TECHNICAL .INFORMATION SERIES · 2012. 3. 26. · TECHNICAL .INFORMATION SERIES Title The Effects of Glass Bead Peening on Anodization Coating Discoloration. NO. , Date June 1974

. TECHNICAL .INFORMATION SERIES

Title The Effects of Glass Bead Peening on Anodization Coating Discoloration.

NO. ,

Date June 1974

Author F. Maltby E. Hanley

Abs t rac t : Studies were made on the effects of glass bead peening on

Subject Classification

Impact - Anodization

modization c o a t i ~ g discoloration. The study evaluated-a range of fac tors n i m a r i l y related to: (1 ) the physical peening p rocess (es ) and ( 2 ) the : lectro-chemical anodization process . Analytical t e s t s w e r e m a d e on rarious peened/anodized samples t o de termine the cause of anodization liscoloration, t .

Class I Filed Technical Documents /NO. Pages

Clear anodic coatings caQe be made on peened aluminum by at l e a s t two methods:

(a) As previously determined - etching in hot acid/czust (b) ~ l e c t r o - c h e m i c a l etching in hot sulphuric acid.

Anodization discolorat ion af te r peening is caused by t r a n s f e r of .constituent elements f r o m the bead t o the aluminum surface . The peening-finishing pre-anodize p rocess does not s e e m feas ib le for heavy indus t r ia l application. At th is t i m e the cos t savings would be marginal. However, a t some future t ime , m o r e s t r ingent waste disposal, o r energy u s e regulations m a y necessi tate a review of the process .

Unclassified

. Information Prepared For Sales Department

Tests Made By F. Maltby

Conclusion

R & D ~ a b o r a t o r ~

,? - I - . sec t ion R & D

M r , O T L e a r y Mr, Gehret t Mr. b k ~ u t c h e n Mr. Balcar Mr, Ritter

8

32

Corporate Headquarters: Industrial R0ad.P. 0. Box 14 Carlrtodt. New Jersey 07072. Tel. (201) 933-3355, ~ i b l c : POTTERS CARLSTADT

THE EFFECTS OF GLASS BEAD PEENING ON ANODIZATION COATING DISCOLORP.TION

Contents

- Introduction

Statement of

Objective of

Statement of

Procedure:

Page

Page

Page

Page

Problem

Study

~xperimental Approaches to Problem . Page

Sample Prkparation: Peening Equipment, General

1. Specimens for Approach One 2. Specimens for. Approach njo

Miscellaneous 4

Anodization: Equipment, General 1. Specimens for Approach One 2. Specimens for Approach Two

~ n a l ~ t i c a l Tests.

Page 9

L

Page 1'2 Discussion of Results

Page 20

.Page 21 Recommendations

INTRODUCTION

Aluminum and aluminum al loys possess a unique combination of p roper t i e s - l ightness, high s trength t o weight rat io , cor ros ion res is tance , ductility, ease of machining, finishing and relat ive economy.

In 1972 aluminum shipments f o r a l l applications totaled 1 2 billion pounds. Of t h i s total. about 3 billion pounds were used in the building and construction industry and another 2 bill ion pounds by the transportation industry. In 1972, it is estimated that about 700 million pounds of aluminum f o r m s and sheets w e r e finished by anodization, p r imar i ly fo r applications in these two industr ies .

By 1977 this will probably i n c r e a s e t o over a billion pounds a year . Anodization, o r anodic coatings, p r e s e r v e the mechanically o r chemically t r e a t e d sur face , can provide color , enhance co r ros ion resis tance, inc rease su r face hardness and can impar t s e v e r a l other useful character isgics t o aluminum.

The word " a n o d i ~ a t i o n ' ~ de r ives f r o m the eiectrochemical coating p rocess wherein the aluminum f o r m o r pa r t i s made the I1anode" ( o r positive electrode). A 15% solution of sulphuric ac id is general ly used a s an electrolyte , and sheet lead a s a cathode. By cont ro l .of voltage, (15-20v) cu r ren t (12 a m p e r e s p e r sq. ft. ) and electrol+e tempe?ature, a n oxide (Al2o3) i s fo rmed on the

sur face of the i m m e r s e d aluminum (anode). This oxide i s somewhat s i m i l a r to a natural a luminum oxide except- that the oxide l a y e r impar ted by anodiza- t ion will normally be up t o s e v e r a l thousand t imes thicker . Archi tec tura l C lasses I and II Anodic Coating must have a minimum thickness of 0.4 mils . o r 0.7 mils . respectively. These coatings can be c l e a r o r colored. Besides sulphuric acid other ac ids , (chromic, oxalic, phosphoric) and var ious propr i - etary electrolytes a r e used t o produce e i ther c l e a r o r a var ie ty of inherent anodization colors . T h e s e colors range through black, various g rays , bronzes, and golds.

