Anodized Aluminum Alloys

7/30/2019 Anodized Aluminum Alloys

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Anodized Aluminum Alloys

Insulator or Not

Donn G. BellmoreSr. Reliability AnalystUniversal Instruments Corporation

P.O.Box 825, Binghamton NY, 13902Phone Number: 607-779-7471

Fax Number: 607-779-5256

E-mail: [email protected]

Abstract: The Automated Handling Equipment (AHE) Industry presently uses expensive, exotic andenvironmental unfriendly conductive or dissipative finishes such as Silver Impregnated Anodize, Black Electroless Nickel, Black Chrome, and Black Copper to control ESD. The purpose of this study was todetermine if Anodized Aluminum would provide a sufficient range of dissipation. This paper describes the

result of controlled experiments of several plating suppliers and types of plating. When implemented,Dissipative Anodized Aluminum will provide a conservative annual savings of at least $1,000,000 to UniversalInstruments Corporation.

IntroductionIn the manufacturing of Automated HandlingEquipment (AHE), ESD control has to be designedinto the product. It is very difficult and expensive toadd ESD control solutions to the finished design.When designing ESD controls in AHEs, materialselection is very important. If the surface is very

conductive, charges may be dissipated too quicklyproducing high current that can damage componentsthrough various methods. If the surface is toodissipative or insulative, charges may be produced orinduced by various methods that can also damageproduct. Assemblies that either guide, handle, or arein close proximity (6 inches) to the path of customer’sproduct have to be made of materials and coatings thatwill wear well and provide a path to ground within 1ohm [1]. Anodized aluminum has always beenthought of as insulative in nature and if an inspectorwanted to make sure the aluminum was anodized, the

inspector would use an ohm meter and point probesthat would read infinity (over range) if the coatingwas indeed anodic. Also, from an electricalperspective, relative to ground, anodic finishes areinsulative.

Anodized aluminum has always been thought of as aninsulator and therefore detrimental to components andother product when used as a guide or path in some

way, or in handling the product. In assemblies,additional ground straps have been used to provideadditional ground paths for anodized aluminum.Engineers and designers in order to provideconductive surfaces use a number of finishes withresistance in the conductive range (< 105 ohms) [2].Some of the finishes are expensive, some are

environmentally unfriendly, and some do not wear aswell as anodic finishes or coatings. Black Chrome,Black Copper, Silver Impregnated Anodize, andBlack Electroless Nickel are all more expensive thananodized aluminum finishes (50% to 200% more)while black chrome is environmentally unfriendly andblack copper does not wear as well as anodize. Silverimpregnated anodic surfaces have been used inassemblies to reduce the resistance to a lowdissipative range (<107 ohms). In assisting to stabilizea silver impregnated anodizing process, it wasobserved that typical anodized plating was within thedissipative range (< 1012 ohms) [2]. If anodizedaluminum could be used as a dissipative surface inAHE, the annual savings would be 1 to 1.5 milliondollars, which is considered a conservative estimate. It was decided to conduct this experiment to definethe electrical characteristics of anodized aluminum.



1 The Anodizing ProcessAnodized aluminum is a multi-step process. Theobject to be anodized is immersed in an acid bath.The bath is an electrolyte and the object to beanodized is the electrically positive part or anode.Direct current is then applied and hydrolysis takesplace releasing a high concentration of oxygen in auniform manner. The oxygen reacts with the

aluminum anode to form a greatly thickened, hard,and porous film of aluminum oxide.

The object is then immersed in a bath of dye or otherpigments or special electrolytes to obtain the specificcolor and then sealed by immersion into a bath of boiling water and other chemicals, closing or fillingthe pores in the aluminum oxide. Depending on theprovider’s process and the application, there may beother steps involved such as cleaning or etching priorto anodizing, rinsing, and drying in between the mainsteps.

Typically, there are 3 types of anodize withmodifications available in each type. Chromic acidand sulfuric acid produces films typically from .0002to .0007 of an inch. Hard Anodize or Hard Coatconsists of a modified sulfuric bath, which produces aharder, more dense, and thicker coating. HardCoating is usually opaque and will vary from gold tobronze or gray to black. Its thickness is usually .0005to .0015 inches thick. Silver Impregnated Anodize isa process in which the anodized aluminum prior todying and sealing is placed in a chemical or

electrolytic bath and silver is impregnated in the poreswith the intent of providing a path through the anodiclayer to dissipate electrical charge.

