Alloy composition chloride concentration

It is in this area that most work has been carried out, particularly in relation to corrosion resistance in sulphuric acid solutionsBourelier etal. and Raicheff etal. investigated the inhibitive effect of chloride ions on corrosion in sulphuric acid. The inhibition efficiency was found to depend on the alloy composition, alloy surface and chloride concentration. The more aggressive the environment, the greater the inhibition efficiency. Yagupol skaya etal studied the effect of iodine additions to sulphuric acid on the corrosion resistance of Ni and Ni-Fe alloys. Again there was an inhibitive effect caused by the halide ion. 

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. 

From an environmental perspective, the chemical composition and pH value of an electrolyte are most critical to the corrosion performance of Mg and its alloys. Any other factors which affect these two variables can certainly influence the corrosion. A general rule is that Mg and its alloys are more corrosion resistant in a high alkaline and low chloride concentration solution. Many chemicals in solution can significantly affect the corrosion resistance of Mg alloys which has been summarized previously (Song, 2006 Song and Atrens, 1999) and will not be repeated in this chapter. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

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... 

Chloride corrosion of steel is controlled by the environment, the concentration of oxidizer, the structural characteristics of metal, the composition of the alloy, and the presence of dissolved oxygen. The chloride ions adsorb on the outer surface of the passive film, permeate through the passive oxide film, and interact with the underlying metal. 

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