**3.5. Passivity breakdown**

**3.4. The causality of wear-corrosion synergism**

90 Metallic Glasses - Properties and Processing

taken individually and independently, namely wear and corrosion.

• The dissolution of the metal in the corrosive medium;

• The formation of a new compound that may contribute to the breakdown process;

• And the restoration of the protective film known as "repassivation" process.

The synergy between wear and corrosion has recently attracted increasing attention to improve materials used in systems where tribocorrosion plays a role. However, wear-corrosion synergy still seems to be a developing topic of discussion as no convincing expression is available yet. A positive or a negative (antagonism) synergistic effect can occur in most cases where surface interactions interfere with tribocorrosion phenomena. It intervenes especially (in a positive way) by increasing the wear (volume) when the mechanical process affects the electrochemical process and *vice versa*. In these situations, the total wear (volume loss) will be very different and greater than the sum of the mechanical wear in the absence of corrosive environment and the loss of material by corrosion in the absence of any mechanical stress. A negative effect of synergy, however, will occur when the total wear is less than the sum of the two protagonists

Madsen et al. [37, 38] critically reviewed the measurement of wear-corrosion synergism, and proposed a group of penetration rate equations to quantify the wear and corrosion processes and the wear-corrosion synergism. Their results showed that the wear-corrosion synergism is of great extent for alloys, such as AISI 316 stainless steels, which depends on the formation of a film of passive layers for their corrosion resistance, sometimes only a few atom layers thick, resulting from an interaction between the material and the surrounding environment [37, 38]. On the contrary, the synergism was limited for alloys, such as low alloy steels (e.g. amorphous steels), which do not depend on the presence of a passive film for their resistance to a corrosive environment. The causality of this synergistic effect has been explained in part for some passive materials. The presence on their surface of passivation layers and the ability of their surface, even in a deformed or partially damaged state, due to the sliding contact by the counter-body or a third-body, to be rehabilitated (by forming reaction layers with a thickness of a few nm, such as oxides, solid precipitates, adsorbed layers or passive surface films) is at the origin of the increment of that synergy. Dense oxide layers, precipitates, or passive films may play a protective role in isolating the underlying metal from a direct contact with the surrounding corrosive environment, thereby protecting the metal from a corrosion increment, but not necessarily their mechanical wear. In particular, one of the possible explanations could be related to the mechanical or chemical shear strength of these formed layers. This can likely be one of the causes of the incremental or decline effect of the wear-corrosion synergism of passivating metals. This is mainly true in the case of stainless steels and other alloys containing chromium. Their passive surface film formed in the ambient air or in contact with an aqueous solution has a thickness of a few nanometers but gives them a high resistance to corrosion. The sliding of a hard counterbody material on such a surface is likely to damage that passive film what is known as a "depassivation" process by which the bare material is exposed to the corrosive environment [8, 10, 13, 32, 44]. Various but essentially electrochemical processes can then compete on these bare surfaces [8, 10, 13, 32, 44], namely:

Most of engineering and biomedical metals and alloys oxidize and frequently passivate "spontaneously" in contact with the ambient air, and with suitable aqueous media to form "natural" thin passive surface oxide films. Alternatively, alloying is also considered to be, inter alia, the most extensively used method for enhancing the passivity of base metals [45]. Passive films that growth on most surface alloys are found to be of two types: discontinuous, and continuous [46]. Discontinuous films are porous and are formed at the metal/solution interface from the reaction of metal cations with species in solution. They have a thickness of up to 1 mm and are less protective. Continuous films, on the other hand, are tenacious and thin (nm's to μm's thickness range). They have high resistances (≥10<sup>6</sup> ohm.cm−2) and support a high electrical field. They are called barrier protective layers, which serve to prevent current flow, and corrosion (i.e., dissolution). Commonly, the passive film on most metals and alloys consist both continuous, and discontinuous layers, with discontinuous film forming the outer layer, and the continuous film forming the inner barrier protective layer. A typical example of a bi-layer passive film formed on Fe-based amorphous alloy and interacting with anions adsorption in various aqueous solutions [47] is depicted in **Figure 4**.

