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Effects of alloyed molybdenum on the kinetics of repassivation on austenitic stainless steels
Corrosion Science, Vol. 24, No. 5, pp. 463--478, 1984 Printed in Great Britain.
0010-938X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
EFFECTS OF A L L O Y E D M O L Y B D E N U M ON T H E KINETICS OF R E P A S S I V A T I O N ON AUSTENITIC STAINLESS STEELS P. I. MARSHALLand G. T. BURSTEIN Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Abstract--The kinetics of repassivation of Type 316L austenitic stainless steel in aqueous solutions after in situ generation of the metal surface have been obtained. The results are compared with those obtained
previously from similar experiments on Type 304L stainless steel. In alkaline and slightly acidic solutions both alloys repassivate at a rate controlled by ion migration through the growing oxide film under high electric field. The presence of alloyed molybdenum accelerates the rate in the early stages of film growth but in the later stages the kinetics for the two materials are identical. In strongly acidic solutions dissolution of the iron component of the oxide film accompanies film growth. This constructive dissolution process occurs at a rate controlled by the diffusion of iron ions through the oxide matrix. While the presence of molybdenum does not affect the diffusion coefficient significantly, it does allow the release of iron ions to the solution to commence at a far earlier stage of film growth, thereby hastening the onset of passivity. The associated kinetic parameters are presented and the implications of the proposed mechanisms discussed.
INTRODUCTION
THE EFFECTSof alloyed molybdenum in improving the passivity and pitting resistance of austenitic stainless steels is a long documented phenomenon.l-4 Despite the many researches aimed at understanding the precise role played by molybdenum in these processes 5-7 no definitive mechanism has yet been provided. One of the intriguing aspects of the phenomenon is that relatively little Mo is required to improve the corrosion resistance quite markedly as is exemplified by the comparison between Types 304L and 316L stainless steels. 1,4 Some higher grades of stainless steels exist, however, where the Mo content can be as high as 6.5 wt%.8 It is interesting to note that some nickel based alloys also contain considerable quantities of alloyed Mo and the effects may be considered similar. 9 Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) have both demonstrated the presence of Mo in the passive film on stainless steels containing the element although the outer surfaces (i.e. at the film-electrolyte interface) of such films are apparently depleted in Mo. 6,7,10,11 The same is true for the passive films on nickel-molybdenum alloys in which Mo is found immediately below the film surface.12'13The fact that Mo is depleted from the outer surfaces of such films is consistent with equilibrium thermodynamics,14 at least for acidic and alkaline solutions, in that Mo(VI) is quite soluble over a wide range of potentials. The fact that Mo continues to exist within the films reflects only the non-equilibrium nature of the film in situ. Analysis by XPS has shown that at least some of the Mo in the passive Manuscript received 3 October 1983. 463
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P.I. MARSHALLand G. T. BURSTEIN
film is in the f o r m o f M o (VI) 15 and that the metal side o f the metal-film interface may, in fact, be enriched in metallic M o . 6'11 Such observations must be interpreted with caution, h o w e v e r , since it can sometimes be difficult to distinguish inherent properties of the thin oxide films f r o m processes which have b e e n induced by the analytical technique.13 In particular, since no M o is o b s e r v e d on the o u t e r surface o f the film, such analyses must be m a d e on the interior o f the film, access to which is generally gained by argon ion b o m b a r d m e n t . This process can p r o d u c e drastic changes in oxide film c o m p o s i t i o n s ) 3 T h e actual presence o f M o in the film is, however, unambiguous. C u r r e n t ideas of the role of oxidized M o in the passive films of b o t h stainless steels and nickel based alloys suggest that M o m a y allow thicker films to be f o r m e d t h e r e b y reducing the passive current density 5 and that it m a y hold the oxide lattice t o g e t h e r m o r e tightly, 16 also reducing the passive current density. These theories m a y also explain the i m p r o v e d resistance o f the alloys to chloride-induced pitting since dissolution o f the film would then be m o r e difficult and pit nucleation retarded. Since the o u t e r surface of the film is free of MO 6'7'10'11 such retardation must occur only w h e n the o u t e r surface has already b e e n penetrated. T h u s it would in fact be p r o p a g a t i o n of the pit t h r o u g h the film which is retarded, rather than nucleation itself. Since p r o p a g a t i n g pits develop an internal electrolyte of low p H 17 the role of M o m a y well be in retarding acid attack of the film. Alternatively, it has b e e n p r o p o s e d that m o l y b d a t e anions m a y be f o r m e d by anodic oxidation of M o from the alloy. 7"15 That M o O 2- is in itself an inhibitor of pitting has b e e n d e m o n s t r a t e d by adding it to a chloride-containing electrolyte.IS All the a b o v e theories are largely qualitative in nature and few n o n - s t e a d y state kinetic m e a s u r e m e n t s have b e e n made. In previous papers 18,19'2°we have described the kinetics of growth of passive films on 304L stainless steel surfaces which were freshly g e n e r a t e d in situ. In neutral and alkaline solutions the films grow with no significant e n r i c h m e n t of any alloying element. In acidic solutions, 2° h o w e v e r , dissolution o f the Fe c o m p o n e n t of the film occurs the kinetics of which are c o u p l e d with those o f film growth. T h e film t h e r e b y b e c o m e s enriched in c h r o m i u m , and p e r h a p s Ni as well. Passivity is thus achieved by a film containing essentially c h r o m i u m oxides, at a rate controlled by the electric field across the film and the p H of the electrolyte. This p a p e r describes the effects o f alloyed M o on these processes as f o u n d by examination o f 316L austenitic stainless steel. E X P E R I M E N T A L METHOD The alloy used for the present results was an austenitic 316L stainless steel of composition (in wt%): Cr 16.8, Ni 12.4, Mo 2.4, C 0.03, Fe bal. It was used in the solution annealed condition. Apart from the Mo content, the composition is comparable with the previously used 304L stainless steel s-2o(Cr 18 3, Ni 10.6, C 0.03, Fe bal.); this enables comparison of the results of the two steels to be made. The kinetics of repassivation of 316L stainless steel were examined using the scratched rotating disc electrode under potentiostatic control. The technique has been described in other papers. 2°'2~It consists of generating a scratch rapidly in a rotating disc electrode made of the test material while the metal is held under potentiostatic control in the electrolyte solution. The scratch, which is created using a diamond stylus, was --1.5 mm long by --30 p,m wide by - 3 p,m deep. This depth is sufficient to penetrate completely the previously formed passive film on the disc thereby exposing bare metal to the electrolyte. The consequent current transient is recorded on a bank of transient recorders set to record the initial event and as much of the current decay as possible. A typical anodic current transient was presented earlier.IS Repassivation kinetics are defined in terms of the current density flowing from the scratch, i(t), as a
Effects of alloyed molybdenum on the kinetics of repassivation of austenitic stainless steels
465
function of the charge density that has flowed from the scratch, q(t), both measured at time t after scratching. These are given by i(t) -
1 [l(t) - Ib] 2~rrtoyt c
(1)
and q(t)
_12rrrtoytc Ii [l(t) -
Ib]dt
(2)
where to is the electrode rotation rate (100 Hz in this work), y is the scratch width (30/zm), r is its distance from the centre of rotation and tc is the contact time of the stylus ( ~ 1 ms). I(t) is the total absolute current flowing from the whole electrode, including the scratch, at time t and Ib is the absolute base current flowing from the rotating disc before scratching. The integral defined by equation (2) is thus measured by the area under the current-time transient. Transients were measured at ambient temperature (19 + 2°C) as a function of electrode potential for de-aerated electrolytes of varying pH. These electrolytes were: 1.0 M K O H , pH 14; 0.5 M H O A c , 0.5 M NaOAc, p H 4.8 and 3 electrolytes made from HCIO4 and NaCIO4 of pH 1.8, 1.4 and 0 to a total CIO4 concentration of 1.0 M. All chemicals were of a.r. grade except for NaCIO4 and were made up with double distilled water.
