to the theory of anticorrosion alloying of brasses

0033-1732/05/4102- © 2005 Pleiades Publishing, Inc.0138

Protection of Metals, Vol. 41, No. 2, 2005, pp. 138–145. Translated from Zashchita Metallov, Vol. 41, No. 2, 2005, pp. 149–157.Original Russian Text Copyright © 2005 by Kondrashin.

INTRODUCTION

α

- and

β

-Brasses are commonly (and effectively)prevented from selective corrosion by alloying themwith a third element. The right element (metal or non-metal) in amounts from hundredths of fractions to 1 to4% can radically change the resistance of the alloy toselective destruction, only slightly affecting the totalcorrosion losses. The classic example is arsenic; whenadded to commercial

α

-brasses in small amounts(0.02–0.04 at. %), it usually prevents their dezincingeven in very corrosive media [1–3]. Similar but not sopronounced effects are exhibited by aluminum, tin, andsome other metals modifying the corrosion propertiesof

α

- and

β

-brasses [1–4].Construction of a physicochemical theory of anti-

corrosion alloying is particularly based on the conceptof the blocking effect of atoms of an alloying constitu-ent

L

on the reactive surface sites of a solid. As a result,the anodic dissolution is inhibited, which is favorablefor metal protection [5–7].

The protective effect of arsenic in

α

-brasses is mostfrequently explained by the formation of surface filmsconsisting of As

0

or As

2

O

3

which inhibit copper redepo-sition [8, 9]. According to another viewpoint [10],arsenic is involved in a heterogeneous redox cycle toreduce corrosion products (Cu

2+

ions) to Cu

+

:

3Cu

2+

+ As

0

3Cu

+

+ As

3+

; (1)

the resulting arsenic ions oxidize copper atoms to theoxidation state +1:

3Cu

0

+ As

3+

3Cu

+

+ As

0

.

Since the reductive potential of Cu

+

ions is lower thanthat of Cu

2+

ions, the former do not deposit to the sur-face of arsenic brass. This is tantamount to the preven-tion of dezincing.

Thus, the effect of an alloying constituent is ulti-mately associated with a change in the electrochemicalproperties of the surface of a corroding metal. However,

in terms of thermodynamic equations derived in [11],the main function of an alloying constituent seems to bea bit different. In this study, the role of an alloying con-stituent such as arsenic, which affects the selective pro-cesses of anodic dissolution and the corrosion of single-phase brasses, was considered in terms of thermody-namics.

THERMODYNAMICS OF THE ALLOYING EFFECT

Dissolution of a pure metal

B

0

can significantly dif-fer from dissolution of a metal

B

with even a very smallcontent of element

L

provided that they constitute ahomogeneous

B

,

L

phase and the latter is nobler than thebasic metal. For a pure metal with the equilibriumpotential , there is an immediately adjacent

range in which an arbitrarily small positive potentialshift causes anodic dissolution. This becomes evident ifone takes into account that an arbitrary run of theanodic reaction (

d

ξ

B

> 0

) on a pure metal

B

0

B

z

+

+ (2)

is accompanied by no changes in the intense parametersof the metal phase. In contrast, the range of the anodicpotentials of metal

B

containing a noble element

L

shows a gap within which metal

B

does not dissolve[12]. In other words, the true equilibrium potential ofthe electrode

B

z

+

/

B

–

L

and the potential range of thedissolution of the

B

,

L

phase are not in contact. The gapwidth depends on a number of factors discussed below.Note only that the gap itself is gradually created by thedissolution process; as the result, anodic voltammo-grams can show signs of a transient state.

A qualitative pattern of the thermodynamics of thealloying effect is as follows. The concentration changesin the basic metal

B

and the alloying constituent

L

in anear-surface area of the

B

,

L

phase are displayed in

EB

z+/B0

zBe

To the Theory of Anticorrosion Alloying of Brasses

V. Yu. Kondrashin

Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006 Russia

e-mail: [email protected]

Received October 15, 2003

Abstract

—The protective effect produced by alloying was interpreted in terms of the gradient energy of thenear-surface diffusion zone formed as a result of the anodic dissolution or self-dissolution of ternary phases.The general reasons for a changed tendency of such phases toward selective anodic dissolution and selectivecorrosion compared to nonalloyed phases were revealed. The method for quantitative estimation of the protec-tive effect produced by small amounts of arsenic added to

α

-brass was proposed. A number of alloying elements(As, P, Ni, and Au) to

α

- and

β

-brasses were found to be similar in the nature of the protective effect.

