al anodizing

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LITERATURE SURVEY

The chromic acid anodizing process for aluminum and aluminum alloys was

developed by Bengough and Stuart [28]. They carried out anodizing in electrolytic bath

containing chromic acid solution or materials producing chromic acid at anode. In their

process, carbon cathode was used and temperature was 40C. They were quite successful

in producing corrosion resistant oxide coating for corrosion prevention of aluminum and

aluminum alloys.

Later on they developed another process for anodizing of aluminum and its alloys

to make them corrosion resistant to sea water [29]. The degreasing of aluminum was

done by washing with a solvent and then washing thoroughly with hot water. After this,

the aluminum was anodized in chromic acid bath with carbon cathode and temperature

was not less than 40C. The voltage across the electrolytic bath was raised to about 40 V

in the course of 15 minutes and this voltage was maintained for about 35 minutes, then it

was raised to 50 V in the course of 5 minutes and maintained for about 5 minutes. The

anodized surface was then rinsed in distilled water and was desiccated in air. The oxide

coatings produced by this method were corrosion resistant to sea water.

Bengough and Sutton studied anodizing of aluminum and its alloys for the

prevention of corrosion [30]. The best oxide coating on various alloys was obtained by

using 3% chromic acid aqueous solution and 40C temperature. Although this method

was not successful for aluminum alloys containing 5% or more copper content, because

oxide film was broken down at 30 V. However this process was used to anodize

aluminum silicon and aluminum zinc alloys, although Al-Si alloy containing 7.5-

8.75 % silicon caused high current consumption. Anodic oxidation was followed by

dipping in molten lanolin, a 15% solution in benzene or into a lanolin suspension, thus

providing the best protection against water-line corrosion.However health and environmental problems does not allow using of hexavalent

chromium due to its poisonous and carcinogenic causing effects and this process is

progressively restricted even banned [40-43]. Thus the alternate of anodizing process in

chromic acid is essential and vital problem in engineering areas and various problems

concerning environment.


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Bengston developed a new method for the anodizing of aluminum and its alloys

[13]. He used sulphuric acid electrolytic solution for the anodizing of aluminum. This

process was useful for producing anodic coloured film and the sealing of pores of oxide

coating at a temperature of 80-100 C was used to increase the resistance to chemical

attack.

Marceau et al successfully developed anodizing process using phosphoric acid to

obtain adhesively bonded Al or Al alloy parts [31]. This method was effective for

increasing the environmental stability of adhesively bonded aluminum structures in

service. They used this method chiefly as a pretreatment for use of organic or plated

coating. Now this process is normally used to manufacture adhesively joined aluminum

parts in aircraft industries and other industries [44-47]. It has also been used in printing

and plating processes [48-53].

Fukuda and Fukushima studied the effect of addition of aluminum sulphate or

magnesium sulphate in sulphuric acid anodizing of aluminum [54]. They found that the

anodic coating surface became microscopically more even by the addition of the sulphate

concentration. The porosity near the surface was also decreased. They also found that the

uniformity of the film was decreased by increasing the sulphate concentration.

Arowsmith and Clifford developed a method for adhesive bonding of aluminum

[55]. They carried out hard anodizing using sulphuric acid electrolyte to form thick and

corrosion resistant surface and this coating was accompanied by limited phosphoric acid

dissolution. They also studied the surface structure and its chemical properties and found

it fitting for adhesive bonding.

Wong and Moji developed boric /sulphuric acid anodizing as a replacement for

chromic acid anodizing [56]. They sealed the porous coating using hot dilute dichromate

solution to accomplish acceptable corrosion resistance. They found that the oxide coating

had properties as good as or superior to coatings applied in chromic acid anodizing

Lockheed Martin introduced thin film sulphuric acid anodizing (TFSAA) process.

All the parameters are the same or close to those of the (BSAA) process with the removal

of boric acid [57,58]. The anodizing conditions were: 3-5% wt. sulphuric acid electrolyte

solution, 25-30 C temperature, 15V and 25 minutes anodization time. The anodized

aluminum was sealed in 5% wt. sodium dichromate solution at 85-95C temperature for a


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period of 20 minutes. This process required low cooling and current requirement than

Type II anodizing. Both the boric/sulphuric and thin film sulphuric acid anodizing

processes use environmentally caring electrolytes and relatively their treatment and

disposal is cheap. These processes were planned to make thin oxide coating on Al to

increase corrosion resistance and paint adhesion with low fatigue loss. The oxide film

mass was in the range from 200 to 700 mg/ft2, and it was about 1 to 3 microns thickness.

Various researches have revealed that boric/sulphuric acid anodizing and thin film

sulphuric acid anodizingprocesses generate the anodic coatings suitable and fitting forthe military and aircraft industry requirements and specifications[59-64].

Griffen and Askins studied surface preparations without chromium for aluminum

adhesive bonding [65]. They evaluated the chrome free surfaces for their ability to impart

both stability and acceptable property requirement to bonded aluminum joints. They also

conducted various property checking tests with coated and bonded samples. They found

that coated surfaces without chromium were equivalent to phosphoric acid anodizing

process.

Ikegaya investigated post treatment of anodic oxide film [66]. This involved

dipping the aluminum anodic oxide film in a water bath (having a specific resistance >

3000 cm and pH = 6-8 and heated to 40-60 C) for 3-30 minutes. This method was

used to apply a coating to prevent water adsorption to the film and to decrease the acid

content in the film. They found that the cracking of the coating material in high

temperature baking was prevented.

Surganov et al. examined the dissolution of anodic oxide coating at the initial

stage of anodization of aluminum in aqueous solution of organic acids with different

dissociation constants; oxalic, malonic acid citric acids [67]. They found that the

dissolved aluminum content appeared in significant amounts (0.04 0.1 g/cm2)

practically right after applying the anodic potential Ea to the studied sample.

Panitz et al. studied the formation of barrier anodic film on 6061-T6 Al alloy in

electrolytes containing different ethanol to water ratios [68]. They prepared by dissolving

five mixture compositions of ammonium tartarate in water and diluted with different

amounts of ethanol .The effect of electrolyte temperatures within the range of 18-38C

was investigated. They found that the best dielectric coatings and the shortest processing


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time occurred for the 100% water ammonium tartarate electrolyte ,while the second best

coating and the processing time occurred for the 98% ethanol, 2% water plus ammonium

tartarate electrolyte composition.

