Alloys passivity

Oxide, hydroxide and basic salts of aluminum are less soluble at pH values of about 7 than those of zinc [17], which explains the easy passivatability. Galvanic anodes of aluminum alloys are primarily employed in the area of offshore technology. The anodes work in relatively pure seawater flowing with a high velocity so that by using suitable alloys, passivation phenomena are rare. Their low weight is particularly favorable in view of a service time of 20 to 30 years. 

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 ... 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

In terms of the Pourbaix potential/pH diagrams, the theoretical scale compares the potentials of immunity of the different metals, while the practical scale compares the potentials of passivation. But this is not enough either. The real scale depends on the environment with which the structure will be in contact during service. Passivity, as we have seen, depends on pH. It also depends on the ionic composition of the electrolyte, particularly the concentration of chloride ions or other species that are detrimental to passivity. Finally, one must remember that construction materials are always alloys, never the pure metals. The tendency of a metal to be passivated spontaneously can depend dramatically on alloying elements. For example, an alloy of iron with 8% nickel and 18% chromium (known as 304 stainless steel) is commonly used for kitchen utensils. This alloy passivates spontaneously and should be ranked, on the practical scale of potentials, near copper. If... 

K. Tachibana and M.B. Ives, Selective Dissolution Measurements to Determine the Nature of Films on Nickel-Molybdenum Alloys, Passivity of Metals, The Electrochemical Society, 1978, p 878-897... 

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

As shown in Fig. 4.15, the anodic polarization curves obtained for steels with chromium content between 3.54% and 19.20% indicates that increasing the chromium content enhances the Fe-Cr-Ni alloy passivation ability by decreasing the critical current density of the alloys from 10 to 10 pA/cm [49]. The passive current also decreases from 50 pA/cm for 3.54% Cr alloy in sulfuric acid to 10 pA/cm for the alloy with 19.2% Cr. 

Depending on the potential, chloride concentration in the electrolyte, structure of the alloy, passive film thickness, and the chemical composition of the corrosion environment, pits may appear in various morphologies as illustrated in Fig. 7.4. 

Minimum hydrogen ion concentration, that initiates alloy passivation is at Jcrit and Eppi... 

The two principal alloys of aluminum usually employed as sacrificial anodes are Al-Zn-Hg and Al-Zn-I. These are exclusively used in seawater, which is a major disadvantage. As well, they spark when struck with rusting iron and therefore may not be very useful in the petroleum industries. In addition, these alloys passivate when operated in the cold. This problem is worse in muddy environments. Another limitation to their usage is that they produce poisonous dissolution products. 

In homogeneous single-phase alloys, passivity usually occurs at and above a composition that is specific to each alloy and that depends on the environment, as explained in Section 6.8. For Ni-Cu alloys, the critical composition is 30-40% Ni for Cr-Co, Cr-Ni, and Cr-Fe alloys, it is 8%, 14%, and 12% Cr, respectively. The stainless steels are ferrous alloys that contain at least 10.5% Cr. Stainless steels are passive in many aqueous media, similar to the passivity of pure chromium itself, and they are the most important of the passive alloys. 

Alloy Passive Current Range of Corrosion Breakdown... 

Figure 64. Sequence of ISS spectra from an Fe53 - Ni47 alloy passivated in I mol/L NaOH (rp = 5 min, p= 0.44 V), during depth profiling [211]...

These inhibiting spedes often occur as natural process stream constituents or contaminants and need not be intentionally added to achieve complete titanium alloy passivation. However, because the... 

No hydrogen uptake and embrittlement problems occur when titanium is coupled to fuUy passive materials in a given environment. These compatible materials may include other titanium alloys, passive stainless steels, copper alloys, and nickel-base alloys, depending on conditions. 

Song Qian, R. C. Newman, R. A. Cottis, and K. Sieradski, Computer simulation of alloy passivation and activation, Corros. Sci.31 62 (1990). 

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