EXPERIMENTAL RESULTS In line with results obtained previously for 304L stainless stee118-20the repassivation kinetics are plotted as log i(t) as a function of q(t) -1. The rationale behind these plots is the fact that the repassivation rate of 304L stainless steel is consistent with the film growth kinetics being controlled by ion conduction through the growing oxide under high electric field. Thus i(t) = A exp [Bh--~)]
(3)
where V is the voltage drop across a film whose thickness is h(t) at time t and A and B are parameters related to the energy barrier through which the mobile ions pass. 22,23Appropriate rearrangement of equation (3) gives 18 log i(t) = log A + B z F p ( E - Eg) 2.3Mq(t)
(4)
where p is the density of the film, M its molecular weight, z the charge number on the current carrying ion, Eg is the equilibrium potential between the metal, the oxide and the solution and E is the electrode potential. Figure 1 shows the repassivation kinetics of 316L stainless steel for several values of E in electrolytes at pH 14 and 4.8 according to equation (4). After an initial period in which the plots are curved a linear region is encountered, consistent with equation 4, which extends over some 2-3 orders in i(t). The intercepts of Fig. 1 provide the parameter A from equation (4). These are listed in Table 1 and are invariant with E and pH, at least for 4.8 ~ pH -< 14. Differentiation of equation (4) gives 0 log i(t) _ B z F p ( E - Eg). Oq(t) -1 2.3M
(5)
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TABLE 1. pn 14.0
FILMGROWTHPARAMETERSA and B from equation 4 for repassivation of 316L stainless steel E[mV(NHE) ]
-155 -55 +45 +145 +245 4.8 +245 +445 +645 +845 +1005 0.0 +645 +745 +845 + 1045 +1145 Mean 304LIow current density data (Ref. 19).
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Plots of equation (5) in terms of the high field gradient as a function of E are shown in Fig. 2. These, too, are linear as predicted, and their slopes provide the value of B under the appropriate assumptions of z, p and M. The values used are the same as those assumed for 304L stainless steel, 18 with z = 2, p = 5.7 g cm -3 and M = 70 g mol -~. The calculated values of B are also listed in Table 1; these too, are independent of pH. The intercepts of Fig. 2 provide Eg (see equation 5) and are given in Table 1. Similar data obtained for repassivation of 304L stainless steel 18'19 showed two consecutive regimes in which equation (4) was obeyed; the first was a faster process occurring at lower values of q (t) and the second a slower one occurring at higher q (t). The present results on 316L steel are quantitatively consistent with the second of these regimes. A comparison of data obtained under identical conditions for the two steels is shown in Fig. 3. The low i(t) [high q(t)] kinetics for 304L steel are followed for the 316L steel extending through both the low and high current density regions. To show the similarity in these kinetics, corresponding mean values of A and B, which describe the linear regions of Figs. 1 and 3 completely, are also tabulated for the low i(t) kinetics of 304L steel in Table 1. The values of Eg determined for the two steels are plotted in Fig. 4 as a function of pH. The Eg/pH data for 316L steel lie, within the scatter of data points, on the Eg/pH line for 304L steel. Kinetic plots according to equation (4) are presented for 316L steel repassivating in acidic solutions in Fig. 5. At p H 0 the data still conform with equation (4) but the gradient is very much steeper than those observed at higher pH and the data show a far lower A value. The data for p H 1.4 and 1.8 show curvature in Fig. 5. To show the comparison between the behaviour for 316L and 304L steels, Fig. 6 shows the kinetics of repassivation for the two steels measured under identical conditions of E and pH. There is now a distinct difference between the repassivation rates of the steels.
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DISCUSSION
Repassivation in alkaline and slightly acidic electrolytes It might well be expected that the repassivation rates of 304L and 316L stainless steels be similar since the Mo content of the latter is quite small (2.4 wt %, equivalent to 1.4 at%). This is, in fact, only true when the film has already grown to a certain thickness, dictated by the value of q (t) at which repassivation of 304L steel enters its second phase (see Fig. 3). The identity of these results indicates that the added Mo has negligible effect on the rate of growth of the film and probably on the film composition and structure as well, since variation in these properties would be expected to give measurable variation in A, B and Eg. Table I (and ref. 19) show that such variation as may occur is not measurable by this technique. Nevertheless, in the early decay region, where the film is still very thin [dictated by lower values of q (t)], the kinetics of repassivation of the two alloys are different. The early decay regime for 304L steel has not been fully explained, 1s'19and 316L steel shows no evidence of this regime (see Fig. 3). A consequence of this is that it actually requires more charge to reduce the bare surface current density to a particular value in this regime than is required by 304L steel, implying that the film must have had to acquire a greater thickness. This is seen by inspection of Fig. 3. For example, at pH 14, E = -155 mV(NHE), the charge density required to reduce the scratch current density to 0.1 A cm -2 is 5.75 mC cm -2 for 304L and 8 mC cm -2 for 316L. One can speculate 18that 304L steel forms two different films: the first formed occurs at high i(t) and is converted entirely into the second in the low i (t) region. Under this assumption 316L steel shows no evidence of the first formed film and all the linear regions of Fig. 1 relate only to growth of an oxide film similar to the second film of 304L steel. It is, however, quite possible that a film equivalent to the first formed on 304L does grow on 316L but its kinetics are lost in the very high current density regions of these high field plots. This would imply that alloyed Mo has accelerated formation of the first film. Inspection of Fig. 3 shows that the very high current density regions of 316L steel are consistently faster than those for 304L steel and the explanation seems sound. We thus conclude from this that the presence of Mo accelerates the onset of passivity, even though a greater charge density (and therefore thicker film) is required to do so. The possibility that Mo actually dissolves in the electrolyte giving rise to the increased charge densities for 316L steel must also be considered. Provided that Mo is not being preferentially oxidised in comparison with the other elements in the matrix then the difference in charge density for the two steels is far greater than that which would be accounted for by oxidation of Mo alone. Thus in Fig. 7 we plot this difference q ( t ) d i f f ---- q ( t ) 3 1 6 L -- q ( t ) 3 0 4 L
(6)
as a function of q (t)316L,where q(t)316Land q (t)304Lare the charge densities measured at constant t for 316L and 304L steels respectively. The graph shows that q(t)diff is a considerable fraction of q ( t ) 3 1 6 L and could not possibly be associated with oxidation of Mo alone. Repassivation of the two steels in these alkaline and slightly acidic solutions is thus kinetically similar once the initial period has been surmounted.
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Repassivation in strongly acidic electrolytes The repassivation kinetics of 316L steel in strongly acidic electrolytes are different from that in more alkaline solutions (see Fig. 5 and Table 1). This is demonstrated in the plot of repassivation kinetics for both 304L and 316L steels at pH 0, Fig. 6. The 304L kinetics have been analysed 2° in terms of film growth by equation (4) in the early regions, showing both kinetic regimes as described above. At a certain film thickness however, dissolution of the iron component of the film occurs causing the graph to swing to far higher values of q(t) than those expected from equation (4). The broken line in Fig. 6 thus represents the rate at which the film on 304L steel is actually growing and the corresponding solid line gives the film growth kinetics coupled with the iron dissolution kinetics. The kinetics of dissolution of Fe from the film were obtained 2° by considering the difference between the experimental data and the extrapolated lines. It should be noted that the 316L data in Fig. 6 actually tend towards the 304L data at very low current densities but at high current densities there is a large charge density difference. The data for 316L steel indicate that a similar dissolution process is occurring but that it is occurring at a rate far higher than that for the 304L steel, so high in fact, that the point at which it commences lies in the very high current region of the graph and is thereby obscured. Since the point at which dissolution commences during repassivation of 304L steel is pH dependent 2° (it occurs at constant electric field which decreases as pH increases) it is reasonable to expect that a higher pH may enable the appropriate electric field to be measured. For pH values up to 1.8 this field has not been found. The data measured at pH 1.4 and 1.8, however, do show curvature in the high field plots (Fig. 5), consistent with the region below -0.01 A cm -2 for 304L steel at pH 0. The lack of a suitable buffer solution of pH slightly greater than 1.8 prohibits examination of the full pH dependence. In light of the fact that for pH -> 4.8 the two steels grow similar films by similar kinetics (see above) it is possible to analyse the kinetics of repassivation of 316L steel in the strongly acidic electrolytes by assuming that the film growth kinetics are the same as for 304L steel (with the appropriate small adjustment in the value of B, see Table 1). The difference in the predicted film growth kinetics and the actual data for
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q(t)-t(cm z C-') FIG. 8. Experimentally obtained repassivation kinetics of scratched 316L stainless steel. (rq) pH 1.8, E = +1045 mV(NHE), (A) pH 0.0, E = + 745 (NHE). The broken line and dotted line show predicted filmgrowth kinetics for the above respectively.
