" Technical Applications and Theoretical Backgrounds of Secondary Electrolysis of Anodized Aluminum

By T. SAT0 and K. KAMINAGA

Summary Electrolytic coloring, electrodeposition

coating and several other surface finishing

techniques for aluminum h a w been carried

out by secondary electrolysis of anodized

aluminum. The theoretical backgrounds of

these surface finishing techniques are usuiilly based on the "pore filling theory",

"fliiw theory" and "current recovery

phenomena".

1. In t roduct ion

Electrolytic coloring, electrodeposition

coating and several other surface finishing

techniques for anodized aluminum have

been performed by applying secondary and

sometimes tertiary electrolysis to anodized

aluminum. Upon applying secondary

electrolysis to anodized aluminum, thc

understanding of (1) the pore filling theory,

(2) flaw theory and (3) current recovery

phenomena is essential. Accordingly, the

technical applications and theorctic:rl

backgrounds of the secondary electrolysis

of anodized iiluminum ;ire reported in the

succeeding sections.

2. Examples of Technical Applicat ions

O F

of Secondary Electrolysis of Anodized

Aluminum

Several surface finishing techniques for

the secondary and tertiary electrolysis of

anodized aluminum have been known.

These techniques using the multistage

electrolysis are typically shown in Fig.1.

There are three surface finishing

techniques for the secondary anodic

electrolysis of anodized aluminum as shown

in the following:

(1)Thc secondary anodic electrolysis of

anodized aluminum in an anionic water

soluble coating material leads to the

formation of a film layer over anodized

aluminum. This surface finishing is called

electrodeposition coating, and has been

widely applied to aluminum building

materials.

(2)The secondary anodic electrolysis of

anodized aluminum in aqueous sodium oleic

acid solution results in the deposition of

oleic acid in the pores of anodized

aluminum, which thereby becomes

lubricative. This method has been

employed for the surface finishing of

industrial aluminum parts.

(3)Application of high voltage to the

1

367

PRIMARY SECONDARY & TERNARY TECHNICAL

ELECTROLYSIS ELECTROLYSIS APPLICATIONS

ANODIZING M

HZSO4 BATH

(1)ELECTRO DEPOSITION COATING

(2)DEPOSITION OF OLEIC ACID

(3)SPECIAL ELECTROLYSIS

CAPACITORS

(1)ELECTROLYTIC COLORING

BY CATHODIC CURRENT

(1)ELECTROLYTIC COLORING

BY ALTERNATING CURRENT

r@i (1)MULTI-ELECTROLYTIC + COLORlNG( 1)

r81 r@i (1)MULTI-ELECTROLYTIC

3 COLORING(2)

M E

I J I I

Fig1 Secondary and ternary electrolysis of anodized aluminum.

2

368

secondary electrolysis of anodized

aluminum in neutral aqueous solutions

leads to the formation of thick barrier

layers at the pore bottoms of anodized

aluminum; accordingly, these films have

been used for electrolytic capacitors for the

special use.

When anodized aluminum is subjected

to cathodic electrolysis in an aqueous

metallic salt solution, the metal is deposited

in the pores of anodized aluminum, which is

thereby colored. This surface finishing

technique has been adopted by part of

companies for coloring aluminum building

materials since 20 years before.

The AC electrolytic coloring of anodized

aluminum in aqueous metallic salt solutions

has been more nidely employed in

companies worldwide than the above-

mentioned c;ithodic electrolytic coloring

method.

If anodized ;iluminum undergoes

secondary anodic electrolysis or secondary

AC electrolysis and is followed by tertiary

AC electrolytic coloring in i\ nickel biith,

then anodized aluminum is electrolytically

colored into various kinds such as blue,

green and red. The commercialization of

this surface finishing technique has been

investigated in a pilot plant sciile.

While these various electrolytic coloring

techniques a re being applied, spotty pealing

off of anodized aluminum sometimes takes

place. This phenomenon i s ciilled "spalling",

and it is important from the industrial

point of view to prevent spalling from

occurring. Another phenomenon to be paid

attention to during the secondary

electrolysis of anodized aluminum is its

"chemical dissolution". Whether o r not

voltage is applied to anodized aluminum in

its secondary electrolysis, part of anodized

aluminum is dissolved by the chemical

dissolution power of an electrolytic bath,

resulting in the degradation of the film

performance of anodized aluminum. This

means that chemical dissolution of anodized

aluminum should be inhibited on its

secondary electrolysis.

3. Theoretical Background Common t o Film Structures of Anodized

Aluminum and All the Types of Secondary Electrolysis

As to the film structures of anodized

aluminum, the Keller, Murphy and Wood

models have been proposed, but the

changes in anodized aluminum have often

been discussed by using the Keller model.

These film structure models are shown in

Fig. 2. Quantitative investigation has also

been carried out on the film structures of

anodized aluminum. The thickness of the

barrier layer of anodized aluminum

depends on anodic oxidation voltage. Since

the barrier formation rate of sulfuric acid

film is 10 AN. Then, 15-volt anodizing

voltage ciin produce the barrier thickness

3

369

Fig.2 Models of film structures of anodized aluminum.

of 15O&lOhi x 15 volt). Moreover, since

the pore wall thickness of anodized

aluminum is two times the barrier layer

thickness, the former anodically oxidized at

15 volt becomes 300A (150A A: 2). The

result of observation with an electron

microscope shows that the pore size of

anodized aluminum range from 100 to 150

A with the number of pores of anodized

aluminum is about 76 billion/cm2. These

values are given in Fig. 3. The thickness of

a porous layer depends on time of

electrolysis. Figure 4 shows the differences

of film structures by different anodizing

voltages. When anodic oxidation voltage is

lower, the barrier layer and pore wall of

Fig.4 Different film structures by different

anodizing voltages

4

370

anodized aluminum a re thinner and its

number of pores is greater. On the

contrary, when anodic oxidation voltage is

higher, the barrier layer and pore wall of

anodized aluminum a re thicker and the

number of pores is smaller. If these kinds of anodized aluminum are subjected to

secondary electrolysis, then the film

structures of anodized aluminum change

the state of secondary electrolysis.

Upon performing secondary anodic

electrolysis o r secondary A C electrolysis, if

secondary anodic voltage is higher than

primary anodic voltage, the thicker barrier

layer shown in Fig. 5 (A) is formed at the

pore bottom of anodized aluminum.

phenomena will be explained in Section 4.

