PASSIVITY OF ALLOYS

6.8 PASSIVITY OF ALLOYS

Several metals, such as chromium, are naturally passive when exposed to the atmosphere, and they remain bright and tarnish - free for years, in contrast to iron and copper, which corrode or tarnish in short time. It is found that the passive property of chromium is conferred on alloys of Cr – Fe, provided that ≥ 12 wt.%

Cr is present. Iron - base alloys containing a minimum of 12 wt.% Cr are known as the stainless steels .

Typical corrosion, potential, and critical - current density behavior of Cr – Fe alloys is shown in Figs. 6.9 – 6.11 . Note in Fig. 6.11 that i critical for passivation of Cr – Fe alloys at pH 7 reaches a minimum at about 12% Cr in the order of 2

μ A/ cm 2 . This value is so low that corrosion currents in aerated aqueous media easily achieve or exceed this value, illustrating why > 12% Cr – Fe alloys are self - passivating. In addition, the passive fi lm becomes more stable with increasing chromium content of the alloy. Several other alloy systems exhibit critical compositions for passivity, as was fi rst described by Tammann [36] . Examples of approximate critical compositions

Figure 6.9. Corrosion rates of chromium – iron alloys in intermittent water spray at room temperature. [ Reprinted with permission from W. Whitman and E. Chappell, Ind. Eng. Chem. 18 , 533 (1926). Copyright 1926, American Chemical Society .]

PASSIVIT Y OF ALLOYS

Figure 6.10. Potentials of chromium – iron alloys in 4% NaCl. [ Reprinted with permission from H. Uhlig, N. Carr, and P. Schneider, Trans. Electrochem. Soc. 79 , 111 (1941). Copyright 1941, The Electrochemical Society .]

Figure 6.11. Critical current densities for passivation of chromium – iron alloys in deaerated 3% Na 2 SO 4 at pH 3 and 7, 25 ° C [8] ; data in 10% H 2 SO 4 , room temperature, are from R. Olivier,

102 PASSIVIT Y

determined from plots of i critical versus alloy composition are 35% Ni – Cu, 15% Mo – Ni, 8%Cr – Co, and 14% Cr – Ni. Critical alloy compositions for passivity have also been observed in three - and four - component alloys — for example, Fe – Cr –

Ni – Mo [37] , Fe – Ni – Mo, and Cr – Ni – Fe [38] . Critical compositions based on i critical values are not sensitive to the pH of the electrolyte in which the polarization data are measured (Fig. 6.11 ). On the other hand, critical compositions based on corrosion data usually vary with the environ-

ment to which the alloys are exposed. In 33% HNO 3 , for example, the critical chromium concentration for passivity of Cr – Fe alloys decreases to 7% Cr, whereas in FeSO 4 solution it increases to 20% Cr; only in aqueous solutions of about pH 7 is the critical composition equal to 12%. This effect of environment is to be expected because whether an alloy becomes passive depends on whether the corrosion rate equals or exceeds the specifi c i critical for passivity in the given

environment. As seen in Fig. 6.11 , increasing chromium content reduces i critical ; on the other hand, decreasing pH and increasing temperature increase i critical . The corrosion rate, in turn, depends on factors of metal and environment affecting the rate of both cathodic and anodic reactions.

It can be shown from fi rst principles [39] that i critical = K (H + ) λ where K and λ are constants, the values of which depend on the anion. The cathodic reaction rate in an aerated solution when controlled by oxygen reduction, on the other

hand, is given by the diffusion current i 1

diff

corresponding to the reaction 2 O 2 +

H 2 O → 2OH − − 2e − . For an unstirred air - saturated solution, it can be shown that diff i

= 0.039 mA/cm 2 (0.39 A/m 2 ). Hence, a critical pH exists for each alloy com- position at which i critical diff = i and above which, but not below, passivity is stable. The observed critical pH values, for example, of 18% Cr, 8% Ni, and of 12% Cr

stainless steels in 0.1 M Na 2 SO 4 [40] are 1.4 and 5.0, respectively, in close agree- ment with the calculated values. With iron, for which i critical is much higher than for the stainless steels, the critical pH is approximately 10.