PROTECTION OF METALS

Vol. 41

No. 2

2005

TO THE THEORY OF ANTICORROSION ALLOYING OF BRASSES 139

Fig. 1a. The curves suggest the presence of the diffu-sion zone in the solid and the enrichment of the surfacewith element

L

. This fact is fundamental in the thermo-dynamics of the alloying effect. Indeed, in the ioniza-tion of

B

from the

B

,

L

phase

B B

z

+

+ (3)

the potential-determining anodic process (3) creates apermanent diffusion zone, continuously redistributingthe constituents within the intermetallic phase. How-ever, the Gibbs energy of the phase with the diffusionzone is higher than that of the same phase without con-centration gradients. This energy excess (gradientenergy) can be available only from process (3), whichshould be accompanied by an additional increase in theanodic (quasiequilibrium) potential of the electrode.

Let us calculate the energy contribution from theconstituent

L

to reaction (3). Let an elementary run

d

ξ

B

> 0

of this reaction be accompanied by dissolutionof a metal layer with a thickness

dy

, the diffusion zone

δ

remaining constant (Fig. 1b). The content of element

L

at any point of the diffusion zone is higher than in thebulk of the phase. The surface concentrating of

L

involves an increase in the Gibbs energy (

dG

L

> 0

),while the function

c

B

=

c

B

(

y

)

shows an opposite changein the concentration and a decrease in the energy of theprocess (

dG

B

< 0

). The total energy effect is positive:

(4)

If the chemical potentials

µ

B

and

µ

L

within the diffusionzone are determined only by the concentrations of

B

zBe

dGB L–grad dGB dGL 0.>+=

and

L

and are independent of the location of the phasearea under discussion,

1

then the quantity (4) can be cal-culated as the difference between the energies of forma-tion of the

B

,

L

phases of the initial composition (

y

=

∞

)and of the composition at the surface of the dissolvingelectrode (

y

= 0) [11]:

(5)

According to the general principles of electrochemicalthermodynamics, the following equation can be writtenfor the potential-determining reaction (3):

This equation defines a part of the total shift of thequasiequilibrium anodic potential of the

B

,

L

phasefrom the analogous potential of the pure metal

B

0

. Thepart is due to the contribution of the gradient energy (5)to the energy of reaction (3):

(6)

The rest of the shift is trivial. The chemical potentialof the basic metal in an alloy is lower than that of thepure metal:

µ

B

< , (7)

taking into account that the surface is enriched with ele-

1

This is possible when there is no internal mechanical stress in thediffusion zone [13].

dGB L–grad ∆fGB L– ∞( ) ∆fGB L– 0( )–[ ]dξB.=

dGB L–grad zBF∆EB L–

grad dξB.⋅=

∆EB L–grad 1

zBF--------- ∆fGB L– ∞( ) ∆fGB L– 0( )–[ ].=

µB0

00 δ δdy δ + dyy

CL(∞)

CB(∞)

CB(0)

CL(0)

CB , CL CB , CL(a) (b)

y

Fig. 1. (a) Schematic distribution of the concentrations of B and L upon the selective dissolution of the B,L phase and (b) directionsof the concentration gradients (marked with arrows) in the dissolution of a layer dy at a constant thickness of the diffusion zone.

140

PROTECTION OF METALS Vol. 41 No. 2 2005

KONDRASHIN

ment L, one can write

(8)

The total shift of the quasiequilibrium anodic potentialis the sum of Eqs. (6) and (8):

(9)

This sum gives a gap width between the equilibriumpotential of the electrode Bz+/B–L and the potentialabove which the anodic dissolution of the B,L phaseoccurs.