The effect of barrier layer thickness on

cathodic electrolysis appears in the

electrodeposition of metal ions into the

pores of anodized aluminum. When the barrier layer is thicker, metal deposition

into the pores i s uneven as shown in Fig. 6,

but even when the barrier layer is thinner.

This problem will be discussed in detail in

Section 6.

(6 1 ( € 3 )

Fig.5 Formations of thicker barrier layers

Conversely, if secondary anodic voltage is

lower than primary anodic voltage, the

state of secondary electrolysis varies with

the kind of a n electrolytic b:ith. In neutral baths, no change occurs due to secondary

anodic electrolysis. In acidic baths,

however, the special change called "current

recovery phenomena" takes place. This

I

Fig6

thinner barrier layers Metal depositions on thicker and

The difference in the thickness of porous

layers of anodized aluminum gives

influence on all the types of secondary

electrolysis. In the secondary anodic

electrolysis of thicker anodized aluminum,

the insides of anodized aluminum pores

become partially acidic, but partially

alkaline in cathodic o r AC electrolysis. For

thinner porous layers, the pH value of bulk

solutions and anodized aluminum pores a re

neirrly identical.

Other problems common to all the types

of secondary electrolysis are (1) both ionic

and electronic conductions in barrier layers,

5

371

(2) the existence of "flaws" on the barrier

layers and (3) the chemical dissolution of

pore walls. Anodized aluminum is an oxide film

consisting mainly of alumina, which is an

insulating material having greater band

gaps for electronic conduction. Therefore,

the current flows through the barrier layer

of anodized aluminum, as shown in Fig. 7, is

ionic, and not electronic. Since the film

Fig.7 Ionic current through barrier layer

formation of anodized aluminum is carried

out by the ionic current due to AI3+ and

02-, no problem occurs even if electronic

current does not flow. However, if

electronic current does not flow in the

barrier layer of anodized aluminum, then

electrolytic deposition of metal ions into the

pores of anodized aluminum does not take

place. Practically, a surface finishing

technique by which metal ions a rc

electrolytically deposited into the pore of

anodized aluminum has been commercially

estiiblished as the "electrolytic coloring

method". In other words, electronic

current does not flow in the barrier layer of

anodized aluminum theoretically, but it

flows practically. According to Vermilyeal),

the electronic current flowing in the barrier

layer occurs due to "flaw" existing in it.

Various phenomena associated with the

secondary electrolysis of anodized

aluminum can not be properly explained

unless the concept - "The site of an oxide

film where electronic current flows is called

flaw." - is introduced. When anodized aluminum is left

immersed in sulfuric acid, its pore walls

begins to dissolve as shown in Fig. 8, and

anodized aluminum finally disappears.

Fig.8 "Chemical dissolusion" and "field

assisted dissolusion" of anodized aluminum

This phenomenon is called the "chemical

dissolution" of anodized aluminum. In

chemical dissolution, the pores of anodized

aluminum are widened and dissolved, so this way of thinking is called the "pore

widening theory". Although the degree of

chemici~l dissolution of anodized aluminum

is low in both weakly acidic and neutral

baths, it takes places in all kinds of

secondary electrolysis in all the electrolytic

baths. The chemical dissolution of

anodized aluminum is affected by the kind

of electrolytic bath, bath concentration and

6

372

bath temperature. If the degree of the

chemical dissolution of anodized aluminum

is high, then its corrosion resistance, wear

resistance and hardness decrease, and

thereby the film performance of anodized

aluminum degrades. In the secondary

electrolysis of anodized aluminum,

therefore, its chemical dissolution must be

inhibited.

Table 1 gives the theoretical

backgrounds for performing various kinds

of secondary electrolysis of anodized

aluminum in a variety of baths. These are

explained in Sections 4,5 and 6.

4.Theoretical Background for Secondary Anodic Electrolysis.

On performing the secondary anodic

electrolysis of anodized aluminum, it is

useful to understand the following facts and

theories:

(1) formation of thick barrier layers at the

pore bottoms of anodized aluminum,

(2) the "pore filling theory" developed by Dekker and Middelhock,

(3) the anodic decomposition reaction of water,

(4) the generation of "flaw",

( 5 ) the effect of difference between

"monoprotic acid" and "polyprotic ;icid"

on the formation of oxide films,

(6) within the pores of anodized aluminum,

(7) neutralization reaction within the pores

acidification due to anodic reactions

of anodized aluminum,

(8) the difference between the "field

assisted dissolution" and "chemical

dissolution" of anodized aluminum,

(9) the formation of new porous films, and

(10) the "current recovery phenomenon"

found by Murphy.

If secondary anodic voltage higher than

primary anodic voltage is applied to

anodized aluminum, then thick barrier

layers are formed in every electrolytic bath.

In particular, the application of high

voltage to anodized aluminum in neutral

baths causes special changes. The

secondary anodic electrolysis of anodized

aluminum in the neutral baths under

constant current density gives the voltage-

time curve shown by the polygonal line (A)

in Fig. 9. Vertical voltage rise originates

u c1 0 5 4

-

Fig.9 Voltage vs. time curves during

anodizing aluminum and oxide films

from the electrical resistance of the barrier

layer of anodized aluminum, and the

subsequent voltage rise shown by the

oblique line originates from the formation

of thick barrier layers at the pore bottoms

7

373

All Bathes

Neutral Bathes

(pH2-pH9

Acidic Bathes (PH<2)

Table 1 Theoretical Backgrounds of Secondary Electrolysis of Anodized Aluminum

Anodic Electrolysis I Cathodic Electrolysis I AC Electrolysis

(1)Effects of Thicknesses of Barrier Layer and Porous Layer (2)Ionic Conductance and Electronic Conductance Through Barrier Layer (3)Existance of "Flaw" on Barrier 12ayer (4)"Chemical Dissolusion" of Oxide Film

(2)"Pore Filling Theory" (3)Anodic Decomposition of Water (4)Generation of "Flaw" (5)Hurmful Ions for Anodizing (6)Acidification in the Pores (7)Neutralization Reaction i n

the Pores

(1)Formation of Thicker

(8)"Field Assisted Dissolution"

(9)Formation of New Porous

( 10)"Current Recovery Phenomena"

of Barrier Layer

Layer

(1)Cathodic Reduction of

(2)Cathodic Reduction of

(3)Electrodeposition of

(4)Alkalinization in the

(5)Formation of Metal

(6)Destroy of Barrier

(7)"Spalling" of Oxide

(8)Spalling with Na' ,K* ,

H' Ions

Dissolved oxygen

Metal

Pores

Hydroxide in the Pores

Layer

Film

NH; . and etc.

