ESTUDIO DE CASOS: ALGUNOS RESULTADOS PROVISIONALES

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 i passive (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.).

PASSIVIT Y OF ALLOYS 103

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 .)

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.

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] .

* 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)].

PASSIVIT Y OF ALLOYS 105

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 3 d ⁸ 4 s 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+ nat.% Y 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

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

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.

* 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 OF ALLOYS 107

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