In many applications, par t icu lar ly in the archi tec tura l and t ranspor ta t ion a r e a s . anodized coating co lo r control is important. . In a building with ex te r io r aluminum facade panels, a l l decorat ive t r i m and window f r a m e extrusions m u s t be carefully controlled and matched in color. ~ n o d i i e d aluminum t r i m and functional p a r t s used in t ransportat ion applications

- m u s t a l so meet specif ic co lo r s tandards. It i s thus important that a n anodize p rocess impar t e i ther a c l e a r coating o r a specific reproducible color to the aluminum.

Anodize colors o r defects should not resul t f r o m a process variable. If they do, the resultant color m a y be objectionable' o r in ter fere with the specific color o r hue tha t i s sought.

STATEMENT OF PROBLEM

bfost of the aluminum anodized for archi tec tura l o r t ransportat ion applications is processed (with s o m e variat ions) in a manner s imi la r to tha t s h o r n in t he following flow chart.

I' I I I - . . I - I Inhibited I U Mechanical .

finish Rock . cjeon

I r i

brighten Rinse

Fig. 3. Flow chart o! &ration, t~rpically employed in anodizing

For archi tec tura l applications the chemical finishes (used p re l imina ry t o anodizing) shown in Figure 3, have two principal purposes: t o c lean the aluminum surface in prepara t ion f o r subsequent finishes, and t o etch and thereby roughen the na tura l sur face t o obtain diffuse reflectivity. The etch procedure i s a l s o used t o remove the die extrusion m a r k s and /o r mill rolling l ines inherent in the aluminum forming processes .

There a r e numerous p r o p r i e t a r y compounds, acid mixtures and caust ic eolutions f o r etching aluminum p r i o r to anodizing. However, t h e m o s t ' commonly used solution i s hot caust ic (Sodium Hydroxide - 5 t o 670 in water @ 1 4 0 - 1 5 0 ~ ~ . ). . h t caustic solution i s probably used about 80 t o 90% of the t i m e , with various ni t r ic -acet ic -phosphoric, o r s u l ~ h u r i c -chromic acid compounds "ccounting for the remaining 10 o r 20%.

noth caustic a n 4 acid p rocesses etch me ta l f r o m the aluminum surface . In the hot caustic p rocess , f r o m 2 t o 4% by weight of the aluminum is removed. This m c a w an avcrage l o s s of 3 pounds for every 100 pounds of a l u m i m ~ m

processed. a . . In addition t o the metal l o s s , the etching process requires e l e c t r i c a l power ( immers ion hea ters ) .a secondary desmut operation, and con- s iderable quantities of r inse water. The spent caustic solution a l s o presents an ecological waste disposal problem. Processing with ac id compounds, part icular ly chromic acid, resul t in somewhat s m a l l e r meta l lo s s , but present s i m i l a r power and waste disposal problems.

Part of the pre-anodizing t rea tments of aluminum a r e for the purpose of cleaning (nonorganic so i l s ) and removing &e/rolling l ines and i m p a r t - ing a diffuse tex ture to the surface. With proper process control a l l of these objectives can be per formed by one g lass bead pPening operat ion. Hence it was fel t that a considerable saving i n regard to t ime , power, metal , chemicals and waste disposal could be achieved by u s e of g la s s bead peening a s a pre-anodize t reatment . In view of the l a r g e tonnages of aluminum f o r m s that a r e -anodized, t h e marke t potential of g la s s beads f o r this application, if real ized, would b e considerable.

C

- t i i -* 32 A l imited a m o u i t of pre l iminary work, utilizing g lass bead peening a s an aluminum p r e -anodize t rea tment , was done at the Pot te rs Indus t r ies Inc. Research Labora tory i n 1971 and 1 972. However, this init ial work indicated that the g lass bead peening of aluminum before anodizing impar ted a g r a y hue t o a normally c l ea r anodized coating. The shade of g r a y va r i ed s o m e - what in gradation with bead s i ze , p r e s s u r e impingement angle and a lumi - num alloy. The g r a y color thus impar ted would now be considered objectionable on visual grounds f o r mos t applications i n the a rch i t ec tu ra l and transportat ion a r e a s .

a - OBJECTIVE . O F STUDY

This study was then a multi-purpose effort t o determine i f possible:

1. Methods o r processing procedures f o r the glass bead peening and/or anodizing techniques that would eliminate g r a y (anodize) color. Ideally these procedures should be applicable within the genera l f ramework of existing anodizing equipment and techniques. They would util ize glass bead peening t o eliminate some cur ren t pre-anodize p;ocedures and resu l t in an overa l l cost saving.