(1)"Faradic Current" and " Non- Faradic Current "

(2)Changes of Anodic Peak Current and Cathodic Peak Current with the Lapse of Time

(3)Impeadance Equivalent Circuit of AC Eletro- lysis

(4)Formation of "AC Oxide Film" (5)Different Characteristics of

"DC oxide Film" and "AC Oxide Film"

(6)Recovery Phenomena of Altern- ating Current

of rinodized aluminum. These thick barrier

layers are formed by the ionic conduction

of AI3+ and 02- ions through the existing

barrier layer, and can be written as the

chemical reaction formula (1):

2AI + 302- -> AI203 + 6e ..... (1)

On the other hand, the primary anodic

electrolysis of aluminum metal in the

neutral baths under constat current density

results in the voltage-time curve shown by

t h e linear line (B) in Fig. 9. The state of

barrier-type film formation in this case can

be represented by Fig. 5 (B). The difference

in gradients between the lines (A) and (B) in Fig. 9 stems from that in the areas of the

barrier Iilyers. From the difference in

gradients, the tronsport numbers of 02-

ancl AP+ ions, pore size of ilnodized

aluminum and thickness of the porous layer

can be calculated. The rese:irch method of

this type was proposed by Deklcer and

MiddeIhock2) about 25 years ago, and hns

been called the "pore filling theory" or

"pore filling phenomena".

Lately, however, Baizuldin3) has insisted

that the pore filling theory could be applied

only to thin oxide films. He reported that

the secondary anodic electrolysis of thick

anodized aluminum in the neutral baths

under constant current density led to the

polygonal line shown in Fig. 9 (C). For

thick anodized aluminum, thick harrier

layers are formed at the pore bottoms of

anodized aluminum up to lOOV anodic

voltage in accordance with the pore filling

theory proposed by Dekket et al.; however,

when anodic voltage exceeds lOOV, "flaw"

is generated in the pore bottoms of

anodized aluminum, and thereby anodic

voltage rises slowly.

Generally, the current flowing through

the barrier layer of anodized aluminum i s

an ionic one due to 02- and AI3+ ions, but

electronic current sometimes flows through

the barrier layer. The site where this

electronic current flows is named "flaw" by

Vermilyeaf.). If electronic current flows

through the barrier, the anodic reaction

represented by Formula (2) takes place in

the pore bottoms of anodized aluminum.

2H20 -> 02+ 4H' + 4e ..... (2)

The reaction given by Formula (2) is

said to be the anodic decomposition of

Ivilter, by which oxygen gas and hydrogen

ions are generated from the pore bottoms

of anodized aluminum; the generation of

hydrogen ions makes the aqueous solutions

in the pores of anodized aluminum acidic.

The secondary anodic electrolysis of

anodized aluminum, therefore, leads

sometimes to the occurrence of the

chemical reaction (1) only, and sometimes

to the occurrence of (1) followed by (2).

The difference between these two types can

9

375

be easily distinguished on the basis of

difference between the voltage-time curves

shown in Fig. 9.

When anodized aluminum undergoes

constant-current anodic electrolysis in an

electrodeposition paint bath, the resulted

anodic voltage-time curve becomes

polygonal as shown by Fig. 9 (C)4). In

other words, when anodized aluminum is

subjected to anodic electrolysis in the

electrodeposition paint bath, thick barrier

laycrs are first formed in accordance with

the pore tilling theory, and then the

electrodeposition paint molecules are

neutralized by hydrogen ions generated due

to the anodic decomposition reaction of

water, forming finally paint films. The

chemical reaction i n this c:iw can be

represented by Formulii (3):

R(COO-), + nH+ -> R(CO0H)n ..... (3)

The chemicnl reaction given by Formula

(3) is schematically shown in Fig. 10.

Nitmelv, for electrodeposition coating over

Fig.10 Mechanism of electrophoretic

deposition coating

the surface of anodized aluminum, it is

necessary that flaws are formed at the pore

bottoms of anodized aluminum and that the

anodic decomposition reaction of water by

electronic conduction takes place in the

barrier layers. Further, the

electrodeposition paint is neutralized by

hydrogen ions generated by the anodic

decomposition reaction of water;

accordingly, electrodeposition coating is

not an anodic reaction, but a neutralization

reaction. To cause the anodic

decomposition of water in anodized

aluminum pores, an anodic voltage of lOOV

or more should be applied; consequently,

applied voltage for electrodeposition

coating is as high as 150V.

When anodized aluminum undergoes

anodic electrolysis in sodium oleic acid

aqueous solution, the resultant voltage-time

curve is polygonal as shown in Fig. 9 (C).

In this case, oleate anion is also neutralized

by hydrogen ions generated by the anodic

decomposition reaction of water, with the

chemical reaction being as Formula (4).

The authors reported in SUIUFIN '94

that the anodic electrolysis of anodized

iiluminum at +200V in 0.lgll tartaric acid

made it a opaque gray film. The voltage-

time curve obtained in this case also

showed a polygonal line given in Fig. 9 (C).

10

376

This is because secondary anodic

electrolysis a t high voltage caused the

anodic decomposition of water, making the

solutions within the pores of anodized

aluminum acidic, and because this acid

formed new porous oxide films between

anodized aluminum and aluminum metal.

Photograph 1 shows the cross section of this

opaque grey film taken by the electron

microscope.

