Nickel-chromium-molybdenum alloys corrosion potentials

The stainless steels (types 303, 316, and 316LVM) as well as the cobalt-nickel-chromium-molybdenum alloy MP35N are protected from corrosion by a thin passivation layer that develops when exposed to atmospheric oxygen and which forms a barrier to further reaction. In the case of stainless steel, this layer consists of iron oxides, iron hydroxides, and chromium oxides. These metals inject charge by reversible oxidation and reduction of the passivation layers. A possible problem with these metals is that if the electrode potential becomes too positive... 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

Manufacturers of nitinol products are known to electropolish the surface of the nitinol wire to produce an oxidized surface layer that contains only titanium and oxygen. This reduces the potential for nickel allergy and toxicity problems. The alloy MP35N used in the CS/SF device is a quaternary of cobalt, nickel, chromium, and molybdenum, and is known to have high strength and corrosion resistance. Additionally, this metal has been found to be biocompatible and MR compatible. These are all desirable properties for an implantable device, but its nickel content should be noted and considered when dealing with patients who are allergic to nickel. 

From a corrosion standpoint, the roles of various alIo)dng elements in the cobalt-base alloys parallel those seen in the nickel-base alloys. Chromium, molybdenum, and tungsten, for example, are highly soluble in both atomic forms of cobalt. Chromium is added to most of the commercially important alloys, and provides passivity over a wide range of potentials. Molybdenum and tungsten enhance resistance to corrosion within the active regime. 

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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