316L steel then provide the rate of dissolution of the iron c o m p o n e n t of the growing oxide film. To demonstrate this we show in Fig. 8 experimentally obtained repassivation kinetics of 316L steel and the predicted film growth kinetics using the mean value of A from Table 1, the m e a n value of B (for 316L steel) from Table I and the Eg value shown in Fig. 4. We then consider the charge density difference, Aq(t), between that observed for 316L steel [q(t)316L] and that obtained from the constructed film growth line [q(t)film ] as
Aq(t) -- q(t)a16L -- q(t)fitm (7) each measured at constant t. Plots of Aq(t) versus t ]/2 for the p H 1.8 electrolyte are shown in Fig. 9. The graphs conform to linearity and pass through the origin, at least for the time scale considered. The gradients of these lines do not show any systematic variation with potential, but rather, are scattered about a m e a n of aAq(t)
0t--~r~j316L = (6.5 + 3.5) X 10 -3 C c r n -2 s -1/z.
(8)
This parabolic rate constant is quantitatively comparable with that obtained for 304L steel 2° for which
aAq(t)]
0 - - ~ - J304L
=
(2.9 + 1.3) X 10 -3 C cm -2 s -1/2.
(9)
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The value was found to be independent of E and pH, within the scatter of the data. These dissolution kinetics are interpretable in terms of rate control by non-steady state diffusion of iron ions through the oxide lattice. The boundary conditions and analysis of this solution to Fick's second law were presented previously 2° and the solution is given by
Aq (t) = 2zFrr t/2o 1/2(t - to) 1/2Co
(10)
where Co is the bulk concentration of Fe ions in the oxide lattice, D is their diffusion coefficient in the oxide and to is the time after scratching at which dissolution commences. The fact that the kinetics of 316L pass through the origin means that to is very small and is therefore not detected, either in Fig. 9 or in Fig. 5. Equation (10) allows computation of D using equation (8). In line with that for 304L steel, assuming z = 2 (Fe 2÷) and Co = 0.059 mol cm -3 (from the density of the alloy) we obtain D = 2.5 x 10 -13 cm 2 s -1, which, within the scatter, is again similar to that for 304L steel. 2° We can now derive the electric field at which this dissolution process commences in line with the measured fields for 304L steel. 2° To do this we plot Aq(t) as a function of q(t)316L and extrapolate to Aq(t) = 0. Under these conditions q(t)316L = q(t)tilm (see equation 7). Typical plots are shown in Fig. 10 and they are linear. At Aq(t) = 0 the value of q(/)316L, denoted by qd provides the electric field in terms of (E - Eg)/qd at which deviation from high field film growth would be expected to be observed. A plot of this electric field is shown in Fig. 11 as a function of pH, together with the corresponding data measured for 304L steel.2° The field for 316L steel is considerably higher for the same pH. The scratch current density which flows at this field is dictated by equation (4): for the p H 1.4 and 1.8 electrolytes it has a value of - 0 . 1 1 A cm -2, which is in fact, still just within the curved regions at very high current in the high field plots (see Fig. 5). Thus a less acidic electrolyte would be required to observe the field at which deviation occurred, similar to that observed for 304L steel at p H 0 (see Fig. 6). Figure 10 shows a further important feature. The values of qd [for which Aq(t) = 0] all lie above 4 mC cm -2 and since this anodic charge density is associated
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FI~. 10. Plot of the excess charge Aq (t) (above that necessary for film growth) flowing from the repassivating 316L stainless steel electrode as a function of the total charge flowing q(/)316L- At Aq(t) = 0, q(/)316L is the charge density at which deviation from high field kinetics occurs (qd). ( I ) pH 1.8, E = +655 mV(NHE). (~7) pH 1.4, E = +655 mV(NHE). (IS]) pH 1.8, E = +855 mV(NHE). ( V ) pH 1.4, E = +855 mV(NHE). (©) pH 1.8, E = + 1055 mV(NHE).
exclusively with film formation, and no dissolution is involved to this point, the film coverage on the scratch surface must be far greater than a monolayer, even if a generous monolayer charge density is assumed. (In previous work ~8'19'2° we estimated that - 0 . 5 mC cm -2 of anodic charge is required to produce a monolayer of oxide.) Thus the dissolution of iron which follows occurs only from the film itself, and not directly from the bare metal surface. This deduction is consistent with observations made for repassivation of 304L stainless steel. 2° It is possible to apply the same analysis to repassivation of 316L steel in the pH 0 electrolyte. Plots of Aq (t) against t lr2 are shown in Fig. 12. The diagram is not directly comparable with Fig. 9 (for the pH 1.8 electrolyte). It shows a very rapid rise to high values of Aq(t) followed by tailing off as t increases. I n this case the rate of iron dissolution is so great that through most of the time documented in Fig. 12 the growing film is essentially depleted of iron, and the rate of further dissolution is now controlled by the rate of film growth itself. We can demonstrate that this is true by plotting the value of Aq(t) as a function of q(t)316L (similar to Fig. 10) and showing that the charge density which represents the amount of film dissolved, is an approximately constant proportion of the total charge density involved and that this fraction is what would be expected if it represented most of the iron component of the entire film. The data are shown in Fig. 13. In Fig. 13(a) it is seen that Aq(t) is linearly dependent on q (t)316Land the lines pass through the origin. The ratio of Aq (t)lq (t)316L is thus independent of q(t)316L as is shown in Fig. 13(b). Thus for these conditions a constant proportion of the total charge density passed at any time has dissolved in the
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solution. The fact that the data show a small dependence on electrode potential may arise simply from scatter in the original current transients or may represent the fact that a little iron does remain in the film. One can estimate what fraction of the total anodic charge density is associated with oxidation of Fe only by assuming the oxidation number of each of the metal ions in the film. To obtain an idea of this we assume the following charge numbers: Fe(II), Cr(III), Ni(II) and Mo(III). Under these circumstances a fraction of 0.64 of the total anodic charge density evolved is associated with oxidation of Fe, and this is also marked in Fig. 13(b). Clearly, if some or all of the Fe is in the form of Fe(III), this fraction is greater. It is nevertheless seen
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476
P.I. MArtSNALLand G. T. BURS~IN I 0.03
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Fro. 13. Plot of dissolution charge [Aq(t)], Fig. 13(a), and the ratio hq(t)lq(t)3t6L, Fig. 13(b), as a function of the total measured charge q(l)316L for repassivationof 316L stainless steel at pH 0.0. (O) E = +845 mV(NHE). (I-1)E = +945 mV(NHE). (x) E = +1045 mV(NHE). from Fig. 13(b) that for reasonable assumptions of these charge numbers, the proportion of the film being dissolved into the acidic electrolyte of p H 0 does indeed represent approximately the entire iron component. It is striking that the repassivation kinetics measured under these conditions, where the dissolution component is so fast that its rate is controlled simply by the rate of growth of the iron free film, still follows equation (4) (see Fig. 5), although the associated parameters have been apparently changed. This phenomenon is, however, easily derived. Using the same symbols as in equation (7), we rewrite equation (4) as
[ B z F p ( E - E~)]. i(t) = A exP i Mq(t)sm J
(11)
Under conditions where all the iron dissolves from the film as fast as the film is being formed then Aq(t) is a constant fraction, f, of q(/)316L, and using equation (7)
[ B z F o ( E - Eg) 1i(t) = A exp [Mq(t)316L(1 -- f ) ]
(12)
Thus Fig. 5 still shows linearity but its slope has been modified by the factor
Effects of alloyed molybdenumon the kineticsof repassivationof austenitic stainlesssteels
477