Photo.1 Cross section of opaque grey film

The reason why anodized aluminum

must undergo hot water rinsing and pure

water rinsing prior to the electrodeposition

coilting of anodized aluminum can be

explained on the basis of the "difference of

monoprotic and polyprotic acids for anodic

electrolysis". Monoprotic acid means one

having only one hydrogen ion such its HCI, HNO3 CH3(COOH) etc., whereas

yolyprotic acid means one having two or

more hydrogen ions such as H2S04,

H3PO4, (COOH)2 etc. It is said that

anodic electrolysis of aluminum in aqueous

monoprotic acid solutions does not form

anodic oxide films. This is because anodic

electrolysis does not form proton space

charge in the barrier layer of anodized

aluminum as shown in Fig. 11. In HNO3,

- . ( A 1

Fig.11 Formations of proton space charge

in barrier layers

for example, it dissociates into NOS- and

H+ ions in water. Since NO3- ion does not

have H+ ion which can be emitted into the

barrier layer, no proton space charge is

formed in the barrier layer, resulting in no

formation of anodic oxide films. In H2SO4

and H3P04, on the other hand, they

dissociate into HSO4- and H+, and H2PO4-

and H+ ions, respectively. As shown in Fig.

11, HSO4- and H2PO4- emit H+ into the

barrier layers and form proton space

charges. Therefore, the anodic electrolysis

of aluminum in H2SO4 and H3PO4 baths

can form anodic oxide films. One thing to

which attention should be paid is that

anodized aluminum can be formed by the

anodic electrolysis of aluminum in sulfuric

acid baths, but that it can not be formed by

the anodic electrolysis of aluminum in

11

377

N q S O 4 baths. This is because Na2SOj

dissociates in water into Na+ and S042-.

The latter ion, S042-, does not have the H+ ion to be emitted into the barrier layer;

accordingly, anodic oxide films can not be

formed. The S042- ion destroys the

barrier layer, causing thereby pitting

corrosion. Although HSO4- ions generally

form the anodized aluminum films, it

should be noted that S042- ions destroy the

barrier layer.

Therefore, the hot water rinsing of

anodized aluminum followed by pure water

rinsing prior to the electrodeposition

coating of it is performed to remove S0j2 - ions contained within the pores of anodized

aluminum. If a great amount of ~ 0 4 2 - ions

iirc l d t 11 ittiin the pores of' i\nodizcd

aluminum, then the abnormal deposition of

electrodeposition paint takes place. Figure

12 gives the voltage-time curves when

anodized aluminum is subiected to

Fig.12 Effects of Na2SO4 on secondarj

anodizing of oxide film in neutral aqueou!

solution

electrodeposition coating in the

electrodeposition paint baths. When the

concentration of Na2S04 in the

electrodeposition paint bath becomes about

20 ppm, the barrier layer of anodized

aluminum is destroyed, and so bath voltage

does not become higher, but abnormal

deposition of electrodeposition paint takes

place, instead.

The same is true for oxalic acid and

oxalates. Oxalic acid dissociates into

C2O4H- and H+ ions, and C2O4H' anions

can form oxide films. Sodium oxalate,

however, dissoocate into 2Na' and C2042-

ions. Because c ~ O 4 ~ - anions have no

protons, they destroy oxide films.

Not only sulfates and oxalates but also

sulfuric acid and oxalic acid in their

extremely low concentration baths cause

two-stage ionic dissociation, generating

S042- and c ~ O 4 ~ - ions; accordingly, the

barrier layers are destroyed at the time of

anodic electrolysis. The reason why pitting

corrosion occurs during the anodic

electrolysis of aluminum in extremely low-

concentration sulfuric acid and oxalic acid

baths can be ascribed to S042- and

C2042- ions generated by two-stage

dissociation.

The same is true for extremely low-

concentration tartaric acid baths, where

tartaric acid undergoes two-stage

dissociation and form harmful ions that can

not emit protons. Therefore, if anodized

12

370

aluminum is subjected to secondary

electrolysis in 0.1 - g/I tartaric acid bath (at

60°C ), a great number of flaws occur in

the barrier layer, making the anodic

decomposition reaction of water vivid. As

a result, solutions in the pores of anodized

aluminum become acidic, forming new

porous oxide layers as shown in Photo. 1. In 10 - g/i tartaric acid hath, however, its

ionic dissociation takes place in one stage,

so the secondary anodic electrolysis of

anodized aluminum forms only thick

barrier layers a t the pore bottoms of

anodized aluminum without any change in

the appearance of anodized aluminum.

Therefore, even if baths itre neutral, the

pore filling phenomenon does not occur in

neutral baths containing ;inions generated

from monoprotic acids, those from two-

stiige-dissociated diprotic iicicis (Sod2-,

O d i c acid and tartaric acid ions etc.) and

those from three-stage dissoci;ited triprotic

acids (Pod3- etc.), but flaws ;ire generated

in the barrier layers. Owing to these flaws

the anodic decomposition reaction of \viiter

takes place.

Anions destroying the biirrier layers

also affect A C electrolytic coloring in

nickel baths. If AC voltage higher than the

primary anodic voltage of iinodized

aluminum is applied to anodized aluminum

in weakly acidic electrolytic coloring baths,

the barrier layers of anodized aluminum

become a little thicker, iind at the same

time flaws are generated due to SOq2'

anions etc. which hinder the formation of

oxide films. These flaws may be the sites

for the cathodic reduction of metal ions. In

the case of tin baths, anions generated from

stannous sulfate consist of the great amount

of HSO4- ions and the small amount of

Sod2- ions. Consequently, the destruction

of the barrier layers by Sod2- ions is minor,

but the field assisted dissolution of the

harrier layers by the great amount of H+

ions is large.

Baizuldin3) reported that flaws were

generated in the secondary anodic

electrolysis of thick anodized aluminum,

iind in this case these flaws may also be

generated due to ~ 0 4 2 - ions remaining in

the pores of anodized aluminum.

Remarkable flaw generation makes the

corrosion resistance of anodized aluminum

lower. Therefore, the expectation that the

high- \rOltiige secondary anodic electrolysis

of iinodized aluminum in neutral baths

makes the barrier layers thicker and

improves its corrosion resistance is not

;ilw:rys correct.

The commentaries described above

relates to the secondary anodic electrolysis

of anodized aluminum in the weakly acidic,

neutral or weakly alkaline baths. The

results of the secondary anodic electrolysis

of iinodized aluminum in acidic bath are

different from those obtained above.

Neither the pore filling phenomenon nor

13

379

anodic decomposition reaction of water

takes place. In acidic baths, the secondary

anodic electrolysis forms the secondary

porous layers. For example, the secondary

anodic electrolysis of sulfuric acid film in

oxalic acid baths leads to the formation of

oxalic acid film between the sulfuric acid

film and aluminum metal; namely, a

double-layer film consisting of sulfuric acid

film and oxalic acid film is formed on the

aluminum metal. In the same manner, the

secondary anodic electrolysis of sulfuric

acid film in phosphoric acid baths lead to

the formation of the double-layer films

consisting of sulfuric acid film and

phosphoric acid film. The latter double-

layer films have been industrially used as

the interference coloring method.