. 2. Corollary t o "1. " - determine if possible the mechanism whereby pre-anodize g lass peening impar t s a gray color t o anodized coatings. :

4 8

STATEMENT O F EXPERIMENTAL A P P R O A C I E S TO PROBLEM * -- -

Prepara to ry to the s t a r t of fhe anodize-peening program, approaches and objectives w e r e examined. In discussions between Mr . J. Ri t te r , Direc tor of Research E a r l Hanley and F r e d Maltby, it was decided that the problem would be investigated by two different approaches.

The f i r s t approach would deal with a range of factors p r imar i ly re la ted t o the pre-anodize g lass bead peening processes . This would encompass variations in: e

1. Material . (various aluminum alloys).

2. Peening impingement angle.

3. Bead size.

4. A r c height ~ e e n i n g intensity.

Mr. Hanley's previous work had indicated that any o r a l l of these ma te r i a l / p rocess variations were , o r could be fac tors in the degree of anodization discoloration. F o r this phase'of the investigation we fixed on one s e t anodization process . The 1 570 sulphuric acid (electrolyte) anodization -process was selected. This p r o c e s s is - the one mos t frequently u s e d by industry f o r anodization.

The second approach would invest igate a range of factors p r imar i ly related t o the anodization p rocess (es) . F o r th is s e r i e s of t e s t s 100 specimens of 6063 T-5 alloy w e r e prepared. The specimens were a l l peened t o saturat ion t o one specific a r c height peening intensity. A s e r i e s of experiments related

- to the anodization p r o c e s s ( e s ) could then be made. These experiments would - re la te to:

1. Effects of t empera tu re of the anodizing bath.

2. Effects of entrapped s m u t during anodization.

3. - Coating c l a r i ty in anodizing baths other than 15% sulphuric acid.

4. Effect of var ious pre-anodizing sur face t reatments . .

I 5. Poss ib le effects on anodization due t o change in sur face mic ro - s t r u c t u r e a n d / o r t r a n s f e r of constituent elements f r o m peening beads to peened surface. .

Essentially then, the f i r s t approach would deal with factors re la ted to the glass bead peening processes and the second approach would deal

*

with factors related to the anodization p rocess (es).

PROCEDURE 4 - Sample Prepara t ion

Genera l Procedure and Equipment Used

General Procedure:

Peening was done with four Ballotini bead s i z e s - B . D, AD, and AH. A sieve anlaysis was made on each bead s i z e p r io r t o use. F igures 2 and 3 a r e copies o f t h e s ieve analysis.

0

Arc height peening intensity curves were made fo r each bead s i z e , angle and pressure . These sixteen curves a r e shown in ~ i g u r ' e s 4 t h r u 7. Unless s tated otherwise, a l l specimens used in the p r o g r a m were peened to saturation. The five aluminum alloys selected fo r t rea tment in the anodization p rogram w e r e AA1100, AA2024, AA5052, A ~ 6 0 6 1 and AA6063. Nominal compositions, t e m p e r and some typical applications a r e l is ted in Figure 1 . All of the extruded and s h e e t aluminum were 0.125 if thick. Where necessary , the ma te r i a l was shea red to a convenient specimen size, generally 0. 75 f f -1e 0 ' ' wide and f r o m 5.0"-9.0" in length. The specimens w e r e identified with a punched code number on one end and degreased in M. E. K. . All specimen peening was done with the Model 1 O O P Dry Honer, manufac- tured by the Vacu-Blast Inc. of Belmont, California. This is a d i rec t p r e s s u r e unit. Templa tes w e r e used to adjust nozzle angle and distance. L a t e r program developments necessi tated the u s e of high density ' fJu s i ze g lass beads and 250 u s i z e s t e e l shot. A r c height peening intensity curves were a lso made f o r these mater ia ls . See Figure 8.

Prepara t ion of Specimens - Approach One F o r this portion of the p r o g r a m 48 samples were used fo r each grade of aluminum. Since t h e r e w e r e five g rades , a to ta l of (5x48) 240 specimens were prepared f o r the ini t ia l phase of the program. The bead s i z e and process fac tors for each grade of aluminum a r e indicated in Figure 9 l Each grade of aluminum was peened with four different bead s i zes a t two different angles and two different a r c height peening intensities. L a t e r on a s the p rogram de'velop- ments dictated, an additional 68 specimens were peened.

Pre-anodization Trea tmen t s - Approach One

Each peened t e s t spec imen then had o r was given one of t h r e e sur face t r ea tmen t s p r i o r t o anodization:

1. As peened 2. Hot caust ic etch 3. Hot b r i gh t etch

A 6% solution of sodium hydroxide a t 6 6 ' ~ . was used f o r calist ic etching. A solution of 8096 phosphoric, 570 n i t r i c acid, 5% acet ic acid a n d 10% water at 104°C. was used for the acid 'bright dip. Immers ion t i m e s in both etches var ied f r ~ m 2 t o 4 minutes. T e s t s t o d.etermine the effects of va r ious su r face i r r egu la r i t i e s (produced by p r o c e s s e s o ther than g l a s s bead peening) on anodization color w e r e made. T h e s e included peening with a diamond tipped vibra- tool , scr ibing and incis ion of var ious l ine pa t te rns . T e s t s t o de te rmine the effects of su r f ace ab ras ion with s e v e r a l types and s i z e s of ab ras ive g r i t w e r e a l so made.