A similar effect is observed when element L isadded to the B,A phase. However, since the concentra-tion gradients of L and A are opposite in sign, theirenergy contributions are antagonistic (Fig. 2). Thus, theelement L breaks (or even breaks up) the link betweenthe partial ionization reactions of constituent A and B.This role can be quantitatively described by analogywith Eq. (6):

(10)

∆EB L–surf 1

zBF--------- µB

0 µB 0( )–[ ].=

∆EB* ∆EB L–grad ∆EB L–

surf .+=

∆EB A– L–grad 1

zBF--------- ∆fGB A– L– ∞( ) ∆fGB A– L– 0( )–[ ].=

The energy (10), which is negative for nonalloyedB,A phases [11], decreases in absolute value or reversessign for a phase containing element L. The other part ofthe shift of the quasiequilibrium potential is due to ine-quality (7) and is quantitatively defined in Eq. (8):

(11)

where µB(0) is the chemical potential of the constituentB at the surface of the dissolving B, A, L alloy. As thesurface becomes enriched with the alloying constituent,

µB(0) decreases and increases. As the result,the sum of Eqs. (10) and (11) in the alloyed phasedecreases in absolute value or becomes positive.

Below, it will be demonstrated that an increase inthe anodic potential of the phase can significantlyreduce its tendency toward selective dissolution.

ALLOYING EFFECT OF ARSENIC IN COPPER AND α-BRASS

A Cu–Zn–As system and, as a model, a Cu–As sys-tem are convenient for the study of a phenomenon ofanticorrosion alloying. Arsenic is well soluble in cop-per (α-phase): 6 at. % at 200°C [14]. In α-brasses, itssolubility is lower but sufficient to create protectiveconcentrations [15]. It is also significant that additionof arsenic changes the rate-limiting step of the anodicprocess on neither copper nor α-brass: in concentratedchloride electrolytes, the process is controlled by diffu-

sion mass transfer of complex ions CuC , CuC , or

CuC from the electrode surface to the bulk of thesolution [15, 16]. Arsenic oxidizes during the self-dis-solution of arsenic brasses in aerated chloride media togive insoluble Cu2AsO4OH, Cu3AsO4(OH)3, andCu5(AsO4)2(OH)4 as a black deposit covering the alloy[17]. No soluble arsenic compounds in the anodic oxi-dation of alloyed brasses were detected [15]. Indeed,the standard electrode potentials for “soluble arsenicform–free arsenic” redox systems range from 0.25 to0.65 V [18]. These values are higher by 0.2 to 0.8 Vthan the typical anodic potentials of copper and α-brass.

The partial thermodynamic parameters of the basicmetal in the Cu–0.1As phase (the number in front of Asindicates the arsenic content (at. %)) virtually do notdiffer from the analogous parameters of the pure metalCu0. This follows from the plots of the equilibriumpotentials of the Cu+/Cu0 and Cu+/Cu0.1As electrodesvs. the concentration of the potential-determining ions(Fig. 3). In both cases, there is an equilibrium (Nernst)plot of E vs. log[Cu+] with a slope of 2.3RT/F; thepotentials of the arsenic electrode are higher than thepotential of pure copper by at most 1 mV. The alloyingeffect does not yet manifest itself since the concentra-tions of arsenic in both the bulk of the phase and at its

∆EB A– L–surf 1

zBF---------= µB

0 µB 0( )–[ ],

∆EB A– L–surf

l2– l3

2–

l43–

0 dydy

CB(∞)

CB(0)

CL(∞)

CA(∞)

CA(0)

CL(0)

CB , CA, CL

y

Fig. 2. Directions of the concentration gradients (markedwith arrows) in the dissolution of a layer dy at a constantthickness of the diffusion zone in the B,A,L phase.