Upon performing the secondary anodic

electrolysis of anodized nluminum in acidic

baths, if secondary anodic electrolyzing

voltage (V2) is lower than primary anodic

electrolyzing voltage (Vl), then the famous

"current recovery phenomena" take place.

Figure 13 shows the crplunation of the

current recovery phenomena which is given

by MurphyS). If anodic voltage is reduced

Fig.13 "Current recovery phenomena" by

Murphy

to V2 volt while aluminum undergoes

electrolysis with anodic voltage being V1

volt, then anodic current does not flow for

some time, and after T min. anodic current

(i2) begins to flow. The time T shown in Fig.

13 is called current recovery time, which is

a transient time required for changes in the

film structure of anodized aluminum. The

changes a re shown schematically in Fig. 14.

The thickness of the barrier layer and pore

Fig.14 Changes of film structures of

anodized aluminum by "current recovery

I)henomena".

size of anodized aluminum depend on

anodic voltage during anodic electrolysis.

The coating ratio of the barrier is 10hilv.

Therefore, higher anodic voltage results in

oxide films with thicker barrier layers,

thicker pore walls and greater pore sizes,

whereas lower anodic voltage leads to oxide

films with thinner barrier layers, thinner

pore walls and smaller pore sizes.

Accordingly, if anodic voltage is reduced to

V2 volt while anodic electrolysis proceeds

14

380

at VI volt, then "anodized aluminum with

thicker barrier layers and greater pore

sizes" must change "anodized aluminum

with thin barrier layers and smaller pore

sizes". The transient time required for the

changes in the film structures of anodized

aluminum is current recovery time T.

Conversely speaking, current recovery for

modized aluminum enables its barrier

layers and pore sizes to be thin and small,

reslmtively. The current recovery

phenomena have been industrially applied

to multi-electrolytic coloring of anodized

aluminum. If the barrier layers of

anodized aluminum are made thin by

utilizing the current recovery phenomena,

metal is deposited uniformly in the pores of

anodized aluminum during electrolytic

coloring, so that anodized aluminum is

multicolored with blue, green, red etc.

Attention should be paid to the "field

:issisted dissolution" and "chemical

dissolution" of anodized aluminum when it

is subjected to secondary anodic

electrolysis. "Field assisted dissolution"

should be taken into account only in

secondary anodic electrolysis in acidic

baths, but attention should be paid to

''chemical dissolution" when anodized

aluminum is immersed in electrolytic baths

as ivell as during all secondary electrolysis.

When anodized aluminum undergoes

secondary anodic electrolysis in neutral

baths, the pore filling phenomenii and

anodic decomposition of water take place.

However, when anodized aluminum is

subjected to secondary anodic electrolysis

in strongly acidic baths, the dissolution of

the barrier layers is accelerated on account

of the effect of electric field, and thereby

new anodized aluminum films are formed.

The high-speed dissolution of the barrier

layers due to the effect of electric field is

called the field assisted dissolution of

anodized aluminum. On the other hand,

whether or not voltage is applied to

anodized aluminum, it is dissolved by the

chemical dissolving forces of electrolytic

baths as long as anodized aluminum is

immcrsed in them. The difference between

the field assisted dissolution and chemical

dissolution of anodized aluminum is

schematically shown in Fig. 15.

The theoretical backgrounds of the

secondary anodic electrolysis of anodized

. . .- . .

I 1

Fig.15 "Chemical dissolution" and "field

assisted dissolution"

15

38 1

aluminum explained in this section are

summarized in the left column of Table 1.

5. Theoret ical Background of Secondary C a t h o d i c Electrolysis

Compared with anodic electrolysis

explained in the preceding Section, the

number of papers regarding the basic

researches of the cathodic and AC electrolysis of anodized aluminum itre so

small that there exist few famous theories.

On performing the secondary cathodic

electrolysis of anodized aluminum, the

understanding of the following items is

important:They are

(1)two hypotheses on the cathodic

reduction of H+ ions,

(2)two cathodic reactions taking 1)lilce in

sulfuric acid baths,

(3)the alkalification of solutions contained

within the pores of anodized aluminum due

to the cathodic reactions,

(4)the deposition of metals and formation

of metal hydroxides in metallic salt baths

due to the cathodic reactions,

(5)difference between the destruction of

barrier layers and spalling of anodized

aluminum, iind

(6)the effect of the thickness of anodized

aluminum on the cathodic reactions.

The cathodic reduction reaction of H+

ions is the simplest electrochemical

reaction in the field of electrochemistry,

and ciin he written by the Formulil (5 ) .

2H+ + 2e -> H2 ..... (5 )

The cathodic reduction reaction of H+

ions which occurs on metal plates such as

platinum is a simple electrochemical

reaction as shown in Fig. 16 (A), but that

taking place within the pores of anodized

aluminum, as shown in Fig. 16 (B) and (C),

is not simple. The first problem lies in

whether H+ ions are cathodically reduced

on the barrier surfaces of anodized

aluminum or they undergo ionic conduction

through the barrier layers and cathodically

reduced in the interface between the I

Fig.16 Cathodic reduction of H+ ions and

formation of AI(OH3)

barrier layers and aluminum metal.

Unfortunately, there is no way at present to

confirm the adequacy of above two

hypotheses. The second problem is the fact

that the impedance of the barrier layer of

anodized aluminum changes remarkably.

Figure 17 shows the voltage-time curves

16

1 382

when l o p anodized aluminum is

subjected to secondary cathodic

electrolysis in 0.01 -mol/l H2SO4 bath at - 2.5 to - 0.25 A/dm2. Cathodic voltage

r

Fig.17 Cathodic electrolysis of anodized

aluminum in 0.01 moUl H2SO4 bath at

variou current densities

ranges from minus several 10V to minus

1OOV. In addition, over voltage in the

cathodic reduction of H+ ions on the metill

plate is minus several V. In the cathodic

reduction of H' ions within the pores of

anodized aluminum, cathodic voltage is

extremely high, and this arises from the

high impedance of the barrier layers of

anodized aluminum. The third problem

relates to the diffusion of H+ ions on the

surfaces of the barrier layers at the pore

bottoms of anodized aluminum. In the

citthodic reduction of H+ ions on the metal

plate, H' ions in the bulk solution diffuse

fast on the surface of the cathode; however,

in the case of the anodic aluminum

electrode with its pore size and length being

1SOA and l o p respectively, the diffusion

of H + ions on and their supply to the

barrier layer surfaces are not always fast.

