CA2023502A1 - Electrolytic method for coloring anodized aluminum - Google Patents

Abstract

WAX/dpv 1736a ABSTRACT

The invention provides an improved electrolytic process for producing anodized metal substrates such as aluminum or aluminum alloys colored using optical interference effects. In particular, the invention pertains to a novel electrolytic method of modifying the anodic barrier layer. The method results in all colors of the visible spectrum Without the need for anodizing in a separate phosphoric acid-based electrolyte.
The modification procedures consists of treating the metal substrate with a sequence of direct and alternating currents. The alternating current is symmetrical, the voltage of the positive half-wave equal to that of the negative half-wave.
A preferred red sequence is direct current, alternating current, direct current. As a result, metallic oxide is deposited within the enlarged pores to a defined height, thus achieving the decomposition of light and obtaining the different colors of the visible spectrum. The modification step can be performed in the anodizing bath, the electrocoloring bath, or in a specific bath with an acid electrolyte.

Description

~0~61/7000 W~/dp~
8/s~so TMPROVED ELECTROL~YTIC MET~OD FOR
COLORING ~ODIZED A~UMINUh~
BACK~ROU~JD OF THE INV~TION
.
The color~ng of alumtnum by fo~mation o~ anodic cxide films and electrolytlc de~ositio~ of ino~g~nic particle~ thereln ha6 been known for many years and compr~ses several well-defl~ed ~te~. F1rs~, s~odization of alumlnum or other light metal produaes a porous oxide film (porous 2nodic layer) on th~ metal under alt~rna~lng or direct curr~nt flow in an acid b~th ln which th~ metal is suspendQd. The b~h gener~lly coneains suluric, oxal~c, phos~horic or chromlc acid.
ln the s~bse$uent el~ctrocolorlng process, lnorgan~c material, usually a metal, is deposited in the potes of th~ anodic oxide ~ilm by the passage o~
an el~ctric curren~, usually alternating cur~ent, between ~he anod~zed aluminum s~b~tra~e and a counterolectrode, which counterelectro~ ugually consists o gr~phl~e or stainle~s ste~l, although nickel, copper, and ~in electrodes can also be used. The depo~ltion of inorganic materl~l ~unctions to giv~ ~he metal a colored appeara~e, the a~a~ent aolor due to optical inter~erence efec~s. In a porou5 anotic aluminum oxide film, the pore~ are evenly ~paced apa~ and ther~ is a barrier layer o~ al~lmlnum oxide ~ett~reen ~he ~ottom _ .................................... .

~2~2 B0561~7000 WAK/dpv 8/9/gO
1736a of the pore and the ~ur~ace of the metal. Ino~ganic metallic p~gments d~posited ln the pores of the anodic film result in light ~eing scattQred both from the lower ends of the indlvidual deposits and from the aluminum/al~minum oxide interface. ~he color pxoduced depends upon the dlfference i~
optical pa~h langth r~sultlng from separation of the two ligh~ scattering surfaces. e.g. the ends of th~
deposits and the al~minum/aluminum oxide lntsrface.
~he pore di~meter ant bdrrier layer thickness are directly related to the applied anod~zin~ voltaga.
Increase in the size o~ the ~eposits and ch~nges ln the colors produced can be achleved by modi~ication of the porzs adjacent to the bsrriex layer. ~n order to o~tain coloring by optical ~nterference e~fec~s, however, i~ i8 nece~6ary to ~rovide anodised me~al tn which the deposited particles are constra~ned to have an average size of at least 260 A at a 3eparation di~tance f~om t~e alumi~um/aluminu~ oxite lnterface of the or~er of 300-700 A.
Although slectrolytic coloring pe~mit6 colors to be obtained, the repertoire of colors produced i~
often limlted to bronzes, blacks and reds, Furthermore, it is often necess~ry ~o have a coloring bath for ~ach color. ~herefore, t~e ma~ority of procedures for anodizing aluminUm pleces W~K/dpv 1736a only pro~uce lim~ted co10r6 due to the higher C08tS
lnvolved in having multiple color~ng ~a~hs. In addit~on, mos~ conventional ~nodlzlng p~ocedures use a double anod~zlng proces6, exemplifled by methods ~tillzlng both 6ul~huric acld and phospho~ic acid ~nodizing solutlons tO modify the pore~ of the bs~rier layer, ~se of a s~con~ ~cidic bath ~olution, such a5 phos~ho~ic acid, i5 disadvantageous because it i~creases the likelihood of contamina~ion by phosphate io~s in the slectrocolo~ing proce~s, Contami~ation by phos~horic acid in this mann~ may prevent the Qffectlve sealing of the finAl ~oduct and lead to the g~adual loss of color th~ough weathering.
Other electroly~lc procedures use complex wave forms such ~s a~ymmetric ~ine wAves to incre~s~ thQ
guality of the final product ~y producing more consi~tent colors, but these wave forms raquire expensiv~ equlpment.
~ t ls impo~tant, therefore, to devalop an electrolytic proce~ that can produce a wide vs~i~ty o~ colo~s quic~ly and eiciently wit~out the use of unnecessa~y baths ~nd sophisticated electrical ~quipment which maXe ~he process mo~e compl~c~ted and sub~tantially increage the C08t.
The maln ob~ect of the lnvention i~ to ob~ain the range of colors of tho visible spectrum by , 2023~02 .

