Mechanlam of the anodic dissolution of the ARMCO iron in HClO₄ solutions in mixed water-alcohol solvents. II

A C I /\ U N I V E R S I T A T I S L O D Z I E I I S I S POLIA CH2UICA 5, 1985

Stenisiaw Rooanowski, Hanryk Scholl

MECHAHISM OP THE ANODIC DISSOLÜTION OP THE ARMCO IRCH IN HC104 SOLUTIONS IH MIXED WATER-ALCOHOL SOLVEHTS, II

ARitCO iron anodic diaeolution in HCIO. solutions in mixed water-alcohol solvents has been investigated. On the basis of Prunkin’s isotherm the adsorptive effects of the particles involved in the electrode reuotion de­ termining step has been discussed.

In the proposals of the iron aleotrochamical corrosion me­ chanism in aqueous acidic electrolyte solutions, without spe­ cifically adsorbing ions or molecules, hydroxyl ions play an important role. According to the metal surface properties the catalyzed or noncatalyzed mechanisms of the anodic dissolu­ tion of the Iron in H-,30. or HCIO. solutions have been

descrt-r , 2 4 4

bed [1-7].

Discrepancies in literature data concern the application of the electrodes with different pre-traataents of the working surface for the use of the method of iron zero charge potenti­ al values (j^E g » 0)[«3 and capacitance curves determination [4,8-10 ].

On the basis of the measurements of iron RDE impedance in H^SO^ solutions E p e l b o l n at a 1. [11-17] the influ­ ence of the hydrogen adsorption and of the active complexes FeOH+ on the mixed mechanism of the anodic reaotion in the range of the polarization potentials characterized by so-cal­ led "negative Impedance have been defined. The investigation of a physical model confirms the influence of Warburg’s impoda- nce on the process under discussion, but the electrochemical layer impedance cannot be included as a constant capacitance in a parallel connection with faradaic impedance.

DlBCUJBSlOn

The investigation of the iron corrosion in aoidid solutio­ ns in mixed aqueous - nonaqueous solvents encounters additio­ nal difficulties for the following reasonst

1° adsorption of the nonaqueous solvent molecules on the electrode surface. If the solvent is not Involved in creating active complexe«» then its participation is described by means of various models used in inhibited reactions descriptions;

2° an appearance of the solvent moleoules on the electrode ourface leads to the PflE shift and to the influence of the polycrystalline metal surface energetic heterogeneity on the potential shift. Theorefore the charge "anisotropy" determines the adsorption anisotropy which limits the inhibitor action. Thti proposals of the formal equations taking into aocount the total sum of the adsorption, kinetic, chemical and electroche­ mical effects, aad the mechanical interlocking are reported in

ths works of Antropov's aad Parsons*schools;

3° the nonaqueous solvents molecules are involved in crea­ ting of the active complexes. So far the reaction mechanism of few solvents, especially of the mixtures HgO - DMSO [19,20] HgO - 0£F [PI ] , H20 - MeOH [21 ] and HgO - EtOH [18,22] has been put fc.’ward.

The me Kanism of the anodic dissolution reaction suggested by other a u ;hors [ 22 ] , is based on the participation of the active complexes of ?eOR+ type in the rate determialng step (RDS). Using these proposals we have*

rc + HOH ; = s Fe( HOH )a(j„ O a )

r, + R O H î = = £ P e ( R O H ) ad8

v (1b)

(ROH » MeOH; EtOH; 1-PrOH; 2-PrOH)

Fe(HOH)a(je » (^®°H”)a<jB + H+ (2a)

--- ---- -. - - -— ^

Mechanism of the anodic dissolution of the AHMCO 83

CP#°H ~)ada - 0 ° H>ads + • (3 a ) O e 0 R “ ) a d s -C P eO R )ad8 + e _, (3 b )

(PeOH)adfl + F«(H0H)ad e [(P eO H ) • ?<H0H)]ad8

-\ C4a)

CPeOR)adB + Fe(ROH)adB - [(FeOR) • Fe(ROH)Jad8 ■

_h

The electrode reaction rate determining step in mixed wa- ter-alcohol solvents has the fora of the following equation acoording to [22Ji

Iw + ia — ► PeOH+ + FeOR+ + Pe(HOH)adB+ Pe(RGH^da+2e U ) and finally*

PeQH* + PeOR+ + 2H+ 2 P«2+* eolv. + HOH + ROH ( 7 ) The reaction mechanlam of the iron anodic dissolution do­ es not solely depend on the solvent dipoles adsorption but al­ so on the sum of interactions of adsorbed active complexes wi­ th the electrode and between the neighbouring solvent molecu­ les.