The fourth problem concerns the formation

of aluminum hydroxide colloid within the

pores of anodized aluminum as shown in

Fig. 16 (D). The pores of anodized

aluminum immersed in the sulfuric acid

bath contains H', OH- and AI3+ ions. If

this anodized aluminum undergoes

secondary cathodic electrolysis, H+ ions

become H2 gas, making thereby the insides

of anodized aluminum pore alkaline. As a result, AI3+ and OH- ions existing in the

pores of anodized aluminum cause a neutralization reaction, leading to the

formation of colloidal AI(OH)3. Further,

the continuation of the cathodic reaction

changes the colloid layer of AI(OH)3 into

i1n electrically resistant gel layer. The

increase in cathodic voltage in Fig. 17

partly stems from the formation of the gel

layer consisting of aluminum hydroxide.

The fifth problem is the destruction of the

barrier layers and spalling of anodized

aluminum due to the cathodic reaction. As

shown in Fig. 17, cathodic voltage reaches a

maximum, and then drops because of the

destruction of oxide films and occurrence

of spalling. The elucidation of the

mechanism of these phenomena is still in

the hypothetical stage.

As to the cathodic reduction reactions of

anodized aluminum in the sulfuric acid

baths, the cathodic reaction of H+ ions

shown in Formula (5 ) illone has been

17

383

discussed in this Section, but there exists

another reaction; it is the cathodic

reduction reaction o f dissolved oxygen in electrolytic baths, and can be represented by Chemical Formula (6).

0 2 + 4 H + + 4 e -> 2H20 ..... (6)

The above cathodic reaction does not

generate hydrogen gas, and takes place at

higher cathodic voltage than that of the

cathodic reaction 2H+ + 2e -> H2.

Accordingly in the cathodic electrolysis of

anodized aluminum shown in Fig. 17, the

cathodic reactions of ( 5 ) and (6) take place

simultaneously. When anodized aluminum undergoes

secondary cathodic electrolysis, the

destruction of the barrier layers and

spalling of anodized aluminum occur.

Figure 18 shows the voltage-time curves,

Fig.18 Cathodic electrolysis in 0.01 M &

1.0 M H2SO4 bath

when lOpn thick anodized aluminum was

subjected to secondary cathodic electrolysis in 1.0- and 0.01- mol/l H2SO4 baths. In both cases current density was - 0.5 A/dm2. In 0.01- mol/l H2SO4 bath,

cathodic voltage reaches -9OV, followed by

rapid decrease. Many spotty peelings with

about I-mm diameter were observed on the

surface of anodized aluminum. The peeling

of the oxide film is called "spnlling". The

generation of spalling is explnined in the

Brace's book "ANODIC COATING

DEFECTStf6). However, when anodized

aluminum underwent secondary cathodic

electrolysis in 1.0- moM H2SO4 bath,

resultant cathodic voltage was about -5V, and no spnlling of the oxide films was

observed. The observation of this anodized

aluminum with an electron microscope revealed the destruction of the barrier.

layers of about 1 O O p n from the results

given in Fig. 18, it was found 'that the

secondary cathodic electrolysis of anodized

aluminum sometimes causes spalling, and

sometimes the fine destruction of the

barrier layers. The fine destruction of the

barrier layers may arise from the

concentration of cathodic current to flaws

or from hydrogen gas bubbles cathodically

reduced in the interface between the

barrier layers of anodized aluminum and

aluminum metal. In the case of spalling, on

the other hand, the size of a peeled oxide

film is about 1 mm in diameter, so that the

384

18

mechanism of spatting differs from that of

fine peeling. This can be understood from

the difference in cathodic voltage shown in

Fig. 18. Spalling is considered not to occur

in irccordance with the cathodic reaction

2H+ + 2e -> H2, hut to occur in accordance

with the cathodic reaction 0 2 + 4H+ +4e - >2H20. Spalling, therefore, takes place at

higher cathodic voltage. Since the 0 2

molecules in the barrier layers are

consumed by the cathodic reaction 0 2 + 4H+ +2e ->2H20, anodized aluminum miry

peel off in a spotty state. Moreover, if the

cations such as Na+, K+ and NH4+ coexist

in electrolytic baths, spiilling takes place

more easily, hut the reason in unknown.

The results of experiments shown in Fig.

18 endorse the empiriciil filct thiit spiilling

is ciis\. to occur in the weakly acidic nickel

bath but difficult in the strongly acidic tin

bath.

The secondiiry ciithotlic clcctrolysis of

iinodized aluminum in the aqueous solutions

of metallic salts leiids to the deposition of

the metals in the pores of iinodized

irluminum, resulting thereby in coloring of

irnodized iiluminum. This technique is

ciilled the cathodic electrolytic coloring

method or DC electrolytic coloring method,

iind has been industrially used since 20

years before. However, the secondiiry

cathodic electrolysis of anodized aluminum

in the aqueous solutions of metallic salts

iilSO forms metal hydroxides in the pores of

anodized aluminum. The cations such as

AI3+ and Mg2+ are not reduced to metals

by cathodic electrolysis in aqueous

solutions. This is the reason why aluminum

and magnesium are produced by molten

salt electrolysis. The cations AI3+ and

Mg2+ existing in the pores of anodized

aluminum form hydroxides such as

AI(OH)3 and Mg(OH)2 within the pores by

the cathodic electrolysis of anodized

aluminum. Since these metal hydroxides

a re electrically resistant, they tend to

decrease cathodic current during cathodic

electrolysis.

