Influence of surface pretreatment and electrocoating parameters on theadhesion of cathodic electrocoat to the Al alloy surfaces

C.M. Reddy, R.S. Gaston, C.M. Weikart, H.K. Yasuda*

Surface Science and Plasma Technology Center, Department of Chemical Engineering, University of Missouri, Columbia, MO 65211, USA

Received 20 October 1997; revised version received 21 September 1998; accepted 24 September 1998

Abstract

Adhesion of cathodic electrocoat films to the aluminum alloys 2024-T3 bare and Alclad 2024-T3 with different pretreatments and withdifferent cathodic electrocoat process parameters was investigated. The pretreatments studied were acetone wipe and alkaline cleaning. Thecathodic electrocoat process parameters studied include variation of cathodic electrocoating voltage and time. Adhesion performance wasevaluated by measuring the delamination time and percent delamination of the electrocoat from the alloy surface by placing the smallspecimen of the sample in theN-methyl pyrrolidinone (NMP) solution at 60°C until the film lifts off or for 2 h whichever comes first. NMPtimes for electrodeposited film delamination from alkaline cleaned surfaces were found to be higher than the acetone wiped and or those ofas-supplied metal surfaces. There was not much effect of acetone cleaning of these alloy surfaces on the adhesion performance of thecathodic electrocoat. The voltage–current (of cathodic electrocoating process) relationships for alkaline cleaned surfaces were also found tobe significantly different from the other two types of surfaces. The NMP times of cathodic electrocoat delamination at lower cathodicelectrocoating voltage and lower electrocoating times were higher than those at higher cathodic electrocoating voltage and electrocoatingtimes for alkaline cleaned 2024 bare surfaces. Electrocoat thickness developed on the surfaces during the electrodeposition processincreased with increasing electrodeposition voltage and time as anticipated. 1998 Elsevier Science S.A. All rights reserved

Keywords:Cathodic electrodeposition; Paint adhesion; Aluminum alloys; Surface pretreatments; NMP test

1. Introduction

Good adhesion of polymers to the metal surfaces is animportant parameter in the protection of the metal fromcorrosion and mechanical stress. Adhesion improvementof metal polymer bonds has been a topic of research forthe past several years [1–4]. There have been several pre-treatment methods developed in the past to improve theadhesion of the paint to the metal surface [2]. The adhesionof polymers depends on the characteristics of the metalsurfaces which include surface roughness, surface contami-nants, nature of chemical bonds on the surface, etc. [3,4]before the polymer film is applied. Mechanical interlockingof the polymer with porous surfaces was the one of thefocused studies in the improvement of the adhesion ofmetal polymer interfaces [5]. In the course of several

years, many pretreatments have been investigated toimprove the adhesion of polymer to the metal surfaces.The pretreatment processes developed ranged from the sur-face cleaning to the surface conversion into different oxidesto improve the adhesion of the paints to the metal surface[1]. The adhesion performances of polymer metal bondswere found to be better for the anodized surfaces of Alalloys because of their porous column structure.

Cathodic electrocoating has been widely used in automo-tive, industrial and appliance areas in recent years in thecorrosion protection system as a primary layer coating ortop coat [6]. The chemistry of cathodic electrocoating andelectrodeposition parameters has been a subject of severalinvestigations [7–11]. Cathodic electrocoating has severaladvantages including high throw power, superior corrosionprotection, high coating utilization (.95%), low level ofpollution (aqueous system), easy to automate, etc. whichmakes this system an attractive coating system. Cathodicelctrocoating is a fairly simple process and can be used in

Progress in Organic Coatings 33 (1998) 225–231

0300-9440/98/$ - see front matter 1998 Elsevier Science S.A. All rights reservedPII : S0300-9440(98)00068-X

* Corresponding author.

different scales. High throw power makes the cathodic elec-trocoating process attractive in the automobile industry asthe electrocoat penetrates into cavities and pores of curvedshaped parts.