(1 - f ) - l . The factor f w o u l d have a value of 0.64 if the charge numbers on the ions were as described above. It can be equal to the atom fraction of iron in the alloy only when the charge numbers on all the oxidised metal atoms are equal. The loss of the iron component of the growing oxide films on these steels means that the remaining components of the film must rearrange so as to bring about passivity, since the final state of the scratched metal is a return to the state of passivity. We have termed this 'constructive dissolution '2° in order to distinguish it from dissolution processes which result in rapid corrosion of the material. Not only is the dissolution p h e n o m e n o n common to both 304L and 316L stainless steels, but the r a t e of dissolution is the same for both alloys: it is controlled by the diffusion rate of iron ions in the film to the film electrolyte interface. The presence of Mo in the alloy (and consequently in the film) has no detectable effect on this diffusion rate, as described by the calculated diffusion coefficients given above. The presence of Mo in the steel does, however, have a pronounced effect on the electric field across the film at which this dissolution process commences. It allows dissolution of Fe from the film to start at a far earlier stage in the growth of the film (given by the higher electric field), and it is, perhaps, surprising that such a small level of Mo in the alloy produces such a large effect. Because of the fact that iron is dissolved from the film at an earlier stage when Mo is present the enrichment of Cr in the film also occurs at an earlier stage and thus the metal acquires its passivity more rapidly, since it is ultimately the chromium oxide which passivates the metal. For this reasoning to be valid one would expect that the passive film would always retain at least some of the Mo, and this is consistent with surface analyses of such films. 5'6A1 The outer surface of the final passive film has however, been found to be free of M o 6'7'1°'11 and this is probably also lost by dissolution. If the field across that region of the film which contains Mo is a little stronger than the outermost region which may be free of Mo then relaxation of the field by virtue of film growth will occur more quickly at the outer surface than it would if all the film were free of Mo. Since dissolution of Fe commences at a particular field (dependent on the electrolyte pH) then this field would be reached at an earlier stage for repassivation of 316L steel than for 304L steel, consistent with observation. Such behaviour would not significantly affect the assumptions made above since these effects are confined only to the outermost layer of the film. The same arguments apply to pitting of these materials by chloride ions. It is widely accepted that attack of the material by CI- causes acidification of the local electrolyte within the pit. If it is assumed that the attack by CI- occurs primarily on the Fe ions within the film then depletion of the Fe ions by an alternative path within the film should retard further attack. This path is provided by the acid dissolution of the Fe component. The presence of Mo accelerates this process allowing an incipient pit to repassivate by chromium oxides more rapidly. Thus pitting of austenitic stainless steels is rendered more difficult. Moreover, such a mechanism accounts for the observed synergistic effects of Mo with Cr in improving passivity and pitting resistance.5'11'16 In particular since the presence of Mo allows more rapid dissolution of the Fe component of the oxide, alloys of Fe and Mo which do not contain Cr may well have poorer corrosion resistance than Fe itself, and this, too, is consistent with previous r e p o r t s )
478
P.I. MARSHALLand G. T. BURSrEIrq
CONCLUSIONS 1. R e p a s s i v a t i o n o f 316L stainless steel is c o n t r o l l e d b y o x i d e film g r o w t h u n d e r high electric field. T h e a s s o c i a t e d k i n e t i c p a r a m e t e r s a r e s i m i l a r to t h o s e o b s e r v e d for t h e l o w e r c u r r e n t kinetics o f 304L stainless steel. 2. T h e h i g h e r c u r r e n t r e p a s s i v a t i o n kinetics o f 304L steel ( w h e r e t h e film is still thin) a r e n o t o b s e r v e d for r e p a s s i v a t i o n o f 316L steel. F o r a l k a l i n e s o l u t i o n s this p r o v i d e s an a c c e l e r a t e d initial r a t e o f r e p a s s i v a t i o n . 3. In acidic e l e c t r o l y t e s r e p a s s i v a t i o n o f 316L stainless steel b y o x i d e film g r o w t h is a c c o m p a n i e d b y d i s s o l u t i o n o f t h e i r o n c o m p o n e n t o f t h e film. T h e r a t e o f this p r o c e s s , which is c o n t r o l l e d b y t h e diffusion coefficient o f i r o n ions in t h e m i x e d o x i d e lattice, is t h e s a m e as t h a t o b s e r v e d for 304L stainless steel. T h e p r e s e n c e o f M o in t h e o x i d e has no significant effect o n this r a t e . 4. T h e d i s s o l u t i o n o f t h e i r o n c o m p o n e n t o f t h e o x i d e film d u r i n g r e p a s s i v a t i o n o f 316L stainless steel c o m m e n c e s at a c o n s t a n t e l e c t r i c field which is d e p e n d e n t u p o n t h e p H o f t h e e l e c t r o l y t e . T h e p r e s e n c e o f M o in t h e steel raises t h e e l e c t r i c field ( w h e n c o m p a r e d with 304L steel) a n d thus d i s s o l u t i o n c o m m e n c e s e a r l i e r in t h e r e p a s s i v a t i o n p r o c e s s . In o t h e r w o r d s , at a n y p a r t i c u l a r stage d u r i n g r e p a s s i v a t i o n m o r e F e has d i s s o l v e d f r o m t h e film o n 316L steel t h a n f r o m t h e c o r r e s p o n d i n g film on 304L steel. 5. This c o n s t r u c t i v e d i s s o l u t i o n p r o c e s s , w h i c h is a i d e d b y t h e p r e s e n c e o f M o , a c c e l e r a t e s t h e o n s e t o f passivity a n d thus i m p r o v e s t h e r e s i s t a n c e to c h l o r i d e i n d u c e d pitting corrosion of the material.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
15. 16. 17. 18. 19. 20. 21. 22. 23.
REFERENCES Y. M. KOLOTYRKIN,Corrosion 19, 261t (1963). M. A. S~EXCHER,J. electrochem Soc. 103,375 (1956). J. HARVATHand H. H. UaLIG, J. electrochem Soc. 115, 791 (1968). B. E. WILDEand N. D. GREEN,Corrosion 25,301 (1969). K. SUGIMOTOand Y. SAWADA,Corros. Sci. 17,425 (1977). I. OLEFJORDand B. O. ELFSTROM,in 'Eurocor 77', p. 21. Society of Chemical Industry, London (1977). H. OGAWA,H. OMATA,I. ITOHand H. Or,ADA,Corrosion 34, 52 (1978). A. J. SEDRICKS,Corrosion of Stainless Steel, p. 9. John Wiley, New York (1979). W. BETI'ERIDGE,Nickeland its Alloys, p. 61. Macdonald and Evans Ltd., London (1977). J. B. LUMSDENand R. W. STAEHLE,Scripta Met. 6, 1205 (1972). I. OLEFJORD,Mater. Sci. Engng. 42, 161 (1980). G.T. BURSTEINand T. P. HOAR,Corros. Sci. 17,939 (1977). G. T. BURSTEIN,Mater. Sci. Engng. 42, 207 (1980). M. POURBAIX,Atlas of Electrochemical Equilibria in Aqueous Solution, p. 272. Pergamon Press, Oxford (1966). K. SUGIMOTOand Y. SAWADA,Corrosion 32,347 (1976). E. A. LIZLOVSand A. P. BOND,J. electrochem Soc. 118, 23 (1971). G. W. PETERSOH,G. C. SOLTZand K. MAIRS,Corrosion 30, 366 (1974). G. T. BURSTEINand P. I. MARSHALL,Corros. Sci. 23, 125 (1983). P. I. MARSHALLand G. T. BURSTEIN,Corros. Sci. 23, 1219(1983). G.T. BURSTEINand P. I. MARSHALL,Corros. Sci. 24,449 (1984). F. P. FORD,G. T. BURS~INand T. P. HOAR,J. electrochem Soc. 127, 1325 (1980). M. J. DI~NAM,in Oxides and Oxide Films (ed. J. W. DIGGLE),Vol. 1, p. 91. Marcel Dekker, New York (1972). T. P. HOAR, in Modern Aspects of Electrochemistry, No. 2 (ed. J. O'M. BOCKRIS),p. 265. Butterworths, London (1959).
Corrigenda
In ref. 18, equation 11 and the abscissa of Fig. 6 should contain the term 2.3/q rather than 1/2.3q as indicated. For Fig. ld the ordinate scale should be calibrated as 50, 100, 150 ~.A.
0010-938X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
EFFECTS OF A L L O Y E D M O L Y B D E N U M ON T H E KINETICS OF R E P A S S I V A T I O N ON AUSTENITIC STAINLESS STEELS P. I. MARSHALLand G. T. BURSTEIN Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, U.K. Abstract--The kinetics of repassivation of Type 316L austenitic stainless steel in aqueous solutions after in situ generation of the metal surface have been obtained. The results are compared with those obtained
previously from similar experiments on Type 304L stainless steel. In alkaline and slightly acidic solutions both alloys repassivate at a rate controlled by ion migration through the growing oxide film under high electric field. The presence of alloyed molybdenum accelerates the rate in the early stages of film growth but in the later stages the kinetics for the two materials are identical. In strongly acidic solutions dissolution of the iron component of the oxide film accompanies film growth. This constructive dissolution process occurs at a rate controlled by the diffusion of iron ions through the oxide matrix. While the presence of molybdenum does not affect the diffusion coefficient significantly, it does allow the release of iron ions to the solution to commence at a far earlier stage of film growth, thereby hastening the onset of passivity. The associated kinetic parameters are presented and the implications of the proposed mechanisms discussed.