WAK/dp~
8/9/g o 1736a ele~troly~ic colo~ng of aluminum or ot~er metal~ in a simple and unifo~m manner witho~t contamln~ting the anodizing and electrocoloring bath~ with ~hosphoric acl~. A furthe~ ob~ect of tho ~nvention ls to provlde an improved proce~ fo~ modifying th~
anodic ~arrier usin~ as f~w sepa~ate ~aths ~s posstble.
~ ~art~cular, one objQct of thi~ inv~nt~on provide6 ~or barrier layer modification ln the electrocolor~ng ~ath 50 that all thQ colors are obtained in ~ha same tank, thus elimi~at~ng a second anodizing treatment.

$UMMARY OF THE tNVEN~IO~

This inv~ntion per~ains to an im~rovad process for the electrolytic colo~ing o~ a metallic substra~ such as aluminum or alum~num alloys. It has ~een discovered that ~he a~plication of direct and al~ a~ing cu~rent in a defined sequence will modify ~he ~srrier film ~nd the pores, to produc~
bo~h a wider range ant increased bri~htness of colors. The u~e of a defined sequence o~ direct a~d al~e~na~ing current to modify the pores of the anodic aluminum ox~de fil~ and ba~ie~ ~ilm is designed to mo~e precisely control the dimQn~ions of ~aid f~lms without th~ need for a second anodizing 202~02 Bo561/7000 WAX/dpV
8/9/so 1736~

bath of phosphor~c acid. The improved ~e~hod uses inexpen~ive and ea~ily o~tained equlpmen~.
In one aspect of the invention, a process for the Qlectrolytic coloring of metallic substrat~ 15 provi~ed by ~he ~ollow1ng steps:
~ a) developing a porou~ anodic f11m on ~he substra~e ln a sulphuric acid electrolytic b~th;
(b) mo~fying the anodic b~rrler film ~y sequentially applying to the substrate a ser~es of volta~es; a firs~ vol~age o~ direct current, a secon~ ~oltage of alt~rnating curr~nt, and, optionally, a ~hird voltage of dlrect current;
(c) ~lectrolytically depogiting an amount of tnorga~ic materlal in the pores prevlously modlfied in step (b).
In a ~r~errad embodiment, the alternat~ng current uæed in the modifying method i8 rymmetrical 80 that the peak voltage of the ~08itiVQ half-waYe i8 equal to the peak ~oltage of the n~gative half-wave. The ftnal direct current applt~ation i~
dasigned to redissolve any electrolytically-deposit~d inorgan~ o material an~ to i~sure uniformity of the barrler film.
In another as~ect of th~ vention, the modlflc~sion s~ep as outlined above can be performed in either the sulphuric acid ano~izi~g bath, ln a ~eparate modific~tion b~th, or Fre~erably, in the elect~ocoloring bat~ itself.

_.
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2~3~02 B~561/7000 WAK~dpv ~/9/~o 1736a In a ~urther ~SpQCt of this in~ention, direct current i~ eptlonally ~pplled to the metsl substrate af~er ~he electrocoloring step to radis~ol~e the inorganlc deposit6. Thls procedure allow~ for fine scale ad~ustment of ~he color ~one, resulting in more pr~cise control of th~ colors of the final alumlnum or other metalllc product.
An aluminum article having an anod~c oxlde coat~ng on it~ surfa~e ls also descr~bed, said article produced according to the following process:
a. d~velopng a porou~ anodic film on the subs~rat~ in a sulphuric w id ele~trolytic bath:
b. modifying th~ anodic ~arrier film by segu~ntially applying to the substrate a f~rst volta~e of direct cuxren~, a s~cond volt4ge of altexnating current and a thlr~ voltage of d~ct current:
c. ~lectrol~tically depositing an amou~t of inorganic metallic matQrial in an elèctrocoloring bath, a ma~erlal deposited w~hin the pore~ of the oxid$zing layer.

Brief~Descri tion of the Drawln~s FIG. 1 is a sch~ma~lc illustration o the ~nodic laye~ formed on ~he su~tra~e du~ing the a~odizatlon StQp.