The phenomenon of the self-acting adsorption of the mix­ ture of reacting and electrode Inactive substances is descri­ bed by general Prumkin’s isotherm [24]»

B i*c i*fi " exp''~2 ^ ' aiiei^ Ce) 1 1

where» f 1 - the reacting substance activity officient defined by Debye - Hflckel theory,

- characteristic constant of reacting substance in­ teraction with others adsorbed molecules (later­ al repuleive attraction)

and . .

B 1 * Ba ' exP [ - T ^ - 9 » « ) 2] (9) wherei Ba - adsorption rate oonstant

cpa - maximum adsorption potential

_ Ke-0 " *81 f<n s

r

-and where* Kq-0j - integral capacitances for given co­ verage degrees respectively,

rm - max. Gibbs* surface excess

e

1

- (e

1

♦

©2

♦ ... en)

(n)

’

«ten we substitute [9] into [8], we obtain*

0 , » Bm • c1 • r,(l - Qj) exp[2 ( 2 a1l© i)3 •

‘ * exp [(1 Y j ~ ((12)

Independently of eq [12] we get the reaction current equation*

i - KBm • c 1 • f, • (1 - £ 6 t) exp(2 S a1i0 l)*

p, F

• exp J.1 - c O ^ (y> - y j - (13) Ke-0 " X9i f,. '\2i

The Bo-oalled "salting out" takes place as the result of the interaction between ions and solvent molecules , i.e. the activity of organ!o substances in the solution is Increased and as • consequence the rate of the reaotlon nay also incre­ ase

[

].

The transition from one solvent Into another under const Is accompanied by the change of the charge, which leads to the potentials change (the reaction plane potential) and to the rate change* The same situation takes place for the quan­ tities and <pm .

In the range of medium coverage degrees C 0.2 < < 0.6) the reaetion rules become more oomplioated and It Is neoosoary to take into consideration the terms (1 - £6^) and exp (ZEa^O^).

The Increase in 6^ should bring about the decrease in the reaction rate together with the decrease in (1 - ^ 0^). Simultaneously, the exp(2^!-a1i 6^) value increases. The total effect depends on a ^ , so with the increase in e.j both the reaotlon slowing down and spending up may ocour under con­ stant potential.

The increase in the amount of adsorbed moleculss which co­ ver the eleotrode surfaoe leads to the considerable decrease in the reaotlon rate. It may be explained by the so-called "great coverage effects". I.e. In the adsorbed molecule takas up more place on the surface than In the equilibrium conditio­ ns. This is closely connected with the increase in molecule preorientation energy and with reaorganization of tho adsor­ ptive layer what in consequence leads to the descrease in the rate of the reaction under coverage degrees 6 * 1 .

For the discussion about the mechanism of the iron anodic dissolution reaction let us designate!

A. for solution In watert

coverage degrees, 8 ^ > e pe0a+| e 12 a 8Pe(H0H)ad8 lateral repulsive attraction constansi

a n - of PeOH+ complex S12 ” °f Pe(H0H)ad* and w„ - yeGH+ complex activity in water,

B, for solutions In wator - alcohol solventst coverage degrees« 9 2y a 0 ^ ^ + 5 Q 22 B e PeCHOH)ad#

6 23 * 0 PeOR+ { 9 24 H 9 p«CROH) &d8 taleral repulsive attraction constanst

a^ 1 - of FeOH+ complex j a22 - P* HOH adB a23 “ of y#0R+ complex 1 a2ij - Fe RCH adg activities«

s_ - of PeOH+ complsx;

1 +

s - PeOR complex; 3

Let us write anodic reaction ourrent equation expressed by equation ( 5a ~) applying formula ( 13)«

•wi . wk . vb • V [1 - (e., ♦ e J ] » * p [2 * ( a ^ e ^ ♦ *12e i2^ *

♦ m 1 •

• exp [(1 - a 5a)RT (W

SP00r r "

‘

, ! v > v ^ (ww

v

2]

2RT W T^ 7Corr and analagously for equation (5b):

ai+ - ak . aBn • V sa3 {[i - (e21 ♦ e22 + q23 ♦ e 24)]}.

