0033-1732/03/3901- $25.00 © 2003

MAIK “Nauka

/Interperiodica”0077

Protection of Metals, Vol. 39, No. 1, 2003, pp. 77–80. Translated from Zashchita Metallov, Vol. 39, No. 1, 2003, pp. 84–87.Original Russian Text Copyright © 2003 by Revenko, Kozlova, Astakhov, Chernova, Bogdashkina.

Composite electrochemical coatings (CEC) basedon many metals have come into wide use in engineer-ing. The disperse particles of a dielectric (the secondphase—Al

2

O

3

, TiO

2

, ZrO

2

, and so on) involved inCECs improve their wear resistance, ductility, hard-ness, strength, and other physical characteristics,including the surface structure, thereby enhancing theprotection of articles (parts) against corrosion.

The mechanism of involving various disperse parti-cles during electrical deposition from a suspension hasits particular features. The formation of CEC is basedon implanting particles of the second phase measuringfrom 1–10 nm to several

µ

m into a cathodically depos-ited metal. Compared with electroplating purely metal-lic coatings, this process depends not only on electrol-ysis conditions, such as temperature, stirring, etc., butalso on dispersity, quantity and nature of particles. Thisallows one to vary the physicochemical properties ofcomposite coatings [1–3, 7]. Earlier, it was noted thatiron-based, electrochemical composite coatings are dis-tinguished by their enhanced hardness, wear and heatresistance [4].

It is the absence of literature data on the effect of thedisperse phase Al

2

O

3

on the corrosion and electrochem-ical behavior of the iron-based composite coatings thatpromoted the idea of performing this work.

When choosing concrete investigation conditions,we took into consideration the available data on theeffects of various factors on the obtention of optimumproperties of CEC with Al

2

O

3

on steel.

The investigation objects were cylindrical speci-mens (10 mm high and 7–8 mm in diameter) made of45 type steel with electrodeposited iron and composite

coatings applied from an iron plating bath with inclu-sions of aluminum oxide in the form of disperse pow-der.

Prior to applying coatings, the steel surface was pre-pared by a procedure described earlier in [5]. In orderto obtain coatings with high adhesion, the tested speci-mens, after grinding, degreasing with soda lime, andwashing with water, were anodically treated for 3 minin a chloride iron-plating electrolyte at 50–60 A/dm

2

and a temperature of 60

°

C. After washing in water atroom temperature, the slurry was removed from thespecimens by similar, but lasting only 1 min anodicetching in 30% H

2

SO

4

solution at room temperature,followed by washing them in water at a temperature of60

°

C. Before starting the iron-plating operation, eachcathode was held for 1 min in an electrolyte withoutpassing current through it, for the purpose of removingthe passive film.

The iron coating was applied in a simple chloridebath containing 500 g/l FeCl

2

⋅

4H

2

O at a chosen cur-rent density of 20 A/dm

2

, pH 0.8–1.0, and a tempera-ture of 40

°

C. The current efficiency determined by cou-lometry amounted to 94–96% depending on thecathodic current density.

For cleaning the electrolyte from Fe(III), it wasboiled with armco iron chip and subjected to electroly-sis at pH 0.6–0.8;

D

c

= 20 A/dm

2

, and a temperature of60

°

C. The content of Fe(III) was qualitatively con-trolled by using a 10% KCNS solution as indicator, andquantitatively by titrating with 0.05 N solution of ascor-bic acid.

Corrosion and Electrochemical Behavior of Composite Electrolytic, Iron-Based Coatings

V. G. Revenko

1

, T. V. Kozlova

1

, G. A. Astakhov

1

, G. P. Chernova

2

, and N. L. Bogdashkina

2

1

Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, Moldova

2

Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia

Received May 21, 2001

Abstract

—The corrosion and electrochemical properties of composite electrolytic iron coatings containingAl

2

O

3

on the steel substrate are investigated (in comparison to the purely iron ones) in 0.05 M Na

2

SO

4

and 5%NaCl solutions. Adding a finely dispersed Al

2

O

3

phase to the composition of iron coatings leads to someimprovement in their corrosion characteristics, that is, a positive shift of

E

cor

, and lowered anodic dissolutioncurrents. The corrosion rate of the composite coating is lower than that of the purely iron one. The effect is ana-lyzed.