The process parameters of the electrodeposition processhave been largely determined by the bonding polymer char-acteristics [10]. Several investigators have found that thecathodic electrocoated films provide superior corrosionprotection of the metal. The variation of adhesion strengthsof the electrocoat to the metal surfaces with electrodeposi-tion parameters as well as the pretreatments of the metalsurfaces have not been elucidated in the past. It has been acommon practice to conversion-coat aluminum surfaces toachieve good adhesion of the primers or paints. These con-version-coating processes use health hazardous materials,which are under severe Environmental Protection Agencyregulatory restrictions. It is the motivation of this investi-gation to develop environmentally benign processes foraluminum alloys, which could possibly replace the hazar-dous conversion coatings to achieve similar or better adhe-sion performance.

Aluminum alloys 2024-T3 bare and Alclad 2024-T3 havebeen used in the aircraft industry because of their highermechanical strengths. Pure aluminum, which has high cor-rosion resistance properties, has limited applicationsbecause of its low mechanical strengths [12]. By addingsmall quantities of alloying elements the mechanicalstrengths of aluminum are increased several fold. Althoughmechanical strengths have been improved the addition ofalloying elements reduces the corrosion resistance of thesemetals drastically. The chromate conversion coating pro-cess, which has excellent corrosion protection property,has been used for the past several years to protect themetal from the corrosion [13]. The use of chromates hascome under severe restrictions because of their healthhazards and necessitated a need for environmentally benigncorrosion protection processes [14].

Electrodeposition is an excellent process because of itslow level of pollution. The adhesion of the electrodepositedpolymer metal bonds is an important factor in the protectionof the metal from corrosion. The nature of the oxide on themetal surface plays a very important role in the adhesionperformance of the metal–polymer interface systems. In thepresent study we have looked into the adhesion strengths ofelectrodeposited films on the smooth surfaces of aluminumalloys AA 2024-T3 bare and AA Alclad 2024-T3 which areused for aircraft building. Cathodic electrocoating is beinginvestigated for the application of airplane constructionmaterials as this process has several advantages which canbe exploited for our benefit. The objective of this study is toinvestigate the adhesion strengths of the cathodic electro-coat to different pre-cleaned smooth surfaces of aluminumalloys at different electrodeposition conditions such as elec-trocoating voltage and time. A simple pretreatment likealkaline cleaning was studied as a possible alternative toreplace the hazardous conversion coatings.

2. Experimental

2.1. Materials

The Al alloys panels, with size 3× 6 × 0.034 inch, usedfor the present study were 2024-T3 bare and Alclad 2024-T3 procured from Q-Panel Lab Products. The cathodic elec-trocoat used was a mixture of 44 wt.% resin emulsion(BASF U32CD033A), 8 wt.% electrocoat pigment paste(BASF U32AD290), 48 wt.% DI water and 4 vol% electro-coat additive (BASF 20CD0043). Turco 4215S was used asalkaline cleaner for the chemical cleaning of the Al alloyssurfaces. The solvents, acetone andN-methyl pyrrolidinone,were procured from Fisher Scientific The thickness of theelectrocoat films was measured by Elcometer 355 with anon-ferrous probe.

2.2. Experimental procedures

The cathodic electrocoating was carried out on threedifferent kinds of panels: (i) without cleaning, (ii) acetonewiped and (iii) alkaline cleaned at three voltages 170, 200and 250 V and at different electrocoating times from 0.5 to4.0 min. In the case of without cleaning panels, the panelsof alloys of both kinds were used as supplied by Q-Panel.When observed visually the panels of 2024-T3 bare hadsome kind of protecting layer and ink marks of the panelidentification tag printing. Alclad 2024-T3 panels had shinysurfaces with panel identification ink marks. In the case ofacetone wiped panels, the panels were wiped with acetoneusing tissue paper (Kimwipes , Fisher Scientific) to cleanthe ink marks and loose organic matter on the surfaces ofthe panels. In the case of alkaline cleaning panels, thepanels were immersed in the alkaline bath (about 4 l solu-tion) for about 25 min, or until panel becomes water breakfree when rinsed with DI water, and rinsed with DI waterand air dried. The composition of the alkaline cleaner solu-tion was maintained such that a water break free surface isobtained while rinsing with DI water after immersing in thealkaline bath for a definite time. Water contact angles of allthe surfaces were measure by a sessile drop method beforepanels were used for electrodeposition. The electrodeposi-tion was carried out in a 1 gallon electrocoat bath by usingthe substrate as cathode and a stainless steel strip (1.5× 10inch) as anode. Darrah Digital DC power source withvariable voltage facility was used for the electrodepos-ition.