Danford investigated the corrosion resistance of aluminum alloy 6061-T6 by

different anodizing processes [69]. The anodized aluminum was obtained by both

impregnated anodized coating and hard anodized coating (sealed in water and sealed in

dichromate). He investigated the corrosion protection at both pH 5.5 and pH 9.5 with 28

days exposure period in 3.5% NaCl solution (25 C) for each sample. He found that

corrosion resistance for all samples was superior at both pH. He found that hard

anodized, hydrothermally sealed coating at pH 9.5 provided the best corrosion protection.

Kim et al. investigated the growth and structural properties of the aluminum oxide

thin coating grown on aluminum substrates by anodizing using phosphoric acid at 35-75

V (D.C)[70]. They investigated the surface and depth profiles of anodized thin films of

alumina by scanning electron microscope (SEM), Auger electron spectroscopy (AES)

and secondary ion mass spectroscopy (SIMS). The pits were formed on the anodized film

surface upon increasing the anodization time. They also found that the surface was

restored to its mirror like surface as revealed by SEM.

Shular et al. investigated the location and estimation of air emissions from sources

of chromium [71]. They updated the technical information and presented a new emission

data upon which emission factors were based for hexavalent chromium emissions from

cooling towers and chromium electroplating operations. They also included process

descriptions for 5 kinds of plating/anodizing operations, emission control techniques for

reduction of chromic acid missed from plating operations, updated information about the

distribution of industrial process cooling towers that use Cr-based water treatment

chemicals, new information about comfort cooling towers, emission reduction techniques

for chromium emissions from cooling towers equipped with low and high-efficiency drift

eliminators.

Adachi et al. developed a method for the manufacturing of aluminum foil

electrode for electrolytic capacitor [72]. They impregnated an etched aluminum foil with

hot pure water, anodized in aqueous citric acid, depolarized by aqueous NH3, heated, re-


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anodized in aqueous phosphoric acid, heated, re-anodized again and washed to give a

formed foil showing large electrostatic capacitance.

Adachi et al. devised another method for the manufacture of aluminum foil

electrode for electrolytic capacitor by a different method [73]. They impregnated an

etched aluminum foil with hot pure water, anodized in aqueous citric acid, depolarized by

aqueous ammonia, heated, re-anodized in aqueous adipic acid, heated, re-anodized again

and washed to give a formed foil showing large electrostatic capacitance.

Adachi et al. developed another method for the manufacture of Al foil electrode

for electrolytic capacitor [74]. They impregnated an etched aluminum foil with hot pure

water, anodized in an aqueous mixture of citric acid and boric acid, depolarized by

aqueous ammonia, heated, reanodized in the aqueous mixture and washed to give a

formed foil showing large electrostatic capacitance.

Okubo et al. studied the microstructure of anodic oxide films on aluminum by

pulse current technique with negative component [75]. They used electron microscopic

study to provide a better understanding of cells and pores in aluminum anodic oxide

films. They carried out anodizing process in 15 wt. % sulphuric acid (20 C, 2 A/dm2, 5

minutes), 5 wt. % oxalic acid (30C, 2 A/dm2, 5 minutes), 8 or 10 wt. % chromic acid

(50C, constant 60 V, 30 minutes) bath using 13.3 Hz rectangular a.c. with various duty

factor (ratio of post. component in one cycle). The rear surface (boundary between oxide

layer and aluminum metal) of film stripped in a HgCl2 solution were shadow-cast with

Au-Pd ion sputtering technique, and were observed by SEM. TEM images of pore

structure were given of cross section 40 nm (in thickness) of perpendicular to film surface

using ultramicrotome. They concluded that the cell size, pore diameter and pore

branching has increased with increase in duty factor in oxalic acid and chromic acid bath.

Albort developed electrochemical method for control of the anodization process

[76]. The method evaluated the quality of the anodic coating of aluminum anodization.

The anodization was conducted with an 8.5 wt % solution of chromic acid, c.d. 0.15-0.3

A/dm2

and temperature 35C. The coefficient of corrosion of the sample in millimeters

per year was determined.

Hoshino et al. determined chromium contents in anodic oxide coatings formed on

Al in chromic acid [77]. They analyzed the chromium content in 9 conventional oxide


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films formed in chromic acid bath by instrumental activation analysis. The anodic

oxidation conditions used were as follows: chromic acid electrolyte 30-90 g/L, bath

temperature 40-50C, bath voltage 20-60 V and overall current 1.5-4.0 C/cm2. They

found that the chromium content in the films was 0.010-0.020% and discussed relations

between the chromium content and electrolytic parameters.

Liu and Yang carried out high finishing hard anodizing of aluminum alloys to

obtain a thick and hard alumina layer [78]. They have described the operational

conditions (different sulphuric acid concentrations, temperature, c.d, oxidation time and

electric potential) for high-finishing hard anodizing of hard aluminum, ultra-hard

aluminum and wrought aluminum and properties of anodic films. This process had some

drawbacks like poor brightness and high fragility of the oxide layer, easy peeling of the

layer and electrochemical corrosion during oxidation.

Anon developed a method for anodizing of aluminum using alkaline bath

(hydrazine fluoride bath) containing amino acids [79]. He found that in the absence of

amino acids (glycine or taurine) , uneven oxide films were formed and thickness of the

film was only ~ 9 m. In the presence of aminoacids in the electrolytic bath, 15-16 m

thick uniform oxide films were obtained in 30 minutes electrolysis. He also determined

scratch hardness as 14.8 and 27.4 when taurine and glycine, respectively were present in

the bath.

Fenzl developed a method for improving the corrosion resistance of anodically

produced oxide layers on aluminum alloy materials [80]. He carried out a two step

sealing process after anodization of aluminum. The metal part was dipped in a potassium

dichromate or sodium dichromate bath and, after an intermediate rinse, subjected to a hot

water bath.

Sergeev developed a new electrolyte for anodizing articles of aluminum and its

alloys [81]. He added dimethyl sulphate and an antistatic agent to an electrolyte

containing sulphuric acid, oxalic acid and a fluoroplast suspension for anodizing of

aluminum articles which were used especially for vibrational support of diamonds. He

also found an improvement in the sliding properties of aluminum.