INTRODUCTION
THE EFFECTSof alloyed molybdenum in improving the passivity and pitting resistance of austenitic stainless steels is a long documented phenomenon.l-4 Despite the many researches aimed at understanding the precise role played by molybdenum in these processes 5-7 no definitive mechanism has yet been provided. One of the intriguing aspects of the phenomenon is that relatively little Mo is required to improve the corrosion resistance quite markedly as is exemplified by the comparison between Types 304L and 316L stainless steels. 1,4 Some higher grades of stainless steels exist, however, where the Mo content can be as high as 6.5 wt%.8 It is interesting to note that some nickel based alloys also contain considerable quantities of alloyed Mo and the effects may be considered similar. 9 Auger electron spectroscopy (AES) and X-ray photo-electron spectroscopy (XPS) have both demonstrated the presence of Mo in the passive film on stainless steels containing the element although the outer surfaces (i.e. at the film-electrolyte interface) of such films are apparently depleted in Mo. 6,7,10,11 The same is true for the passive films on nickel-molybdenum alloys in which Mo is found immediately below the film surface.12'13The fact that Mo is depleted from the outer surfaces of such films is consistent with equilibrium thermodynamics,14 at least for acidic and alkaline solutions, in that Mo(VI) is quite soluble over a wide range of potentials. The fact that Mo continues to exist within the films reflects only the non-equilibrium nature of the film in situ. Analysis by XPS has shown that at least some of the Mo in the passive Manuscript received 3 October 1983. 463
464
P.I. MARSHALLand G. T. BURSTEIN
film is in the f o r m o f M o (VI) 15 and that the metal side o f the metal-film interface may, in fact, be enriched in metallic M o . 6'11 Such observations must be interpreted with caution, h o w e v e r , since it can sometimes be difficult to distinguish inherent properties of the thin oxide films f r o m processes which have b e e n induced by the analytical technique.13 In particular, since no M o is o b s e r v e d on the o u t e r surface o f the film, such analyses must be m a d e on the interior o f the film, access to which is generally gained by argon ion b o m b a r d m e n t . This process can p r o d u c e drastic changes in oxide film c o m p o s i t i o n s ) 3 T h e actual presence o f M o in the film is, however, unambiguous. C u r r e n t ideas of the role of oxidized M o in the passive films of b o t h stainless steels and nickel based alloys suggest that M o m a y allow thicker films to be f o r m e d t h e r e b y reducing the passive current density 5 and that it m a y hold the oxide lattice t o g e t h e r m o r e tightly, 16 also reducing the passive current density. These theories m a y also explain the i m p r o v e d resistance o f the alloys to chloride-induced pitting since dissolution o f the film would then be m o r e difficult and pit nucleation retarded. Since the o u t e r surface of the film is free of MO 6'7'10'11 such retardation must occur only w h e n the o u t e r surface has already b e e n penetrated. T h u s it would in fact be p r o p a g a t i o n of the pit t h r o u g h the film which is retarded, rather than nucleation itself. Since p r o p a g a t i n g pits develop an internal electrolyte of low p H 17 the role of M o m a y well be in retarding acid attack of the film. Alternatively, it has b e e n p r o p o s e d that m o l y b d a t e anions m a y be f o r m e d by anodic oxidation of M o from the alloy. 7"15 That M o O 2- is in itself an inhibitor of pitting has b e e n d e m o n s t r a t e d by adding it to a chloride-containing electrolyte.IS All the a b o v e theories are largely qualitative in nature and few n o n - s t e a d y state kinetic m e a s u r e m e n t s have b e e n made. In previous papers 18,19'2°we have described the kinetics of growth of passive films on 304L stainless steel surfaces which were freshly g e n e r a t e d in situ. In neutral and alkaline solutions the films grow with no significant e n r i c h m e n t of any alloying element. In acidic solutions, 2° h o w e v e r , dissolution o f the Fe c o m p o n e n t of the film occurs the kinetics of which are c o u p l e d with those o f film growth. T h e film t h e r e b y b e c o m e s enriched in c h r o m i u m , and p e r h a p s Ni as well. Passivity is thus achieved by a film containing essentially c h r o m i u m oxides, at a rate controlled by the electric field across the film and the p H of the electrolyte. This p a p e r describes the effects o f alloyed M o on these processes as f o u n d by examination o f 316L austenitic stainless steel. E X P E R I M E N T A L METHOD The alloy used for the present results was an austenitic 316L stainless steel of composition (in wt%): Cr 16.8, Ni 12.4, Mo 2.4, C 0.03, Fe bal. It was used in the solution annealed condition. Apart from the Mo content, the composition is comparable with the previously used 304L stainless steel s-2o(Cr 18 3, Ni 10.6, C 0.03, Fe bal.); this enables comparison of the results of the two steels to be made. The kinetics of repassivation of 316L stainless steel were examined using the scratched rotating disc electrode under potentiostatic control. The technique has been described in other papers. 2°'2~It consists of generating a scratch rapidly in a rotating disc electrode made of the test material while the metal is held under potentiostatic control in the electrolyte solution. The scratch, which is created using a diamond stylus, was --1.5 mm long by --30 p,m wide by - 3 p,m deep. This depth is sufficient to penetrate completely the previously formed passive film on the disc thereby exposing bare metal to the electrolyte. The consequent current transient is recorded on a bank of transient recorders set to record the initial event and as much of the current decay as possible. A typical anodic current transient was presented earlier.IS Repassivation kinetics are defined in terms of the current density flowing from the scratch, i(t), as a
Effects of alloyed molybdenum on the kinetics of repassivation of austenitic stainless steels
465
function of the charge density that has flowed from the scratch, q(t), both measured at time t after scratching. These are given by i(t) -
1 [l(t) - Ib] 2~rrtoyt c
(1)
and q(t)
_12rrrtoytc Ii [l(t) -
Ib]dt
(2)
where to is the electrode rotation rate (100 Hz in this work), y is the scratch width (30/zm), r is its distance from the centre of rotation and tc is the contact time of the stylus ( ~ 1 ms). I(t) is the total absolute current flowing from the whole electrode, including the scratch, at time t and Ib is the absolute base current flowing from the rotating disc before scratching. The integral defined by equation (2) is thus measured by the area under the current-time transient. Transients were measured at ambient temperature (19 + 2°C) as a function of electrode potential for de-aerated electrolytes of varying pH. These electrolytes were: 1.0 M K O H , pH 14; 0.5 M H O A c , 0.5 M NaOAc, p H 4.8 and 3 electrolytes made from HCIO4 and NaCIO4 of pH 1.8, 1.4 and 0 to a total CIO4 concentration of 1.0 M. All chemicals were of a.r. grade except for NaCIO4 and were made up with double distilled water.
EXPERIMENTAL RESULTS In line with results obtained previously for 304L stainless stee118-20the repassivation kinetics are plotted as log i(t) as a function of q(t) -1. The rationale behind these plots is the fact that the repassivation rate of 304L stainless steel is consistent with the film growth kinetics being controlled by ion conduction through the growing oxide under high electric field. Thus i(t) = A exp [Bh--~)]
(3)
where V is the voltage drop across a film whose thickness is h(t) at time t and A and B are parameters related to the energy barrier through which the mobile ions pass. 22,23Appropriate rearrangement of equation (3) gives 18 log i(t) = log A + B z F p ( E - Eg) 2.3Mq(t)
(4)
where p is the density of the film, M its molecular weight, z the charge number on the current carrying ion, Eg is the equilibrium potential between the metal, the oxide and the solution and E is the electrode potential. Figure 1 shows the repassivation kinetics of 316L stainless steel for several values of E in electrolytes at pH 14 and 4.8 according to equation (4). After an initial period in which the plots are curved a linear region is encountered, consistent with equation 4, which extends over some 2-3 orders in i(t). The intercepts of Fig. 1 provide the parameter A from equation (4). These are listed in Table 1 and are invariant with E and pH, at least for 4.8 ~ pH -< 14. Differentiation of equation (4) gives 0 log i(t) _ B z F p ( E - Eg). Oq(t) -1 2.3M
(5)
466
P.I. MARSHALLand G. T. BURSTEIN
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FIG. 1. Repassivation of scratched 316L stainless steel by high field ion conduction (equation 4). (O) pH 14.0, E = -355 mV(NHE). (0) pH 14.0, E = -255 mV(NHE). (A) pH 4.8, E = +445 mV(NHE). (&) pH 4.8, E = + 1045 mV(NHE).
TABLE 1. pn 14.0
FILMGROWTHPARAMETERSA and B from equation 4 for repassivation of 316L stainless steel E[mV(NHE) ]
-155 -55 +45 +145 +245 4.8 +245 +445 +645 +845 +1005 0.0 +645 +745 +845 + 1045 +1145 Mean 304LIow current density data (Ref. 19).
log(A) (A cm -2 )
106B(cmV -t )
-5.5 -6.7 -5.7 -4.6 -6.0 -5.0 -5.7 -5.4 -6.2 -6.8 -7.1 -8.1 -10.5 -8.3 -11.2 - 5 . 9 + 0.5
6.1 6.7 5.4 5.4 6.3 4.7 5.6 5.1 3.1 5.9 28 26 25 25 31 4.4 + 2.2
Eg [mV(NHE)]
-800
-100
-25
Effects of alloyed molybdenum on the kinetics of repassivation of austenitic stainless steels I
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E [ mV(NHEII Fro. 2. High field kinetic slope, O log i(t)/Oq(t)-t, as a function of potential, E, for pH 14 (O) and pH 4.8 (A). Intercept with the abscissa gives Eg for 316L stainless steel in these electrolytes.