' ' ~

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2~35~2 B0561~7000 WAK/dpv 8/s/so 1736~

FI~. 2 illustrate~ the effect o the altarnat~ng curr~nt treatment of th~ modificatlon s~ep, serving to increase the d~ame~r of the pore by forming a cavity.
~ illustrst~ depo~ltion of inor~an~c material in ~he pores of the anod~c la~er during the elactrocoloring step.

DETAt~EI) DESCRIPT~ON OF THE INV~TION

Refarring to FIGS. 1-3. the anodizing process and the improved me~hod o~ the p~e8~nt in~ntlon are schematically illustrated . Befo~e tha metal subst~ate 10 is ~u~jectad to the ano~izing process, it i~ preparQd ~sinq co~ventional metho~s ~or achi~v~n~ a uniform, smooth and ~tt~active finish.
I~itial trea~men~s can compri6~ d~greasing, matting, polishing, ~i~sing and neutraliz~ng, Thc prQpared piece 16 then ~po~tted in the anod~c ox~dation tank 11 which tank gonera}ly contalns an acid ~olution comprl~ng sulphuric acid 12. ln ~ome cases, addl~i~e~ can be used in th~
sulphuric acid bath to dlminlsh ~h~ dtssolu~ion strength of the elect~olytQ. Other acids or acid mlxtures such as a mixture 4~ sulphuric acid and chromic acid can al60 be us~d in the con~entio~al anodizing bath.

2023~02 Bo561/7000 WAK/dpv a~s~o 173~a T~e su~strate lo is then sub~ected to an anodizing ~low of direct 13 curront wherein the ~ubstrate is the posit1v~ el~ctrode (anod~) ~nd el~ct~odes 14 m~de o~ aluminum, carbon, lead, stainless ~teel and the llke, are the negative electrode ~ ca~hode) .
In ~hls anodizing step, if the substrate 10 is aluminum, an anodic l~yer 16 i8 formed on the substr~te (~IG. 1). The layQ~ 16 i5 pO~OUS, containing a plu~ality of eveAly spaced pore~
the distance between the bottom of a pore 18 and the subctr~te lo, ~eing defined as the ~arrier film 20.
The thickness of the laye~ ~6 an~ t~e length and depth of t~e pores 18 will vary dependi~g on many variables such as time, which will determ~ne the pore thickness: voltage, which will determine t~e barrier film 20 size; temperature, which will determine the diameter of the pore in additio~ to the dis~olution rate of the anodic laye~, and current ~ens~ ty .
The ty~cs of current used tc develop t~a anodic layer 16 a~e n~t critical to the function~ng of this in~ention. Direct current, alterna~ing current; or alternating current w~ th dirQct cu~rent components, either in sine, sqUarQ, or ~ulsed waves, in any of their fre~uencie~, ca~ b~ employed in this conventional anodizin~ st~p. ln gene~al, di~ect 2023~2 ~0561~7000 WAK/dpv ~/9 /9 0 1736a _g_ curr~nt voltages ~n the range of 16-22 volts axe u~ed in sulfuric acid-based electrolyt~s dep~nding upon the strength and t~mperature of the acid.
~ene~ally, the thickness of the r~sulting ~arrier film ~ ~ on the order o 10 A per volt ~ppli~d, Typicslly, in sulfurio acid anodizing bath 12 the electrolyte contains 15-20% ~y w~lght) sulfuric acid at a temperasure of 20aC and a voltage of 17-18 volts. In no~mal sulfuric acid anodizng, ~e pore diameters 19 are in the rango of 1S0-180 P~
(15-18nm). The barrier f~lm thickness 20 is typically about e~ual to ~he por~ diam~ter lg in the anodiza~lon step. Thes~ same conditions hold true with mixed sulfuric acid-oxal~c acid ~lectrolytes 12.
The operatlng range~ which m~y ~e used mogt effectlvely in thi~ anod~sng step are tho~e in which the ~ulphuric acid el~ctrolyte ha~ a concentration of 50 to 250 g~l, tem~e~atur~s ~ange from -5 to 40C, and D.C. voltages range from 5 to S0 vol~s with preferred voltages of 15-20 volts D.C. The time during whlch the curr~nt 18 applied may v~y ~om 1 to 100 m~nut~s.
It 16 essential tha~ ths anodlc laye~ 16 has a conslstent thic~ness, height of barrier film 20, and diameter of pore 19. There~ore, esta~lished cond~tlons mus~ be ma~nta~ned wsthin a narrow tolerance ~ange. A~y va~ation may induc~ a ~23~2 Bo561/7000 WAX/dpv a/s/so 1736a differ~nt color from that de~ired. T~is will result bacause ~he color depends upon the th~cXness of the anodic lay~r 16 and, especially, on the thicknos~ of the ~ar~ier film 20, i.e., the d~etance betwoen the su~st~ate 10 and the bottom of the pore 18. Whan inorganic material i6 deposited wi~h~n the pores 18 under alternating currQnt condlt~on~ by alectro~olo~ing of thQ porou~ ahodic layer 15 (FIG.
Sl, the barrier film 20 d~st~nce ~ill dir~ctly influence the wa~elQngth of visible light, thus producing a wa~elength corre~ponding to a qiven color of the visible spectrum by opt~cal i~terference.
Mod~ficatio~ of the anodlc layer 16 (FIG. 2) is performed i~ an a~id elect~olyte 12 whi~h pexmi~s the flow of the current through the bar~iar film 20 a~d ~ub~e~uent ~o~mation of hydrogen within the poras 18. Generally, th~s is ~one 'oy modifying the pore wall~ sur~ounding the ba~rier film 20 to form a ca~ity 22 with a volume and dimensions proportional to the temperatu~, co~centratlon, voltaga, treatment time, and the l~ke. When the cavity 22 is formed, enlarging of the ~ol~mo o~ the bo~tom of ~he pore 18 determines the level o~ color which ~ay be obtained when inorganic material~ are deposited on the pores 18 in the subsequent ~lectrocoloring step. ~or example, in a ~mall ca~ity, the barrier . _ .
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-, :.