Prepara t ion of Specimens - Approach Two or this phase of the p rogram, 100 spec imens of 6 0 6 3 ~ 1 5 a luminum w e r e

. prepared . The spec imens w e r e peened t o sa tura t ion with AH beads at a 60° impingement angle and 6. distance at 25 psi.

ANODIZATION - . EQUIPMENT, AND G E N E M L

Anodizing Equipment and Procedures

The power source for anodization was a motor-generator se t with a controlled output f rom 0 t o 40 volts, d i rec t current . The anodization tank was f iberglass , o r a two l i t e r beaker. TWO sheet lead catllodes, each placed a t 3*11 distance on either side of the anode, were used. The electrolf ie was agitated with a glass motor driven s t i r r ing impel ler . F o r most experiments, cu r ren t density of 1 2 amperes pe r square foot was main- tained b y a force of 1 5 to 20 volts. However, for approach two, EMF a s high a s 3 3 volts were used. A voltmeter , a m m e t e r and t i m e r and the rmo- m e t e r were used to monitor a l l tests .

Anodization - A ~ p r o a c h One t

All specimens anodized for the approach o n e phase of this p rogram util ized r 15% solution of svlphuric acid a s an electrolf le (20-2S0c). The E. M. F- was 1 5 t o 20 volts (D. C. ) and current density 12 a m p s per square ft.

Anodization - Approach Two . \ In the approach two phase, the effects of four different electrolytes w e r e

-

evaluated :

A11 Anodization at 1 ~ O C -20°C For 20 Minutes

Electrolyte . .

18 Volts 1. 1570 Sulphuric Acid 2. 10% Sulfosalicylic Acid t 1% Sulphuric Acid 20 Volts

. 3. Oralic Acid (90 ~ m s /L) t Sulphuric Acid (1. 5cc-L) 33 Volts 4. 15% Sulphuric Acid t 1% Oxalic Acid 25 Volts

A s e r i e s of experiments surface t rea tments af ter

were made to evaluate the effects of various peening, but before anodization. .

'

PRE-ANODIZE TREATh4ENT(S)

Sodium Hyd roxide, 6 oz. gallon @ 60-65OC. . Immersed till gassing s t a r t s , Rinse

Phosphoric Acid, 5.6 bz. gallon. Near boiling. I m m e r s e d f o r 3 0 Rinse seconds.

Anodize

Sodium Hydroxide, -602. gallon@ 6 0 - 6 5 O ~ .

Nitr ic Acid, @ 20°c. 50% by

Immersed t i l l gassing volume. Immers.ed s tar t s . Rinse two minutes. Rinse Anodize

Sulphuric Acid-1 5% ' Nitr ic Acjd, @ 8

by weight, @ 3 2 ' ~ . 2 0 O ~ . 507; by Immersed 2 minutes. Rinse volume. Rinse Anodize

In one experiment dealing with the possibili ty of discoloration due to -entrapped smut, 2 g r a m s pe r* l i t e r of gluconic acid was added to the 15% sulphuric acid electrolyte. The gluconic -acid was added t o t e s t the effect of a sequestering agent.

ANALYTICAL TESTS .

To de termine the effects of peening/anodization variables on anodization coatings, var ious analytical t e s t s were made. A spectrographic ana lys i s was made of the coatings of seve ra l samples from approach two. X - r a y diffraction photographs w e r e made on samples f r o m approaches one and two. Anodization thickness was determined both optically and by a weight lo s s method. Changes i n weight due t o peening and a c i d h a u s t i c etching were a l s o determined on representat ive groups of samples . R. M. S. determinations of sur face roughness were m%de, with a prof i lometer on various specimens. .

DISCUSSION . O F RESULTS

Pre l iminary t o a discussion*of the resul t s obtained f r o m approach one - a brief recapitulation of the specimen preparation. F ive aluminum alloys were peened a t angles of 60' and 90°. Four bead s i z e s , "Bl1(30-40), D(50-80). AD(70-140) and AH(170-325) w e r e used a t two a r c height peening intensities. All specimens, except a s noted, w e r e peened to saturation.