78

PROTECTION OF METALS

Vol. 39

No. 1

2003

REVENKO

et al

.

Finely ground, high-purity aluminum oxide (EB-99M-2 grade white electrocorundum), which is exten-sively used to prepare high-quality CECs, served as dis-perse phase. The bath was continuously stirred with amagnetic stirrer, in order to maintain particles in sus-pended state and to evenly distribute them over thecathode surface.

The deposition temperature of 40

°

C allowed apply-ing coatings with a fairly high content of Al

2

O

3

[2].Above this temperature, the solution viscosity substan-

tially decreases, and the proportion of inclusions in thecoating drastically falls (from 7 to 2%) because of sed-imentation depletion of the bath of coarser particles anddeteriorated sorption of the remaining particles by thecoating. Moreover, at a low temperature, the secondphase is especially effective in counteracting the forma-tion of cracks and the growth of locked stresses.

The Al

2

O

3

micropowder was added to the iron-plat-ing bath in an amount of 10 and 20 g/l, without chang-ing the electrodeposition conditions. In choosing theconcentration of the disperse phase, we considered thedata from [2] that already 20 g/l of powder in the sus-pension ensure the limiting content of the second phasein the coating, whereas a somewhat smaller proportionof inclusions results in the optimum mechanical prop-erties of the coating. The excessive content of Al

2

O

3

inthe bath leads to increased roughness and decreasedstrength and hardness because of localized nonuniformdislocation stresses in the coating. At the aforemen-tioned concentrations of suspensions, the content of thesecond phase in CECs can vary within 1.5–4 times. Acurrent density of 20 A/dm

2

prevents both large andsmall particles from prevailing in the coating.

According to the conclusions [4–7], the processparameters (

D

c

, pH,

T

) used in the present study pertainto the number of optimal ones. In all the cases, the coat-ing thickness was about 0.2 mm without any traces ofcracks. The specimens had metallic luster. No forma-tion of dendrites and scabs was noticed.

The adhesion of coatings was assessed from a net ofscratches. The electrochemical investigations and cor-rosion tests were carried out according to the proceduredescribed in [6] in unstirred 0.05 M Na

2

SO

4

and 5%(1.16 M) NaCl solutions at room temperature from 18to 21

°

C. The potentials are converted to the standardhydrogen scale (s. h. s.). Prior to performing electro-chemical and corrosion tests, the surface of specimenswas degreased with an alcohol–ether mixture, washedwith distilled water, and dried with filter paper.

Active dissolution is seen in anodic potentiody-namic (4 mV/s) curves of the specimens with both ironand composite coatings (Figs. 1a, 1b). Lower anodiccurrent densities at CEC specimens compared to ironones may be ascribed, though not completely, toscreening the matrix by disperse particles. In sulfuricacid solution, the free corrosion potential (

E

cor

) of spec-imens with composite coatings (compared to iron coat-ings) is shifted in the positive direction (by about0.04 V), whereas anodic current in a potential rangefrom –0.1 to 0.6 V is lowered by a factor of 1.5–2. Theobserved effects are pronounced more drastically in a5% NaCl solution (Fig. 1b), where the positive shift of

E

cor

is close to 0.1 V. Concurrently, anodic currents atthe electrodes with CEC are approximately three to five

1

2

3

–0.2

0.2

0.6

0 2 3

E

, V (SHE)(a)

(b)

–0.2

0.2

0.6

0 3

2

log

i

[A/m

2

]

1

2

,

3

log

i

[A/m

2

]

Fig. 1.

Anodic potentiodynamic (4 mV/s) curves for 45 typesteel with (

1

) iron and (

2

,

3

) composite electroplatesobtained from iron-plating baths, containing (g/l) (

2

) 10 and(

3

) 20 of Al

2

O

3

. The curves are recorded in (a) 0.05 MNa

2

SO

4

and (b) 5% NaCl solutions. Points at the

Y

-axis

E

,V (SHE) indicate free corrosion potentials (SHE) at theimmersion moment.

–0.1

1

1

–0.3

–0.5

2 3 4 50.1

τ

, min

E

cor

, V (SHE)

1

'

1

''

22

'

2

''

Fig. 2.