Belousova investigated anodization finishing of aluminum alloy articles in

alkaline bath [82]. He found that thermo-mechanical strength was increased when the


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aluminum or aluminum-alloy articles were anodized for 30-40 minutes at the c.d. 1.5

1.7 times that of the oxide film formation, followed by the principal anodization in

alkaline bath at electric arc potential, decreasing the c.d. at 150-170 m A/dm2-min and

then heat treatment for 1 hour at 200 C.

Xu studied the effect of pore enlargement treatment on the structure and other

characteristics of aluminum coating [83]. He used transmission electron microscopy

(TEM), microhardness meter and SRV friction machine to study the microscopic porous

structure of oxide coating formed on aluminum using oxalic acid electrolyte. He also

studied the change of oxide film porous structure after the pores in the oxide film were

enlarged and its effect on the mechanical performance.

Surganov et al. investigated the growth and dissolution of oxide coating of

aluminum using oxalic acid electrolyte [84]. They determined dependence of the anodic

potential, thickness of the anodized metal layer, and content of dissolved aluminum by

using atomic emission plasma spectrometry. They resolved three characteristic stages

which were inherent to the dependencies. They found that anodic oxide dissolution

occurred at the beginning of the anodic process.

Yashimura et al. carried out anodizing of aluminum in organic alkaline bath using

ethylene-diamine alkaline bath containing ammonium fluoride and ammonium carbonate

[85]. They found that the optimal concentrations were ammonium fluoride 0.25,

ammonium carbonate 0.1 and ethylenediamine 0.1 mol/L. They also tried five polyhydric

alcohols for studying their effects on anodizing of aluminium, and found that propylene

glycol addition increased the thickness of anodic film by ~ 2 m. Also the hardness of

anodic oxide film was better than that obtained from fluoride bath, but the scratch

hardness was inferior to that obtained from sulphuric acid bath.

Schnyder and Koetz investigated spectroscopic ellipsometry and XPS of oxide

coating formation aluminum in sulphuric acid [86]. They monitored the growth and oxide

layers dissolution on aluminum electrodes in 3M H2SO4 by using ex-situ spectroscopic

ellipsometry and ex-situ XPS. The XPS results indicated that the porous oxide, as well

the barrier oxide, grown in tartaric acid could be described as -Al2O3 containing no

major amount of water. Coatings grown in 3M H2SO4 hold ~ 1.5% anions. Spectroscopic

ellipsometry revealed the growth of a porous layer on top of a dense interphase layer.


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They also found that the film thickness was directly proportional to the anodizing time

and applied current density.

Surganov and Poznyak studied the kinetics of aluminum dissolution during

electrochemical anodization in a tartaric acid electrolyte [87]. They found that the

dissolution of aluminum occurred from the amount of application of an anodic potential

on the sample. They explained the nature and interrelation of the kinetic dependencies of

the anodic potential and the mass of the dissolved aluminum on the base of the concept of

ejection of Al+3

ions from the surface of barrier oxide film into the electrolyte and by the

electrochemical dissolution of the oxide.

Floyd and Maddock developed an alternate finishing method of aluminum by

resin-seal anodizing and provided a source of contributing corrosion protection to the

aluminum without causing environmental problems [88]. They employed traditional

sulphuric acid anodizing, but modified the final hydrothermal sealing step by using a

resin containing bath in place of steam, water or water solution generally used. The resin

particles filled the porous structure of the oxide coating. This resin-sealed surface acted

successfully as a primed surface and could be used without paint or top coated. They

found that the process was environmentally friendly and the finished product was weight

and cost savings by this method. They used successfully resin-sealed anodizing on the C-

5 cargo aircraft and this process has been accepted for use in selected C-130 aircraft

applications.

Le and OBrien developed a new anodizing process for producing a high

emittance coating on aluminum and its alloys [89]. They performed anodizing of

aluminum or aluminum alloy substrate surface in an aqueous M2SO4 solution at elevated

temperature (30 C) and by a step wise c-d-procedure, followed by sealing the resulting

anodized surface in water at 200 F and then air dried. They found that the resulting

coating has a high IR emissivity and solar absorptivity and a relatively thick anodic

coating.

Wolf et al. developed a process for sealing of aluminum anodic coating obtained

from a chromic acid bath [90]. They performed the sealing of the oxide layer formed by

chromic acid electrolyte by using a bath composition of de-mineralized water, potassium

or sodium dichromate 8-12g/L, pH adjusted to 4.5 to 6.5 by NaOH addition, bath


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temperature 75-85C, and immersion time sufficient to make sure coating hydration rate

of 8-15% for simultaneously obtaining good corrosion resistance and paint adherence.

Danford carried out the study for comparison of chromic acid and sulphuric acid

anodizing [91]. Because of federal and state restrictions on the use of hexavalent

chromium, he compared the corrosion protection of 2219-T87 aluminum alloy by both

Type I chromic acid and Type II sulphuric acid anodizing. Corrosion measurements were

made on large, flat aluminum alloy sheet material with an area of 1 cm2

exposed to a

corrosive medium of 3.5% sodium chloride at pH 5.5. He carried out both a.c

electrochemical impedance spectroscopy and the d.c. polarization resistance techniques.

He found that the corrosion protection obtained by Type II sulphuric acid anodizing was

superior and no problems would result by substituting Type II sulphuric acid anodizing

for Type I chromic acid anodizing.

Golab et al. examined the anodic coating formed on aluminum alloy AlMg2 [92].

They obtained anodic coating at 1-3 A/dm2

and 5-20C temperature in oxalic acid baths.

The tribolic parameters of the oxide layers were determined and compared with those of

PTFE. To obtain an optimal sliding behaviour, the layer thickness must be 80-100 m,

the microhardness > 4200 MPa, combined with a low roughness. They found that the

optimal tribolic parameters were obtained at 1 A/dm2, 5C and 6 hours oxidation time.

Zhou developed an improvement of corrosion resistance of anodized aluminium

by chromic acid anodizing process [93]. He carried out anodization in a bath containing

30-60 g/L chromic acid solution and the temperature, voltage and time used were 35-

37C, 18-22 V and 30-40 minutes respectively. The anodic oxide film obtained was 37.7-

56.0 mg/dm2. He found that no corrosion was observed after 5% NaCl solution was

sprayed continuously for 336 hours and the adherence was also good. The chromium

concentration of chromium containing waste water was reduced, so the energy used for

treating waste water was decreased by using this method.

