Corrosion phenomenon Stability

It is apvparent from the brief review and case study that oxide systems could be quite helpful to reduce or slow down the corrosion phenomenon of metallic systems. However, it could be dependent on the oxide system employed. Importantly the stability of oxide phases is very vital as it could undergo several changes during the operation depending on the temperatures of application environment. Dispersion of Si02 showed marked enhancement in the oxidation and corrosion resistance of the hybrid coatings at high temperatures. 

After 5 days of aging treatment, grain boundaries appeared on SEM micrographs. Surface modifications is assumed to be due to formation of volatile aluminium hydroxides, as expected by themodynamic modelUng [9], As the corrosion phenomenon remains limited to the surface, it is not affecting the stability of the foam in the bulk. As a proof, mechanical resistance of alumina foams is kept constant before and after aging the compressive crash strength of 10 ppi alumina foams with 85% apparent porosity has been measured at 2 MPa. 

In the preceding sections, various types of fluctuations and instabilities essential to corrosion were examined. As a result, it was shown that a corrosion system involves various kinds of problems of stability and instability. Unlike thermodynamic equilibrium systems, in nonequilibrium systems like corrosion systems, a drastic change in the reaction state should be defined as a bifurcation phenomenon. 

From a chemical viewpoint, the interior of a crystal and its surface can be looked upon as if they were different individuals, comparable to two modifications of a substance or to a metal which is under mechanical stresses. The phenomenon of stress corrosion reveals that the part under tension differs in its chemical reactivity from the part under compression. Different modifications of silica can have very different reactivities. Therefore, it must be expected that the surface film can affect the apparent stability of a solid. Indeed, the surface structures of some hydrated salts are in equilibrium with the ambient atmosphere and prevent these crystals from losing water (efflorescing), even if the bulk of the crystal has a higher water vapor pressure than the atmosphere. The formation of a fresh highly asymmetrical surface may also cause substances to detonate. 

When metals are placed in a solution of an oxidizing agent, it is common to find that corrosion does not occur although the oxidation of the metal is thermodynamically very favourable. Indeed, this phenomenon was first observed over one-hundred years ago when it was found that iron dissolved in dilute, but not in concentrated nitric acid it was suggested that in concentrated nitric acid, a thin layer of a non-porous and insoluble oxide film was formed. This protection by a surface film is termed passivation and explains the apparent stability of metals such as Al, Cr, Ni and Pt in damp air. 

With hydrolysis constants of = 10 and 10 one obtains for the saturation eoneentrations 4.8 and 4.2 M, pH values of 4.3 and 2.4 for saturated NiCl2 and FeCl2 solutions, respeetively. These caleulations show that the precipitation of hydroxide within a eorrosion pit may be prevented for these metals by acidification. However, as diseussed before, passivation should nevertheless be possible for many teehnically important metals. For Fe, Ni, and other metals, especially in acidic media, passivity eannot be explained on the basis of thermodynamic equilibria and Pourbaix diagrams. In these eases passivity is a kinetic phenomenon. Otherwise these metals should not show passivity in a strongly acidic environment. Thus, different explanations are required for the stability of a corrosion pit. 

This phenomenon is restricted to the stabilized grades of steel, such as 321 and 347. The knife-line attack occurs immediately adjacent to the weld and shows as a thin line of intergranular corrosion. It results from intergranular corrosion like the weld decay. It may be noted that weld decay develops at some distance away fi"om the weld. The following is the mechanism suggested for the knife-line attack of stabilized steels. 

If, however, the cerinm salt is very solnble, this limits its ability to provide long-term corrosion inhibition. " It can also disrapt the stractirre of the gel or of the overlying paint layer. Organics snch as benzotriazole can also adversely affect the stability and barrier properties of hybrid gels, so this is not a phenomenon confined to inorganic anti-corrosion additives. 

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