Time dependence of the free corrosion potential ofsteel with (

1

,

2

) iron and (the indexed figures) compositeelectrolytic coatings deposited from the iron-plating bathsdoped with (

1

',

2

') 10 g/l and (

1

'',

2

'') 20 g/l of Al

2

O

3

. Testsolutions: (

1

,

1

',

1

'') 5% NaCl and (

2

,

2

',

2

'') 0.05 MNa

2

SO

4

.

PROTECTION OF METALS

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No. 1

2003

CORROSION AND ELECTROCHEMICAL BEHAVIOR OF COMPOSITE ELECTROLYTIC 79

times in a potential range from –0.2…+0.1 V and eventen times (at the zero potential) lower than in the caseof iron coatings.

A change in the content of Al

2

O

3

in the bath withina range from 10 to 20 g/l unnoticeably affects

E

cor

, aswell as anodic dissolution currents, in the sulfate solu-tion, so that anodic curves

2

and

3

coincide. However,in the chloride solution, the same increase in the con-centration of Al

2

O

3

in the plating bath results in agreater (0.14 V) shift of the corrosion potential and astronger decrease in the anodic dissolution currentcompared to the iron coatings: from 5 to 15 timeswithin a range from –0.1 to 0.1 V and 40 times at 0.0 V.For all the kinds of coatings, the changes in

E

cor

in chlo-ride (Fig. 2, curves

1

,

1

', 1'') and sulfate (Fig. 2, curves 2,2', 2'') solutions are of the same type, and indicate someactivation, which is probably associated with the break-down of the oxide film at the electrode surface. Theeffect of the disperse phase in the iron coating on Ecor

proves to be stronger in Na2SO4 solutions (a shift of0.10–0.12 V). In NaCl solution, the shift is insignifi-cant. Yet, in the sulfate solution the above shift of Ecor isconsiderable only just upon the immersion. For a spec-

imen from the bath containing 20 g/l of Al2O3, it equals0.08 V; but in 5 min, it becomes equal to Ecor of a spec-imen electroplated in the bath with 10 g/l of Al2O3. Byand large, both the potentials remain more positive thanthose for the iron coatings.

Anodic chronoammetry showed (Fig. 3) that CECsdissolve some more slowly than iron coatings at poten-tials of 0.1 and 0.3 V, which is probably associated withthe screening of the surface by Al2O3 particles. How-ever, an increase in the concentration of Al2O3 in theiron-plating bath insignificantly affects the anodic dis-solution currents in the electrolytes investigated.

The results of immersion corrosion tests of the com-posite and iron electroplates in 0.05 M Na2SO4 and 5%NaCl solutions are shown in Fig. 4. Although the plot-ted curves are of similar shape, the weight losses of thecomposite electroplates (for a 120-h test) are a littlesmaller compared to iron electroplates, that is, byapproximately 4–6 g/m2 in NaCl and 4–10 g/m2 inNa2SO4 solution; the difference is more pronounced forthose composite electroplates that were obtained in thebaths with higher content of Al2O3.

Thus, the addition of a finely dispersed α-Al2O3, toan iron-plating bath results in some enhancement ofelectrochemical and corrosion properties of the coat-ings, i.e., an increase in Ecor, and a decrease in anodicdissolution current. The composite electroplates aremore corrosion resistant than iron ones. An increase inthe Al2O3 content in the plating bath and, accordingly,in the composition of CEC makes their corrosion resis-tance improve.

20

3

2 4 τ, h

0

2

2 4 6

3

logi [A/m2](a)

(b)

1

1' 2'

3'

2

3

1 2

3

1'2'

3'

Fig. 3. Chronoamperograms at (a) E1 = 0.1 V and (b) E2 =0.3 V for the steel with (1, 1') the iron coating and (theindexed figures) CEC deposited from the baths containing(2, 2') 10 g/l and (3, 3') 20 g/l of Al2O3. Test solutions: (1–3) 0.05 M Na2SO4 and (1'–3') 5% NaCl.

4

∆m, g/m2

24

12

20

48 72 96 τ, h0

1'1

2

3

2'3'

Fig. 4. The growth of corrosion weight losses of the steelelectroplated with (1, 1') iron and (the other figures)composite coatings from the iron-plating baths containing(2, 2') 10 g/l and (3, 3') 20 g/l of Al2O3. Test solutions:(1−3) 0.05 M Na2SO4 and (1'–3') 5% NaCl.

80

PROTECTION OF METALS Vol. 39 No. 1 2003

REVENKO et al.

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