Metals surface ennoblement

In an environment with a constant redox condition (e.g., permanently aerated and/or constant pH), a condition not uncommon in industrial and environmental situations, corr could shift in the positive direction for a number of reasons. Incongruent dissolution of an alloy could lead to surface ennoblement. Alternatively, as corrosion progresses, the formation of a corrosion product deposit could polarize (i.e., increase the overpotential, i), for) the anodic reaction as illustrated in the Evans diagram of Fig. 4. Polarization in this manner may be due to the introduction of anodic concentration polarization in the deposit as the rate of transport of dissolved metal species away from the corroding surface becomes steadily inhibited by the thickening of the surface deposit i.e., the anodic half-reaction becomes transport controlled. 

The ennobling of the metal surface is based on the assumption that the conducting polymer, in its oxidized state, will set the metal at a potential within its passive range where the dissolution rate is slow. This generally involves the formation of a thin insulating metallic oxide layer, more or less porous, the effect of which is to protect the metal from a rapid dissolution, and make it behave like a noble metal. To understand this effect it must be recalled that three distinct areas related to active, passive, and transpassive... 

B. Wessling, Corrosion Prevention with an Organic Metal (Polyaniline) Surface Ennobling, Passivation, Corrosion Test Results. Mater. Corros. Korros. 1996,47,439-445. 

The paints must exhibit metallic properties to ennoble the metal surface. 

It was only in 1992-1993 that we finally found out after an in-depth evaluation of the interactions between various metal surfaces and coatings of polyaniline (applied as pure dispersion or as dispersion paints) that together with a remarkable corrosion potential shift (ennobling) and an iron oxide layer formation (passivation) lead to a significant anticorrosion effect [71]. In a study together with Elsenbaumer et al. [72], we discovered that the corrosion rate was reduced by a factor of up to 10,000. The iron oxide that formed between the metal surface and the polyanUine primer coating was determined to be Fc203, later confirmed with even clearer x-ray photoelectron spectra (XPS) [23b]. 

Wessling, B. (1996) Corrosion prevention with an organic metal (polyaniline) surface ennobling, passivation, corrosion test results. Werkst. Korros. (Mater. Corros.), A7, 439-445. 

More recently, attention has been directed to the "ninth form of corrosion, biologically influenced corrosion, which includes studies on an area referred to as "ennoblement. The presence of biofilms on metals and alloys immersed in natural seawater produces a complex, heterogeneous chemistry along the metallic surface. It has usually been observed that passive alloys such as aluminum, stainless steels, nickel-base alloys, or titanium show an increase to more noble (electropositive) potentials or ennoblement of several hundred millivolts with exposure time in natural seawater, thus magnifying the potential differences that may exist between dissimilar alloys [26,55-64]. Ennoblement is likely caused by the formation of microbiological films, which increase the kinetics of the cathodic reaction [55-63],... 

Suitable coating of metals with pure polyaniline from dispersions and the subsequent interaction with the metal surface results in a significant shift in the corrosion potential of the coated metals in the direction of noble metals ( ennoblement"). This entire process can also be regarded as a passivation of the metal by polyaniline" (see Figs. 19.68-19.72). 

With other metals, in particular with aluminium and its alloys, the ennobling mechanism is not clear-cut. Recent work by Yan et al. [60] carried out with PPy doped with electroinactive 1,3-benzene-disulfonic acid and electroactive sodium 4,5-dihydroxy-l,3-benzene-disulfonate and using a two-compartment cell similar to that of Rammelt et al. [64] seems to prove that the anodic protection model cannot be applied to the aluminum surface. Although the recharging of PPy by O2 was demonstrated, they found that a small area of exposed aluminium (AA 2024-T3) simulating a defect in a coating and immersed in dilute Harrison s solution (0.35 wt% (NH4)2S04 + 0.05 wt% NaCl in water) did not... 

Applications like corrosion protection by ennobling and passivation, and manufacture of printed circuit board surfaces composed of the organic metal polyaniline (Ormecon) are based exclusively on this unique set of properties and use most of the properties in parallel. Here, this new materials class is able to offer a performance that cannot be matched by any other material or by another conductive polymer. 

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