The electrodeposition was carried out in galvano-poten-tiostatic mode as follows: the panel was immersed in theelectrocoat bath by using a paper clip and the DC powersource was switched on. The current was controlled under 1A in the initial stages and the voltage was slowly increasedto maintain the current at 1 A as the electrodeposition pro-ceeded. Once the current drops sharply (within 1 min), thevoltage was raised to the predetermined value and main-tained throughout the remaining time. Electrocoating

226 C.M. Reddy et al. / Progress in Organic Coatings 33 (1998) 225–231

times were controlled by automatic function on the DCpower supply. The electrocoat deposited panels were thenrinsed with deionized (DI) water to wash off the loose elec-trocoat from the surface. Panels were dried in air for 30 minand cured in an oven for 30 min at 300°F, which is thecuring temperature recommended by the manufacturer ofelectrocoat materials. This curing temperature is the lowerside when compared to automotive cathodic electrocoat cur-ing temperature. For aluminum, lower electrocoat cure tem-peratures are preferred as higher temperatures have adetrimental effect on the mechanical and chemical proper-ties of Al alloys.

2.3. NMP test

The test specimens of 0.5 inch diameter were punched outof the cured panels and used for theN-methyl pyrrolidinone(NMP) test. The samples were punched with a hand punchby taking care to minimize the mechanical damage to thespecimen surface and edges. The NMP test, first developedby van Ooij et al. [15], is a very good method for distin-guishing the adhesion strengths of the electrodepositedpolymer films on the metal surfaces. The NMP test hasbeen used to distinguish the adhesion strength of the electro-coat to the substrates. The NMP test was performed as fol-lows: first specimens were punched out of the curedelectrocoated panels and were placed in the NMP solutionwhich was preheated to 60°C and a stop watch was started.The NMP solution temperature was maintained at 60°Cwhile closely observing the specimen for delamination.When the total electrocoat film lifts off from the specimen,the time was noted as NMP time otherwise the specimenwas left in the NMP solution for 120 min. Percent adhesionof electrocoat film was noted by visual observation for thespecimens, which lasted 120 min in the NMP solution with-out total delamination of the electrocoat film.

3. Results and discussion

The adhesion of paints and organic coatings to the metal-lic substrates has been studied for years to understand thenature of the adhesion and many attempts have been madeto improve the adhesion. Bond strengths of polymers on Alalloy surface vary depending on the nature of the surface,the surface roughness and chemical bonds before the poly-mer was placed on the substrate [4]. Various chemical pre-treatments have been found to have varying performanceon the corrosion protection of metals [16,17] by the electro-coat deposition. Mainly conversion coatings were em-ployed to improve the adhesion performance of metal poly-mer bonds. The adhesion performance of polymer metalinterfaces was attributed to mechanical interlocking. Adhe-sion performance of smooth surfaces was thought to bepoor for lack of roughness disregarding the role of oxideon the surface.

3.1. Electrocoating process and electrocoat thickness

Cathodic electrocoating of different surfaces, withoutcleaning, acetone cleaning and alkaline cleaning, was per-formed at different voltages and different electrocoatingtimes. As this process was carried out by current–voltagecontrol mode operation, the current was maintained at 1 Awhile increasing the voltage as the current drops. Typicalelectrodeposition voltage–time and current–time relation-ships for electrodeposition on 2024-T3 bare surfaces areshown in Figs. 1 and 2. As seen from voltage–time relation-ship, the deposition process is different on acetone wipedand alkaline cleaned surfaces.