Dima and Anicai studied the physical (mechanical, thermal) and electrical

properties of aluminum anodic films obtained by continuously anodization of aluminum

wires and aluminum sheets using an electrolyte based on boric acid, oxalic acid ,citric

acid and isopropyl alcohol [94]. They found that the thickness of aluminum anodic oxide

layers was 5 1, 10 1, for aluminum sheet, respectively and 5 1, 10 1, 15


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1 for aluminum wire. Aluminum oxide films ensured a breakdown voltage of minimum

200 volts, for coils having a curvature radius > 12.5 mm and operating temperature up to

500C.

Gaskin et al. carried out investigation of sulphuric/boric acid anodizing as a

replacement for chromic acid anodizing [95]. This research was under-taken to find the

effectiveness of sulphuric /boric acid anodizing process on 2044-T3 aluminum adhesive

bond durability. They analyzed three processes in this study: a phosphoric acid process

and two sulphuric-boric acid processes (the patented process that was developed at

Boeing and a process that was developed at Rohr). They found that the sulphuric-boric

acid anodize process developed by Rohr gave results that were comparable to the

phosphoric acid anodize process.

Xu and Zhang developed a method for rapid anodizing of aluminum [96]. They

obtained optimization of rapid anodizing of aluminum in sulphuric acid solution by using

a bath containing a composite corrosion inhibitor comprising organic acids, alcohols,

inorganic salts and a small amount of surfactants. They also studied the effect of the

corrosion inhibitor on current density, anodization time, temperature and impurity content

and tested the properties of the oxide film.

Zeng developed alternating current anodization of aluminum and its alloy in

sulphuric acid solution [97]. He introduced the characteristics and recent development of

a.c. electrochemical oxidation in sulphuric acid. He emphasized on the mechanism of a.c.

electrochemical reduction and its technology and comprehensive comparison was made

between a.c. and d.c. electrochemical oxidations.

Ishmuratova et al. used an electrolyte containing sulphuric acid to improve the

corrosion resistance and increasing the breakdown voltage of an anodic coating on

aluminum and its alloys [98]. The electrolyte used additionally contained cerium sulphate

in the following proportions: sulphuric acid 180-200 g/L, cerium sulphate 0.5 10.0 g/L.

Kallenborn and Emmons developed thin-film sulphuric acid anodizing as a

replacement for chromic acid anodizing [99]. They studied a more dilute (5% by wt.)

sulphuric acid anodizing process which produced a thinner coating than Type II or III

with nickel acetate as a sealant. They evaluated this process in regard to corrosion


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resistance, throwing power, fatigue life and processing variable sensitivity and found it

promising as a replacement for the chromic acid process.

Noguchi and Yoshimura studied the result of organic acid salts on the anodizing

of aluminum in organic alkaline baths (benzylamine-fluoride base) [100]. They

conducted investigations to determine optimum bath compositions for the anodizing of

aluminum in organic alkaline baths (benzylamine-fluoride base) involving ammonium

fluoride and different organic acid salts (ammonium formate, ammonium acetate,

ammonium oxalate, ammonium citrate). They found that uniform films were formed

during anodizing in baths containing organic acid salts but non-uniform films were

formed in baths without these additives. They also observed that the anodic oxide coating

produced in 0.1 mol/L benzyl amine bath containing 0.1 mol/L ammonium citrate and 0.1

mol/L ammonium fluoride showed the highest corrosion resistance.

Minch developed a new method with epoxy adhesives and coatings with basic

bulk properties providing dissimilar metal separation and necessary corrosion protection

to produce an oxide surface receptive to the formation of strong and durable bonds for

anodizing of aluminum [101]. This innovation provided a two steps electrolytic process

which included, phosphoric acid solution anodizing the aluminum firstly and then later

on further anodizing with a boric/sulphuric acid electrolyte solution. He obtained a

product with two anodized regions. The phosphoric acid produced primary outer region

was characterized by open pores particularly suited for bonding with adhesives &

sepoxy primers. The next base regions formed by boric/sulphuric acid solution provided a

thick, hard, corrosion resistance section.

Fang studied anodizing of aluminum in sulphuric/oxalic acid system and

properties of anodized film of aluminum [102]. His results showed that the oxide film

became thicker than that in pure sulphuric acid electrolyte. He observed that the film

thickness was affected by the electrolyte concentration, current density and oxidation

time. The conditions of aluminum anodizing in sulphuric acid-oxalic acid system were:

electrolyte concentration 15% sulphuric acid + 1% oxalic acid, current density 1.0 A/dm2

and oxidation time 60 minutes. The results obtained helped to understand the aluminum

anodizing process mechanism and showed an improvement in the properties of the anodic

coating of aluminum.


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Bopp investigated boric/sulphuric acid anodizing on 2219 aluminum alloy to

provide sufficient coating thickness due to alloys high copper content 5.8-6.8%[103].

The coating thickness and weight were crucial to ensure sufficient corrosion protection.

He carried out the testing on a small section of this alloy to prove that the proper coating

thickness and weight were achievable and met a minimum coating weight of 21.5

mg/dm2. Boeing Company used this process for anodizing of the Air Force KC-135

CFM-56 engine leading edge cowling which was being corroded in high salt

environment.

Fujino et al. developed anodization of aluminum in barium hydroxide alkaline

bath and absorption of carbon dioxide from air was prevented by setting carbon fluoride

membrane on the surface [104]. They compared the properties of the anodized film with

those produced in other alkaline baths such as sodium carbonate and sodium hydrate

baths and in sulphuric acid baths. Further more sodium aluminate and sodium tetraborate

were added to barium hydroxide bath in order to improve hardness and abrasion

resistance of the soft film produced in the alkali and the effects were examined. They

found that the films obtained in barium hydroxide baths were superior to those with the

same thickness (8m) obtained in sodium hydrate and sodium carbonate baths in the

hardness and the resistance to alkali corrosion. The film obtained in sodium tetra borate

added to barium hydroxide bath was found to have the highest hardness and greatest

alkali resistance. They attributed these properties to the formation of cell structure from

SEM observation of the film surface and were thought to be related to the composition

ratio of aluminum & oxygen and film depth.

Surganov and Poznyak studied the aluminum dissolution in the initial stages of

anodization in a boric acid solution [105]. They investigated the influence of the anodic

voltage scanning rate on dissolution of aluminum during its potentiodynamic anodization

in 0.1 M boric acid solution.

