Fathi Habashi
Montana
School
of
Kinetics of Corrosion of Metals
Mines Butte
For better understanding of corrosion processes, it is essential a t first to consider the simple cases of corrosion when no insoluble products, scales or films are formed. Once the mechanism of these processes is well established, it may be possible to understand corrosion processes in general. Corrosion processes in this category (no insoluble end products are formed) can he divided into two broad groups: (1) corrosion with no hydrogen evolution, and (2) corrosion with hydrogen evolution. Hydrogen is liberated in those processes when H+ ion is acting as depolarizer (electron acceptor). If, however, the solution contains a depolarizer other than H+ ions, as for example Oz, H202,etc., then corrosion proceeds with hydrogen evolution. It is the purpose of this paper to report that all these corrosion processes follow a single kinetic law.
concentration, the rate depends only on the concentration of the acid, i.e., on the concentration of hydrogen ions, and is independent of the concentration of the depolarizer. At high acid concentration, the rate is independent of the acid concentration, depending only on the concentration of the depolarizer. Further, the ratio of the concentrations a t which the rate changes its dependence is a constant (0.12), as can he seen from the points where the lines change slope in Figure l(a). Table 1.
Corrosion Reactions with N o Hydrogen Evolution
Examples
Mplecular complexmg agent
Zinc in dilute acetic acid Iron in dilute HCI Copper in aqueous PITH3 Cop er in aqueous et&lenediamine Copper, silver, and rold in CN-solution
Acid
Corrosion With N o Hydrogen Evolution
Three groups of reactions of this type can he identified. These are shown in Table 1, and the kinetics of these processes are plotted in Figures 1-3. In all these reactions, the aqueous phase is composed of t,wo components: depolarizer (02, H202, etc.) and H+ ions or complexing agent (NHa, CN-, etc.). These processes have two things in common: depending upon experimental conditions, the rate of corrosion is controlled by only one of the components of the aqueous phase; arid there is always a constant ratio between the concentrations of the two components of the aqueous ~ h a s ae t which the rate changes its dependence from one of the components to the other. The example shown in Figure l(a) illustrates these two points. This shows the corrosion of zinc in acetic acid in presence of KN03 as depolarizer. At low acid
Depolarizer
Medium
Anion complexing went
Ref.
KXO?
(1)
H?02 O2 O?
(3,
O2
(6-9)
(b)
Mechanism. The electrochemical theory of corrosion assumes that when a metal comes into contact with water to which oxygen or any depolarizer is added, the depolarizer takes up electrons a t one part of the surface (the cathodic zone) while the metal gives them up at another (the anodic zone) (10). The cathodic reduction of oxygen for example, a t the surface of the metal may lead to the formation of either hydrogen peroxide or hydroxyl ions as follows: O2
--
+ 2Hp0 + 2ec H202+ 20H40HO2 + 2H90 + 48-
-
The anodic reaction may he represented as follows: 31
31"+
+ ne-
0 0 Moles11 HCI
a .c
0
I
I
0.02
004
I
006
I
I
008
ACETIC ACID C O N C E N T R A T I O N I M a l e s l l )
((11 Figure 1.
Corrosion of metals in acid,.
0
v
I 008 H,O,
0 24 0 32 CONCENTRATION (Males 1 1 0 16
Ibl
lo) Zinc in acetic acid in the presence of KNO, at 25'C(I1. (b) lronin HCI in the presence of HIOl at 2S°C 121.
318 / Journal of Chemical Education
4)
(6)
OZ PRESSURE
(ATM.1
Ibl
Corrosion of copper in molecular complexing .hove). 10) Ammonio (3). Ibl NHCHCHcNHs 151.
Figure 2.
Or
PRESSURE
agents
(left a n d
(ATM.1
Id
The liberated metal ion hydrolyzes forming insoluble compounds', as follows: M"+ + n HnO M(OH), + n H+
-
The insoluble hydrolytic products form a thin 6lm which adheres firmly on the anodic zone (Fig. 4), thus preventing or slowing down further dissolution of the metal. If,however, the aqueous phase contains an acid or a complexing agent, these insoluble products will be dissolved as soon as formed. Kinetic Consideration. Like any other heterogeneous process, the surface of the metal in contact with an aqueous phase is thought to be covered by a thin layer of a stagnant liquid (the Nernst boundary layer), through which the reactants have to diffuse before 1 Evans and Davies (11) described experiments in which pure metallio einc was suspended in distilled water through which g ion per oxygen was continuously bubbled. After 4 days, liter of Znz+ was detected, and this amount did not increase. This was in agreement with the coneentmtion predicted from the solubility product of einc hydroxide.
reaching the metal surface. If the rate of the chemical reaction at the interface is much higher than the rate of diffusion, the process will be diffusion-controlled. On the other hand, if the rate of chemical reaction is much slower than the rate of diiusion, then the process will be chemically-controlled. The rate of the cathodic reaction can be given in its simplest form, when first order kinetics are followed, by the equation: Vz = ktA,[DI (1) where k,
velocity constant' surface area. of the cathodic zone [Dl = concentration of the depolarizer
A1
= =
The rate of the anodic reaction can be given by the 1 When the rate is diffusion-controlled, the velocity constant = D/6 where D = the diffusion coefficient of the reagent, and 6 = the thickness of the Nernst boundary layer. When the rate is chemically-controlled, k = A ecEIRT the familiar Arrhenius equation. A diffusion-controlled process is always first order; a ehemically-controlled process can be any order. However, the discussion here is limited to processes which follow first order kinetics, established by either mechanism.
