Nickel alloys passivity

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Metals and alloys vary in their ability to resist crevice corrosion, and this applies particularly to those that rely on passivity for their resistance to corrosion. Titanium and high-nickel alloys such as the Inconels and Hastel-loys are amongst the most resistant, but even these will be attacked under highly aggressive environmental conditions. 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Amorphous Fe-3Cr-13P-7C alloys containing 2 at% molybdenum, tungsten or other metallic elements are passivated by anodic polarisation in 1 N HCl at ambient temperature". Chromium addition is also effective in improving the corrosion resistance of amorphous cobalt-metalloid and nickel-metalloid alloys (Fig. 3.67). The combined addition of chromium and molybdenum is further effective. Some amorphous Fe-Cr-Mo-metalloid alloys passivate spontaneously even in 12 N HCl at 60° C. Critical concentrations of chromium and molybdenum necessary for spontaneous passivation of amorphous Fe-Cr-Mo-13P-7C and Fe-Cr-Mo-18C alloys in hydrochloric acids of various concentrations and different temperatures are shown in Fig. 3.68 ... 

Pitting of nickel and nickel alloys, as of other metals and alloys, occurs when passivity breaks down at local points on the surface exposed to the corrosive environment, at which points anodic dissolution then proceeds whilst the... 

In practice, pitting of nickel and nickel alloys may be encountered if the corrosive environment contains chloride or other aggressive ions and is more liable to develop in acidic than in neutral or alkaline solutions. In acidic solutions containing high concentrations of chloride, however, passivity is likely to break down completely and corrosion to proceed more or less uniformly over the surface. For this reason nickel and those nickel alloys which rely on passivity for their corrosion resistance are not resistant to HCl. 

Nickel and nickel alloys possess good resistance to sea-water in conditions where the protective properties of the passive film are fully maintained. As pointed out above, Ni-30 Cu Alloy 400, in contrast to its behaviour in acidic solution, normally forms a protective film in neutral and alkaline environments, including sea-water this alloy and its age hardening variant... 

The wide range of corrosion-resistant nickel alloys that are produced commercially is capable in practice of handling most types of acid. Since the nickel-alloy range includes some that are corrosion resistant by virtue of their relative nobility and others that owe their resistance to passivity, alloys suitable both for hydrogen-evolving acids and for more oxidising acids are available. Table 4.27 contains a summary of data mainly derived from laboratory corrosion tests to illustrate the behaviour of individual alloys in some common mineral and organic acids. 

In terms of the Pourbaix potential/pH diagrams, the theoretical scale compares the potentials of immunity of the different metals, while the practical scale compares the potentials of passivation. But this is not enough either. The real scale depends on the environment with which the structure will be in contact during service. Passivity, as we have seen, depends on pH. It also depends on the ionic composition of the electrolyte, particularly the concentration of chloride ions or other species that are detrimental to passivity. Finally, one must remember that construction materials are always alloys, never the pure metals. The tendency of a metal to be passivated spontaneously can depend dramatically on alloying elements. For example, an alloy of iron with 8% nickel and 18% chromium (known as 304 stainless steel) is commonly used for kitchen utensils. This alloy passivates spontaneously and should be ranked, on the practical scale of potentials, near copper. If... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

In the case of the nickel alloys, the stability of the passive layer is a problem. The alloys depend on the oxide films or the passive layers for corrosion resistance and are susceptible to crevice corrosion. The conventional mechanism for crevice corrosion assumes that the sole cause for the localized attack is related to compositional aspects such as the acidification or the migration of the aggressive ions into the crevice solution [146]. These solution composition changes can cause the breakdown of the passive film and promote the acceleration and the autocatalysis of the crevice corrosion. In some cases, the classic theory does not explain the crevice corrosion where no acidification or chloride ion build up occurs [147]. 

K. Tachibana and M.B. Ives, Selective Dissolution Measurements to Determine the Nature of Films on Nickel-Molybdenum Alloys, Passivity of Metals, The Electrochemical Society, 1978, p 878-897... 

Although most metals display an active or activation controlled region, when polarised anodically from the equilibrium potential, many metals and perhaps even more so alloys developed for engineering applications, produce a solid corrosion product. In many examples the solid is an oxide that is the stable phase rather than the ion in solution. If this solid product is formed at the metal surface and has good intimate contact with the metal, and features low ion-conductivity, the dissolution rate of the metal is limited to the rate at which metal ions can migrate through the film. The layer of corrosion product acts as a barrier to further ion movement across the interface. The resistance afforded by this corrosion layer is generally referred to as the passivity. Alloys such as the stainless steels, nickel alloys and metals like titanium owe their corrosion resistance to this passive layer. 

It is possible for the passivation (oxide) layer on the surface of a metal to be continuously removed or not allowed to develop, by erosion from particulate matter or gas bubbles. Not only is the surface eroded but the removal of the protective oxide layer allows corrosion to take place. The problem is accentuated by the presence of an obstruction or debris, on the metal surface that diverts and accelerates the flow near the surface along a defined path. Sato et al [1977] report experimental data on erosion-corrosion resistance of condenser tubes fabricated from various cupro nickel alloys. They suggest that high iron bearing cupro nickels are superior in respect of erosion corrosion by clean sea water. 

FIGURE 21-11 Auger sputtering profiles for the copper-nickel alloys shown in Figure 21-8 . 1, passivated sample H. nonpassivated sample C. chemically etched sample representing the bulk material. (Adapted from G. E. McGuire et al J. Electrochem. Soc., 1978. 125. 1002. Reprinted by permission of Ihe publisner, the Electrochemical Society, Inc.)... 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

Some SRB may cause loealized corrosion on stainless steels, nickel alloys, aluminium, zinc and copper alloys. Mechanisms of sulphur-assisted corrosion, with emphasis on Fe- and Ni-based materials, have recently been reviewed by Marcus [6.17]. The review includes the fundamentals of enhanced dissolution, retarding or blocking of passivation, and passivity breakdown. 

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 compositions. Practically, this distinction is observed in the specification 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). 

Figure 6.14. Values of critical and passive current densities obtained from potentiostatic anodic polarization curves for copper-nickel alloys in N H2SO4, 25°C [42]. (Reproduced with permission. Copyright 1961, The Electrochemical Society.)...

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 unfilled rf-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 unfilled. 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. 

The catalytic effect of copper-nickel alloys as a function of composition for the reaction 2H H2 is shown in Fig. 6.17 [53]. Above 60 at.% Cu, the filled d-band is less favorable to hydrogen adsorption hence, favorable collisions of gaseous H with adsorbed H are less probable, and the reaction rate decreases. The similarity to passive behavior of copper-nickel alloys, which also decreases above 60 at.% Cu, can be noted. The parallel conditions affecting passivity and catalytic activity support the viewpoint that the passive films on transition metals and their alloys are chemisorbed. 

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