T h r e e specimens were anodized for eve ry s e t of conditions. In e v e r y c a s e the sur face of the specimen anodized i n f i e "as peened" condition

' tukned gray. The intensity of the g r a y h e r e varied somewhat with the alloy and peening p rocess factors. The "graying" effect was m o s t pronounced on specimens of the 2024T-3 aluminum peened with AH beads, a t a 60° impingement angle and the higher a r c height peening intensity. Specimens of 1100 aluminum alloy pe,ened with ItB" beads

0 at a 90 impingement angle, a t the lower a r c height peening intensity had the l e a s t discoloration. In every case. where the specimen(s ) were given a hot caus t ic o r acid bright etch previous to anodization, the "gra - ingn discoloration did not occur.

Caustic o r ac id etching h a s a dual effect on the peened aluminum - surface -skin meta l removal and leveling of the a spe r i t i e s produced by peening. Essent ial ly then, the removal a n d / o r leveling of the sur face skin from peened aluminum eliminates the discoloration i n subsequent anodization. The anodization discoloration mechanism m u s t then be related to some change i n the proper t ies of the aluminum surface skin produced by - peening. T h r e e p roper t i e s could b e effected:

1. Physica l - Grain s ize , orientation, induced s t r e s s due t o compres - sion, etc.

2. Chemical - Change i n surface composition due t o t r a n s f e r of bead m a t e r i a l e lements t o aluminum surface.

3. Optical - Change i n diffuse o r specular reflectivity due to changes in su r face texture.

Any one, o r a combination of these fac tors a r e known t o effect the color and/or t r anspa rency of anodized coatings. .

Consider Number One. Glass bead peening cold works and induces com- p ress ive s t r e s s e s on the aluminum surface . It might be postulated t h a t th is cold work converts the normally crys ta l l ine s t ruc tu re of t he aluminum sur face t o a s e m i - c rystall ine (c rystal i te) o'r a totally axnorphous s t ruc ture . Consider N u m b ~ r Two. Glass bead peening may a l t e r the chemical composi of the alurninum surface. Some of the constituent elements of the peening

beads may be t r a n s f e r r e d to/into the aluminum su r face skin. Cons ide r Number Three . The random sukface t ex tu re induced by peening and subsequent anodization m a y effect the specular and /o r diffuse rcf iect ivi ty of the sur face by sca t te r ing l ight a t angles f a r f r o m the specular beam.

. A series of exper iments re la ted to each of t hese t h r e e possibi l i t ies w e r e made. F i r s t , t he possibil i ty that the effects of cold work ( compress ion , amorphous s t r u c t u r e ) might discolor the anodize coating. A group of specimens w e r e peened f o r var ious t i m e in te rva ls up to saturat ion. The object was t o de t e rmine i f , o r the deg ree of cold work effected anodize color. 0

MATERIAL - 6063 T-5 ALUM 25 P. S. I. 60' IMP. ANGLE - 6 . 0" DISTANCE

ALL ANODIZED AS P E E N E D

Peening Bead T i m e

Ident . Size Seconds Appearance Af te r Anodization No discolorat ion Slight discolorat ion

.- Slight discolorat ion Modera te No discolorat ion Slight d is colorat ion Slight discolorat ion Modera te

This exper iment would indicate that t h e r e was some relat ionship between the degree of s u r f a c e cold work and discoloration. However, the exper - imen t was t o s o m e degree co r re l a t ive with fac tor two s o another exper i - men t was made. Two spec imens w e r e peened and the rma l ly t r e a t e d as follows :

MATERIAL - 6063 T-5 ALUM PEENED T O SATURATION AT 25 P. S. I. 60' IMP. ANGLE 6.0" DISTANCE. T R E A T E D AS PEENED

Bead He at . Ident. Size T r e a t m e n t Appearance After hnodizat ion

45 B 40 m i n - 9 7 0 ~ ~ . Q. Mode r a t e Discolorat ion 46 B 120 rnin-990°F

f i r n a c e Cool Modera te Discolo rat ion

Since the ma te r i a l was completely annealed a t tempera tures capable of recrystal l izat ion, 6ffects due t o cold work were obliviated. Th i s would rule out cold work a s a c a u s a t i w factor in the discoloration of specimens 45 and 46.

L

Sections of seve ra l specimens (1 1221, 11 222, 11 223) were mounted and metal- . lographically polished and etched. The specimens were examined through a

microscope a t magnifications up t o 450x. Although the specimens were p r e - pared p r imar i ly to determine anodize coating thickness, i t was a l so possible to examine the anodize subsurface to a sce r t a in the degree of g ra in distortion

- due t o cold work. Virtually no gra in distortion was visually evident a t magnifications up to 450x, on the acid, caustic etch o r a s peened specimen.

