Nickel-iron alloy films

Interest in passivity started with the studies of Faraday [1] and Schonbein [2] over 150 years ago. The lack of metallic corrosion in the case of iron immersed in certain solutions was attributed to either the presence of an oxide film or an electronic change in the metal. This basic argument has persisted in various forms to this day, although the majority of scientific evidence suggests protection by a three-dimensional oxide film. Much has been published on passivity and its breakdown over the last 50 years. This chapter does not attempt to cover all the literature but concentrates on work over the past 10-15 years, emphasizing the passivity of iron, nickel, iron-chromium, and iron-nickel alloys in aqueous environments. Examples are given fi om the authors and other selected laboratories. 

A similar method of test was used at the International Nickel Company s Corrosion Laboratory at North Carolina. The specimen discs are mounted on insulated vertical spindles and submerged in sea-water, which is supplied continuously to the tank in which the specimens are immersed. The maximum peripheral speed of the spinning disc is about 760cms , and the characteristic pattern of attack is shown in Fig. 19.3a. Studies of variation of depth of attack with velocity indicate that at low velocities (up to about 450 cm s ) alloys such as Admiralty brass, Cu-lONi and cupro-nickel alloys containing iron maintain their protective film with a consequent small and similar depth of attack for the diflferent alloys. At higher velocities the rate increases due to breakdown of the film. 

When constructing electrolyzers for this process it is rather difficult to find suitable materials which can resist the effects of fluorine as it attacks most metals even at normal temperature fortunately continuous fluoride coatings are formed on the surface of some metals which protects them against further corrosion at least to a certain extent. Such metals are iron, nickel, Monel metal, aluminium and its alloys, magnesium and especially electron one of its alloys. However, the protective films are only stable at lower temperatures. At elevated temperatures a violent reaction proceeds between the fluorine and the metal. Monel metal and copper have relatively the best resistance against fluorine at elevated temperatures. These metals, therefore, were widely used to construct electrolyzers. In more recent designs, copper was replaced by steel or electron. 

Bardolle and Benard (26) first showed in studies of the oxidation of iron that small nuclei formed in thin oxide films. More recently a number of studies have been carried out on nucleation and growth during the Initial oxidation of iron (27), copper (28,29, 30), nickel (31), nickel alloys (32, 33), and silver (34). Since the number, size and shape of die nuclei varied with the face exposed at the surface, these results support the above general conclusion that the nature and die rate of oxidation depend on the crystal face exposed at the surface. No estimates of the variation of rate with face have been made with this type of study. These results will be referred to later under Section 4 dealing with topography. 

The technique may be understood in terms of metallic passivity, i.e. the loss of chemical activity experienced by certain metals and alloys under particular environmental conditions as a result of surface film formation. Equations 15.2 and 15.3 suggest that the application of an anodic current to a metal should tend to increase metal dissolution and decrease hydrogen production. Metals that display passivity, such as iron, nickel chromium, titanium and their alloys respond to an anodic current by shifting their polarisation potential into the passive regon. Current densities required to initiate passivity are relatively high [Uhlig and Revie 1985] but the current density to maintain passivity are low, with a consequent reduction in power costs [Scully 1990]. 

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. 

A passive film, usually consisting of only a few molecular layers, such as found in iron, nickel, chromium, and iron- and nickel-based alloys with passivation capacity... 

Aluminum brass resists high-velocity waters (impingement attack) better than does admiralty metal. Cupro-nickel alloys are especially resistant to high-velocity seawater when they contain small amounts of iron and sometimes manganese as well. For the 10% Ni cupro-nickel alloy, the optimum iron content is about 1.0-1.75%, with 0.75% Mn maximum for the analogous 30% Ni composition, the amount of alloyed iron is usually less (e.g., 0.40-0.70% Fe accompanied by 1.0% Mn maximum) [46]. It is found that supplementary protective films are formed on condenser tube surfaces when iron is contained in water as a result of corrosion products upstream or when added intentionally as ferrous salts. Accordingly, the beneficial effect of iron alloyed with copper-nickel alloys is considered to result from similar availability of iron in the formation of protective films. 

In the case of many metals showing the ability to be passive, and of importance for engineering alloys, e.g. iron, nickel and chromium, the passive film, in acidic solution, does not result from thermodynamic equilibrium, but from the fact that the dissolution rate of the oxide in the acidic solution is slow. Figure 3-2 b shows a schematic potential-pH diagram for metal-water... 

The alloy composition (and microstructure) has strong effects on all the aspects of passivity that have been described above chemical composition and thickness of the passive film, electronic properties, structure, and kinetics of formation. The influence of alloyed elements on the electrochemical characteristics of passive systems can be seen in Fig. 3-16. This is the same current-potential curve as in Fig. 3-1, on which the two major effects of alloyed elements are indicated lowering of the dissolution current in the active region and at the active-passive transition, and broadening of the passive region. A third effect, not illustrated in Fig. 3-16 but which will be discussed later, is the improvement of the resistance of the alloy to passivity breakdown and localized corrosion. For iron-based alloys, these beneficial effects are obtained with chromium, molybdenum, nickel, and nitrogen. 

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