Three kinds of anodized aluminum with

the thickness of 2, 10 and 20pm unclerncnt

cathodic electrolysis under the same

conditions, but the Stiites of ciIthotlic

electrolysis varied with film thickness. I n

other words, the secondary electrolysis of

iinodized irluminum is affected not only by

the conditions of liquid phases hut iilso by

those of solid phases. The reason is that

when iinodized aluminum is thin, the supply

of H+ ions from hulk solutions is fast

despite the decrciise in H+ ions within the

pores of nnodizetl irluminum due to the

cathodic reaction, and that thereby the

solutions within the pores do not become

iilkdine. When modized aluminum is thick.

however, the supply of H+ ions from the

bulk solutions is not fast, so that the

solutions within the pores of anodized

iiluminum become locally alkiilinc. In

19

385

addition, in secondary anodic electrolysis

and secondary AC electrolysis, the

thickness of anodized aluminum affects

electrochemical reactions.

The explanations given to secondary

cathodic electrolysis in Section 5 can be

summarized as shown in the middle column

of Table 1. Similar to secondary anodic

electrolysis shown in Table 1, in the

secondary cathodic electrolysis of anodized

aluminum, different electrolytic conditions

result in different states of cathodic

reactions.

6. Theoretical Background of Secondary AC Electrolysis

Upon performing the secondary AC

electrolysis of ilnodized itluminum, the

understanding on the following items is

important:

(1)tlifference between the case where no

electrochemical reaction takes place while

AC is flowing and the case where it takes

place; difference between "Faradic

current" and "non-Faradic current",

(2)tlistinction between "the Case where

cathodic reactions alone take place" and

"the case where both the anodic and

ciithodic reactions take place",

(3)distinction between the case where

porous oxide films are formed nnd the CilSe

where they a re not formed,

(4)distinction between the case where either

anodic nor cathodic peak current

decreases and the case where they do not

decrease,

(5)difference in film structures and film

compositions between oxide films obtained

by AC electrolysis and those by DC electrolysis,

(6)formation of colloid and gel layers,

(7)AC current recovery phenomena, and

@)impedance equivalent circuits for AC

electrolysis. If sine-wave AC voltage is applied to

anodized aluminum in electrolytic baths,

three kinds of alternating current wave

forms, depending on the differences in pHs

and bath compositions, are observed on the

oscilloscope as shown in Fig. 19. The

application of low AC voltage to anodized

aluminum often leads to the current wave

form shown in Fig. 19 (A). Although a

phase appears between AC voltage a n d .

Fig.19 Current wave forms by AC

electrolysis of anodized aluminum

20

current, the current wave form is also

sinusoidal in this case. This sine-wave

current is charged and discharged by oxide

films, and is non-Faradic. This means that

although sine-wave A C voltage is applied to

anodized aluminum, no electrochemical

reaction takes place on it when the

altcrnating current wave form is sinusoidal.

In ttnodic and cathodic electrolysis, current

flow causes electrochemical reactions

without fail, but in AC electrolysis,

alternating current flowing through the

systems of electrolysis docs not always

cause electrochemical reactions. The

electrolytic current of this kind is called

"non-Faradic current" meaning "current

which does not always accompany

electrochemical reactions". On the other

hand, "Faradic current" accompanies the

electrochemical reactions. The AC

electrolysis of anodized aluminum in

\ve:iMy acidic electrolytic baths results in

the alternate current have form shown in

Fig. 19 (B). This current wave form

consists of sine-wave non-Fradic current

and cathodic peak current, which serves

the cathodic reduction of hydrogen ions

and metal ions. Cathodic peak current,

therefore, is Faradic (current serving

electrochemical reactions). When anodized

aluminum undergoes AC electrolysis in

strongly acidic baths, a It ern at i n g distorted

current wave form shown in Fig. 19 (C) is

observed on the oscilloscope. Besides sine-

wave non-Faradic current, this alternating

distorted current wave form includes

anodic and cathodic peak currents. The

anodic peak current in this case is one of

Faradic currents caused by the formation

of new oxide films and the anodic

decomposition of water. Consequently,

when anodized aluminum is subjected to

secondary AC electrolysis, the following

three phenomena can be clearly judged by

the observation of alternate current wave

forms with the oscilloscope : (1) whether o r

not ,electrochemical reactions are taking

place, (2) whether cathodic reactions alone

or combined cathodic and anodic reactions

are occurring, and (3) whether o r not new

oxide films are being formed. For

measuring alternate current wave forms,

the Lissajours' figure and differential

Lissajous' figure are also available. These . alternate current measurement methods

:ire shown in Fig. 20.

I

: SYNCHRO- SCOPE SCOPE

( ~ 1 Ollferrniiri currant wave

SCOPE

( 0 ) Oillereiiol Lirsajour' ( b l Lissajou's ligurr liguro

Fig.20 Meagurment methods of alternating

current

21

387

388

Oxide films formed hy anodic peak

current as shown in Fig. 19 (C) are called

"AC anodized oxide films", and have

different properties from those of anodized

aluminum formed by ordinary DC

electrolysis; namely, the pore sizes of AC

oxide films a r e small, and the number of

pores per unit area is great. AC oxide

films often contain sulfur, so they

sometimes give off the smell of hydrogen

sulfide. Therefore, if normal anodized

aluminum is prepared in a sulfuric acid

bath by DC electrolysis and then followed

by secondary AC electrolysis, then "AC

oxide films" are formed under "DC oxide

films". The appeariince of the double-lap-

oxide films is opaque, and they are colored

green when they undergo electrolytic

coloring in the copper bath. This

electrolytic green coloring technique has

already got a patent.')

Since eilrly times AC electrolysis hns

been studied by using impedance equivalent

circuits, and this research method is called

"Faraday Impedance Method". Figure 21

gives the impedance equivalent circuits for

each current wave form shown in Fig. 19.

Ventura8) studied electrolytic coloring

re;ictions by using the Faraday impedance

method, and the results were presented in

SUR/FIN '94.

Although the impedance circuits were

assumed for AC electrolytic reactions, the

calculations for these impedance circuits

Fig.21 Equivalent circuits of AC

electrolysis of anodized aluminum

were very complicated. However, the

utilization of an analog circuit simulator

used recently in the field of the electronic

engineering enables the calculation o f the

impedance equivalent circuits to be

performed easily. Figure 22 shows the

results of calculations authors conducted on

L I

Fig.22 Deformed wave forms of AC

electrolytic coloring by analog circuit

s imul at o r

22

thc impedance equivalent circuits for

electrolytic coloring by using the analog

circuit simulatorg) I'P Spice". When anodized aluminum undergoes

AC electrolysis, A C current wave form

sometimes changes with electrolyzing time,

but sometimes does not,as shown in Fig.23.