The structure of the passive fi lm on alloys, as with passive fi lms in general, has been described both by the oxide - fi lm theory and by the adsorption theory. It has been suggested that protective oxide fi lms form above the critical alloy composition for passivity, but nonprotective oxide fi lms form below the critical composition. The preferential oxidation of passive constituents (e.g., chromium)

may form protective oxides (e.g., Cr 2 O 3 ) above a specifi c alloy content, but not below. No quantitative predictions have been offered based on this point of view, and the fact that the passive fi lm on stainless steels can be reduced cathodically,

but not stoichiometric Cr 2 O 3 itself, remains unexplained.

By the adsorption theory, it is considered that, in the presence of water, oxygen chemisorbs on chromium – iron alloys above the critical composition cor- responding to passivity, but immediately reacts below the critical composition to form a less protective or nonprotective oxide fi lm. Whether the alloy favors a chemisorbed or reaction - product fi lm depends on the electron confi guration of the alloy surface, in particular on d - electron interaction. The electron confi gura- tion theory describes the specifi c alloying proportions corresponding to a favor-

able d - electron confi guration accompanying chemisorption and passivity. It

PASSIVIT Y OF ALLOYS

suggests the nature of electron interaction that determines which alloyed com- ponent dominates in conferring its chemical properties on those of the alloy — for example, why the properties of nickel dominate above those of copper in the nickel – copper alloys at > 30 – 40% Ni.

6.8.1 Nickel – Copper Alloys

Nickel, containing 0.6 d - electron vacancy per atom (as measured magnetically), when alloyed with copper, a nontransition metal containing no d - electron vacan- cies, confers passivity on the alloy above approximately 30 – 40 at.% Ni. Initiation of passivity beginning at this composition is indicated by corrosion rates in sodium chloride solution (Figs. 6.12 and 6.13 ), by corrosion pitting behavior in seawater (Fig. 6.13 ), and, more quantitatively, by measured values of i critical and

passive i (Fig. 6.14 ), [41 – 43] or by decay (Flade) potentials (Fig. 6.15 ) [44] in 1 N

H 2 SO 4 . Corrosion pitting in seawater is observed largely above 40% Ni because pit growth is favored by passive – active cells (see Section 6.5 ), and such cells can operate only when the alloy is passive — that is, in the range of high nickel com- positions. Practically, this distinction is observed in the specifi cation of materials for seawater condenser tubes in which pitting attack must be rigorously avoided. The cupro nickel alloys are used (10 – 30% Ni), but not Monel (70% Ni – Cu).

Similarly, marine fouling organisms are much less successful in establishing themselves on the surface of nonpassive nickel – copper compositions because the

Figure 6.12. Corrosion rates of copper – nickel alloys in aerated 3% NaCl, 80 ° C, 48 - h tests (M.I.T. Corrosion Lab.).

104 PASSIVIT Y

Figure 6.13. Behavior of copper – nickel alloys in seawater [F. LaQue, J. Am. Soc. Nav. Eng. 53 , 29 (1941)] .

Figure 6.14. Values of critical and passive current densities obtained from potentiostatic anodic polarization curves for copper – nickel alloys in 1 N H 2 SO 4 , 25 ° C [42] . ( Reproduced with permission. Copyright 1961, The Electrochemical Society .)

PASSIVIT Y OF ALLOYS

Figure 6.15. Potential decay curves for nickel – copper and nickel – copper – zinc alloys in 1 N H 2 SO 4 , 25 ° C (two time scales). Pure copper behaves like Alloy D [36] .

latter corrode uniformly at rates that release enough Cu 2+ to poison fouling organisms. * But on passive nickel – copper compositions, for which the overall corrosion rate is much less, fouling organisms in general can gain a foothold and fl ourish (Fig. 6.13 ).

