Corrosion duplex films

Duplex films of different metals are formed, if the substrate is passivated by its own ions, and foreign ions deposit from the electrolyte. For example, FeOOH layers are deposited on other metals, for example, Cu or Au. The FeOOH-layer on Cu improves the corrosion stability of Cu. 

Neither the duplex film, nor the oriented portlandite crystals in the interfacial zone were found by all the authors [19, 20]. However, there is a common opinion that this area is enriched with calcium hydroxide crystals and exhibits higher porosity. Consequently, it will have a great impact on the corrosion resistance of concrete. The transition zone of high porosity is the weakest micro-area where the corrosion of concrete will begin [21]. For this reason, the interfacial transition zone became a subject of numerous studies [16,17,22-28]. The constitution of this zone can be easily observed on the model proposed by Rooij et al. [28] (Fig. 6.9). 

Chlorides have probably received the most study in relation to their effect on corrosion. Like other ions, they increase the electrical conductivity of the water so that the flow of corrosion currents will be facilitated. They also reduce the effectiveness of natural protective films, which may be permeable to small ions the effect of chloride on stainless steel is an extreme example but a similar effect is noted to a lesser degree with other metals. Turner" has observed that the meringue dezincification of duplex brasses is affected by the chloride/bicarbonate hardness ratio. 

Porous or nonporous films can be formed but the porous film is the one most widely used for corrosion protection. Porous oxide films are composed of two layers an outer porous layer of duplex layer structure, and an inner nonporous layer (barrier layer). The main compounds of porous oxide films are amorphous AFO, y-AUO, 7-AI2O3, ci-ALO, and other compounds depending on the electrolyte used. 

EIS results indicate that the passive films of AkCoCrFeNi alloys become increasingly thicker and more dispersive with an increasing x. Therefore, Ipass increases with x. As x value increases to 1.00, the inductance effect appears in the equivalent circuit for severe dissolution of A1 and Ni-rich phase. As for the effect of chloride on the anti-corrosion property, chloride eases the passive layer to form metastable ion complexes and further dissolve into H2SO4. With an increasing chloride concentration and A1 content, the metastable ion complexes easily form, allowing Epu to shift to a more active region. Additionally, the microstructure of both C-0 and C-0.25 is single FCC phase, while those of C-0.50 and C-1.00 are duplex FCC-BCC and complex BCC-ordered BCC phase, respectively. 

The passive film formed on austenitic stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit of hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers—nickel, manganese, and nitrogen—all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is foimd in greater abundance in the passive film than iron, which is the major element in the alloy. 

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

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