2 Experiment

2.1 Test Design

All 63 samples had to be clearly identified so thatcorrelations in resistance could be made between

suppliers and the supplier’s process steps at 12 %and 50 % relative humidity. Supplier 1 supplied

45 samples, Supplier 2 supplied 6 samples andsupplier 3 supplied 12 samples. The samplesconsisted of 6”X6”X .030” thick coupons of 6061 T 6Aluminum.

Cleaning of Samples and Probes:All measurement surfaces of the samples andelectrodes were cleaned with 70% isopropyl alcoholand water as needed.

2.2 Material TestedSupplier 1 Samples

6 samples anodized, no dye, and no seal6 samples anodized, dyed (black), and no seal6 samples anodized, dyed, and sealed6 samples hard coat, no dye, and no seal

6 samples hard coat, dyed (black), and no seal6 samples hard coat, dyed, and sealed3 samples anodized, dyed red and sealed3 samples anodized, dyed yellow and sealed3 samples anodized, dyed blue and sealed

Chemistries Used:Clariant’s H3LW Black DyeClariant’s Anodal MS-1 Nickel Acetate SealClariant’s MF Blue A DyeClariant’s Bordeaux RL Red DyeClariant’s Gold L Dye

Supplier 2 Samples

2 samples hardcoat aluminum, no dye, and no seal2 samples hardcoat aluminum, dyed, and no seal2 samples hardcoat aluminum, dyed, and sealed

Chemistries Used:Clariant’s H3LW Black DyeClariant’s Anodal MS-1 Nickel Acetate Seal

Supplier 3 Samples

2 samples anodized no dye, no seal,2 samples anodized baked, dyed (black) no seal2 samples anodized baked, dyed, sealed2 samples of hard coat, no dye, and no seal2 samples of hard coat, dyed (black), and no seal2 samples of hard coat, dyed, and sealed

Chemistries Used:

Clear Nickel Cobalt SealBlack Dye-Sanadol H3LWSulfuric AnodizeHardcoat Anodize

2.3 Equipment Used

Probes: 2.5 inch diameter conductive rubberelectrodes weighing 5 lbs. each.Power supply: Sorensen DRC600.75B



Meter for current : Keithley 199 SystemDMM/ScannerVolt Meter: Fluke Bench Meter, 8050A

2.4 Test Set UpResistance measurements were made in environmentsof 25 degrees C at 12 % and 50 % RH using anEnvironmental Chamber. A voltmeter measured the

voltage output of a power supply and a meter wasused to measure the current. See Illustration 1 fordiagram of set up instrumentation. The resistance was

calculated.The

voltagewas always

increasedfrom zeroto a level

that

produced a significant

current. Both voltage andcurrent were recorded. All samples and probes wereplaced in the chamber as seen in Figures 1and 2 at theprescribed environment and soaked for 24 hours priorto making measurements. Since there were many

measurements to bemade, all of thesamples were placedin the chamber forthe duration of thetest. Threemeasurements were

set up at a time andafter they were madethe chamber was

opened and the next setof measurements setup. One hour after the

humidity level stabilized to the set level, the next setsof measurements were made. Two sets of

measurements, A andB were made persample as illustratedin Figure 3. The

samples werescanned using aScanning ElectronMicroscope (SEM) todetermine the degreeof porosity and

pitting. Since the SEMhas a size limitation,

the samples were cut along the dashed lines indicatedby Illustration 3.By making therecommended cutsthe scanned areaswould be the sameareas contacted bythe measurement

probes. Themeasurement set

and sample number wererandomized to minimize any

inherent variation of the measurement process. Afterthe SEM scans were complete the samples were thenplaced into a salt spray and fog environment for aperiod of 336 hours to determine the quality of theanodized surface.

3. Results3.1 Resistance Measurements

The following tables describe the anodize process andgraphs indicate the average resistances of multiplemeasurements made under the stated conditions.

Table 1

Category Samples Treatment Dyed Sealed

1 6 Anodized2 6 Anodized Black

3 6 Anodized Black Yes4 6 Hard Coat

5 6 Hard Coat Black 6 6 Hard Coat Black Yes

7 3 Anodized Red Yes8 3 Anodized Yellow Yes

9 3 Anodized Blue YesTable 1 lists the samples and treatment from Supplier1. Note that Supplier 1 was the only supplier of multiple colors.

