Amorphous alloys passivation

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 ... 

The passive films formed by the addition of sufficient amounts of valve metals to amorphous nickel-valve-metal alloys are exclusively composed of valve-metal oxyhydroxides or oxides such as TaOjCOH) , Nb02(OH) or TajO,. Consequently, amorphous alloys containing strongly passivating elements, such as chromium, niobium and tantalum, have a very high ability... 

Table 3.S9 Concentration of chromic ion in passive films formed on amorphous alloys and stainless steels in I n HCI at ambient temperature...

As can be seen in Fig. 3.67, the corrosion resistance of amorphous alloys changes with the addition of metalloids, and the beneficial effect of a metaU loid in enhancing corrosion resistance based on passivation decreases in the order phosphorus, carbon, silicon, boron (Fig. 3.72). This is attributed partly to the difference in the speed of accumulation of passivating elements due to active dissolution prior to passivation... 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

A common observation in most cases is that the surface of amorphous alloys, especially those containing Ti, Zr and Mo, is largely covered with inactive oxides which impart low electrocatalytic properties to the material as prepared [562, 569, 575], Activation is achieved by removing these oxides either by prepolarization or, more commonly and most efficiently, by leaching in HF [89, 152, 576]. Removal of the passive layer results in a striking enhancement of the electrocatalytic activity [89], but surface analysis has shown [89, 577] that this is due to the formation of a very porous layer of fine particles on the surface (Fig. 32). A Raney type electrode is thus obtained which explains the high electrocatalytic activity. Therefore, it has been suggested [562, 578] that some amorphous alloys are better as catalyst precursors than as catalysts themselves. However, it has been pointed out that the amorphous state appears to favor the formation of such a porous layer which is not effectively formed if the alloy is in the crystalline state [575]. 

Passivity of aluminum-based amorphous alloys and stainless steels and the catalytic activity of copper-based amorphous alloys... 

X-Ray and SEM measurements indicate that during anodic polarization there is a preferential, fast dissolution of the palladium-rich phases from the crystalline alloys whereas the surfaces of the amorphous alloys remain unchanged (65). Detailed XPS studies revealed that the amorphous alloys are passivated by the formation of a thick, passive film on the alloy surface (65, 96). This film is enriched in the ions of the second metallic element (65, 96, 98, 99), and the activities of the alloys increase almost linearly with the concentrations of the platinum group cations in this surface layer (95), suggesting that these ions are the active sites in chlorine evolution. The fact that the surface film is formed in the gas evolution... 

A. Rossi, B. Elsener, XPS analysis of passive film on the amorphous alloy Fe70Crl0P13C7 the effect of the applied potential. Surf. Interf Anal. 18 (1992) 499-504. 

In tetrahedrally coordinated amorphous alloys the theraputic role of hydrogen in passivating "dangling bonds" is well known. In amorphous silicon without hydrogen (a-Si) the densities of silicon "dangling bonds" as measi ged by electron spin resonance (ESR) are almost always greater than 10 spins cm. With thj addition o hydrogen the spin densities in a-Si H can be as low as 10 spins cm... 

Key words bulk metallic glass, amorphous alloy, Mg-based alloy, general corrosion, passivity, filiform corrosion, hydrogen. 

Mg and possessed a superior corrosion resistance than a couple of Mg alloys and another amorphous alloy Mg65Cu2sYio (Fig. 6.6). Interestingly, the Auger electron spectroscopy (AES) analysis showed that there was no trace of Ga compounds in the passive layer and that it was enriched only with aluminium oxide. However, AES depth profiles suggested the deposition of metallic Ga below the corrosion layer, whieh was further confirmed by the XRD results. It appears that the euhaneed eorrosion resistance was only due to the aluminium oxide enrichment at the surfaee of this alloy. These researeh findings opened up avenues for the development of amorphous alloys with higher aluminium eontent that eould provide not only an improved electrochemical behaviour but superior meehanieal properties as well. 

Inhibition and passivation Corrosion of amorphous alloys Internal oxidation... 

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

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