surface are low. Nor the initial segments of voltammo-grams point to this effect. The anodic potentiodynamicvoltammograms recorded for Cu0 and Cu–0.1As elec-trodes under the same hydrodynamic conditions areshown in Fig. 4. Curves 1 and 2 come out of nearly thesame point and initially follow the same line, without anoticeable difference. However, as the basic metal dis-solves, the curves gradually diverge: curve 2 goestoward higher potentials, which indicates the alloyingeffect of arsenic. Thus, arsenic in such an insignificantconcentration initially affects neither the thermody-namic nor kinetic parameters of the electrode. Its effectmanifests itself during the selective ionization of cop-per, which enriches the surface and near-surface areasof the solid with arsenic. This gives rise to a diffusionzone which can be reproduced only by using the energyof the potential-determining anodic process (seeFig. 1). For this reason, the re-recorded potentiody-namic voltammogram distinctly shows an initial seg-ment of the ideal polarizability of the Cu–0.1As elec-trode: copper does not dissolve over a certain potentialrange (Fig. 4, curve 3). The anodic process restartsbeyond this range and the positive potential shift

remains constant all the way the polarization curveruns.

This shift is especially pronounced for the potentio-static anodic E vs. logi curves in copper-free solution.The voltammograms of the Cu–0.1As, Cu0, α-Cu–30Zn0.05As, and α-Cu–30Zn electrodes under thesame hydrodynamic conditions are shown in Fig. 5(lines 1, 2, 3, and 4, respectively). The negative shift ofline 4 relative line 2 is also of gradient nature and wasdiscussed in [11]. The presence of arsenic in the phasebrings about, for the foregoing reasons, a parallel shiftof both the lines in the positive direction by ca. 10 mV.

Equation (6) relates the level of the surface enrich-ment with arsenic to the first part of the shift of thequasiequilibrium anodic potential of arsenic copper rel-ative to the potential of pure copper. The effectivenessof this correlation can be estimated from the formationenergies of solid Cu–As solutions at low concentrationsof the second constituent. Tentative data on these ener-gies can be obtained from the formation enthalpy of an

intermediate β-phase Cu–25As: ∆f =−107 kJ/mol [19]. To a first approximation, the molarenthalpy of the formation of a solid solution may betaken to be proportional to the mole fraction of the

H298 Cu25As,0

–4.0 –3.5 –3.0 –2.5

–0.05

0

0.05

0.10

E, V

a

b

c

d

2

1

log[Cu+], M

Fig. 3. Plots of the equilibrium potentials vs. the concentra-tion of Cu+ ions for mechanically polished (1) Cu+/Cu0 and(2) Cu+/Cu 0.1As electrodes in 1 M NaCl + 10 M HCl at(a) 12, (b) 25, (c) 40, and (d) 55°C.

1

23

0.05

0.04

0.03

0.02

0 1 2 i, A/m2

E, V

Fig. 4. Potentiodynamic (0.5 mV/s) anodic voltammogramsof mechanically polished (1) Cu0 and (2, 3) Cu 0.1As elec-trodes (1, 2) exposed to 1 M NaCl + 0.01 M HCl + 5 ×10−4 M Cu+ at 25°C; curve 3 refers to preliminary dissolu-tion of 30 atomic layers of copper.

142


KONDRASHIN

alloying element xAs and therefore

(12)

If the Gibbs energies in Eq. (6) are replaced by theenthalpies, the order of magnitude of the gradient con-stituent of the alloying effect can be estimated:

(13)

Equations (12) and (13) show that for zCu = 1 (chlorideelectrolyte), even two- or threefold enrichment of thesurface of the Cu–0.1As electrode with arsenic(xAs(∞) = 1 × 10–3, xAs(0) = (2 – 3) × 10–3) causes an eas-

ily detectable potential shift = 4.4–8.9 mV.Apparently, such a considerable gradient effect is dueto the high formation exothermicity of solid solutionsof the Cu–As system.