. exp [2(a21 . 621 ♦ a220 22 + «230 23 + a24e2A ^

(15) * ex? [ (1 ~ a 5b ^ C\ orr - \ ) '

. -_Ke-o ~ ^ 1 (8 _ s )2]

2RT • P ₀₀ W r j

In RDS we get u> ■ which allows us to the neg- 0 ~corr •irm*

lect last terrae ion equations (14) andv1 5 ).

Experimentally determined values of coefficient b+ [18,22J satisfy the relationship«

(1 - • (1 » a^jj) ■ (1 - cO (16)

which allows ub to divide aquation (14) by eq. (15). So we gett

‘W r ** • *B, ‘ **1 ' **3 [1 - <»21 ♦ °;2 * ♦ V I W^corp ' *" ' • "*1 ^ < » 1 1 * V 1

(17) . exp^2 [(*21 * e21 * a22®22 * a23023 * a24e2*P "

" ^a119 11 + a129 12^ } * exp[~~RT (A w ^corr ~

Among the terms of eq. (17) we can obtain experimentally the values* / w t , (1 - a ) and aJ y corr and to

corr corr c a l c u l a t e A ® ^.

The remaining quantities of equation (17) cannot be dire­ ctly measured and the influence of the adsorption of particu­ lar molecules involved In the reaction (fe) on the corrosion currents values may bo disocused, for particular instances, only after introducing appropriate assumptions*

Conclusions

1° - let ub assume that for snall(< 1 mol %) concentra­ tion of alcohol in water*

0 11 S ®21 110(1 0 1 2 * 0 2 2

(19) a11 * a21 311(1 a12 3 a22

flBl 3 wa, (20)

l}corr m 3k * 3sm * % i1 ~ (811 * 6 12 * 9 23 + Wicorr "k * Wß» " f911 * 012>'

. exp[2(a23-923 ♦ «24 Q 24)] • (21)

. e xp [ ^ Ta --(A Js»c orr “ A w ) J

At the «am« time, 833 and e24 In the seoond tern of eq* (21} are very snail enough to neglect then* And we obtain*

*1 _{corr --- a ---- 1 . exp [2 a?,e2, ♦ a ^ Q ^ J .}S* ’ 8 s m * r o ft 1

corr ra ( 2 2 )

. exp[-^-^-f^(AjyCorr * A i ^ ]

As it oan be seen from above equation the reaction determi­ ning step depends on the adsorptive interaction between PeOR and Fe(RC«)adB and on the activity of the complex PeOR*.

2° - let us assume, for small water ( < 1 mol %) concen­ tration in alcohol, thatt

®24 ^ 9 22 » 0 25'1>921 ^

and &21 a ^ (24)

*24 a a22 8 3 1

a 1

Then, equation (17) will take the form*

si ®k • 8B_ . *a, [1 - (e23 + 6 24}] 1corr _2____ 5_____ 1 , ______ ti----.

v.

. exp{2[2(a23e 23 ♦ ^ 6 2U} * (a116 i1 * *1291 2 ^ } *

. exp p 1- - ^ ^ A *^corr - A j V l )]

(25) (26)

Contrary to the conclusion in point 1°, there is no expll- oite to the question whether water molecules in alcohol inhibit the eleotrode reaction. Equation ( 27 ) contains the suns of fa­ ctors which desoribe the characteristic adsorptive interactions in water and in water- alcohol mixtures. The literature data are quoted the inhibition of this type in the case of the anodlo re­ action of ateel .'25] or titanium [26} In solutions In the same mixed solvents.

However, one should aocept the results of oorroslon investi­ gations In anhydrous aloohol since It la probable that esters may form as the additional components of the solution.

Simultaneously, strong water inhibition in aprotic solvents has been also observed £27].

3° The adsorptive conditions at the interface become fixed in a wide range of concentrations of the water - alcohol mixtu­ res (1 - 99 mol, % of alcohol). The difficulty in interpreta­ tion of the process has previously been explained by discussion of the uselulness of eq.(1 3 ) for the ooverage degress 0 . 2 < ©

<

.

The assumptions made in points 1° and 2° cannot be made in this ease. However, solvation processes have a considerable ef­ fects on the reaction which constitutes the final stage of the Iron anodic dissolution (eq. (7) ).