2~23~02 wAx/dp~
s/9~9o 1736a film ~0 b~tween the inorganic ma~erials daposited ~nd the metalllc subs~rate 10 i~ small, r~aultin~ in a short wavelen~th with the ~ppearance of violaceous colo~. If the above distance i5 incrsased, other colors will begin to emerge.
The barri~r film 20 i8 ~ semi-conduc~or which rests~s the passage o~ the current. This re~stance will ~e d~rectly proportional to the thicXnQ~ of the layer.
It has b~en discovered that the app~ication of di~ect and alternating current 17 in a deflnod seguenoe will modify ~he barrier film 20 and the ~ores 18, to produce both a wi~er range and inCrQaSed ~rlg~tAe85 of colors. The use of a defined saquence of dir~ct and alternatlng current to modify the pores 18 of the anodic layer 16 and barrier film 20 is deslgned to more pr~clsely control the dimensions of said components withou~
the need for a s~cond anodizing bath of phosphoric acid. Us~ of phosphoric acid is problema~c since lt c~n be ca~ied over as a ~ontaminant into subs~guent treatments and maXes thQ metallic substra~e more difficult to 8eal. Thus, weather-resistance can be impaired. The lmpro~ed method us~s inexpensive and easily o~tainsd eguipmenS.
The term "~lternaSing current" d~no~es a type o current ~ary~ng between poslti~e and n~g~tive _ _.
. .

2023~02 so56l/7ooo WAX/dpv 9/~/90 1736a polarity, ~vin~ a po6i~ivo and nega~lve cycle alternately. Alternatin~ current can be a pure sl~e wave or it can ~e modifi~d in any other wsve form.
In a particularly preferr~d embodiment, the A.C.
voltage is symmetrlcal. Th~ term "symm~trical"
refer~ to the mede of ap~lication of alternating current as to well as the ~alues thereof. The term is meant to deno~e an alt~nating currant ir. wh~ch the peak voltage of the negative half-wa~e i8 equal ~o the peak voltage of ~he ~06itive half-wav~.
In a partl~ularly prsferr~d embod~ment of this m~thod, the anodic layer 16 a~d bar~ier film 20 are modi~ied by sequentially applying to the ~ubst~ate 1~ a tripa~tite voltage sequence comprisi~g a fi~st voltage of dixect cur~ent, a second voltag~ of symmetrical alternating current, followed ~y a th~rd voltage of direcS cur~ent.
The efsct of ~he diferent voltage treatments on mod~ication of the anod~c ~ilm i5 not yot fully under~tood. It ls probable that the firss direct current application prov~de~ for a un1form barrlsr film thickness, while the alternat~ng current ser~es to inc~e~se the ~iameter of the ~ore l9 at the bottom of the pore lB by formation of a cavity ~2.
Formation of the cavity tends to reduce the size of ~he barrler Çilm 20. Typically, alternati~g current tre4tment~ in conventional procodures lea~ to 2023~Q2 BOs61/7000 WAKJdpv 8/s~go 1736~