Plots of equation (5) in terms of the high field gradient as a function of E are shown in Fig. 2. These, too, are linear as predicted, and their slopes provide the value of B under the appropriate assumptions of z, p and M. The values used are the same as those assumed for 304L stainless steel, 18 with z = 2, p = 5.7 g cm -3 and M = 70 g mol -~. The calculated values of B are also listed in Table 1; these too, are independent of pH. The intercepts of Fig. 2 provide Eg (see equation 5) and are given in Table 1. Similar data obtained for repassivation of 304L stainless steel 18'19 showed two consecutive regimes in which equation (4) was obeyed; the first was a faster process occurring at lower values of q (t) and the second a slower one occurring at higher q (t). The present results on 316L steel are quantitatively consistent with the second of these regimes. A comparison of data obtained under identical conditions for the two steels is shown in Fig. 3. The low i(t) [high q(t)] kinetics for 304L steel are followed for the 316L steel extending through both the low and high current density regions. To show the similarity in these kinetics, corresponding mean values of A and B, which describe the linear regions of Figs. 1 and 3 completely, are also tabulated for the low i(t) kinetics of 304L steel in Table 1. The values of Eg determined for the two steels are plotted in Fig. 4 as a function of pH. The Eg/pH data for 316L steel lie, within the scatter of data points, on the Eg/pH line for 304L steel. Kinetic plots according to equation (4) are presented for 316L steel repassivating in acidic solutions in Fig. 5. At p H 0 the data still conform with equation (4) but the gradient is very much steeper than those observed at higher pH and the data show a far lower A value. The data for p H 1.4 and 1.8 show curvature in Fig. 5. To show the comparison between the behaviour for 316L and 304L steels, Fig. 6 shows the kinetics of repassivation for the two steels measured under identical conditions of E and pH. There is now a distinct difference between the repassivation rates of the steels.
468
P . l . MARSHALL and G. T. BURSTEIN
J
I
w
I00
o/°~° •
~ lO-J 5 ,q
KI x
/
o -- 10-2
...x
//
10.3
I
I00
FIG. 3.
I
I
150 q(t.) -I (cm 2 C-I )
200
Repassivation o f 304L ( x ) and 316L ( 0 ) stain]ess steels according to equation (4) in an electrolyte o f p H 14.0. E = - ] 5 5 m V ( N H E ) .
l
4OO
I
I
I
I
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I
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12
14
0
G ar
vZ > E. - 4 0 0
-800
0
FIG. 4.
L
I
I
i
2
4
6
8 pH
I
I0
o
E s as a f u n c t i o n of p H for 304L (C), Ref. 20) a n d 316L ( O ) stainless steels.
Effects of alloyed molybdenum on the kinetics of repassivation of austenitic stainless steels
I
I
I
4-
I
lO-I
"F
fa
I
•
/
E"
'E
'
10.2
g
/
o
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,J
.,J /"
rl t?
10"3
.kn
I
I
Ioo
I
2()o
30O
q ( t ) -I ( c m 2 C -I)
FIG. 5. Repassivation of scratched 316L stainless steel by high field ion conduction (equation 4) in acidic electrolytes. (~7) p H ] .4, E = +45 mV(NHE). (t-I) pH 1.8, E = + ]045 mV(NHE), (+) pH 0.0, E = +745 mV(NHE).
ioo
}
i0-1 A
9
/
<~ ~a i0-~=
r
&
f
/ ,,, f•~,
,,
//
i 0 -3
/
¢
,
10-4 I
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150
q l t l -I I c m 2 C-II FIG. 6.
Repassivation kinetics according to equation (4) for 304L ( A ) and 316L ( A ) stainless steel, pH = 0.0, E = +645 mV(NHE).
469
470
P . I . MARSHALLand G. T. BURSTEIN
DISCUSSION
Repassivation in alkaline and slightly acidic electrolytes It might well be expected that the repassivation rates of 304L and 316L stainless steels be similar since the Mo content of the latter is quite small (2.4 wt %, equivalent to 1.4 at%). This is, in fact, only true when the film has already grown to a certain thickness, dictated by the value of q (t) at which repassivation of 304L steel enters its second phase (see Fig. 3). The identity of these results indicates that the added Mo has negligible effect on the rate of growth of the film and probably on the film composition and structure as well, since variation in these properties would be expected to give measurable variation in A, B and Eg. Table I (and ref. 19) show that such variation as may occur is not measurable by this technique. Nevertheless, in the early decay region, where the film is still very thin [dictated by lower values of q (t)], the kinetics of repassivation of the two alloys are different. The early decay regime for 304L steel has not been fully explained, 1s'19and 316L steel shows no evidence of this regime (see Fig. 3). A consequence of this is that it actually requires more charge to reduce the bare surface current density to a particular value in this regime than is required by 304L steel, implying that the film must have had to acquire a greater thickness. This is seen by inspection of Fig. 3. For example, at pH 14, E = -155 mV(NHE), the charge density required to reduce the scratch current density to 0.1 A cm -2 is 5.75 mC cm -2 for 304L and 8 mC cm -2 for 316L. One can speculate 18that 304L steel forms two different films: the first formed occurs at high i(t) and is converted entirely into the second in the low i (t) region. Under this assumption 316L steel shows no evidence of the first formed film and all the linear regions of Fig. 1 relate only to growth of an oxide film similar to the second film of 304L steel. It is, however, quite possible that a film equivalent to the first formed on 304L does grow on 316L but its kinetics are lost in the very high current density regions of these high field plots. This would imply that alloyed Mo has accelerated formation of the first film. Inspection of Fig. 3 shows that the very high current density regions of 316L steel are consistently faster than those for 304L steel and the explanation seems sound. We thus conclude from this that the presence of Mo accelerates the onset of passivity, even though a greater charge density (and therefore thicker film) is required to do so. The possibility that Mo actually dissolves in the electrolyte giving rise to the increased charge densities for 316L steel must also be considered. Provided that Mo is not being preferentially oxidised in comparison with the other elements in the matrix then the difference in charge density for the two steels is far greater than that which would be accounted for by oxidation of Mo alone. Thus in Fig. 7 we plot this difference q ( t ) d i f f ---- q ( t ) 3 1 6 L -- q ( t ) 3 0 4 L
(6)
as a function of q (t)316L,where q(t)316Land q (t)304Lare the charge densities measured at constant t for 316L and 304L steels respectively. The graph shows that q(t)diff is a considerable fraction of q ( t ) 3 1 6 L and could not possibly be associated with oxidation of Mo alone. Repassivation of the two steels in these alkaline and slightly acidic solutions is thus kinetically similar once the initial period has been surmounted.
Effects of alloyed molybdenum on the kinetics of repassivation of austenitic stainless steels I
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Repassivation in strongly acidic electrolytes The repassivation kinetics of 316L steel in strongly acidic electrolytes are different from that in more alkaline solutions (see Fig. 5 and Table 1). This is demonstrated in the plot of repassivation kinetics for both 304L and 316L steels at pH 0, Fig. 6. The 304L kinetics have been analysed 2° in terms of film growth by equation (4) in the early regions, showing both kinetic regimes as described above. At a certain film thickness however, dissolution of the iron component of the film occurs causing the graph to swing to far higher values of q(t) than those expected from equation (4). The broken line in Fig. 6 thus represents the rate at which the film on 304L steel is actually growing and the corresponding solid line gives the film growth kinetics coupled with the iron dissolution kinetics. The kinetics of dissolution of Fe from the film were obtained 2° by considering the difference between the experimental data and the extrapolated lines. It should be noted that the 316L data in Fig. 6 actually tend towards the 304L data at very low current densities but at high current densities there is a large charge density difference. The data for 316L steel indicate that a similar dissolution process is occurring but that it is occurring at a rate far higher than that for the 304L steel, so high in fact, that the point at which it commences lies in the very high current region of the graph and is thereby obscured. Since the point at which dissolution commences during repassivation of 304L steel is pH dependent 2° (it occurs at constant electric field which decreases as pH increases) it is reasonable to expect that a higher pH may enable the appropriate electric field to be measured. For pH values up to 1.8 this field has not been found. The data measured at pH 1.4 and 1.8, however, do show curvature in the high field plots (Fig. 5), consistent with the region below -0.01 A cm -2 for 304L steel at pH 0. The lack of a suitable buffer solution of pH slightly greater than 1.8 prohibits examination of the full pH dependence. In light of the fact that for pH -> 4.8 the two steels grow similar films by similar kinetics (see above) it is possible to analyse the kinetics of repassivation of 316L steel in the strongly acidic electrolytes by assuming that the film growth kinetics are the same as for 304L steel (with the appropriate small adjustment in the value of B, see Table 1). The difference in the predicted film growth kinetics and the actual data for
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q(t)-t(cm z C-') FIG. 8. Experimentally obtained repassivation kinetics of scratched 316L stainless steel. (rq) pH 1.8, E = +1045 mV(NHE), (A) pH 0.0, E = + 745 (NHE). The broken line and dotted line show predicted filmgrowth kinetics for the above respectively.