The paper is supported within the framework of MR.1.11 Problem. References 1. P. H 1 1 b e r t, Y. U 1 y o a h i, G. B 1 c h k o r n, W. J. L o r e n s , J. Electrochem. Soc., 1.18, 1919 (1978) 2. K. F. B o n h o a f f e r, Z. H e u s 1 e r, Z. Phys. Chem. N. §• 390 (1356) Z. H e u s 1 e r, Z. P. Blektrocbem., 62, 582(1961) 3. J.O'M. B o c k r i s , A . K. M. R e d d y “M o d e m Electroche­ mistry", vol. 2., Plenum Rosetta, NY 0973)

Eleotrochia. Acta, 325(1961); J. o'm. B o o k r 1 a , H» K 1 t a, J, Electrochem. Soc., 108. 676(1961 )i J. O'M. B o c k r i s , D . D r a z i c , Electroohlm. Acta 2» 293

(

9. A. A k i y a m a , R. E. P a t t e r s o n , Ken Höbe, Cor-rosion-HACE, 26, 51 (1970)

10. R, V a l d y a n a t h a n , N. H a c k e r a a n, Electro- chin. Acta 16, 2193 (1971)

•11. I. E p e l b o i n , il. K e d d a m , Ph. M o r e l , "The Pro­ cess of Adsorption in the Course of Iron in an Acid Medium" - 4th Int. Congress on Metallic Corrosion, Sept. 1969, Amsterdam, NANE-Houston, p. 143 12. C. G a b r i e l l i , M . K e d d a m , J-C. L e s t r a d e , C. r. 271C, 1 (1970) 13. I . E p e 1 b o 1 n, «. K e d d a m, J. Electrochwn. Soc., 117. 1052 (1970) 14. I. E p e l b o i n , M. K e d d a m , H. T a k e n o u t l , J. Apr lied Electrochem., 2, 71 (1972)

15. I . E j a 1 t o i n, Ph. ¿ ¿ o r e l , H. T a k e n o u t l , J. El«istrochem. Soc., 118. 1282 0971) 16. I. E e l b o i n , C. G a b r i e l i 1, M. K e d d a m , J-C. L e s t r a d e , H. T a k e n o u t l , J. Electrochem. 3oc. 112» 1632 (1972); C.r. 276 C . 145 (1973); Electro­ n i c . Acta 20. • (1975) 17. C« G i o r i e l 1 i, M. K e d d a m , H. T a k e n o u t i , Vu Quacp; Klnh, 2, B o u r e l i e r , Electrochim. Acta 24,

61 (1979)

18. H. S c h o l l , Acta Univ. Lodz», in press

19.

21. C. A. F a r i n a , G. F a i t a, F. 0 1 i v a n i, Corro- aion Sci., 18, 465 (1978) 22* A. L. B a o a r e 1 1 a, A. L. S u t t o n, J. Electroohem, Soo. 122, 11 (1975 )* ibid 122, 1636 (1975) 23« K. S c h w a b « , J* Electroohem. Soo*, 122. 1636 (1975) 24. B. B. D a m a s k i n , 0. A. P e t r i i , "Vvedenie v «1«-

ktrokhlmitcheskuiu kinetiku", Moskva (1975)

25. V. I. V.ig d o r o v i t c h , t. E. T a y g a n k o v a , K. V. O s i p o v a , T. K o r n e e v a , Zastch. Mot., 594 (1973) 26. A. C « r q u e t l i , F . M a z z a , Corrosion Sol., 1^, 337 (1973) 27. V. S o h m 1 d t, Zaatoh, Met. 608 (1969). University of Łódź, Instltut« of Chemistry 90-136 Łódź, POLAND

Stanisław Romanowski, Henryk Scholl

MECHANIZM REAKCJI ANODOWEGO ROZTWARZANIA ŻELAZA ARMCO W ROZTWORACH HC104 W MIESZANYCH ROZPUSZCZALNIKACH

WODNO-ALKOHOLOWYCH. II.

Przedyskutowano propozycje mechanizmu reakcji anodowego roztwarzania żelaza ARMCO w kwaśnych roztworach w mieszanych rozpuszczalnikach woda-alkohol. W oparicu o lzoterme Prumkina przedyskutowano adsorpcyjne efekty cząstek biorących udział w decydującym stadium reakcji elektrodowej.