The second, additional part of the potential shiftobeys Eq. (8). It is calculable from the plot of µCu vs.xAs, which requires the knowledge of the plot of∆f GCu−As vs. xAs. An acceptable estimation of Eq. (8)may be based on the fact that the concentration of thealloying constituent is much lower than the concentra-tion of the metal acting as a solvent. In such a phase, the

∆fH298 Cu As–,0 107

xAs

0.25---------- kJ/mol( ).–=

∆ECu As–grad 1

zCuF----------- ∆fHCu As– ∞( ) ∆fHCu As– 0( )–[ ].=

∆ECu As–grad

chemical potential of the basic metal is

µCu = – RTxAs,

where is its chemical potential in its inherent phase[20]. Therefore, as long as the condition xAs(0) � xCu(0)

is met, ∆ is directly proportional to xAs(0).According to the experimental data, at xAs(0) = xAs(∞) =1 × 10–3, the equilibrium electrode potential is shiftedby no larger than 1 mV (see Fig. 3). Therefore, theaccumulation of arsenic at the surface2 should lead to aproportional increase in the potential shift according tothe condition

(14)

The constituents of the anodic potential shift for theCu–0.1As phase, which are expected to arise in copperionization and enrichment of the surface with arsenic,are given in table. Calculations were performed underthe assumption that the alloying element is inoxidizableand its atoms do not leave the original phase during theanodic dissolution, concentrating in the surface atomicmonolayer. Such a model of the process corresponds toan extremely high level of the effect since the concen-tration gradients in the solid are as high as possible. As

arsenic accumulates at the surface, both ∆ and

∆ values increase rapidly, the first term beingresponsible for the main part of the effect (80% andmore).

The actual potential shifts are much smaller than thecalculated ones. As the electrode dissolves, the experi-mental value increases from 1 to 10 mV (Fig. 4)and then remains virtually constant. As follows fromthe tabulated data, the latter value corresponds only toa threefold enrichment of the Cu–0.1As phase surfacewith arsenic; in practice, it is reached after the dissolu-tion of 30 to 40 atomic layers of copper. As notedabove, the released arsenic oxidizes to give insolublecompounds and no longer affects the anodic potential.Indeed, the surface of the Cu–0.1As phase darkens after20 to 30 atomic layers have been removed and turnsdark gray upon removal of 100 monolayers. This coloris characteristic of arsenic compounds reported in [17].The impossibility of confining all atoms of the alloyingelement to the diffusion zone of the dissolving B,Lphase was also reported in [21, 22]. For example, theanodic process for the In–0.1Ag alloy is accompaniedby the phase transformation of silver and deposition ofsilver crystals at the electrode surface [21].

2 Under the assumption that it is contained only in the solid α-phase of the Cu–As system.

µCu0

µCu0

ECu As–surf

∆ECu As–surf xAs 0( )

xAs ∞( )---------------- 1 mV( ).×=

ECu As–grad

ECu As–surf

∆ECu*

–1 0 1

–0.05

0

0.05

0.10

E, V

12

3

4

logi, Ä/m2

Fig. 5. Anodic voltammograms of (1) Cu–0.1As, (2) Cu0,(3) Cu–30Zn0.05As, and (4) Cu–30Zn electrodes in 1 MNaCl + 0.01 M HCl at 25°C (rotating disks, 15 rps).



Note that arsenic-alloyed α-brasses do not substan-tially differ from arsenic copper as regards the characterand level of the alloying effect [15].

The dominant role of the gradient energy in the phe-nomenon under study explains the known experimentalfact that the protective effect is inherent only in arsenicas part of a brass. When added to an electrolyte solu-tion, arsenic becomes ineffective [15, 16]. This result iseasy to understand when repeating the analysis ofFig. 1b for cL(∞) = 0: here the gradient energy (4) van-

ishes and hence ∆ = 0.

ALLOYING EFFECT AND STEADINESSOF THE UNIFORM DISSOLUTION OF BRASSES

An increase in the anodic potential of an alloyedbrass compared to the analogous potential of a simple,two-component phase (Fig. 5, lines 3, 4) is due to aninsignificant tendency of the former toward selectivedestruction. The possible dissolution of any (bothalloyed and nonalloyed) alloy at potentials that arelower than the ionization potential of its noble constit-uent from its own phase ( < 0) can be interpretedin terms of increased thermodynamic activity of thisconstituent at the surface of the dissolving metal [23].The formation of a new Cu0 phase in the anodic disso-lution or corrosion proceeds by both the surface diffu-sion of copper atoms and their ionization followed byredeposition [24]:

The formation of copper crystals, which are electro-chemically more inert than copper at the brass surface(Cu*), localizes anodic processes, which can result inhazardous localized corrosion of the alloy. Obviously,

EB L–grad

∆ECu*

Cu* Cu0

Cu+ e+

(15)

(16) (17)

uniform dissolution does not lead to such conse-quences.