Potentiostatic anodic polarization behavior of nickel – copper alloys in 1 N

H 2 SO 4 (Fig. 6.14 ) establishes that the passive current density largely disappears above 60% Cu and vanishes completely at about 70% Cu. Polarization curves of alloys containing > 70% Cu or < 30% Ni resemble those of pure copper; hence, such alloys are not passive. Potential decay curves (Fig. 6.15 ) confi rm that a passive fi lm is formed on anodically passivated alloys containing > 40% Ni, but not otherwise. In other words, alloys containing copper above the critical com- position lose their transition - metal characteristics; that is, they no longer contain

d - electron vacancies. In this connection, the magnetic saturation moment, which is also a function of d - electron vacancies in the alloy, becomes zero at > 60% Cu. This observation is interpreted as a fi lling of vacancies in the d band of electron energy levels of nickel by electrons donated by copper. Physicists [45] have made the assumption that, if copper and nickel atoms are considered to be alike, except that copper contains one more electron per atom than nickel, then the 0.6 d -

electron vacancy per nickel atom is expected, as observed, to be just fi lled by electrons from copper at 60 at.% Cu.

* The minimum concentration of Cu 2+ required to poison marine organisms corresponds to a corro- sion rate for copper of about 0.001 ipy or 0.5 gmd [F. LaQue and W. Clapp, Trans. Electrochem. Soc. 87 , 103 (1945)].

106 PASSIVIT Y

One can also start with the alternative assumption that the two atoms main- tain their individuality and that the vacancies per atom of nickel are a function of alloy concentration [46] . In the gaseous state, nickel has the confi guration of

3d 8 4s 2 , corresponding to two d - electron vacancies or to the equivalent of two uncoupled d - electrons in the third shell of the atom. The maximum number of

d - electrons that can be accommodated is 10, corresponding to copper: 3 d 10 4 s . In the process of condensing to a solid and forming the metallic bond, the uncoupled electrons of any single nickel atom tend to couple with uncoupled electrons of neighboring atoms. This results in a smaller number of electron vacancies in the solid compared to the gas, accounting for the measured 0.6 vacancy or 0.6 uncoupled electron per nickel atom. If we assume that the intercoupling of d -

electrons increases with proximity of nickel atoms in the alloy and is a linear function of nickel concentration, then the vacancies per nickel atom can be set equal to 2 − (2 − 0.6) at.% Ni/100, corresponding to two vacancies for 0% Ni and

0.6 vacancy for 100% Ni. The alloy loses its transition - metal characteristics beginning at the composition for which the total number of d - vacancies equals the total number of donor electrons (one per copper atom), or at.% Ni (2 − 0.014 at.% Ni) = 1 × at.% Cu. Setting at.% Cu = (100 − at.% Ni) and solving, the critical composition is found to be 41 at.% Ni. This value corresponds closely to the observed value derived from magnetic saturation data.

This model was checked by alloying small amounts of other nontransition elements Y, or transition elements Z, with nickel – copper alloys and noting the specifi c compositions at which i critical and i passive merged or at which Flade poten- tials disappeared. Non - transition - metal additions of valence > 1 should shift the critical composition for passivity to higher percentages of nickel, whereas transition - metal additions should have the opposite effect. For example, one zinc atom of valence 2 or one aluminum atom of valence 3 should be equivalent in the solid solution alloy to two or three copper atoms, respectively. This has been confi rmed experimentally [47] . The relevant equations become

at.% Ni ( − 2 0 014 . at.% Ni ) =× 1 at.% Cu +n at.% Y and at.% Ni ( − 2 0 014 . at.% Ni ) + v at.% Z =× 1 at.% Cu where n is the number of electrons donated per atom of Y and v is the number

of vacancies introduced per atom of Z. By plotting 1 × at.% Cu − at.% Ni (2 − 0.014 at.% Ni) with n at.% Y, a straight line is predicted of unit negative slope for nontransition - element additions. If plotted instead with v at.% Z for transition - metal additions, a unit positive slope should result. A plot for the alloying additions so far studied is shown by data summarized in Fig. 6.16 [46] . In order for the line to pass through the origin with unit slope, it was necessary to assume approximately 80% instead of 100% dona- tion of valence electrons. This means that valence electrons from copper and