316L steel then provide the rate of dissolution of the iron c o m p o n e n t of the growing oxide film. To demonstrate this we show in Fig. 8 experimentally obtained repassivation kinetics of 316L steel and the predicted film growth kinetics using the mean value of A from Table 1, the m e a n value of B (for 316L steel) from Table I and the Eg value shown in Fig. 4. We then consider the charge density difference, Aq(t), between that observed for 316L steel [q(t)316L] and that obtained from the constructed film growth line [q(t)film ] as
Aq(t) -- q(t)a16L -- q(t)fitm (7) each measured at constant t. Plots of Aq(t) versus t ]/2 for the p H 1.8 electrolyte are shown in Fig. 9. The graphs conform to linearity and pass through the origin, at least for the time scale considered. The gradients of these lines do not show any systematic variation with potential, but rather, are scattered about a m e a n of aAq(t)
0t--~r~j316L = (6.5 + 3.5) X 10 -3 C c r n -2 s -1/z.
(8)
This parabolic rate constant is quantitatively comparable with that obtained for 304L steel 2° for which
aAq(t)]
0 - - ~ - J304L
=
(2.9 + 1.3) X 10 -3 C cm -2 s -1/2.
(9)
Effects of alloyed m o l y b d e n u m on the kinetics of repassivation of austenitic stainless steels 0.010
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t I/a(s I/a) Flo. 9. Dissolution kinetics of iron from the growing oxide film on 316L stainless steel in an electrolyte of p H 1.8. ( 0 ) E = +645 m V ( N H E ) . (13) E = +745 m V ( N H E ) . (11) E = +845 m V ( N H E ) . ((3) E = +1045 m V ( N H E ) .
The value was found to be independent of E and pH, within the scatter of the data. These dissolution kinetics are interpretable in terms of rate control by non-steady state diffusion of iron ions through the oxide lattice. The boundary conditions and analysis of this solution to Fick's second law were presented previously 2° and the solution is given by
Aq (t) = 2zFrr t/2o 1/2(t - to) 1/2Co
(10)
where Co is the bulk concentration of Fe ions in the oxide lattice, D is their diffusion coefficient in the oxide and to is the time after scratching at which dissolution commences. The fact that the kinetics of 316L pass through the origin means that to is very small and is therefore not detected, either in Fig. 9 or in Fig. 5. Equation (10) allows computation of D using equation (8). In line with that for 304L steel, assuming z = 2 (Fe 2÷) and Co = 0.059 mol cm -3 (from the density of the alloy) we obtain D = 2.5 x 10 -13 cm 2 s -1, which, within the scatter, is again similar to that for 304L steel. 2° We can now derive the electric field at which this dissolution process commences in line with the measured fields for 304L steel. 2° To do this we plot Aq(t) as a function of q(t)316L and extrapolate to Aq(t) = 0. Under these conditions q(t)316L = q(t)tilm (see equation 7). Typical plots are shown in Fig. 10 and they are linear. At Aq(t) = 0 the value of q(/)316L, denoted by qd provides the electric field in terms of (E - Eg)/qd at which deviation from high field film growth would be expected to be observed. A plot of this electric field is shown in Fig. 11 as a function of pH, together with the corresponding data measured for 304L steel.2° The field for 316L steel is considerably higher for the same pH. The scratch current density which flows at this field is dictated by equation (4): for the p H 1.4 and 1.8 electrolytes it has a value of - 0 . 1 1 A cm -2, which is in fact, still just within the curved regions at very high current in the high field plots (see Fig. 5). Thus a less acidic electrolyte would be required to observe the field at which deviation occurred, similar to that observed for 304L steel at p H 0 (see Fig. 6). Figure 10 shows a further important feature. The values of qd [for which Aq(t) = 0] all lie above 4 mC cm -2 and since this anodic charge density is associated
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FI~. 10. Plot of the excess charge Aq (t) (above that necessary for film growth) flowing from the repassivating 316L stainless steel electrode as a function of the total charge flowing q(/)316L- At Aq(t) = 0, q(/)316L is the charge density at which deviation from high field kinetics occurs (qd). ( I ) pH 1.8, E = +655 mV(NHE). (~7) pH 1.4, E = +655 mV(NHE). (IS]) pH 1.8, E = +855 mV(NHE). ( V ) pH 1.4, E = +855 mV(NHE). (©) pH 1.8, E = + 1055 mV(NHE).
exclusively with film formation, and no dissolution is involved to this point, the film coverage on the scratch surface must be far greater than a monolayer, even if a generous monolayer charge density is assumed. (In previous work ~8'19'2° we estimated that - 0 . 5 mC cm -2 of anodic charge is required to produce a monolayer of oxide.) Thus the dissolution of iron which follows occurs only from the film itself, and not directly from the bare metal surface. This deduction is consistent with observations made for repassivation of 304L stainless steel. 2° It is possible to apply the same analysis to repassivation of 316L steel in the pH 0 electrolyte. Plots of Aq (t) against t lr2 are shown in Fig. 12. The diagram is not directly comparable with Fig. 9 (for the pH 1.8 electrolyte). It shows a very rapid rise to high values of Aq(t) followed by tailing off as t increases. I n this case the rate of iron dissolution is so great that through most of the time documented in Fig. 12 the growing film is essentially depleted of iron, and the rate of further dissolution is now controlled by the rate of film growth itself. We can demonstrate that this is true by plotting the value of Aq(t) as a function of q(t)316L (similar to Fig. 10) and showing that the charge density which represents the amount of film dissolved, is an approximately constant proportion of the total charge density involved and that this fraction is what would be expected if it represented most of the iron component of the entire film. The data are shown in Fig. 13. In Fig. 13(a) it is seen that Aq(t) is linearly dependent on q (t)316Land the lines pass through the origin. The ratio of Aq (t)lq (t)316L is thus independent of q(t)316L as is shown in Fig. 13(b). Thus for these conditions a constant proportion of the total charge density passed at any time has dissolved in the
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solution. The fact that the data show a small dependence on electrode potential may arise simply from scatter in the original current transients or may represent the fact that a little iron does remain in the film. One can estimate what fraction of the total anodic charge density is associated with oxidation of Fe only by assuming the oxidation number of each of the metal ions in the film. To obtain an idea of this we assume the following charge numbers: Fe(II), Cr(III), Ni(II) and Mo(III). Under these circumstances a fraction of 0.64 of the total anodic charge density evolved is associated with oxidation of Fe, and this is also marked in Fig. 13(b). Clearly, if some or all of the Fe is in the form of Fe(III), this fraction is greater. It is nevertheless seen
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Fro. 13. Plot of dissolution charge [Aq(t)], Fig. 13(a), and the ratio hq(t)lq(t)3t6L, Fig. 13(b), as a function of the total measured charge q(l)316L for repassivationof 316L stainless steel at pH 0.0. (O) E = +845 mV(NHE). (I-1)E = +945 mV(NHE). (x) E = +1045 mV(NHE). from Fig. 13(b) that for reasonable assumptions of these charge numbers, the proportion of the film being dissolved into the acidic electrolyte of p H 0 does indeed represent approximately the entire iron component. It is striking that the repassivation kinetics measured under these conditions, where the dissolution component is so fast that its rate is controlled simply by the rate of growth of the iron free film, still follows equation (4) (see Fig. 5), although the associated parameters have been apparently changed. This phenomenon is, however, easily derived. Using the same symbols as in equation (7), we rewrite equation (4) as
[ B z F p ( E - E~)]. i(t) = A exP i Mq(t)sm J
(11)
Under conditions where all the iron dissolves from the film as fast as the film is being formed then Aq(t) is a constant fraction, f, of q(/)316L, and using equation (7)