. A s e r i e s of experiments w e r e made t o determine if sur face t ex tu re and/or R.'M. S. Mu inch finish were fac tors in the discoloration. A pa t t e rn of l ines was sc r ibed into th ree samples of 6063 T-5 aluminum. The l ine pat tern var ied f r o m wide t o ve ry close para l le l spacind. In some portions of the

0 pattern these l ines were in tersec ted with a s e r i e s of l ines at 90 , and again 0

at 45O, and in t u r n again a t 45 . A portion of the specimen: was ~ e e n e d L

with a diamond tipped vibra-tool. The peening coverage var ied f r o m about 25% t o 1000/o.

6663 T-5 ALUMINUM SCRIBED AND MECHANICALLY PEENED W I T H A DIAMOND

Trea tmen t Before

Ident. Anodization Appearance After Anodization

Caust ic Etch Acid Etch None

L o s s of Specular Reflectivity, Pa r t i cu la r ly at Certain Angles. Same F o r a l l T h r e e Specimens.

After anodization, a l l t h r e e specimens w e r e identical when viewed f r o m the s a m e angle. As might be expected, t h e r e was considerable l o s s of specular reflectivity and change in sur face lus t e r ; however, t h e r e was none of the cha rac te r i s t i c g r a y discoloration associated with the previously anodized specimens. Another experiment t o determine if R. M. S. Mu inch surfac.e finish was related t o anodization discoloration was made. Three specimens were t r ea ted a s follows:

6063 T-3 ALUMINUM ALLOY po0 IMP. ANGLE - 6 .011 DISTANCE

P e e n i n g Over Peen ing Bcad Pccning Bcad Pccning - .

lent. Size ~ r e s g u r e Size P r e s s u r e 19 AH 25 P.S. I. B 5 R S . 1 , 21 AH 25 P.S. I. B 5 P.S.I.

5 P .S . I .

Pre-Anodize Appearance After Trea tment Anodization None Dark G r a y

Caust ic-Etch Clea r Acid Bright Etch Clcar

Overpeening with "B" s i ze beads levels the peened (AH beads) su r face and - lowers the R. M.S. 'MU inch finish. The "as peened" specimen turned d a r k - -

gray upon anodization. These two experiments seemed to rule out su r face texture a s the p r i m a r y anodization discoloration factor.

To determine i f anodization discoloration was singularly related t o a luminum peened with soda l ime g lass beads, a s e r i e s of specimens w e r e peened and t rea ted a s follows:

6063 T-5 ALUMINUM 6.0" DISTANCE - A L L P E E N E D r. T O SATURATION

Spec. Bead Peening Imp. Trea tmen t Previous t o [dent. Composition Size P r e s s u r e ~ ' e ~ r e e Anodization 25 Soda Lime AH 25 6oU As peened - no anodization

3-1

34 I I I t

HD-1 H Ser i e s J )rn-2 11 t I

None Caust ic etch

' ~ c i d bright etch A s peened no anodization None Caust ic etch

r I Acid bright dip

90° As peened - no anodization , 11 t t I t 1 1 I I

2 1 1 t I ~t II None P 3

HD-4 I I

HD-5 I I

SS-1 Steel SS-2 I I

I I 11 r t Acid bright etch I I t t I I Caust ic etch

250 u 25 I 1 As peened - no anodization I I I I I I Not peened, anodized

II I I None ' I I t 1 Acid bright etch - 11 t t Caust ic etch

These specimens w e r e sent t o a n independent consultant f o r a n X-ray dif- f ract ion analysis of the sur faces . In the X-ray diffraction analysis , a scan was f i r s t made on the untreated 6063 aluminum surface. The pre l iminary

J scan establ isheda qmntatative profile of the alloy elements in the 6 0 6 3 aluminum. A s e r i e s of scans were then made on the sur faces of the t r ea ted (peened/ anodized) aluminum. Here i s a tabulation of the resul ts :

ANODIZATION - A P P R O A C H TWO

A brief recapitulation of the specimens and experimental anodization techniques used for approach two - one hundred specimens of 6063 T-5 aluminum were peened a t a 60" impingement angle and a 6.0" distance - - at 25 p. S. I. with AH beads.

. .

These specimens were then anodized with variables related pr imari ly t o the anodization process. These anodination process factors were:

Temperature of the anodization bath.

Entrapped smut during anodization.

Behavior in anodizing 'baths other than 15% sulphuric acid. t 8

Behavior i f ent ire preparatory step i s used. (Pre-anodize, acid-caustic etches).

Again, in this phase of the program, contamination of the surface and possible surface alterations of grain structure due t o glass bead peening were a lsb investigated.

The result of i t em l l l . " was negative. In reference to i t e m "2. ", gluconic acid did have a slight inhibiting effect on discoloration. However, the overall effect was s o slight, that the anodized coating color was st i l l considered unacceptable. The results of i t em "3. " were negative. A variety of anodizing electrolytes other than 15% sulphuric acid also produced discolored coatings. The results of i t em "4. were also essentially negative. A variety of hot caustic and acid etches were t r ied a s a pre-anodization surface treatment of the peened aluminum.