Graph 1

Surface Resistance, Supplier 1

and Humidity

1.0E+05

2.0E+07

4.0E+07

1 2 3 4 5 6 7 8 9

Category #

R e s i s t a n c e

( O h m s )

12 % RH

50 % RH

Graph 1 is the average resistance of 2 measurementsof each of the six samples of each process fromSupplier 1 at 12 % and 50% R.H. Note that samples

B

A B

A

Figure 1:

Environmental Chamber

Figure 2:Measurements & Samples

Illustration 1

Illustration 1



6,7,and 9, which matches the treatment listed in Table1, were higher than the other samples while still well

within the dissipative range.

Table 2


1 2 Hard Coat Yes No

2 2 Hard Coat Yes Yes3 2 Hard Coat No No

Table 2 lists the samples and treatment from Supplier2, which was only Hard Coated, some samples weredyed and some were sealed.

Graph 2 that follows indicates the average resistanceof multiple measurements of the number of samplesand treatments listed in Table 2 at the two levels of Relative Humidity. Note that the resistances were thehighest of all the suppliers. While the resistanceswere the highest of all suppliers samples tested, theywere still well within the dissipative range of surface

resistance even at 12 % R.H.

Graph 2


and Humidity

1.0E+07

1.0E+09

2.0E+09

1 2 3

Category

R e s i s t a n c e

( O h m s )

12 % RH

50 % RH

Table 3


1 2 Anodized

2 2 Anodized Black

3 2 Anodized Black Yes

4 2 Hard Coat

5 2 Hard Coat Black

6 2 Hard Coat Black Yes

Table 3 lists the treatment of the samples provided bySupplier 3, which was both Anodized and Hard

Coated. There were two samples of each treatment.

Graph 3


and Humidity

1.0E+07

5.1E+08

1.0E+09

1 2 3 4 5 6Category

R e s i s t a n c e

( O h m s )

12 % RH

50 % RH

Graph 3 indicates the average resistance of multiplemeasurements of the number of samples andtreatments listed in Table 3 at the two levels of Relative Humidity. While at the 12% R.H. theresistances tended to be higher, they are still wellwithin the dissipative range.

3.2 Salt Spray

Table 4, the following table, indicates the number of defects in the finish after subjecting the samples to asalt spray/fog environment up to 336 hours with onlyminor defects seen. While the Automated HandlingEquipment (AHE) is not typically specified for use ina harsh environment, the salt spray is a goodaccelerated test to determine if there are any defects inthe finish that may have had a direct impact on theresistance measurements. After 72 hours, there wereno defects realized.

Table 4

Sample Treatment Dyed

72

Hrs.

336

Hrs.31a Anodized Black 0 3

15a Anodized Black 0 3



38a Anodized Red 0 3

41a Anodized Yellow 0 2

44a Anodized Blue 0 0

5t Anodized Black 0 1

6t Anodized Black 0 0

31a Hard Coat Black 0 2



34a Hard Coat Black 0 250m Hard Coat Black 0 1

51m Hard Coat Black 0 1

11t Hard Coat Black 0 1

12t Hard Coat Black 0 1



3.3 Statistical Significance of Resistances

Statistical Significance testing was completedusing Statgraphics and is available but notincluded in this report except for graphs 4 – 7.

Scatterplot by Supplier 1 Process Treatment

R e s i s t a n c e ( O h m s )

Process

H a r d 1

H a r d C o m p 1

H a r d D y e d 1

A n o C o m p 1

A n o .

D y e d 1

A n o d i z e 1

0

1

2

3

4

5(X 1.E8)

Graph 4

Graph 4 displays the difference in resistance betweenthe different process treatments of Supplier 1.

Scatterplot of Supplier 1, Process and Humidity


Humidity (RH)

50% 12%y

0

2

4

6

8

10(X 1.E7)

Graph 5

Graph 5 displays the difference in resistance of thesamples supplied by Supplier 1 at the two levels of humidity. The difference in resistance is minimal.

Scatterplot of the 3 Supplier Differences, 12 % R.H.


Supplier 1 Supplier 2 Supplier 30

1

2

3

4(X 1.E9)

Graph 6

Graph 6 displays the difference between the 3suppliers at 12 % Relative Humidity. Note that thedifference between Supplier 1 and Suppliers 2 and 3are significant while the difference between Suppliers2 and 3 are not significant.

Scatterplot of the Supplier Differences, 50 % R.H.


Supplier 1 Supplier 2 Supplier 3er0

4

8

12

16

(X 1.E7)

Graph 7

Graph 7 displays the difference between the 3suppliers at 50 % Relative Humidity. Note that thedifference between Supplier 1 and Suppliers 2 and 3are significant while the difference between Suppliers2 and 3 are not significant.

