufacturers Association (AAMA), military specifications(MIL), and AeroSpace Material Specifications (AMS). See Table II fortypical examples.

Other Test Requirements

Salt spray resistance tests are also required as standard quality-control practices.Comprehensive testing procedures listed in the following standards andspecifications evaluate the performances of the composite film consisting of theconversion coating and paint topcoat.ASTM D 1730: Standard practices for preparation of aluminum andaluminum-alloy surfaces for painting.ASTM B 449: Standard practice for chromate treatments on aluminum.MIL-C-81706: Chemical conversion materials for coating aluminum andaluminum alloys.MIL-C-5541: Chemical conversion coatings on aluminum andaluminum alloys.AAMA 603.8: Voluntary performance requirements and test proce

dures for pigmented organic coatings on extruded aluminum.AAMA 605.2: Voluntary specification for high-performance organic

coatings on architectural extrusions and panels.AMS 2473: Chemical treatment for aluminum-base alloys, generalpurpose coating.

Table II. Coating Weight Classifications

What then are the critical factors in a coating system? Under film corrosionprotection is an independently quantifiable property of prepaint treatments.You can measure the unpainted salt spray or humidity resistance and assessperformance and quality.On the other hand, top coat paint adhesion, as it relates top pretreatmentquality can sometimes be difficult to measure. This property is jointly dependenton the two coating systems considered. A third factor is substrate metal.Alloy quality significantly affects the overall performance of the coating system.Other factors being equal, paint quality should be reviewed for inclusion inquality planning, especially if an adhesion problem occurs. Significant qualitydefects may be contributed by high levels of low-molecular-weight resinand additive fractions in the formulation. These fractions usually have poorwetting properties. This results in weak boundary layers along the underlyingsubstrate. Closely related to the adhesion properties of the paint system ismoisture permeability. Corrosion and loss of adhesion is accelerated by poor

permeability properties of the paint topcoat