Passivation oxidizer concentration effect

Niobium like tantalum relies for its corrosion resistance on a highly adherent passive oxide film it is however not as resistant as tantalum in the more aggressive media. In no case reported in the literature is niobium inert to corrosives that attack tantalum. Niobium has not therefore been used extensively for corrosion resistant applications and little information is available on its performance in service conditions. It is more susceptible than tantalum to embrittlement by hydrogen and to corrosion by many aqueous corrodants. Although it is possible to prevent hydrogen embrittlement of niobium under some conditions by contacting it with platinum the method does not seem to be broadly effective. Niobium is attacked at room temperature by hydrofluoric acid and at 100°C by concentrated hydrochloric, sulphuric and phosphoric acids. It is embrittled by sodium hydroxide presumably as the result of hydrogen absorption and it is not suited for use with sodium sulphide. 

Under the effect of oxidizing agents, a metal may become passivated even when not anodically polarized by an external power source. In this case, passivation is evident from the drastic decrease in the rate of spontaneous dissolution of the metal in the solution. The best known example is that of iron passivation in concentrated nitric acid, which had been described by M. V. Lomonosov as early as 1750. Passivation of the metal comes about under the effect of the oxidizing agent s positive redox potentiaf. 

The intent of this paper is to present a methodology for estimating, from available information on concentrations and deposition velocities, the potential effects of anthropogenically derived acidic substances on indoor surfaces. Surface accumulation rates are derived that are applicable to all types of indoor surfaces. The discussion of the possible effects of the accumulated substances will concentrate on zinc and aluminum surfaces because data exists on the behavior of these metals in indoor environments (0. Aluminum forms a passivating oxide which protects against corrosion in most environments, while zinc is expected to corrode at a roughly linear rate over its lifetime. 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

Effect of oxidizer concentration on passivity Methods for Spontaneous Passivation of Metals Alloy Evaluation... 

Fig. 4.12 Effect of oxidizer concentration on current density of an active-passive metal.

Chapter 4 presents the fundamentals of passivity the film and adsorption theories of passivity criterion for passivation methods for spontaneous passivation factors affecting passivation, such as the effect of solution velocity and acid concentration alloy evaluation anodic protection systems and design requirements. A fuU discussion on stainless steel composition and crystalline structure, oxidizer concentration, and alloy evaluation is included. The chapter also considers anodic protection to establish a basis for anodic... 

Corrosion resistance of stainless steels results from formation of a passive oxide film which is stable in an oxidising enviromneait. Any physico-chemical instability of this oxide film either as a result of change in the chemistry of the environment or formation of cracks and/or scratches on the metal surface provides conditions for formation of an oxygen concentration cell which can result in localised corrosion. An example of chemical change of the environment leading into oxide film instability mentioned above is the effect of chloride ions. Chloride ions can locally damage the protective film on stainless steels [44]. 

Figure 23. Thickness vs. time during the passive oxide growth for 900 s at 0.80 V vs. Ag/AgCk/ Sat. KCl in pH 8.4 horate solution containing Fe ions at concentration of 0.0, 0.25 and 0.50 mM. Reprint from T. Ohtsuka and H. Yamada, Effect of Ferrous Ion in Solution on the Formation of Anodic Oxide Film on Iron , Corrosion Sci., 40 (1998) 1131, Copyright 1998 with permission from Elsevier Science.

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