Gonzalez et al investigated the effect of triethanolamine addition on sealing time

[106]. They carried out anodizing of aluminum sheets in sulphuric acid solution stirred

with pressurized air and sealed the anodic layers in deionized water containing no

additive or acetate ion or TEA. They also analyzed the efficiency of triethanolamine

(TEA) and sodium acetate additions in accelerating sealing without detracting from its


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quality. They found that immersion for 1 minute in the presence of TEA ensured

compliance with current standards, as measured by the dye spot and acid dissolution

tests. This could result in substantial energy and space savings, because a single sealing

bath could be used to process the output of two or more anodizing baths.

Shino and Hoguchi investigated the anodic oxidation of aluminum in fluoride free

baths containing amino alcohols and organic acid salts [107]. They also studied general

characteristics of oxide coatings.

Xu et al. investigated the structure & composition of anodic coatings of

aluminum (AOF Al) formed in oxalic acid (I) and sulphuric acid (II) solutions by TEM,

IR and TGA [108]. They found that the porous anodic oxide films consisted of a large

number of hexagonal cells having an irregularly round pore in the center of each cell

which penetrated vertically from surface to bottom, while a barrier layer under the film

directly remained in contact with an aluminum substrate in scallop shape.

Shawaqfeh and Baltus investigated the growth kinetics of porous aluminum films

formed in phosphoric acid under galvanostatic conditions [109]. They used scanning

electron measurements (SEM), Faradays law and oxide film mass measurements to

analyze the growth kinetics and obtained film growth rates, pore diameter and porosity.

Current efficiency was also determined from these measurements. They also examined

the effect of c.d. and solution temperature on the oxide film growth rate and morphology.

Mansfeld et al. developed the sealing of boric/sulphuric acid anodized (BSAA)

aluminum alloys in yttrium or cerium salts at high temperature [110]. Dilute chromate

solution, a cold nickel fluoride solution and boiling water sealing was also carried out to

compare the results. They evaluated sealed BSAA aluminum alloy Al 2024, Al 6061, and

Al 7075 for corrosion resistance by recording of impedance spectra while exposing in 0.5

N. NaCl for 7 days. Depending on the corrosion resistance, different time intervals for

exposing were used for samples prepared by different sealing processes. They found that

corrosion protection similar to chromate sealed BSAA aluminum alloys was obtained by

sealing in cerium nitrate and yttrium sulphate solutions.

Gong et al. studied the outcome of chromic acid quantity on the formation process

of anodic porous alumina film [111]. They found that the c.d. of film formation

increased, the time of film formation decreased and the thickness of oxide coating


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decreased with the increasing of chromic acid concentration. They pointed out that the

formation of surface pores was, presumably, due to stress concentration developed during

the formation of the film.

Skoneczny investigated the oxide coating development mechanism on aluminum

in ternary electrolyte [112]. He found that the surface of aluminum and its alloys can be

improved by application of hard anodization using an aqueous ternary electrolyte (SAS).

The SAS electrolyte consisted of among other things, sulphuric acid, adipic acid and

oxalic acid. The electrolyte temperature during oxidation was within the 293-313K range

and anodic current density (c.d) was within the 2-4 A/dm2

range. He also explained the

structural patterns of the oxide coating on aluminum obtained in SAS electrolyte.

Pakes et al. studied the growth of anodic films on aluminum in disodium

tetraborate, so called alkaline anodizing by using transmission electron microscope

(TEM) [113]. They observed that early coating growth proceeded at relatively greater

efficiency under galvanostatic anodizing conditions and porous anodic film was formed

during the following constant voltage period. The penetration of electrolyte species into

the aluminum oxide film established conditions for field assisted dissolution, probably

thermally enhanced and giving a porous anodic film. They described that the cell walls

adjacent to the pores had a feathered appearance which was due to mechanical disruption

of coating composition at pore base under the high filed.

Domingues et al. carried out anodizing of aluminum alloy 2024-T3 using tailored

sulphuric/boric acid electrolyte as an alternative to the anodizing process in chromic acid

[114]. They obtained results showing that the addition of molybdate was necessary to this

bath for satisfactory corrosion results. They used the technique of electrochemical

impendence spectroscopy (EIS) to assess the anodic film properties.

Thompson et al. investigated the effect of boric acid additions to sulphuric acid

for the anodizing of Al 2024-T3 and Al 7075-T6 alloys at constant voltage [115]. They

carried out anodizing after pretreatment by electropolishing, by sodium dichromate

(Na2Cr2O7/H2SO4) etching or by alkaline etching. In their study, the current-time

responses revealed that the concentration of boric acid in 50 g/L sulphuric acid had

insignificant effect. They observed that there was negligible influence of boric acid on the

coating weight, structure of the anodic coating, the thickening rate of the film, or


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corrosion resistance provided by the film of Al 7075-T6 alloy. They found that sealed

anodic films in deionized water or preferably in chromate solution gave improved

corrosion resistance, although not matching the far superior performance provided by

chromic acid anodizing.

Kurita studied the anodizing of aluminum alloys in electrolytic solutions

containing sulphuric acid and oxalic acid or citric acid without decrease of surface

hardness [116]. The aluminum alloys are useful for cylinders of internal combustion

engines etc. He found that the anodized films obtained by this method had good abrasion

resistance.

Lopez et al. obtained anodic coatings in oxalic acid and sulphuric acid baths and

compared their effectiveness to sealing quality control tests [117]. They found that the

coating without sealing obtained in oxalic acid remained virtually unaffected in humid

atmosphere in contrast to coating without sealing obtained in sulphuric acid which are

autosealed. On the other hand, both types of coating aged in a qualitative identical way

after sealing. The TEM study exposed a complex structure of hexagonal cells, consisting

of three distinct zones in the coatings formed in oxalic acid and pores were five times

larger in diameter as compared to films formed in sulphuric acid. They also found that the

alteration of the film morphology, under the electron beam was slower in films obtained

in oxalic acid bath.

Cao et al. studied the anodizing of ZL102 Al cast alloys in boric acid/ tartaric acid

mixture (weak acid anodizing) and obtained a compact, uniform anodic film with violet

colour [118]. They found that the coating development rate increased by increasing the

current density but coating growth was inhibited by increasing the bath temperature,

while corrosion resistance of the film was also decreased with the increasing of bath

temperature.