3.4 Scanning Electron Microscope

The pictures in thissection were taken

with a ScanningElectron

Microscope or

SEM. The reasonfor this analysis was

to rule out thepossibility of porosity or defects influencing theresistance measurements. Photo 1 is a 35 X

magnified image of a untypical sample submittedby Supplier 1. As

indicated by thearrow, a smalldefect, a small

bright spot, wasobserved. Is the

defect a hole andis it sufficient to

cause a decrease in the

platting resistance? In Photo2, the defect ismagnified to 350 X and as seen in the photo, may

be a hole in the anodic layer which may provide ashort circuit path through the anodize. Does the

Photo 2

Photo 1



hole penetrate to the aluminum surface? As seenin Photo 3, a 1000

X magnified image,the hole is ashallow hole and

does not penetratethe anodic layer.

As stated earlier,this sample was nottypical of Supplier

1 since there were veryfew defects observed in the

samples under similar analysis. Compared toother Suppliers, these samples are smootherlooking with fewer pits and porosity. Suppliers

2 and 3 as seen in the following photos appear tobe speckled with

dark black spots.The appearance of

the samples causedquite a concernabout the resistance

measurements.

However, theresistance of the surfaces

was typically higher withSuppliers 2 and 3 than Supplier 1 with therelative smooth surface. Photo 4 is a typical

example of the 35 Xs magnified images of HardAnodized samples from Supplier number 2. As

seen in Photo 4, there appears to be a speckled orgritty surface finish. A higher (1000 X)magnified image can be seen in Photo 5, which

displays pits andporosity in the

anodic layer.There is alsoevidence of

microcracking thatis exhibited as

bright streaks. The

reason for thebright streaks and surfaces

around some of the pitting isthe surface edge retaining a charge from the

electron beam. Cross sectioning of the samplesfrom Supplier 2 as seen in Photo 12 revealed thepits and porosity was limited to the anodic

surface and did penetrate to the substrate. Photos

6 and 7 are typicalof the images

taken from HardAnodized samplesfrom Supplier 3.

As seen in thesephotos, the anodic

surface is muchlike Supplier 2 in

that the same level of

pitting and porosity is present. And as seen inPhoto 7, there is the same level of microcracks as

seen in the samplesfrom supplier 2.There does appear to

be other particles of material on the

surface in addition tothe pitting and

porosity, which isvisible in the some of thecross-sectioned samples.

Spectral Analysis of these particles indicated that

many of the particles were iron inclusions, whichis normal for this

type of aluminumalloy and theplatting process.

Photo 8 and 9 are35 X and 1000 Xs

magnified images

of Hard Anodized samplesfrom Supplier 1. Note that

compared to Suppliers 2 and 3, it appearssmoother or lessgritty and as seen in

Photo 9, the porosityand pitting is much

less than the othersupplier samples.

Also note in Photo 9

that there does not appearto be as many microcracks

as in previous samples.

Photo 3

Photo 7

Photo6

Photo 5

Photo 4

Photo 8

Photo 9





All three of the suppliers’ products would provideadequate dissipation of ESD charges in the typicalenvironments associated with Universal InstrumentCorporation’s products. However, there are someinconsistencies between the different supplier’sprocesses, which prove the requirement for qualifyingthe suppliers. While the each of the suppliers testedare satisfactory, Supplier 1 would be the preferred

supplier for this application due to the overall quality.

Anodized aluminum if not used properly can

cause severe and expensive problems in the EMCarea. If anodized aluminum is being used in

applications such as chassis or mounting plateswhere typically a low impedance mechanicalground is required, additional grounding devices

or conductive assembly surfaces will be required.

The correlation between micro-cracking of the anodiclayer and wear characteristics, the combined effects of

micro-cracking and porosity at high humidity levelswas not in the scope of this work and should beconsidered in future works.

5. Acknowledgements

I would like to Acknowledge Rodger Congdon whoprovided excellent Technician support and conductedthe test and express gratitude to William Rabey andDanielle Lapidus for their continued technical andeditorial support. I would also like to thank MeganPellenz of Anoplate Corporation and Dave Christie,Ed Brothers, Rick Contento, and Jeff Zerilli of IthacaMaterial Research for their technical assistance andanalysis.

6. ReferencesESD Association Standard Practice forProtection of Electrostatic Discharge SusceptibleItems, Automated Handling Equipment. ESD

SP10.1 – 2000

ESD Association Standard Practice forProtection of Electrostatic Discharge SusceptibleItems, Worksurfaces – Resistance Measurements

ESD STM4.1-1997

Machinery Handbook, 24 th Edition

Documents

Anodized Aluminum Alloys