The resistance of the electrodeposited film during thedeposition process is calculated from the voltage–currentrelationships for both 2024-T3 bare and Alclad 2024-T3and are shown in Figs. 3 and 4, respectively. Without clean-ing (which are not shown in Figs. 3 and 4, since they weresimilar to acetone wiped surfaces) and acetone cleaned sur-faces of 2024-T3 bare had the same trend for the current andvoltage during the process while alkaline cleaned surfacesshow lower resistance (Fig. 3). Alkaline cleaning removesthe organic material, which is difficult to remove by acetonesolvent wipe at room temperature, and loose oxide from thesurface, which reduces the resistance to the current flowinitially.

Once the film attains a certain thickness, about 10mm forboth alloys, then the resistance reaches a certain plateauvalue and increase of resistance is very slow. The resistanceof the electrodeposited films reaches its plateau value within1 min of the deposition process. The behavior of alkalinecleaned surfaces of Alclad 2024-T3 is not so much differentfrom the acetone cleaned surfaces at lower voltage (170 V)but a slight difference could be observed at higher voltage(250 V) as shown in Fig. 4. This could be due to the nature ofthe oxide layer on the Alclad 2024-T3 remaining unaltered,since these panels have comparatively lesser organic con-tamination on the surface. From this observation one can seethat the oxide layer is different on the different substrates.

Fig. 1. Typical voltage–time relationships of the electrocoating process for2024-T3 bare. The current was maintained at,1 A at the beginning of theelectrocoating and the voltage was increased to the set value when thecurrent dropped down.

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The thicknesses of electrodeposited films of all threetypes of surfaces of 2024-T3 and Alclad 2024-T3 at differ-ent electrodeposition times and voltages are shown in Figs.5 and 6. The thickness of all types of surfaces increases withelectrocoating time and voltage irrespective of pretreatmenttype. The alkaline cleaned surfaces of both alloys showhigher electrocoat film development than the other two pre-treatment type surfaces. The higher thickness on alkalinecleaned surfaces is easy to understand as the resistance ofthese surfaces is lower (Figs. 3 and 4). The electrocoatthickness and time follow, more or less, a linear relationshipin the lower electrocoating times (less than 1 min) whichcan be seen from Figs. 5 and 6 and the growth of the film isslower after a limiting thickness has developed on the sur-face.

Electrocoat thickness developed at lower electrocoatingtimes is higher for Alclad 2024-T3 than that for 2024-T3bare. This can be explained with the same principle of theconductivity of the surface, which is higher for clad alloy.

Surface wettability for without cleaned surfaces and acetonecleaned surfaces show that acetone cleaning is not removingall the rolling mill oil from the surfaces (see Tables 1 and 2).It appears that acetone wiping probably removes looseorganic matter like ink marks, dust particles, excessive roll-ing mill oil, etc. from the surfaces, and leaves the morestable organic film on the surface which makes the surfacemore hydrophobic.

3.2. Alloys as-received versus acetone cleaned

The NMP times for without cleaning, acetone cleaningand alkaline cleaned surfaces at three different electrodepo-sition voltages 170, 200 and 250 V and eight different elec-trodeposition times, 0.5–4 min, of 2024-T3 bare and Alclad2024-T3 are shown in Figs. 7 and 8, respectively. As can beseen from these figures, adhesion performance was notimproved by acetone cleaning of both surfaces. On bothalloys the differences in NMP times are not significanteither with electrodeposition voltage or with electrodeposi-tion time for acetone cleaned surfaces. This is not surpris-

Fig. 2. Typical current–time relationships of the electrocoating process for2024-T3 bare. The current was maintained at,1 A at the beginning of theelectrocoating and the voltage was increased to the set value when thecurrent dropped down.

Fig. 3. Typical resistance of the electrocoat film during the electrocoatingprocess for 2024-T3 bare. The current was maintained at,1 A at thebeginning of the electrocoating and the voltage was increased to the setvalue when the current dropped down.

Fig. 4. Typical resistance of the electrocoat film during the electrocoatingprocess for Alclad 2024-T3. The current was maintained at,1 A at thebeginning of the electrocoating and the voltage was increased to the setvalue when the current dropped down.

Fig. 5. Electrocoat thickness developed for 2024-T3 bare during the elec-trodeposition process. The current was maintained at,1 A at the begin-ning of the electrocoating and the voltage was increased to the set valuewhen the current dropped down.