Li et al. developed a method for the preparation of porous anodized film and

studied its corrosion resistance [119]. They carried out anodizing of aluminum foil in

sulphuric acid solution , sulphuric-acetic acid solution, sulphuric-oxalic acid solution and

sulphuric-phosphoric acid solution, respectively, at 18C and 20 V for 3 hours and the

anodized films were stripped from substrates by reverse voltage method. They

determined the thickness of the anodized films and studied coating corrosion protection


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37

in media of sodium chloride, sodium carbonate, oxalic acid, hydrochloric acid and

sodium hydroxide. They found that the film prepared in sulphuric acid was the thickest

one and anodic films were stable in neutral media and their corrosion resistance

decreased at pH < 1 or pH > 12.

Wang et al. investigated the influence of concentration of phosphoric acid on

formation of barrier layer of anodic porous alumina film [120]. They found that by

increasing the concentration of phosphoric acid, the time needed to form barrier layer was

shortened and the barrier layer became thinner also. They interpreted this phenomenon

reasonably by the stress concentration that developed in barrier layer.

Wang and Hu carried out anodizing of aluminum alloy (LF2) in a solution

containing sulphuric acid (d = 1.84) 150-200, CK-LY additive 20-35 and Al+3

0.5-20g/L

at 15-35C and c.d 1.5A/dm2 for 20 minutes time [121]. The CK-LY additive contained

organic acid and a conducting salt. They found that this composition widened the

temperature range of anodization process. The results of this process were also compared

with that of conventional sulphuric process and their process was found better.

Mansour et al. studied anodizing of aluminum and some of its alloys in organic

and inorganic acids [122]. They carried out anodizing of Al, Al-Mn alloys and Al-Mg

alloys in acetic acid, butyric acid, citric acid, propionic acid, oxalic acid, maleic acid,

tartaric acid, phosphoric acid and sulphuric acid at 1.0 A/dm

2

and 30 0.1Ctemperature. They found that the anodization of Al, Al-Mn and Al-Mg alloys for 0.5 hour

in citric acid and tartaric acid at c.d. 1.0 A/dm2

resulted an increase in the weight of each

specimen by 0.13-0.0156 g/20 cm2, while the weight increased by (0.6-9.0) x 10

-3g/20

cm2

in the other acids.

Lee et al. examined the effect of additives on formation of porous oxide coating

by anodizing in sulphuric acid [123]. They prepared porous alumina membrane from

aluminum metal (99.9%) using d.c. power supply of constant current mode in an aqueous

solution of sulphuric acid. Different additives like aluminum sulphate, aluminum

phosphate and aluminum nitrate were added to aqueous sulphuric acid electrolyte to

prevent the chemical dissolution of alumina membrane by providing Al+3

ions. They

evaluated the effects of these additives on the formation of porous alumina membrane

under various electrolyte concentration (5-20 wt %) and current densities (10-50 m


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38

A/cm2). They used various techniques like inductively coupled plasma atomic emission

spectrometry, SEM, TEM and electrochemical impedance spectroscopy. They found that

the membrane surfaces were damaged with all the additives except aluminum sulphate

and a uniform surface of porous alumina was obtained with aluminum sulphate additive.

Konno et al. developed a two step anodizing of aluminum to improve the physical

and chemical characteristics of oxide film [124]. This process gave hard and corrosion

resistant oxide coating on aluminum. The process consisted of two steps; the formation of

chromate/phosphate treated layer was followed by sulphuric acid anodizing. This process

gave anodic oxide coating containing Cr (III) and phosphate species mostly in the outer

part of the porous layer. They found that the coating obtained by this process gave a

better corrosion protection to the substrate aluminum from pitting in a chloride medium

than the films formed by conventional anodizing. Serebrennikova et al investigated the

characteristics of porous oxide coating by silver electrodeposition [125]. They compared

the characteristics of different porous aluminum oxide coating in terms of their

electrochemical responses during silver electro-deposition. The cyclic voltametric and

current-time responses during silver deposition showed an increased rate in the following

sequence; sulphuric acid grown films (smallest pores), phosphoric acid grown films

(large pores) and barrier oxide films (no pores), under the same applied voltages. They

found that thicker porous oxide coating formed after longer times of anodizing showed

lower deposition currents, while coating formed at higher voltages (expected to yield

larger diameter pores) resulted in higher silver deposition currents.

Kumar et al. studied white anodizing for space applications using mixture of

sulphuric acid, glycerol, lactic acid and sodium molybdate electrolyte system [126]. They

investigated the effect of anodic film thickness and various operating parameters viz.

electrolyte formulation, operating temperature, applied current density, on the optical

properties of the coating, for the optimization of this process. Atomic absorption

spectroscopic analysis, thickness and micro hardness evaluation were used to characterize

the oxide coatings. They evaluated the space worthiness of the coating by humidity,

thermal cycling, thermo-vacuum performance tests and optical properties measurement

and found it appropriate in space environment for heat control applications.


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39

Kim et al. investigated the structural characteristics of electrochemically designed

porous oxide films on AlMg1 [127]. They changed anodizing conditions in a broad range

(electrolyte: sulphuric acid, phosphoric acid and mixture from these, concentration: 0.3-

2.0 M, voltage: 5-10V, time 30-90 seconds, temperature 25-50C) and characterized the

oxide layers via spectroscopic interference measurements to find the coating thickness.

They also observed the layer morphology by atomic force microscopy (AFM) and

characterized structural parameters of the anodic coating by electrochemical impedance

spectroscopy (EIS).

Shih and Tzou studied the characteristics of anodized films on aluminum using a

pulsed current in mixed electrolytes of nitric acid-sulphuric acid or boric acid-sulphuric

acid [128]. They proved in film dissolution tests that the anodic film contained two

layers, and the composition of the film was also changed by the anodizing electrolytes.

These results could also be obtained from energy dispersive spectroscopy analysis, when

nitric acid or boric acid was added to an electrolyte of 10% wt. sulphuric acid. They

found that the film thickness and microhardness could increase greatly by addition of a

modifier.

Zhang and Wang investigated the anodizing process and corrosion protection of

7050 T7451 aluminum alloy, which hardly formed the anodic coating in traditional

chromic acid method [129]. They studied the effects of the anodizing voltage to find a

suitable anodizing method. They found that the suitable voltage was greater than 30 V for

this alloy anodized in chromic acid, and deoxidization worsened the corrosion resistance

of the alloy.