ele~ated temperatureY in ~he bath. This, in ~urn, tncreases the rates of reaction. Htgher temp~ratu~e w~ll result ln variable cond~tions wlthi~ the anodizing lay~r so tha~ the ~ore d~amete~s, thQ
cavity ~lm~nsio~s, and the barrier film thickness may not be entirely uniform. The ~inal DC cu~rant t~eatmen~ ~s a~pli~d for a tim~ ~uffi~nt to ad~u~t the thickness of the bar~ier ~llm 20 to the ex~ent necessary to ~orm a film appro~r~ate for the chosen color and to ~n6ure uniformity o~ barriQr ~ilm thickness. ~ho uniformity of the barrier ilm i6 dirQctl~ rolated to the uniormity of ~he color once ~norga~ic ma~erlal8 are depo~ite~ in the pores (Figure 3). 3y not pro~id~ng a final D.C. t~atment to ensu~e uniformlty of barrier f~lm thickness, the result~ng colored metalll~ subgtra~e will often have a spQckled appearance wi~h ralnbow-llke patchee interspersed throughout a color~d bac~ground. The du~a~ion of D.C:. t~oa~ment i~ preferably le~s than about 20 mlnutos. ~he duratlon i8 8t~0ngly dependent o~ the voltage; low voltages will re~u~re a long tlme to adjust the barrier fllm 2a~ Shorter times and high~r voltages re~ult i~ a thi~ner barrler f~lm ~0 and A corre~pondingly ~hort~
wa~reler~gth of light ~roduoed by optical interferencQ~ U~e of a ~eguential treatmen~ of diroct and alternating current s~rves ~reclsQly to 2023~02 ~as6l/7000 ~AK~dpv 8/s/so 1736a control the barrier fll~ distance 20, thu~ enabling more control of the final ~olor~ when inorganic materials are deposlted in ~he ~lec~rocoloring step (F~G. 3). The best results can be obtainod by ollow~ng a trlpartite seguence as d~scribe~ abova.
Other combinations o~ direct an~ alternating current~ can be u~ed, p~ovide~ that the altarnatinq current is symmetrlcal. ~he ~inal D.C. trsatment can be eliminated, bu~ the resultlng colo~ may not be unlform. In all embodiments o~ the improved modiica~ion step6 de~cribed herein. the D.C.
voltage i~ les~ than 20 volts. The alteznating cu~rent is al 80 les~ than 25 volts.
I~ ls impor~ant ~o maintain a con~ant t~mperature wlth a varlation o~ le~ than about 2-34C during the motlficatlon proc~ss. A
temperature of a~out 20~C ~s pref~rred. At temperatu~es m~ch higher than about 30C, ~he a~odic layer 1~ ls rapldly dissolved du~ to the hlgher chemical activity at the higher tempe~a~ures. The pores are then enlarged and mors metal w~ll be depo~ited i~ the sub~eqyent el~ctrocoloring step.
Thi~ re~ults in darXer color~ which may be less de~irable under certain ci~cumstance~.
The time o~ each voltage treatment will dep~nd on the temperature and o~her parameters b~t ~hould be pre~erably lcss than 20 minutes, ~ince at t~s WAK/dpV
8/9 ~9 0 36a beyond ~h~ point, the process becomes less e~flcie~t ~nd co~seguently more expensive.
In a ~ur~h~r embodiment of this mod~fication s~ep, 6p~C~ al ~oltage characteristics are cho~en to overcom~ the electrical resistance of the barrier fllm. A3 mentioned previously, the barrl~r film 20 is a semi-conductor, and ~5 the barrie~ ~ilm 1ncrea~es ln size, the ~lectrlcal resistance o~ the a~odic layer l~ also increases concommitantly.
Therefore, a pr~ferred method o~ applying tho direct c~rrent and $ymmetric al~er~atlng current i8 to apply the ~olta~ in a linearly lncrea61ng ~nner, in o~her word~, in a "~amped" configurat~on. This ramping may be par~icularly important tur~n~ t~e A.C. treatment ~e~nce, since the i~creasing resistance of the barrier film ar the f~lm ~nlarges ~Qnd8 tO distort t~Q ~ymmetric s~e wsve input.
Another impo~tant ~eatur~ of the ~n~ention is that these co~trolled direct and alternating curren~
tr~atmen~s can be p~r~ormed in t~e ano~izing bath (FIG. l) a~ well as in the elect~ocoloring b~t~
(FIG. 3).
Where the anodizing ~lectrolyte is 6ubstantially free o~ metall~c salts typically used in el~ctrocoloring, m~tallic deposits cannot form during the modifica~ion 5tep. Wher~ th~ anodic l~yer an~ barrl~r fil~ are mod~fied by the mothod of , .

