Passivity of alloys

ICP-MS has also been used to measure trace elements in archaeological native silver artifacts [345] in order to identify their geographical origins. The low detection limits provided by ICP-MS allowed analysis of trace elements on 3 to 15 mg of sample. The passivation of alloy steels using acid solutions has been studied by XPS measurements of the solid in combination of ICP-MS analysis of the passivation solutions [346,347]. When bullets are crushed on impact, striations cannot be used for identification. The percentage of antimony, trace element composition, and lead isotope ratios in bullets was measured for forensic evidence [348]. The lead isotope ratios were found to be the most useful evidence. 

N.D. Tomashov, G.P. Chernova, Y.S. Ruskol, and G.A. Ayuyan, Passivation of Alloys on Titanium Base, Proceedings of the Fifth International Conference on Metallic Corrosion, National Association of Corrosion Engineers, 1974, p 248-252... 

Passivity of alloys is a complex topic and the exact behavior of an alloy is hard to predict. Some mechanisms, however, are known and the principle function of some alloying elements is understood. In a first approach, they can be divided into oxide former, for example, Cr, and dissolution moderator, for example. Mo. 

Selected examples of the passivity of alloyed steel. The extreme corrosion resistance is best demonstrated for iron-chromium alloys. As an example in Figure 10.18 passivity plots of these alloys are shown. 

The total system relies on defense in depth through vitrification and immobilization of the nuclear waste within glass, the storage of borated stainless steel baskets, and enclosure in a modern corrosion-resistant alloy. An extraordinary effort has been under way around the world to verify acceptable performance over what had heretofore been unthinkable lifetimes. This has brought about some revolutionary new lines of thinking in the corrosion field—on, for instance, the theory, experiment, and modeling of extremely low corrosion rates the revisitation of such issues as long-term passivity of alloys in complex environments and... 

Experience shows that at least duplicate test specimens should be exposed in each test. Under laboratory tests, corrosion rates of duplicate specimens are usually within 10 % of each other, when the attack is uniform. Occasional exceptions, in which a large difference is observed, can occur under conditions of borderline passivity of alloys that depend on a passive film for their resistance to corrosion. If the rate difference exceeds 10 %, re-testing should be considered, unless it is observed that localized attack is predominant. Corrosion rates are calculated assuming a uniform loss of metal, and therefore when specimens are attacked non-uniformly, the calculated corrosion rates indicate only the relative severity of attack and should not be used to predict the performance of an alloy to the test solution. In such cases, weight loss per unit of surface area may be used to avoid implying a uniform penetration rate. 

While the same basic mechanisms for passivity of pure metals also applies to alloys, the processes involved in the passivation of alloys have an added complexity. In many cases only one component of the alloy has the property of being passive in a particular environment. Alloys such as steiinless steels, which contain highly passive components (chromium in this case), owe their corrosion resistance to the surface enrichment of the passivating component Thus stainless steels resist corrosion in many acidic systems (where iron or carbon steel would be poorly passive or not passive at all) by a passivating oxide film containing Cr predominantly as Cr(III). Surface analytical techniques such as Auger electron and X-ray photoelectron spectroscopies reveal substantial enrichment of chromium in the passivating oxide film on these alloys " . There are only two ways by which this enrichment can... 

Marcus P, Teissier A and Oudar J 1984 The influence of sulfur on the dissolution and the passivation of a nickel-iron alloy. 1. Electrochemical and radiotracer measurements Corrosion Sc . 24 259... 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

APPLICATION OF SURFACE ELECTROCHEMICAL PASSIVATION OF LEAD-ANTIMONY ALLOY FOR A SIMPLE AND RAPID ELECTROCHEMICAL DETERMINATION OF ANTIMONY CONTENT OF ITS ALLOYS... 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

Under these circumstances the metal s surface within the crevice became active and it corroded with the formation of a yellowish-white corrosion product that was identified as being mainly rutile TiOj. On the other hand, a Ti-0- 13Pd alloy was found to be immune from crevice corrosion, since the presence of the palladium facilitated passivation of the metal surfaces forming the crevice. 

This work has been carried out by Marcus and his co-workersand deals with the influence of sulphur on the passivation of Ni-Fe alloys. For sulphur-containing Ni-Fe alloys, sulphur segregates on the surface during anodic dissolution. Above a critical sulphur content a non-protective thin sulphide film is formed on the surface instead of the passive oxide film. 

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 ... 

Fig. 3.68 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 acid of various concentrations and temperatures ...

The effect of metalloids on the corrosion resistance of alloys also varies with the stability of polyoxyanions contained in their films. Phosphorus and carbon contained in iron-chromium-melalloid alloys do not produce passive films of phosphate and carbonate in strong acids, and so do not interfere with the formation of the passive hydrated chromium oxyhydroxide... 

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