Chi et al. studied antibacterial activity of anodized aluminum by depositing silver

[130]. Aluminum samples were anodized in a sulphuric acid bath, followed by silver

electrodeposition into pores of the coated sample by using a.c.current. They tested the

coated aluminum with deposited silver for its antibacterial activity. They found that the

antibacterial rates of the samples with deposited silver were above 95% against the

growth of different bacteria. Transmission electron microscope(TEM) was used to

characterize the morphology of the silver in pores of anodized aluminum. Silver was

assembled in the form of nanowires with the diameter of 10 or 25 nm. as shown by

micrographs.


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40

Ono and Masuko determined the pore diameter of anodic film formed on

aluminum in four major electrolytes by pore filling technique [131]. They found that the

porosity of anodic film decreased with increasing voltage and at an identical voltage, it

increased in the order of the oxide coating obtained in oxalic acid, sulphuric acid,

chromic acid and phosphoric acid solution.

Zuo et al. studied the influence on the corrosion behaviour of three anodized

aluminum alloys, 1070, 2024 and 7075, in sodium chloride solutions by different sealing

methods [132]. They sealed the anodic films by boiling water, stearic acid, potassium

dichromate and nickel fluoride methods and studied the passivation and pitting behaviour

obtained by these sealing methods using the potentiodynamic polarization tests and

surface morphology by using SEM technique. They revealed that good corrosion

protection was provided in acidic solution by the boiling water and potassium dichromate

sealed films, while nickel fluoride sealed film was better in basic solution , while stearic

acid sealed film offered good corrosion protection both in acidic and in basic solutions.

Djozan and Zehni investigated an anodizing method for the inner surface of long

and small bore aluminum tubes and investigated parameters affecting thickness, shape,

porosity and stability of the oxide layer formed[133]. They determined optimum

conditions for electrolyte solution, temperature, applied voltage and anodizing time.

Anodizing was performed both in dynamic and static modes. Chemical characteristics of

anodic film were studied by adsorbed amount of fuchsin and physical characteristics of

anodic film were studied by SEM. They found in their results that the quality of

aluminum oxide layer formed in dynamic mode was better.

Li et al. developed an environmentally friendly coating method (modified

anodizing) by using aqueous sodium bicarbonate electrolyte and studied corrosion

protection characteristics of oxide coatings on an Al-Si alloy [38]. They deposited oxide

coating on Al-Si alloy samples to form hard and corrosion protective coating using three

different techniques: hard anodizing (HA), anodizing (A) and modified anodizing (MA).

The corrosion resistance of the coatings was determined by potentiodynamic polarization

tests. The coating microstructure and chemical composition was studied by SEM and

EDX studies before and after the corrosion test. They concluded that the modified


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41

anodizing is an environmentally friendly coating method which gives a hard oxide

coating having good corrosion protection.

Wang and Wang carried out analysis of porous aluminum oxide coating formed in

mixed electrolyte of phosphoric acid, oxalic acid and cerium salt [134]. They investigated

growth, morphology and chemical composition of the film by using SEM, EDAX and

XPS techniques. Their results indicated that the formation of porous layers in this

solution undergo three stages during anodizing, as in other conventional solution, while

the whole growth rate was non-linear. They also observed that this electrolyte was

sensitive to anodizing temperature, which affected current density to a greater degree.

They found that the corrosion section of film has two distinct layers; outer layer was of

uniform structure and the inner one was about 8 m which bounded the substrate

compactly.

Kaplanoglou et al. studied the effect of different alloys on the anodizing process

of aluminum [135]. They used specimens of AA 5083 and AA 6111 (unheat and heat-

treated) for their investigation in comparison with the pure aluminum during anodizing in

sulphuric acid bath by using electrochemical techniques, SEM/EDX and XRF. They

found that alloy type affected the kinetics of anodizing but the basic aspects and certain

qualitative characteristics of the anodizing mechanism were similar.

Garcia-Vergara et al. investigated the effect of copper on the morphology of

porous anodic alumina [136]. They used sputtering deposited Al-Cu alloy layers and an

Al-Cu/Al bi-layer to study the influence of copper on the structure of porous anodic

alumina films formed galvano-statically in either sulphuric or phosphoric acid. They

revealed in their results that the development of an irregular morphology of pores during

anodizing of the alloy layers, contrasting with the linear porosity of films formed on

aluminum. Further, the rates of film growth and alloy consumption were relatively low,

since oxygen was generated following enrichment of copper in the alloy and

incorporation of copper species into the anodic film. They also found that the linear

morphology was re-established following depletion of the copper in the bi-layer and at

the same time, film growth was accelerated as oxygen evolution diminished. They

considered that the irregular pore morphology was due to the stress-driven pore

development influenced by effects of oxygen bubbles.


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42

Bensalah et al. carried out the optimization of anodic layer properties on

aluminum in mixed oxalic/sulphuric acid bath using statistical experimental methods

[137]. They obtained a four variables Doehlert design (bath temperature, anodic current

density, sulphuric acid and oxalic acid concentrations). They conducted a multicriteria

optimization using desirability function in order to maximize the growth rate and the

microhardness of the anodic oxide layer and to minimize in the same time their chemical

and abrasion resistances. They used dissolution rate of the oxide in phosphochromic acid

solution (ASTM B 680-80) to express the chemical resistance of films. They attributed

the higher abrasion and chemical resistance of the optimum layer with its structure.

Aerts et al. investigated the temperature effect on the porosity and the mechanical

properties of the oxide coating [138]. They evaluated the microhardness and fretting wear

resistance of anodic oxide layers, produced on commercially pure aluminum by

potentiostatic anodizing in sulphuric acid under conditions of controlled convection and

heat transfer in a reactor with a wall-jet configuration, as a function of the electrolyte

temperatures in a wide range from 5C up to 55C. The microstructure information of the

anodic films was obtained by FE-SEM analyses, whereas image analysis of high-

resolution surface images provided quantitative information on the evolution of the

surface porosity as a function of the electrolyte temperatures. They found that increased

mechanical characteristics were directly related to the microstructure. They also

measured local temperatures at the backside of the working electrode (WE) by five

thermocouples (type T), embedded in the sample holder at 5 different radial positions,

these being 15 mm, 10 mm, 0 mm, 5 mm and +15 mm from the centre of the anode

respectively. They pointed out that anodized electrodes had displayed a regular anodizing

behaviour without occurring of local phenomena.