"
ol/
215
5
'
'
I0
'
15~10~'
KCN concentrotion
F:g.,=
3.
.gent
IKCN solulion a l 2SLC.).
Corrorion of metals :n ononic complexing (01 Copper 181. (bl
Silver
191. CI Gdd 181.
Po,=021 ( A T M )
n 0
5 10 K G N CONCENTRATION
(Moleslll (cl
Volume 42, Number 6, June 7 965
/
319
equation: Vz
=
kzA4CI
(2)
where k2 = velocity constantP Az = the surface area. of the anodic zone [C] = concentration of H + ions, or the complexing agents
At the steady state, the rate of the cathodic reaction = the rate of t,he anodic reaction, i.e. k,A,[DJ = MzICl (3) But, since A
=
A,
+ A*
(4)
(where A is the total surface mea of the metal in contact with the solution) therefore:
Under these conditions, the rate depends on both concentrations of C and D. These are the points where t,he rate curve changes its direction. This change takes place at a certain ratio of [C]/[D] as can be deduced from equation (8)
Equation (5), derived theoretically, is therefore in agreement with experimental data and is a general equation for a large number of corrosion processes. Corrosion With Hydrogen Evolution
Evolution of hydrogen during the corrosion of metals may take place either in acidic or in alkaline medium. I n these cases hydrogen ion is acting as depolarizer, and the reactions taking place are:
-
+ + nec Hz
cathodic reaction: 2H+ 2eanodic reaction: M - M e f
At low concentration of D and high concentration of C, the first term in the denominator may he neglected in comparison with the second, and the velocity equation simplifies to: Rste = klAID] (6)
The two cases will be considered separalely. Aeidic medium. Beside acting as depolari~er,~ HC ions also prevent the hydrolysis of metal ions:
The rate of corrosion under these conditions depends only on the conceutration of the depolarizer. This is in agreement with the experimental facts plotted in Figures 1-3. On the other hand, at high concentration of D and low concentration of C, the second term in the denominator may he neglected in comparison with the first, and the rate equation simplifiesto:
preventing the deposition of insoluble products that may block the anodic zone. Such mechanism is shown schematically in Figure 5. The kinetic equation for this process can be deduced from equation (5), by suhstituting the hydrogen ion concentration for [Dl and [CI:
Rate = &A [C]
(7)
The rate of corrosion in this case is only a function of the hydrogen ion concentration or the complexing agent concentration. This is again in agreement with the experimental facts plotted in Figures 1-3.
= k
+
A [H+]
(11)
where k = klkJkl kr. Equation (11) is the usual velocity equation for the corrosion of metals in acids with H, liberation. Thus Straumanis and Chen (12) reported that the corrosion rate of titanium in hydrofluoric acid, (Ti 3 HF + TiF, HZ),in absence
+
+
a Hydrogen overvoltage of the metal may be rate-determining in this process. This discussion applies only to cases of first order kinetics.
A, Calhodlc Area
HydrolytC Products:
M"'+ n H & l z M I O H h + n H f
nydrolytic Products: M "'tn
i Figure 4. Schematic representation of corrosion of metals gen evolution.
320
/
Journal of Chemical Education
HzO=
M iOHl.+n~'
Figure 5. Schematic representation of corrosion of metois in acids with hydrogen evolution.
0 Figure 8.
I
4 6 8 10 12 14 16 18 20 N o O H CONCENTRATION IMoles / I ) R o k of dissolution of aluminum in sodium hydroxide (141.
where K
2
=
[H+] [OH-].
When
H F CONCENTRATION ( M o l e s N ) Figure 6.
Rote of dirrolvtion of titanium in hydrofluoric acid (I?).
of air, increases linearly vith increasing acid concentration as shown in Figure 6. Alkaline medium. An example of this case is the dissolution of aluminum in NaOH. This process follows the same equations for cathodic and anodic reactions; hydrogen ions also act as the depolarizer. The function of OH- is to complex the liberakd metal ion into a soluble form: M(0H). mOH- M(OH),&
+
-
-
l n the case of aluminum, aluminate ion is formed: AI(OHh + OH- Al(OH),This mechanism is shown schematically in Figure 7.
equation (12) reduces to: This coiricides with the experimental results by Straumanis and BrakSs (15), who found that the rate of dissolution of high purity aluminum in NaOH increases linearly with NaOH concentration in the range 0.3 to 3.0 molar. When
equation (12) reduces to Rate
=
kXA
1 [OH-I
(14)
Equation (14) shows that beyond a certain hydroxide ion concentration, the rate of corrosion should decrease with increasing hydroxide concentration. This agrees with the experiment,al results of Balezin and IUimov (14) in their systematic study of the dissolution of aluminum in different alkalies a t variable concentrations. These authors reported that beyond 5 M NaOH the rat,e of dissolution decreases with increasing NaOH, as shown in Figure 8.