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ing, as surface properties of both these alloys are similar ascan be seen from Tables 1 and 2. This indicates that strongsurface contaminants play an important role in the adhesionperformance of electrocoat on without wiped and acetonewiped surfaces.

3.3. Alkaline cleaning of surfaces

From Figs. 7 and 8, it is clearly evident that the alkalinecleaning improves the adhesion of electrocoat by over oneorder of magnitude in the case of 2024-T3 bare and at lowerelectrocoat times in the case of Alclad 2024-T3 than acetonecleaning. The NMP times of more than 2 h is observed forthe alkaline cleaned surfaces of 2024-T3 bare at all threeelectrodeposition voltages 170, 200 and 250 V. As seenfrom Figs. 7 and 8, at lower electrodeposition times adhe-sion performance is better on 2024-T3 bare surfaces for allthree voltages studied. For Alclad 2024-T3, all the alkalinecleaned surfaces at different electrodeposition times showed

similar behavior as that of 2024-T3 bare but the NMP timeswere not as high as those of 2024-T3 bare. The higheradhesion times are attributed to the alkaline cleaning,which removes the strong organic contamination and altersthe oxide layer. This could be seen from the lower resistanceof all the alkaline cleaned surfaces (Figs. 3 and 4). Thisindicates that the strengths of electrocoat film-oxide bondson alkaline cleaned surfaces are much stronger than those onacetone wiped surfaces.

The NMP test is the best method suitable for the differ-entiation of the adhesion performance of E-coat polymerfilms on different metallic surfaces including conventionalchromate conversion coated surfaces. This method cannotbe used for the adhesion performance evaluation of polymerpint films other than E-coat. Therefore the comparison of theadhesion performance of other polymeric paint films to E-coat films on the Al alloys with chromate conversion coat-ing or the pretreatments discussed in this paper cannot bemade. When compared, the NMP times of electrocoat filmson alkaline cleaned surfaces are much higher than the same

Fig. 6. Electrocoat thickness developed for Alclad 2024-T3 during elec-trodeposition process. The current was maintained at,1 A at the begin-ning of the electrocoating and the voltage was increased to the set valuewhen the current dropped down.

Table 1

The average contact angles, NMP times and electrocoat thickness for2024-T3 bare at different treatment conditions

Treatmentconditions

Averagecontactangle(°)

AverageNMPtime (min)

Averagethickness(mm)

Without cleaning 54.5 2.4 26.7With acetone 58.0 2.7 25.4With alkaline: 11.2 100.6 30.1Without cleaning and with

acetone for 170 V58.2 2.0 21.1

Without cleaning and withacetone for 200 V

60.0 2.7 25.4

Without cleaning and withacetone for 250 V

55.5 3.9 31.7

With alkaline for 170 V 10.5 115.6 24.5With alkaline for 200 V 11.5 108.6 28.8With alkaline for 250 V 11.5 77.4 37.0Chromate conversion

coated 170v62.8 1.5 21.0

Table 2

The average contact angles, NMP times and electrocoat thickness forAlclad 2024-T3 at different treatment conditions

Treatmentconditions

Averagecontactangle (°)

AverageNMPtime (min)

Averagethickness(mm)

Without cleaning 40.9 6.9 27.5With acetone 64.6 5.3 29.51With alkaline: 11.4 43.6 29.1Without cleaning or with

acetone for 170 V52 6.2 24.0

Without cleaning or withacetone for 200 V

53.3 5.8 27.4

Without cleaning or withacetone for 250 Vv

47.4 6.5 34.3

With alkaline for 170 V 11.4 51.9 25.36With alkaline for 200 V 10.9 60.4 27.5With alkaline for 250 V 12.2 19.4 33.9Chromate conversion

coated 170 V39.8 1.5 22.0

Fig. 7. Average delamination time in NMP solution versus electrocoating(e-coat) time for 2024-T3 bare without cleaned, acetone cleaned and alka-line cleaned surfaces at different voltages.