[ B z F o ( E - Eg) 1i(t) = A exp [Mq(t)316L(1 -- f ) ]
(12)
Thus Fig. 5 still shows linearity but its slope has been modified by the factor
Effects of alloyed molybdenumon the kineticsof repassivationof austenitic stainlesssteels
477
(1 - f ) - l . The factor f w o u l d have a value of 0.64 if the charge numbers on the ions were as described above. It can be equal to the atom fraction of iron in the alloy only when the charge numbers on all the oxidised metal atoms are equal. The loss of the iron component of the growing oxide films on these steels means that the remaining components of the film must rearrange so as to bring about passivity, since the final state of the scratched metal is a return to the state of passivity. We have termed this 'constructive dissolution '2° in order to distinguish it from dissolution processes which result in rapid corrosion of the material. Not only is the dissolution p h e n o m e n o n common to both 304L and 316L stainless steels, but the r a t e of dissolution is the same for both alloys: it is controlled by the diffusion rate of iron ions in the film to the film electrolyte interface. The presence of Mo in the alloy (and consequently in the film) has no detectable effect on this diffusion rate, as described by the calculated diffusion coefficients given above. The presence of Mo in the steel does, however, have a pronounced effect on the electric field across the film at which this dissolution process commences. It allows dissolution of Fe from the film to start at a far earlier stage in the growth of the film (given by the higher electric field), and it is, perhaps, surprising that such a small level of Mo in the alloy produces such a large effect. Because of the fact that iron is dissolved from the film at an earlier stage when Mo is present the enrichment of Cr in the film also occurs at an earlier stage and thus the metal acquires its passivity more rapidly, since it is ultimately the chromium oxide which passivates the metal. For this reasoning to be valid one would expect that the passive film would always retain at least some of the Mo, and this is consistent with surface analyses of such films. 5'6A1 The outer surface of the final passive film has however, been found to be free of M o 6'7'1°'11 and this is probably also lost by dissolution. If the field across that region of the film which contains Mo is a little stronger than the outermost region which may be free of Mo then relaxation of the field by virtue of film growth will occur more quickly at the outer surface than it would if all the film were free of Mo. Since dissolution of Fe commences at a particular field (dependent on the electrolyte pH) then this field would be reached at an earlier stage for repassivation of 316L steel than for 304L steel, consistent with observation. Such behaviour would not significantly affect the assumptions made above since these effects are confined only to the outermost layer of the film. The same arguments apply to pitting of these materials by chloride ions. It is widely accepted that attack of the material by CI- causes acidification of the local electrolyte within the pit. If it is assumed that the attack by CI- occurs primarily on the Fe ions within the film then depletion of the Fe ions by an alternative path within the film should retard further attack. This path is provided by the acid dissolution of the Fe component. The presence of Mo accelerates this process allowing an incipient pit to repassivate by chromium oxides more rapidly. Thus pitting of austenitic stainless steels is rendered more difficult. Moreover, such a mechanism accounts for the observed synergistic effects of Mo with Cr in improving passivity and pitting resistance.5'11'16 In particular since the presence of Mo allows more rapid dissolution of the Fe component of the oxide, alloys of Fe and Mo which do not contain Cr may well have poorer corrosion resistance than Fe itself, and this, too, is consistent with previous r e p o r t s )
478
P.I. MARSHALLand G. T. BURSrEIrq
CONCLUSIONS 1. R e p a s s i v a t i o n o f 316L stainless steel is c o n t r o l l e d b y o x i d e film g r o w t h u n d e r high electric field. T h e a s s o c i a t e d k i n e t i c p a r a m e t e r s a r e s i m i l a r to t h o s e o b s e r v e d for t h e l o w e r c u r r e n t kinetics o f 304L stainless steel. 2. T h e h i g h e r c u r r e n t r e p a s s i v a t i o n kinetics o f 304L steel ( w h e r e t h e film is still thin) a r e n o t o b s e r v e d for r e p a s s i v a t i o n o f 316L steel. F o r a l k a l i n e s o l u t i o n s this p r o v i d e s an a c c e l e r a t e d initial r a t e o f r e p a s s i v a t i o n . 3. In acidic e l e c t r o l y t e s r e p a s s i v a t i o n o f 316L stainless steel b y o x i d e film g r o w t h is a c c o m p a n i e d b y d i s s o l u t i o n o f t h e i r o n c o m p o n e n t o f t h e film. T h e r a t e o f this p r o c e s s , which is c o n t r o l l e d b y t h e diffusion coefficient o f i r o n ions in t h e m i x e d o x i d e lattice, is t h e s a m e as t h a t o b s e r v e d for 304L stainless steel. T h e p r e s e n c e o f M o in t h e o x i d e has no significant effect o n this r a t e . 4. T h e d i s s o l u t i o n o f t h e i r o n c o m p o n e n t o f t h e o x i d e film d u r i n g r e p a s s i v a t i o n o f 316L stainless steel c o m m e n c e s at a c o n s t a n t e l e c t r i c field which is d e p e n d e n t u p o n t h e p H o f t h e e l e c t r o l y t e . T h e p r e s e n c e o f M o in t h e steel raises t h e e l e c t r i c field ( w h e n c o m p a r e d with 304L steel) a n d thus d i s s o l u t i o n c o m m e n c e s e a r l i e r in t h e r e p a s s i v a t i o n p r o c e s s . In o t h e r w o r d s , at a n y p a r t i c u l a r stage d u r i n g r e p a s s i v a t i o n m o r e F e has d i s s o l v e d f r o m t h e film o n 316L steel t h a n f r o m t h e c o r r e s p o n d i n g film on 304L steel. 5. This c o n s t r u c t i v e d i s s o l u t i o n p r o c e s s , w h i c h is a i d e d b y t h e p r e s e n c e o f M o , a c c e l e r a t e s t h e o n s e t o f passivity a n d thus i m p r o v e s t h e r e s i s t a n c e to c h l o r i d e i n d u c e d pitting corrosion of the material.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
15. 16. 17. 18. 19. 20. 21. 22. 23.
REFERENCES Y. M. KOLOTYRKIN,Corrosion 19, 261t (1963). M. A. S~EXCHER,J. electrochem Soc. 103,375 (1956). J. HARVATHand H. H. UaLIG, J. electrochem Soc. 115, 791 (1968). B. E. WILDEand N. D. GREEN,Corrosion 25,301 (1969). K. SUGIMOTOand Y. SAWADA,Corros. Sci. 17,425 (1977). I. OLEFJORDand B. O. ELFSTROM,in 'Eurocor 77', p. 21. Society of Chemical Industry, London (1977). H. OGAWA,H. OMATA,I. ITOHand H. Or,ADA,Corrosion 34, 52 (1978). A. J. SEDRICKS,Corrosion of Stainless Steel, p. 9. John Wiley, New York (1979). W. BETI'ERIDGE,Nickeland its Alloys, p. 61. Macdonald and Evans Ltd., London (1977). J. B. LUMSDENand R. W. STAEHLE,Scripta Met. 6, 1205 (1972). I. OLEFJORD,Mater. Sci. Engng. 42, 161 (1980). G.T. BURSTEINand T. P. HOAR,Corros. Sci. 17,939 (1977). G. T. BURSTEIN,Mater. Sci. Engng. 42, 207 (1980). M. POURBAIX,Atlas of Electrochemical Equilibria in Aqueous Solution, p. 272. Pergamon Press, Oxford (1966). K. SUGIMOTOand Y. SAWADA,Corrosion 32,347 (1976). E. A. LIZLOVSand A. P. BOND,J. electrochem Soc. 118, 23 (1971). G. W. PETERSOH,G. C. SOLTZand K. MAIRS,Corrosion 30, 366 (1974). G. T. BURSTEINand P. I. MARSHALL,Corros. Sci. 23, 125 (1983). P. I. MARSHALLand G. T. BURSTEIN,Corros. Sci. 23, 1219(1983). G.T. BURSTEINand P. I. MARSHALL,Corros. Sci. 24,449 (1984). F. P. FORD,G. T. BURS~INand T. P. HOAR,J. electrochem Soc. 127, 1325 (1980). M. J. DI~NAM,in Oxides and Oxide Films (ed. J. W. DIGGLE),Vol. 1, p. 91. Marcel Dekker, New York (1972). T. P. HOAR, in Modern Aspects of Electrochemistry, No. 2 (ed. J. O'M. BOCKRIS),p. 265. Butterworths, London (1959).
Corrigenda
In ref. 18, equation 11 and the abscissa of Fig. 6 should contain the term 2.3/q rather than 1/2.3q as indicated. For Fig. ld the ordinate scale should be calibrated as 50, 100, 150 ~.A.