2023~02 B0s61~7000 ~AXJdpv 8/g/so 1736a this lnven~on in the electrocoloring bath itself, the ele~troly~e is ~Ot su~t~ntially free of metal salts a~d pigme~tary deposits can form under altern~ting current conditions. Thus, whon the improved modiflcation step o this invention is performed in thc elQctrocoloring bath, pore modification can ~ommence sim~ltaneously w~th formation of lnorganic depo~its . The spe~lf ~ o volta~e sequence3 d~cribed hersin can, howe~er, b~
employed to mor~ precisely control the barr~er ~ilm th~cknsss and eliminata metal deposition prior to actual electrocolorlng.
~ he al~ernating cur~a~t volt~ge treatment of the modification 8~0p, which treatment would normally deposit unwanted ~etallic pl~ments in an electrocoloring bath, is ~hosen ~o that the exten~
of metallic depos~tlon is Xept to an absolute mi~imum. One w~y to accomplish thi~ i~ to apply ~lternating current so as to provide for an extremely thin barr~ar fllm.
The final DC tr~atment of the modi~icatio~ st~
will cause slight redlssolution of any metallic depo6its that ~ave been in~dvartently formed durin~
the alte~natin~ curr~n~ ~reatment of the modification s~ep ~n the electrocoloring oath. ThiY
step ls advantageous because it pro~id~ a more precise control o~er the barr~er fllm dopth pr~or ~o 202~02 ~as6l/7000 WAK~dp~t 8 ~ 0 11'36a actual elect~ocoloring, thus leading to more definitlon ln the final color produc~ion when electrocoloring does take place under altern~ting curren~. Whe~ the modificatlon step is performed in the electrocoloring bath. the proceture el~minat~s the need for sqp~rata modlfication an~
elec~rocoloring baths and morQ efflclently ~ses available chemic~ls and elec~rical equipment.
In yet another embodiment, the ~mproved modiflcstion step can also be per~orm~d in a completely 6epa~ate ba~h. This separate bath i8 typ~cally an ACi~iC electxolyte containing a carboxylic acld, sulp~ona~ed organic, or inorganic mineral ~c~d such a6 sulphuric acid, oxalic acid, tartaric acid and t~e like.
once the pore~ 18 and the he~ht of the ba~rier f~lm 20 are modified according to the improved p~OCe~5 of this in~en~lon, inor~anic materials are deposited in the thus-enlargBd e~d ~egio~ o~ the pores ln the electrocolor~ng ~tep (FIG, 3).
The goneral procedures used ~n th0 electrocolortng step are convQntional. Inorqanlc materlal 24 ~ con~ained with~n a p~gmented acid~c sal~ 26 wherein the material is a m0tal selected from one or more of tin, nlc~el, cobalt, copper, silver, cad~ium, i~on, lead, manga~ese, ~nd mol~bdenum. Preferably, electrolytlc coloring is .

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. - 2023~02 .

B05~1/7000 WAK~dpv ~/9/9o 1736a perfo~med in a solution o~ mqtal 8alt ~6 having ~
concentration r~tio to sulfuri~ acid of less than 10~ counter-electrode 27 is tmmers~d in the metal salt ~a~h 2~ and connected to the *lt~rnatin~
current BOU~Ce 30, The counter-electrodes can vary with the type of metal sal~ u6sd, In typ~
applica~ions, gr~phl~e, carbon, ni~kel, or stainle~
stael can be used. Sinc~ the color produced ~epen~
on the difference ln optical path rasulting from separatton of the two light scatter~ng surfaces, the separatio~ will depend upon the barrier fllm d~stance 20. To obtain colors in the visible rang~, ~epa~ation between the surfaces o~ the teposits 24 and the subst~ate 10 5ho~1d be in the ran~ o~ about 300-700 A. Resulting colors range from blue-violet due to interfer~nco effects at the shorter wavelsngths and dark gr~en due to interference effects at the longer wavelengths. ln th~ pre~r~d electrocoloring steps, alternating current ~o~t efficiently deposits inorganic piqment 2~ from metallic sal~ ~olut~ons to the bottom of ~he ~ores 1~. Typically, alternati~g ~urrent of le~s th~n 20 vol~s is preferred.
The alternating ~ur~ent will deposit the metal oxide t~ obt~in a desired color, the ~olo~ depend~ng on th~ hQi~ht of the barrier ~llm ~0. ~he t~me durlng wh~ch th~ altarna~ing curren~ treatment is -~