229C.M. Reddy et al. / Progress in Organic Coatings 33 (1998) 225–231

electrocoat film on chromate conversion coated surfaces(average NMP times of 1.5 min for the panel electrocoatedat 170 V for 2 min) of the same alloy (see Tables 1 and 2).This indicates that the adhesion strength of cathodic E-coaton alkaline cleaned surfaces of Al alloys is much strongerthan the same E-coat on chromate conversion coated sur-faces.

3.4. Effect of electrocoating voltage on adhesionperformance

The adhesion performance of the electrocoat films, whichexceeds 120 min of NMP time, is evaluated by the observa-tion of the film adhesion on the test specimen after 120 minof NMP time. The specimens were rinsed with DI waterafter 120 min in NMP solution and visually observed forthe percent adhesion of the electrocoat film to the Al alloysurface and the percent adhesion is recorded from thesespecimen. Figs. 9 and 10 show the percent adhesion ofelectrodeposited films on alkaline cleaned surfaces of2024-T3 bare and Alclad 2024-T3, respectively at differentelectrocoat voltages and electrocoating times.

From these figures it is clearly seen that at lower electro-coating voltages and times the adhesion performance on2024-T3 bare is better than higher electrocoating voltagesand times. Clearly 170 V and lower than 1.5 min of electro-

coating times favor the adhesion performance on 2024-T3bare alkaline cleaned surfaces which have almost 100%electrocoat film adhering to the surface. The electrodeposi-tion times of the process depend on the film thicknessdesired for protection of the material from corrosion aswell as mechanical damage. For higher electrocoating vol-tage (200 V), the adhesion performance of these surfacesdoes not depend on the electrocoating time. This is true inboth alloy surfaces of alkaline cleaned surfaces. Alclad2024-T3 has good adhesion performance on the alkalinecleaned surfaces with elctrocoating voltage of 170 V andelectrocoating time of 30 s.

4. Conclusions

The effect of pretreatments like solvent wipe and alkalinecleaning, electrocoating voltage and time on adhesion per-formance of cathodic electrocoat on aluminum alloys wasstudied. This study indicated that the nature of the oxide onthe aluminum alloy surface makes a huge difference in thecathodic electrodeposition process and adhesion of the elec-trocoat films. Stable organic contamination on the nativesurfaces seems to be limiting the adhesion performance ofelectrocoat films on them.

The process of cathodic electrocoating depends on thenature of the oxide layer on the surfaces. The solventwiped surfaces show higher resistance during the electro-coating, which limits the film thickness development. Alka-line cleaning which takes out most of the organiccontamination also alters the nature of the oxide on thesurface. These surfaces showed lower resistance therebyallowing higher electrocoat thickness to develop. The sur-faces of Alclad 2024-T3, which have thicker films of alu-minum oxide, do not show dramatic differences due to thelower thickness of the organic films.

The adhesion performance of the electrocoat films onalkaline cleaned surfaces of Al alloys 2024-T3 bare andAlclad 2024-T3 showed greater improvement when com-pared to the solvent wiped as well as without cleaned sur-faces.

Fig. 8. Average delamination time in NMP solution versus electrocoating(e-coat) time for Alclad 2024-T3 without cleaned, acetone cleaned andalkaline cleaned surfaces at different voltages.

Fig. 9. Percent adhesion of electrocoat (e-coat) after 2 h in NMP solutionversus electrocoating time for 2024-T3 bare alkaline cleaned at differentvoltages.

Fig. 10. Percent adhesion of electrocoat (e-coat) after 2 h in NMP solutionversus electrocoating time for Alclad 2024-T3 alkaline cleaned surfaces atdifferent voltages.

230 C.M. Reddy et al. / Progress in Organic Coatings 33 (1998) 225–231

From this study it could be concluded that, by monitoringthe nature of the oxide layer on aluminum alloys the adhe-sion performance could be improved by an order of magni-tude when compared with native surfaces. This study showsthat the conversion coatings, which are developed for im-provement of corrosion performance as well adhesion,could be eliminated by a simple process like alkaline clean-ing for improving the adhesion performance of cathodicelectrocoat films.

Acknowledgements

This study was in part supported by DARPA contract AFF33615-96-C-5055.

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