Where (immersion) etching t imes used were of shor ter duration than those used in approach one, the re was no effect in preventing discoloration. In reference to i tem "5. I t , several analytical tes ts were made. Here i s a tabulation of the results of a spectrographic analysis on the'peened and unpeened surfaces of 6063 aluminum. AH (soda l ime glass) beads w e r e u s e d for peening.

Element Unpecned Surface Peened Surface a

Aluminum Silicon Magnesium Iron Copper h4anganese

.. Major

0, X 0. ox 0. OOX

Major 0.x High 0. X

- 0.X 0, ox 0. OOX

The analysis indicates t h e r e is a significant inc rease in the amount of sil icon on the sur face of the peened aluminumr This sil icon is undoubtedly f r o m the s i l ica of the soda l i m e glass.

Another sample of 6063 aluminum was peened with "Jft beads (H s e r i e s , crown ba r ium glass) . The peened s u r f a c e a n d an unpeened su r face w e r e -

examined and compared b y X- ray fluorescence. It was establ ished tha t the peened sur face had t h r e e t o four t i m e s the amount of zinc i n compar ison t o the unpeened surface. Minor amounts of t i tanium and ba r ium w e r e a l s o found on the peened surface. These two analyses verify the r e s u l t s of the X- ray diffraction t e s t s made e a r l i e r i n conjunction with approach one. T h e r e is a t r a n s f e r of the peening bead constituent elements f r o m the bead t o the peened aluminum surface.

4

drys ta l lographic X- ray photographs made on the peened and unpeened surfaces indicate that peening produces a change -in the normal c rys ta l l ine s t ruc tu re of the aluminum. This change could be due t o cold working (peening) s e v e r e enough t o produce a semi-amorphous o r amorphous s t r u c - t u r e on the aluminum surface. However, it could a l s o be due t o the p resence of embedded g las s par t ic les (amorphous) in/on the aluminum surface . In a previous expe r iment peened aluminum samples were completely annealed (recrystal l ized) . The annealing did not el iminate subsequent anodization discoloration. Hence the m o r e likely cause of the amorphous X- ray crystal lographic pa t te rn i s the presence of minute g las s par t ic les in the sur face of the peened aluminum.

. The major constituent of soda-l ime g lass is s i l ica. Smal l amounts of s i l icon (out of solution, not d ispersed i n the base aluminum) will prdduce cloudy anodic coatings. At the 5% l e v e l the anodic coating v a r i e s f r o m d a r k g r a y to black. in color. 5

All of the analyt ical t e s t s run i n both phases of this p r o g r a m indicate tha t constituent m a t e r i a l is t r a n s f e r r e d from the bead to the aluminum sur face during peening.

, . Various methods (ofher than phase one acid/caust ic etching) w e r e invest i - gated t o determine i f the affected aluminum surface could be removed p r io r to anodization.

It was determined that the affected surface could be removed p r i o r t o anodization by a two t i e r sys tem. In this method the specimen i s e lec t ro- chemically t rea ted i n a sequence of two ce l l s , each containing the s a m e electrolyte - 15% sulphuric acid. This method gives c l ea r anodic coatings.

Exposure in the f i r s t ce l l is a t 7 0 ~ - 8 8 ~ ~ . for 5 to 20 minutes at 6 t o 9 volts. This electrochemical e tch removes the affected surface. Exposure in the second cell i s under s tandard conditions for anodizing. No r insing i s required between the f i r s t and second cells. The peened textureaof the pa r t is retained a f t e r anodization.

' I '

CONCLUSIONS

Clear anodic coatings can be made on peened aluminum by at lea'st two methods :

1. As previously determined - etching in hot acid o r caustic.

2. ~ l e c t r o c h e m i c a l etching in hot sulphuric acid.

Either process i s performed after peening, but prior to anodization.

Anodization discoloration, after glass bead peening, i s caused b y the transfer of constituent elements f rom the bead to the aluminum surface. When soda-lime glass impact beads a r e used, the t ransferred silicon i s

- the element that causes discoloration, Other types of beads will also t ransfer constituent elements such a s zinc, titanium, strontium and barium. These elements can also cause varying degrees of coating discoloration.

It can be seen that the post-peening treatment required to eliminate anodization discoloration i s s imi lar to that now used a s a pre-anodize surface preparation treatment. This means that we have arr ived back a t the same point, except that one additional process has been added - glass bead peening. As a consequence, aluminum finishing b y glass bead peening does not now seem feasible 'as a cost saving process. There might be some limited use, o r specific applications where a diffuse o r a peened surface texture, o r a gray hue i s either desired o r not objectionabl

On t h e b a s i s of menta l work is

the r e su l t s obtained f r o m t h i s study, no f u r t h e r expe r i - recommended a t this t ime. The peening -finishing

pre-anodize p rocess does not s e e m feasible fo r heavy indust r i a l appl i - cation.