Figure 23 (A) showns the A C current wave

form when anodized aluminum is subjected

to AC electrolysis in 1.0-molh H2SO4 bath.

FIg.23

with the lapse of electrolyzing time

Changes of current wave forms

The AC current wave form does not change

even if electrolyzing time is made longer.

This means that the formation of AC

anodized aluminum and the cathodic

reduction reaction of H+ ions continue

under given conditions. Figure 23 (B)

shows the change in AC current wave form

when anodized aluminum is subjected to

AC electrolytic coloring in a nickel bath.

Although in the early stage of AC

electrolysis both anodic and cathodic peak

currents itre observed, the anodic peak

current disappears after several second

(the peak current a t t2 in Fig. 23 (B)). This

Pact indicates the formation of thick barrier

layer on anodized aluminum due to anodic

voltage. After the completion of thick

barrier formation, anodic peak current

stopped flowing. In the case of Fig. 23 (B), cathodic peak current also continues to

decrease with AC electrolyzing time, and

finally disappears. The reason why

cathodic peak current decreases when

anodized aluminum undergoes AC

electrolytic coloring in the nickel bath is that nickel is deposited in the pores of

anodized aluminum and that sol - gel layers

of itluminum hydroxide itrc also formed in

the pores. The mechanism of these

reactions is del)ictcd in Fig. 24'O). In the

fAl .- .,

Fig.24 Formation of AI(OH)3 gel in the

pores of anodized aluminum by electrolytic

coloring

AC electrolytic coloring of anodized

:tluminum in the tin bath, decrease in

cathodic peak current with electrolyzing

time is slow as shown in Fig. 23 (C). This is

because the strongly acidic tin bath makes

difficult the formation of the gel layer of

23

389

aluminum hydroxide shown in Fig. 24.

When the pH value of the tin bath is low,

both anodic and cathodic peak currents are

sometimes observed in the AC current wave form during electrolytic coloring. In

this case, the cathodic reduction reaction of

tin proceeds simultaneously with the

formation of AC anodized aluminum in the

tin bath.

As explained in Section 4, Murphy found

that anodic current did not flow for some

time if anodic voltage is decreased while

aluminum is being anodically electrolyzed.

He named this the "current recovery

phenomena" (See Fig. 13) The authors

made a series of experiments on whether or

not the current recovery phenomena

:ipl)ear when low AC yoltage (AC JV) is

applied to anodized aluminum. The results

are depicted in Fig. 25. AC current did not

1 In *..TIME

I

Fig.25 Recovery of alternating current

low immediately after applying 4-V AC

voltage to anodized aluminum. After 30 sec.

smaller sine- wave AC current began to

flow, followed by greater alternating

distorted current after 2 min. Thereafter,

the value of alternating current was

constant. Like the current recovery

phenomena found by Murphy, the existence

of alternating current recovery phenomena

was confirmed. The phenomena made the

barrier layers of anodized aluminum thin.

If this anodized aluminum undergoes AC

electrolytic coloring in the tin bath, then it

colors into blue, green, red etc. - multi-

color electrolytic coloring. The reason is

that since the barrier layers of anodized

aluminum become thin, tin was uniformly

deposited within its pores. This multi-color

electrolytic coloring technique has been

made public as Benitez's patent1 l). When,

DC 6 or 5V is applied to anodize aluminum

instead of AC 4V, no ;inodic current

recovery was observed even after 30 min.

This means that to make the barrier layers

of anodized aluminum thin by using the

current recovery method, application of

low AC voltage is better than that of low

anodic voltage. However, the reason why

the application of low AC voltage is better

than that of low anodic voltage for

shortening current recovery time is

unknown.

The contents of the discussions

described in Scction 6 :ire summarized in

24

390

the right column of Table 1. Like in

secondary anodic and cathodic electrolyses,

in the case of secondary AC electrolysis,

the different electrolyzing conditions result in the different phenomena, which take

place on the different grounds. The means

for inyestigating these differences is the

me;tsurement of alternating current wave

forms by the oscilloscope. The

me;rsurement of current with an AC meter

can not clarify the rezrction mechanism of

A C electrolysis.

7. Conclusions 'The anodic electrolysis of aluminum hiis

been performed for iibout 70 yenrs;

accordingly, miiny themes have been

studied, ;rncl thereby se\rerirl import:int

theories hwe been m:ide public. On the

other hand, I);isic researches regirrtling thc

seconclary cathodic electrolysis itnd

scctjndiir! .A<' elect rolysis of ;inotlizcd

irluininum itre small i n number. Ho~vever,

the electrolytic coloring and

electrotleposition coitting methods, both of

which are rcliitivdy new surface finishing

tecliniques, htrvc been carried out hy the

second:tr> electrolysis of i\nodized

nluminum. The multi-color electrolytic

coloring method, which is ;i hot

tecllnologiciil topic today, has :idoptecl

tertiary or quaternary electrolysis.

Therefore, it is very important to

underst:rntl i n tlcpth the theoretic:il

backgrounds of the secondiiry electrolysis

of anodized aluminum explirinetf in this

paper.

References 1) D. A. Vermilyea; Journal o f

Electrochemical Society, vol.1 IO, p250

(1 963).

2) A. Dekker, A. Middellhack; Journal of

Electrochemical Society, vol.117, p440

(1970).

3) B. M. Baizuldin; Metal Finishing, 1127,

December (1993).

4) K. Kaminaga, T. Sato; Proceedings of

SUIUFIN '95, Session E, plX9 (1994).

5 ) J. F. Murphy; Proceedings of

International Symposium on Anodizing

(Aluminium Federation, UK), p3 (1967).

6) A. W. Brace; "Anodic Coirting Defects --- Their Couses and Curc" (book),

Technicopy Books, Stonehouse, Ghs., UK

( 1 992).

7) Jiipanesc open patent H3-207895 (1991).

8) X. A. Ventura; Proceedings of SUWFIN

'95, Session H, 11363 (1994).

9) P. W. Tuinenga; "SPICE - A Guide to

Circuit Simulation and Aniilysis Using

€'Spice " (book), Prenticc Hall, SJ. USA

(1 988).

10) T. Sitto; Plating and Surfiicc Finishing,

~ 1 . 7 8 , M;rrch, p70 (1991)

1 1 ) European patent 0413589A1 (1990).

25

39 1