If ionization (16) and electron addition (17) arerapid, then the thermodynamic prerequisites for theprocess to follow pathway (15) and pathways (16)–(17)are the same [24]. The probability of the formation ofcritical nuclei of a new phase at the surface of the dis-solving alloy in any of the above pathways is given bythe function [24, 25]

(18)

where Vm is the molar volume of the new phase and σeffis the effective surface tension described in detail in[20]. Function (18) shows a pronounced threshold:when ∆ reaches a certain value, the probability P*jumps and so does the tendency of the phase towardselective dissolution [25] (see Fig. 6, curve prsl). Thesegment pr with a nearly zero probability correspondsto uniform dissolution of the alloy since the formationof nuclei of a new phase at low ∆ values is unlikely.In the segment sl, the probability of nucleation is high,and the selective dissolution followed by the phasetransformation in the surface layer is possible (process(15)). The intermediate area rs is particularly character-ized by pseudoselective dissolution (processes (16) and(17)).

In a chloride electrolyte, the difference between theanodic potentials of copper and the α-Cu–30Zn elec-trode is 35 to 36 mV. In the case of the α-Cu–30Zn0.05As electrode, this difference is reduced to 25to 27 mV (Fig. 5). The potential difference determinedfrom the partial voltammograms of copper ionizationfrom the above α-brasses (i.e., ) is 19 to 20 and 9to 11 mV, respectively. Thus, alloying diminishes theprobability of nucleation, thus stabilizing the uniformdissolution of an arsenic alloy. Nickel and gold intro-duced into α- and β-brasses exhibit analogous effects

P*16πVm

2 σeff3

3RT zBF∆EB*( )2--------------------------------------–

;exp∼

EB*

EB*

∆ECu*

Constituents of the shift of the quasiequilibrium anodic potential of arsenic copper (xAs(∞) = 1 × 10–3; zCu = 1) calculated by Eqs.(13) and (14) and the experimental shifts for a mechanically polished electrode in 1 M NaCl + 0.01 M HCl + 5 × 10–4 M Cu+ at 25°C

Number of thedissolved atomiclayers of copper

Theoretically expected values (mV) from the

voltammogramsxAs(0) , mVaccording to Eq. (13)

, mVaccording to Eq. (14)

Total, mV

0 1 × 10–3 0 ~1 ~1 0–1

1 2 × 10–3 4.4 ~2 6 0–1

2 3 × 10–3 8.9 ~3 12 0–1

3 4 × 10–3 13.3 ~4 17 1–2

5 6 × 10–3 22.2 ~6 28 2–3

10 11 × 10–3 44.4 ~11 55 3–4

20 21 × 10–3 88.7 ~21 110 7–8

∆ECu*∆ECu–As

grad ∆ECu–Assurf

144


KONDRASHIN

[12, 25, 26]. Nickel enriches the surface because ofslow ionization of its atoms [27], while gold has a highredox potential.

Phosphorus is the closest analog of arsenic asregards the protective effect. This element affects thebehavior of α-brasses in the same low concentrations(~0.04 at. %) [28]. Indeed, the standard enthalpy of for-mation of the Cu–25P phase is –150 kJ/mol [19], whichis close to the enthalpy of formation of the β-Cu–25As.Because of this, the formation enthalpy of solid solu-tions in the Cu–P system obeys, to a first approxima-tion, an equation close to Eq. (12):

(19)

The enthalpy of mixing of Ni or Au with copper islower than (12) and (19) by one exponent and more. Toattain the same effect, the concentrations of these ele-ments in brasses should be ~1 at. % [1–4].