Sulka and Parkola studied the temperature effect on well-ordered nanopore

structures grown by anodization of aluminum in 20% wt sulphuric acid at various cell

potentials and temperatures [139]. They performed quantitative analyses of defects and

Fourier transforms (FFT) from SEM images showing that regularity of nano pores

arrangement could be improved by increasing anodizing potential, independently of the

anodizing temperature. They found that the best result in controlled anodization of

aluminum could be obtained at 25 V and the temperature of 1C.


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43

Bartolome et al. investigated the corrosion behaviour of different bare and

anodized aluminum alloys using exposure tests in natural atmospheres with three

different coating thicknesses [140]. They evaluated these coatings for two years exposure

in two natural atmospheres of very different corrosivities, one urban and the other

marine. Shanges in the specimens during exposure were evaluated by several techniques

but direct attention was focused to the direct measurement of corrosion by gravimetry

and its indirect estimation by the comparatively much more sensitive electrochemical

independence spectroscopy (EIS) technique. The results showed that if no demands were

placed on the conservation of its appearance, aluminum may be used without protection

even in an atmosphere of medium or higher corrosivities.

Harscoet and Froelich studied the use of life cycle assessment (LCA)

methodology to asses the environmental benefits of substituting chromic acid anodizing

(CAA) [141]. They carried out a precise literature review to evaluate bath atmospheric

emissions. The results of the performed assessment confirmed that the only way to

efficiently deal with hexavalent chromium compounds is to substitute the electrolyte used

in the bath as the most hxavalent chromium emissions are caused by other stages than the

main. Other specific issues, such as water and energy consumptions have, nevertheless, to

be studied throughout the whole life cycle of the chemical substitute to monitor

performance against chromic acid anodizing.

Zhang et al. studied the bonding strength and corrosion protection of aluminum

alloy by phosphoric acid modified boric acid/sulphuric acid anodizing bath [142]. They

examined the microstructure and topography of the anodic films by using SEM and AFM

techniques and also studied the adhesive strength and corrosion behaviour with lap-shear

test, wedge test and electrochemical technology. Their results revealed that a thicker film

with high porosity and big pores could be produced by this anodizing method. They

found that the porous film was beneficial to improve the durability and lap-shear strength

of the bonding joints and the thicker film provided better corrosion protection.

Gonzalez et al. studied that atmospheric corrosion of bare and anodized aluminum

in a wide range of environmental conditions [143]. They obtained the oxide coating in a

sulphuric acid bath and sealed for 60 minutes in boiling deionized water and

characterized before and after different exposure times by means of visual inspection;


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44

observation with a magnifying glass; and using classic gravimetric techniques. They also

compared behaviour of aluminum in the bare condition and when protected with anodic

films of approximately 7, 17 and 28 m thickness for 42 months of exposure in 11

atmosphere of very different aggressivities, with salinity values ranging between 2.1 and

684 mg Cl-

m-2

d-1

. They concluded that anodizing and sealing prevented the danger of

pitting corrosion even in the most aggressive atmospheres.

Aluminum is a most multipurpose metal. It can be finished in a variety of ways. It

can be made to resemble other metals, or can be finished to have a colourful as well as a

hard, durable finish unique unto itself. Only the imagination limits the finish and colours

possible with anodized aluminum [25]. It is widely used in aircraft industry, aerospace,

construction industries, transport, packaging, ship building and other fields [144-146].

Pretreatment of aluminum provides not only strong tensile strength and durability but

also good corrosion protection. Chromic acid anodizing process with a potassium

dichromate/ sulphuric acid etching procedure was one of the most widely used

pretreatment for structural bonding due to good properties of the films incorporating

some Cr (VI) and Cr (III) [42]. However the use of hexavalent chromium is not allowed

from a health and environmental concerns due to its poinous and carcinogenic effects.

This process is gradually limited even banned [40-43]. Alternate of chromic acid

anodizing process has been vital and critical subject in industrial sectors and

environmental studies. In Europe, there was interest in boric acid anodizing [36], while in

the U.S.A. Boeing developed phosphoric acid anodizing process [31]. A ferric

sulphate/sulphuric acid treatment was used as alternate of dichromate/sulphuric acid

etching procedure prior to phosphoric acid anodizing [147]. Besides phosphoric acid

anodizing, a mixed sulphuric acid/boric acid process, carried at out lower temperature,

has been patented [56]. Thin film sulphuric acid anodizing [57, 58, 99] has also been

developed to replace chromic acid anodizing. The modified boric acid anodizing or boric/

sulphuric acids anodizing processes have been studied by some scientists [148-150].

Zhang et al [142] have developed a process of phosphoric acid modified boric/sulphuric

acid anodizing to improve bonding properties and corrosion resistance.


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Earlier work [20, 151-153] has described the influence of anodizing conditions

(voltage, temperature, composition and acid concentration of the bath) on the anodizing

current density, the porosity and the diameter of the pores of the anodic oxide in

sulphuric acid electrolyte as follows.

1. Both the current density and the coating porosity increase by increasing thetemperature at a given anodizing voltage due to thermal enhanced field

assisted dissolution. The dissolution of the outer oxide surface is also

increased.

2. At a given anodizing voltage the current density increases by increasing thesulphuric acid concentration due to higher solubility of the oxide film. This

higher film dissolution is also responsible for the increase of the pore diameter

and the coating porosity.

3. At a given temperature, the current density, the barrier layer thickness and the

diameter of the pores increase by increasing the anodizing voltage, while the

coating porosity decreases.


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2.1 Plan of Work

Aluminum metal and its alloys are widely used in aviation, aerospace, and other

fields and it is anodized to resist harsh environmental conditions in its service life. The

anodizing of aluminum and aluminum alloys provides not only strong tensile strength and

durability but also good corrosion resistance. Chromic acid anodizing was one of the

most widely used pretreatment of aluminum and aluminum alloys. However the use of

hexavalent chromium is not allowed from a health and environmental concerns due to its

poinous and carcinogenic effects. This process is gradually limited even banned.

Alternate of chromic acid anodizing process has been an essential and critical subject in

industrial sectors and environment studies:

1. In the present work, different electrolyte compositions including acetic acid,

oxalic acid, citric acid, tartaric acid, boric acid and sulphuric acid being

environmentaly safe were prepared to carry out the anodizing of aluminum

samples at different conditions.

2. The anodized aluminum samples were evaluated using different techniques for its

thickness and morphological studies.

3. Corrosion study of the anodized aluminum samples was carried out using

Potentiostat PG30 to find the effectiveness of anodic oxide coating against

corrosion.

al anodizing

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