Although t h e r e might be s o m e cos t savings (perhaps up t o 10%). it would be difficult to "sell" the p r o c e s s wholly on th? bas i s of economic con- s iderat ions. However, a t s o m e future t i m e , m o r e s t r ingent pollution, ecological waste disposal , o r energy use . requirements may necess i t a t e a review of the process .

* REFERENCES

1. Aluminum Standards and Data 1970-1 971.

Standards F o r Anodized Archi tec tura l Aluminum - The Aluminum Associat ion, 750 Th i rd Ave. , New York, New York 10017. July 1971.

A. S. T. M. S tandard Method F o r Measuring Metal and Oxide Coating Thicknesses By Microscopical m a m i n a t i o n Of A C r o s s . Section. A. S. T. M. ~ e s i p a t i o n B-487-68.

Metals Handbook, Volume 11, Heat Trea t ing , Cleaning and F in i s h- ing. Cleaning and Finishing of Aluminum and Aluminum Alloys.

\

The Surface T r e a t m e n t and Finishing of Aluminum and I t s Alloys, Volumes 1 and 11. Four th Edition - S, Wernick and R. Dinner. Robert D r a p e r Ltd. - 1972. a ?. I

Technical P a p e r s - Applied Sess ion Technical P a p e r s - Anodizing Sess ion Aluminum Finishing s e m i n a r - Chicago ,Xl . March 1973, Aluminum As sociation.

Anodized Aluminum, A. S. T. M. STD. 388. Amer ican Society F o r Test ing and Mate r i a l s , Philadelphia, Pa. 19103. F e b r u a r y 1965.

Fac to r s Affecting L o s s of Brightness and Image Clar i ty During Anodizing of B r ight T rim Aluminum Alloys in Sulphuric Acid Elec t ro ly te . W. E. Cooke. Plat ing, November 1962. P. 1157 t o P. 1164.

SIEVE !.NALYSISY

Gradat ion

* .

170 '

Pan

Total

AH BEADS (1'70-325 U.S. Sieve No.:)'

Gradat ion

140

i7 o

200

230

270

325

Pan

Total

*Average of two s i e v e ana ly s i s 0

FIGURE 2

Grada t ion

25

30

40

50

Pan

' SIEVE ANALYSIS"

B BEADS - (30-40 U.S. Sieve No.)

Total

.Gradat i o n

4 0

50

60

70

80

D SEADS - (50-70 U.S. S ieve No. )

Pan

Total

%Average o f two s i eve analysis

FIGURE 3

ARC HEIGHT WORK SHEET PROJECT NO. ALUM A ~ ~ o D I Z A ' T ~ O ~ DATE &JLY 3ED 197'4 ALMEN TEST STRIP I U' AM ~ ' 6 NOZZLE 7/3 Z.'DI'R DISTANCE d .O ' METERI!JG O X F I C E DIA. BEAD SIZE

ARC HEIGHT WORK SHEET

PROJECT NO.PLUM A ~ J ~ D I Z A T ~ O ~ ~ D A T E 3LlL2< 3 U D \97& ALMEN TEST STRIP ' N ' NOZZLE ' 7/32 " DISTAIlCE G. 0 " METERING OSZ?ICE DI.4 . B E A D S I Z Z P , M - I 7 0 - , 9 2 5 .-.I . .

I I '1

! GO*A!!SLC . l ~ ~ w ~ ~ j ~ ~ ~ j ~ ~ T ? I G L E 19 ~ , i : ~ ~ ~ ~ TIME 15 P.S.I. 125 P.S.I. 1 S P . S . 1 . 1 2 5 p . s . 1 . SECONDS

I Ai iC HT. I ARC 3T. ARC HT. 0 ARC HT.

ARC HEIGUT. W3RK SHEET * - PROJECT I:o. ALUM F ~ ~ ~ D D I z A + ~ J D A T E 3 LILY I 5 \ 974 ALMEN TEST S T X P N & A NOZZLE '7/3'2' DXSTAIICE G , a

"METERI!!C O 3 I ? X E 31.:. - r

9 O~A:IGT,E I9 @A::GLE - T I E 2 3 P.S . I . 125 P.S . I . P . S . I . P . S . I . SECOXDS A.EC IiT. ARC HT. ARC HT. I ARC HT.

t 1 i I

ACKNOWLEDGEMENT

The authors wish to acknowledge, with gratitude, the invaluable assistance and guidance of Mr. James R. Ritter, Director of Research and Development of Potters Industries, I n c .