20235~2 305~1/700 WAX/dpv sJ~/so 1736a appl~ed will determine the tone of ~he color, and the barrier film distance 20 w~ll determine the actual color ~tself. For example, a one minute treatment with an alts~nating current ~ill gl~ a lighter tone of color than a fi~o minute treatment, th~ d~fference in tone belng primarily a funct~on o~
the amount of deposit~d i~organic pigmen~od material.
It has bo~n show~ ~hat. after fo~mation of init~al pigment de~o~t~, ther~ ls some increase in re~is~ancQ of ~he barrier f~lm leading to a change in chemical condt~ion6 within the pores that m~y favor the growth of an addltional anodi~ lAyer.
The growth of a further anodic layer ln the alectrocolorinq ~ath i5 a func~ion of the pH value of the electrolyts w~ich must be set at a level which res~lts ln an appropriate rat~ of la~er without excessl~e redissolut~on of the depo~tad ~igment material. Although the pR of the electrocoloring solutlon plays an important role, and should under all circumstances be maintained abcve 0.8, the oxact p~ is not critical to this modif~cation ~tep of the invention. Pr~f,orably, the electrocoloring solution eléctrolyte h~s ~ pH from 0.5 to 2.3.
~ n a further embodiment of this invention, a short anod~c dl~ct current treatment can be employed after t~e alternating current treatment in 23~2 WAX/dp~
8~/90 173~a the electrocolorir.g bath. ~is Ghort DC ~eatment functions to yield th~ same effect as wo~ld a short treatment in the modification step, whgn said step is performed in th~ el~ctro~olorin~ ba~h. The purpo5e o~ this final DC treatmen~ after electrolytic d~positton of plgmen~ under AC
condition~ is to retuce the ln~ensity of the color by redissolving the metal oxide deposlts. ~ho DC
treatment is contlnued for a shor~ time (about 1/2 to 3 minutes). Tho current is at a voltag~ less than ~bout 25 volts. This procedure allows for a fine scale adjustment of the fina~ barrier fllm thiokness and amount o~ depo~lts, r~sulting ln m~ted colors and a more precise control of the flnal aluminum or other metallic colored ~roduct.
The invention is ill~st~ated further by the followlng exampl~s:

This ~xam~le illustrates modification of the anodic lay~r in a separate b~t~ using a tripartite se~uence o d1 rec~ current, alternating curren~, and di~ect current.
Two p~ece~ of 6063 alum~num alloy ~er~ cleaned in a soa~ solution, etched in 5 ~erCent Caustic soda at 60C, desmutted in nitri~ acid solution (1:1) a~d : . .

Bos51/7000 WA~C/dpv 1736a anodized in a sulphuric acld bath at a temperaturQ
of 20~C w~th a direct curreIIt chsrge density of 2 . SAJdm for 30 minutes . Thi~ re~ul~ed in an anodic porous lay~r of at leas~ 18 roicrons.
one of the pieces wa~ rinsed and transf~rred to a modification bath havin~ ~ulphur~c acid ~50 g/l) at 2DC. $he aluminum ~iece was used as a positi~e cathode and lead electrodes were the negative anode. A 3.~. current o~ 16 volts was appli~d for 3 mlnu~Qs. Following this the pi~ce was sub~ected to a ~ymmetrical alte~a~ing currcnt of 4 volts for 3 minut~s, followed by a D.C. current of 3 vol~ for 4 minutes .
The piece was then r~nsed and tran~ferred to an electrocolorin~ bath containing 16 g/l tin sulphatQ~
17 g/1 6ulphuric acid, 2 g/1 phenosulphuric acid.
~e electrode~ ~ere stainle~s 8teel and the aluminum was subjected S4 an alterna~ing current of 18 volts for 4 ~inutes. A b~ight green colo~ was obtalned (Table 1 ) .
Ta~le 1 fur~her sets ouS the dlfferen~ ~olors that c~n bQ o~ta~ned whQn the D.C. curren~ of the modification s~ep i8 temporally varied in accordance with the impro~ed modification procedure o~ the inve~t ion .

, ~023~02 Bos61/7000 WA~pv 8/sJso 1736~

~ABLE
. _ Film Barrier film mod~ 1catlon RQsultin~
Helq~ts ~A) under DC or ~C curr~nt ~minute~) ~olors 400 to 415 2 grey 41S to 490 3 blue 4~0 to 560 4 green S60 to ~80 S yellow S80 to 660 6 red 650 to 700 ~ purple The second of the two anodlz~d pieces wa~
treated ldentically except that t~e three part modlfication step wa~ eliminated. A bronzs color was obta~ned.
.

This Example lllustrates modification of the anodic layer in the elec~rocoloring ba~h using a dual se~uence of direct current and seguence wa~a alternating current.
An al~minum p~ce w~ degreased in ~n ~lkalin~
cleaner and desmutted for lO minutes in a 10~ sodium 2023~

~05~1/7000 WAK~dpv 1736a hyd~oxide solutlon at 60~C. It was then rin$ed, ne~tralized, and then anod~ zed ln a bath comprising 1~0 g/l ~ulphur~c acid and lS gf l alu~inum sulphate at a te~p~raturQ of lg + 0.5C and a direct current of 3 A/~m or ~0 mlnutes with a positive charge.
The ~lect~odes ~ere carbon.
~ he piece was rinsed and transferred to an electrocol~ring bath contalnlng 16 g/l stannou6 sulphate, ~0 g/l n~ckel sulphatQ, 2S g/l sulphur~c acid, 2 g/l phenosulphu~ic acid ~nd 2 g/l c~tric a~ld. The aluminum piece as the positl~e pole was treated to a.direct current of 0.4 A/dm for 3 m~nutes. Stainlsss steel el~ctrode~ we~e t~e negativ~ ~ole. Next, the piece w~s sub~ected to a symmetrical square wave alternAtlng current hav~ng a current dens~ty of ~.5 A/dm for 4 minutes and th~n to a sy~m~trical 6inusoidal alternating current of 18 volts for 3 ~inute~. A green color w~s produced.