COthOdlC Are0
Autocatalytic Corrosion Anodic A r m
Figvre 7. Schematic representation of corrosion of metals in alkalies with hydrogen evolution.
The kinetic equation of this process is also equation ( 5 ) , uhich after substituting the concentrations of the
depolarizer (H+ ion) and the complexing agent (OHion), becomes: Rate = kAA[H+I[OH-] k,[Htl blOH-I
+
When the product of corrosion reacts further with the metal undergoing corrosion, the rate of corrosion of the metal will be accelerated and the process will be autocatalytic. An example of this is the corrosion of copper in dilute sulphuric acid in presence of oxygen. The copper(I1) ion formed reacts with metallic copper to form copper(1) ion which is rapidly oxidized by oxygen to regenerate the copper(I1) ion. The kinetics of two such superimposed processes can be established by cousidering each process separately. The overall velocity equation will be the sum of the velocities of these two parallel reactions, namely, the electrochemical dissolution reaction and the dissolution due to the autocatalytic action, (the catalytic term). The catalytic term in this case can be derived by assuming that the rate depends on the diffusion of the copper@) ion away from the metal interface. But since Cu
+ CUP+= 2cu+
Volume 42, Number 6, June 1965
/ 321
Therefore, the rate should be proportional to the square root of the copper(I1) concentration: Rate
=
K"[Cu2+1'/%
(17)
This can be verified if the experimental data by Kinevsky (15),who studied the effectof copper(I1) ion on the corrosion of copper in sulfuric acid, are plotted in the form shown in Figure 9. In this plot, the linear proportionality between the rate of corrosion and the square root of the copper(I1) ion concentration, as implied by the mechanism suggested, is demonstrated.
kinetic law applied, no matter what metal was undergoing corrosion, what the composition of the aqueous phase, or whether HI was evolved or not, provided only that no insoluble end products were formed. The data can be interpreted on the basis of the electrochen~ical theory of corrosion and the derived rate equation (5) which governs all these processes is in agreement with experiments. In the special case when H+ is acting as depolarizer, equatiou (5) reduces to equation (11) in acidic medium and to equation (12) in alkaline medium. When the products of corrosion react further with the metal, corrosion will be accelerated, and a rate equation of the type given in equatiou (18) is valid. Acknowledgmenl
This study was carried out in part during a fellowship awarded to the author by the National Research Council of Canada, to which the author wishes to extend his thanks. Literature Cited
0
L-Lpl--~~-~' 02
. -. ,
04
[c"'.
06 I+
I
8
10
08 iMOk./l
1'
Figure 9. Effect of sapperlil) on the corrosion of copper in HSOr (2.26 N and under Ph = 1 dm1 11.5).
(Decrease in rate a t high copper (11) concentration is due to increased O2 solubility.) The overall kinetic eqnation will be:
In the initial stage of corrosion, when only small amounts of reaction products are formed, the catalytic term can be neglected, and the rate equation simplifies to equatiou ( 5 ) . When the reaction proceeds further, the catalytic role becomes predominant and equation (18) simplifies to equation (17).
(1) KINO,C. V., AND SCHACK, M., J . Am. Chem. Soe., 57, 1212 (1935). (2) ABRAMSON, M. B., AND KING,C . V., J . Am. Chem. Soc., 61, 2290 (1939). (3) HALPERN, J., J . Electrochem. Soc., 100,421 (1953). (4) H ~ A S F., H Ber. ~ Bumengesellschajt Physik. Chem., 67, 402
- - -- .
(14fi2) \ ,
(5) HALPERN, J., MILANTS,H., AND WIYILES, D. R., J . Electrochem. Soc., 106, 647 (1959). (6) DEITZ,G. A,, AND HALPERN, J., J . Metah 5, I109 (1953). (7) NABASHI, F., In press. (8) KaKovs~Ir,I. A,, AND KHOLMANSKIKH, Yu.B., h. Akad. Nauk SSSR Otd. Tekh. Nauk, Met. i Toplivo, No. 5, 207 (1QROI. \----,. (9) K,\~ovsnrr,I. A., AND KAOLMANSKIKH, Yu. B., IZV. AM. Nauk SSSR Otd. Tekh. Nauk, Met. i Topliuo, No. 5, 97 (1959). (10) EVANS,U. R., "The Corrosion and Oxidation of Metala," 2nd ed., Edward Arnold & Co., London, 1960. (11) EVANS,U. R., AND DIVIES, D. E., J . Chem. SOC.,2607 (19511. s, M. E., r m CHEN,P. C . , J . Electroehem. Soe., 134 (1951). NAN~S, hl. E., AND BRAKSS,N., J . Electrochem. Soc.
Conclusion
In all cases examined for corrosion of metals a single
322 / Journol of Chemical Education
2924a (1962). (15) KINEYSKY, A. I., Zh. P ~ i k lKhim., . 28, 1113 (1955).