Tin seems to contrast with a family of the elementsAs, P, Ni, and Au. The protective effect of tin is a matterof common knowledge (admiralty brass) [1–3],although there is direct evidence for the selective disso-lution of tin and zinc at the initial step of the corrosionof the α-Cu–29Zn1Sn and α-Cu–26Zn4Sn phases inaerated chloride media [29]. The concentration gradi-ents of Sn and Zn become equal in sign, whichexcludes, in terms of the aforesaid concepts, the protec-tive effect. However, here, this effect is of a differentnature, being associated with the formation of SnO2 andother compounds at the surface of tin brass [29]. Appar-ently, a similar reason is valid for the protective effectof aluminum. Discussion of these problems is beyondthe scope of this study.

∆fH298 Cu–P,0 150

xP

0.25---------- kJ/mol( ).–=

CONCLUSIONS

The developed concepts are based on consideringlinks between the partial electrode processes, with theintermediacy of the diffusion zone. Its gradient energy,when combined with the electrochemical energy of thepotential-determining anodic reaction, changes theapparent thermodynamic activity of the potential-deter-mining constituent at the surface of the dissolvingphase. Thus, both the enhanced activity of the constitu-ent and the alloying effect, which is of opposite sign,are explained in terms of the general principle of theeffect of one electrode process on the other [11].

These concepts develop the content of the studies in[5–7] based on the idea that active sites of the solid sur-face are insulated by atoms of an alloying element. Informula (9), the insulation effect is included in the sec-

ond term ∆ , which can be differently interpretedat a microscopic level, depending on the nature of thephase and the electrolyte solution.

At the same time, it is highly improbable thatarsenic catalyzes the reduction of Cu2+ ions to Cu+ ions[10] (see Introduction). According to experimental data(Figs. 3–5), the alloying effect of this element isretained in deoxygenated chloride electrolytes, inwhich Cu+ ions are virtually the sole product of theanodic dissolution of copper. For this reason, the reduc-tion of Cu2+ into Cu+ (process (1)) is excluded. Thereare no grounds to believe that arsenic is of particularnature: similar alloying effects are exhibited by otherelements, although their chemical and electrochemicalproperties are far from identical with those of arsenic.

The developed method specifies the physicochemi-cal features of an element L required to attain a goodprotective effect. First of all, it needs a sufficiently widerange of homogeneity in the B–A–L system. Otherwise,enrichment of the near-surface zones of a solid can benonhomogeneous. At the same time, element L shouldbe electrochemically inert and inoxidizable. Accordingto Eqs. (6) and (8)–(10), the dissolution of element L inthe B,A phase should be as exothermic as possible.Finally, Eq. (10) reveals that L is especially effective inlow concentrations that ensure the largest differencebetween ∆fGB – A – L(∞) and ∆fGB – A – L(0) (provided theaforementioned conditions are satisfied). However, inthis case, the desired protective effect will not bereached until the surface has sufficiently been enrichedin L as the result of prolonged dissolution of the alloy.

REFERENCES

1. Tomashov, N.D., Teoriya korrozii i zashchita metallov(Corrosion Theory and Protection of Metals), Moscow:Akad. Nauk SSSR, 1959, p. 531.

2. Uhlig, H.H. and Revie, R.W., Corrosion and CorrosionControl, New York: Wiley-Interscience, 1985, 3rd ed.

3. Corrosion: Handbook, Shreir, L.L., Ed., London: New-ness–Butterworths.

EB L–surf

p r

s

lP*

0 ∆EB*

Fig. 6. Plot of the probability of nucleation (calculated byEq. (18)) vs. the parameter . ∆EB

*



4. Marshakov, I.K. and Vyazovikina, N.V., Zashch. Met.,1978, vol. 14, no. 4, p. 410.

5. Kolotyrkin, Ya.M., Plaskeev, A.V., Knyazheva, V.M.,et al., Dokl. Akad. Nauk SSSR, 1978, vol. 243, no. 6,p. 1483.

6. Plaskeev, A.V., Knyazheva, V.M., and Kolotyrkin, Ya.M.,Zashch. Met., 1981, vol. 17, no. 6, p. 661.

7. Plaskeev, A.V., Kasparova, O.V., Kolotyrkin, Ya.M.,et al., Zashch. Met., 1984, vol. 20, no. 1, p. 62.

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to the theory of anticorrosion alloying of brasses

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