~XAMPLE,3 This Example illu~trstes modiflcation of the anodic lay~r in the electrocolorlng bath us~ng a dual ~equence of direct current and symmetrical alternating current.
A 6063 aluminum alloy piece was introduce~ into an anodizing bath havin~ an electrolyt~ containlng - ~23502 ~30561/7000 WAK/dpv 8Js/so 1~36a 155 5~1 sul~huric acid, 3 g~l boric acid, 2 g~l glycerin, at a temperature of 24 ~ ~.5C. Lead electrodes were u~ed under ~ulsating direct curr~n~
of 4 A/dm for 40 minutss. ~he plece was then transferred to ~ modifying ~ath and a direct curr~nt of 0.5 A/dm was applied for 5 minutes, ~he piece having a positive charge, ~n an electrolytQ
conta~ni~g 20~ g/l s~lphuric acid. ~h~ plece was next treated by applying a symmetrical alternati~g curren~ mder curren~ den~ity of ~.8 A~dm ~o~ 2 minutes. Flnally, the piece wa~ rinsed and colored in an el~ctrolyte containing 18 g/l stannous sulphate, 1 g/l ascorbic acld, 2 g~l ci~ric acid with ~in elec~rodes and sub~ected to alte~nating curzent at a voltage o~ 18 volts for S minu~es un~il the de~i~ed ~olor gray was obtalned.
Even ~hough the t~vent~on has ~een tescri~éd and shown in con~ection w~th speclfic embodtment~
thereof, lt is und~rst~o~ by tho~e sXilled in the art that modlflcatlons may be made to the invention itself or to any of its applicat~ons mentlonQd hereln and that the ~ame are encompa~sed within the spi~it and 6cop~ of the invention, as defined ln t~e following c}alms.

Claims (20)

1. A process for the electrolytic coloring of a metallic substrate by optical interference effects, comprising:
(a) developing a porous anodic barrier layer on the substrate in an electrolytic bath;
(a) modifying the anodic barrier layer by sequentially applying to the substrate a define voltage sequence of direct current and alternating current;
(c) electrolytically depositing an amount of inorganic material (in an electrocoloring bath, the material deposited) in the pores of the anodic barrier layer by alternating current.

2. The process of claim 1, wherein the defined voltage sequence comprises a first direct current, followed by a first alternating current.

3. The process of claim 2, wherein the first alternating current is symmetrical.

4. The process of claim 3, further comprising modifying the anodic barrier layer by applying a second voltage of direct current.

WAK/dpv 8/9/go 1736a

5. The process of claim 4, wherein the modification step is performed in the bath selected from the acid electrolytic bath and the electrocoloring bath.

6. The process of claim 5, wherein the third voltage of direct current is applied for less than about 20 minutes.

7. The process of claim 6, wherein the direct current voltage is less than about 25 volts.

8. The process of claim 4, wherein the modification step is performed in a separate bath, comprising an electrolyte containing an acid selected from sulphonated acid, carboxylic acid and mineral acid.

9. The process of claim 1 further comprising adjusting the amount of inorganic material deposited in step (c) by further subjecting the substrate in the electrocoloring bath to direct current.

10. The process of claim 8, wherein the applied voltages are ramped.

WAR/dpv 8/g/90 1736a -27-

11. The process of claim 10, wherein the metallic substrate is selected from aluminum and aluminum alloy.

12. In a process for electrolytically coloring a metallic substrate by optical interference having the steps of establishing on the substrate a porous anodic film in an anodizing bath, modifying the pores in the film; and electrolytically depositing an inorganic material in the pores in an electrocoloring bath, wherein the improvement comprises, modifying the porous anodic film by subjecting the substrate to a first voltage of direct current and to a first voltage of alternating current.

13. The process of claim 12, wherein the alternating current is symmetrical.

14. The process of claim 13, wherein the improvement further comprises subjecting the substrate to a second voltage of direct current.

15. The process of claim 14, wherein the modification is performed in the anodizing bath.

WAX/dpv 1736a

16. The process of claim 14, wherein the modification is performed in a separate bath comprising an electrolyte containing an acid selected from sulphonated acid carboxylic acid, and mineral acid.

17. The process of claim 14, wherein the second direct current voltage is applied for less than about 20 minutes.

18. The process of claim 17, wherein the modification is performed in he electrocoloring bath.

19. The process of claim 18, wherein the direct current voltage is from less than about 25 volts.

20. A colored aluminum article having a porous anodic oxide coating on its surface, produced by the method of claims 1 or 12.