The approach that links the improved resistance of amorphous alloys to their ability to promote amorphous passive oxide formation is well accepted by the scientific community. In high-temperature gas working conditions, vitreous or amorphous oxides offer improved

glass interface is one theory accounting for oxide breakdown [50]. Moreover, vitreous oxides on amorphous alloys perform well due to the enhanced bond flexibility, because the vitreous or amorphous material can rearrange to accommodate lattice mismatch and strain between the oxide and the metal [48, 51]. As a result of this flexibility, almost all surface atoms can bond with oxygen or OH without requiring an optimal epitaxial relationship between the ordered metal substrate and oxide. Intolerable changes in oxide/metal misfit strain with halide incorporation is another theory accounting for the rupture of protective

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233 93

In the case of crystalline alloys, such as the highly passivated stainless steels, it should be noted that the improved corrosion resistance associated with the addition of 18% Cr to crystalline iron is attributed, in part, to a change in the protective oxide structure from a well-oriented spinel structure at 0–12% Cr to a non-crystalline structure at 18% Cr [48]. This disordering has been demonstrated by low-energy electron diffraction [53] and scanning tunneling microscopy [54]. A retardation in ionic transport may occur, because non-crystalline films have fewer defects or grain boundaries to enhance ionic movement. In summary, desirable amorphous oxide/amorphous alloy properties include defect minimization, film ductility, bond flexibility, and efficient, rapid film repassivation, which all contribute to improved

**4. Consensus on the need of materials for triboelectrochemistry** 

**5. Evolution of the solutions to triboelectrochemical problems**

The choice of metals and their alloys seems to be more relevant for designing materials over plastics, foams, polymers, and natural materials if the intended application requires a limiting risk factor of deformation and flexibility, hence interest on their application to load-bearing structures. Alternatively, the development of novel metallic-glass-matrix composite materials

The development of tribocorrosion resistance requirements has resulted in the development of a much larger number of materials that did not exist before, especially metallic alloys. In turn, such a development of materials has multiplied case studies, and increased the number and the diversity of corrosion-wear problems. Accordingly, the resolution and the nature of corrosion-wear problems are intimately related to the choice of materials. This does not mean that the tribocorrosion resistance is necessarily the determining parameter in the choice of a given material. Such a choice must, in fact, make it possible to fulfill at best one or more technological functions, and it is quite obvious that, under such conditions, the mechanical properties of the materials, their properties of implementation, their price or their availability are in many cases, parameters are just as decisive in the choice as their only resistance to

oxide films on metals [52].

corrosion resistance [48, 51].

**systems**

tribocorrosion.

**Figure 4.** Schematic diagrams of anions adsorption and characteristics of passive films (bi-layer structure) formed on Fe-based amorphous alloy in (a) sulfate and (b) chloride solutions. (reproduced from Wang et al. [47] with permission from Elsevier Science).

oxidation resistance due to the absence of oxide grain boundaries, which provide a rapid diffusion path for concentration gradient-driven ion movement [46]. In aqueous solutions, ion transport is mostly driven by the electric field across the oxide film. The lack of oxide grain boundaries may lower ion migration rates, rendering the passive film more protective [46, 48].

It is worthwhile to mention that if the passive films on metals, like iron, nickel, and chromium, remained intact, then the corrosion current flowing across the interface under most industrial conditions would be of the order of 0.01–1.0 μA.cm−2, corresponding to corrosion rates of approximately 0.15–15 μm per year [49]. For most practical situations, metal loss rates of this order are of no concern, so that our automobiles, bridges, aeroplanes, and industrial systems would last for periods extending well beyond the current design lifetime. Unfortunately, passive films do not remain intact, and corrosion rates of many orders of magnitude greater than those indicated above for fully passive substrates are commonly observed, particularly if the attack occurs locally.

Passivity breakdown can occur for a variety of reasons, including mechanical straining of the substrate metallic alloy, the frictional dissipated energy in tribological contacts required for micro-cracking, the presence of thermal stresses within the oxide due to differences in thermal expansivity, compressive stresses in the oxide growth (Pilling–Bedworth ratio), fluid flow and cavitation, transpassivity polarization, and chemically-induced phenomena. In particular, the rapid transport and accumulation of cation vacancies at the oxide/metal glass interface is one theory accounting for oxide breakdown [50]. Moreover, vitreous oxides on amorphous alloys perform well due to the enhanced bond flexibility, because the vitreous or amorphous material can rearrange to accommodate lattice mismatch and strain between the oxide and the metal [48, 51]. As a result of this flexibility, almost all surface atoms can bond with oxygen or OH without requiring an optimal epitaxial relationship between the ordered metal substrate and oxide. Intolerable changes in oxide/metal misfit strain with halide incorporation is another theory accounting for the rupture of protective oxide films on metals [52].

In the case of crystalline alloys, such as the highly passivated stainless steels, it should be noted that the improved corrosion resistance associated with the addition of 18% Cr to crystalline iron is attributed, in part, to a change in the protective oxide structure from a well-oriented spinel structure at 0–12% Cr to a non-crystalline structure at 18% Cr [48]. This disordering has been demonstrated by low-energy electron diffraction [53] and scanning tunneling microscopy [54]. A retardation in ionic transport may occur, because non-crystalline films have fewer defects or grain boundaries to enhance ionic movement. In summary, desirable amorphous oxide/amorphous alloy properties include defect minimization, film ductility, bond flexibility, and efficient, rapid film repassivation, which all contribute to improved corrosion resistance [48, 51].