PASSIVIT Y OF ALLOYS

Figure 6.16. Plot of excess electrons or d - electron vacancies in nickel – copper alloys at their critical compositions versus electron vacancies, or electrons donated by alloying additions, unit slope.

from other nontransition elements are presumably donated in major part to nickel, but not entirely. Assuming 0.8 electron donor per copper atom in binary nickel – copper alloys, the critical nickel composition below which the d - band is fi lled becomes 35 at.% instead of 41 at.% as calculated earlier. * This value is consistent with the composition at which i passive and i critical intersect in Fig. 6.14 .

Values of n for germanium, aluminum, and zinc shown in Fig. 6.16 are 4, 3, and 2, respectively, in accord with their normal valence. For gallium, a valence of 2 is in better accord with the other data, refl ecting perhaps the known tendency of gallium to form chemical compounds having a valence lower than 3, but other explanations are also possible. For iron and cobalt, the number of vacancies per

atom is equated to v g − (v g − v s ) at.% Z/100, where v g and v s are d - electron vacan- cies per atom in the gas and solid, respectively. Values of v g for iron and cobalt

are 4 and 3, and for v s they are 2.2 and 1.7, respectively.

There are no observed phase changes or major discontinuities in the thermo- dynamic properties of nickel – copper alloys at 60 – 70% Cu, whereas chemisorp- tion on any metal is known to be favored by an unfi lled d - band confi guration

[48] . The good agreement, therefore, between observed and predicted critical

* The calculated number of d vacancies per Ni atom at this composition equals 2 − 0.014 × 35 = 1.51, which is close to the value 1.6 assumed earlier [33, 39] .

PASSIVIT Y

alloy compositions supports not only an effect of electron confi guration on pas- sivity, but also an adsorbed structure of the passive fi lm.

6.8.2 Other Alloys

Because present - day theory of the metallic state does not treat the situation, the electron confi guration of alloys made up of two or more transition metals with relation to their passive behavior is not as well understood as for the copper –

nickel system. Nevertheless, useful simplifying assumptions can be made. For example, the most passive component of an alloy is assumed to be the acceptor element, which tends to share electrons donated by the less passive components.

Accordingly, for stainless steels, the d - electron vacancies of chromium are assumed to fi ll with electrons from alloyed iron [41] . At the critical composition at which vacancies of Cr are apparently fi lled, which occurs for alloys containing less than 12% Cr, the corrosion behavior of the alloy is like that of iron. Above 12% Cr, the d - electron vacancies of chromium are unfi lled, and the alloy behaves more like chromium.

The critical compositions for passivity in the Cr – Ni and Cr – Co alloys, equal to 14% Cr and 8% Cr, respectively, can also be related to the contribution of electrons from nickel or cobalt to the unfi lled d - band of chromium [49] . In the ternary Cr – Ni – Fe solid solution system, electrons are donated to chromium mostly by nickel above 50% Ni, but by iron at lower nickel compositions [50] . Similarly, molybdenum alloys retain in large part the useful corrosion resistance of molybdenum (e.g., to chlorides) so long as the d - band of energy levels for molybdenum remains unfi lled. In Type 316 stainless steel (18% Cr, 10% Ni, 2 –

3% Mo), for example, the weight ratio of Mo/Ni is best maintained at or above 15/85, corresponding to the observed critical ratio for passivity in the binary molybdenum – nickel alloys equal to 15 wt.% Mo [51] . At this ratio or above, passive properties imparted by molybdenum appear to be optimum.