Ennobling mechanism

However, as previously described, this ennobling mechanism is only based upon steady-state considerations, and it does not adequately take into account electrochemical kinetics, which, depending on the experimental conditions (polymer, substrate, aeration of the environment... [41]), makes the protection efficiency vary considerably. 

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... 

The self-healing effect, described in this section, is in our opinion an extension of the ennobling mechanism in which the release of anions, concomitant to polymer reduction, is used in order to control to some extent the electrolytic medium around the scratch and thus facilitate its passivation. Studies in this direction are stiU scarce and further work is needed to explore this concept. Some of the work described in Section 16.2.2 probably concerns self-healing properties, but this concept was not highhghted by the authors. It should be reanalysed in the light of recent work more focused on self-healing effects. 

Another mechanism associated with the coating of an active metal with a conducting polymer in the oxidized state is the so-called ennobling mechanism (Spinks et al, 2002). This is a form of anodic protection according to which the oxidized polymer will set the underlying metal in its passive state. To achieve passivation, several parameters come into play, namely the type of metal, pH of solution and nature of the anions close to the metal interface. This mechanism has been accepted for ferrous substrates coated with PANi in acidic media (Lu et al, 1995 Ahmad and McDiarmid, 1996 Cook et al, 2004), with protonated PANi being able to provide high current density for passivation of defects above the Flade potential. This effect has also been demonstrated for PPy-coated iron and steel (Michalik and Rohwerder, 2005), but not for polymethylthiophene-coated... 

His wife, Marie-Anne, studied drawing with the great painter Jacques-Louis David in order to transcribe her husband s laboratory notes and illustrate them. David s ravishing portrait of the Lavoisiers hangs in the Metropolitan Museum of Art in New York. The son of Marie-Anne s longtime lover founded the E. I. Du Pont de Nemours Corporation in Delaware. Marie-Anne later married the American physicist, Benjamin Thomson. Later ennobled as the Count of Rumford, Thomson demonstrated the mechanical nature of heat. Marie-Anne had an excellent eye for scientific talent. 

P. Chandrasekaran and S.C. Dexter, Mechanism of Potential Ennoblement on Passive Metals by Seawater Biofdms, Paper 493, Corrosion 93 (Houston, TX), NACE International, 1993... 

In addition to these mechanisms, there is a mentioning of enzymatic mechanism where hydrogen peroxide (produced as a result of oxidation of glucose) can cause ennoblement of stainless steel, for more details see [42]. 

Little et al [1] have pointed out that ennoblement in fresh and brackish water is related to the microbial deposition of manganese whereas in seawater, this phenomenon may be ascribed to depolarisation of the oxygen reduction reaction that may occur, in effect, due to some of the proposed mechanisms mentioned above such as mechanisms 1,2, and 4. For example, it is well known that the oxygen reduction potential shifts positive (about 60 mV) for each decrease in pH unit and such a decrease produces a noble shift of 35 to 40 mV on stainless steel electrodes in seawater [41, 43], see also [6, 7]. Figure 4.5 shows how the increase in potential due to biofilm formation can endanger the material to pitting. 

The study of the mechanisms of the ennoblement of stainless steel is complex, as seawater constitutes an enviromnent in which numerous parameters can act. 

Figure 11 Hypothetical mechanism of the stainless steel ennoblement in natural seawater [32],...

P. Chandrasekaran and S. C. Dexter, Mechanism of potential ennoblement on passive metals by seawater biofilms, CORROSION 93, New Orleans, paper No. 493, NACE, 1993. 

Hypothetical mechanism of the stainless steel ennoblement in natural seawater. (From Dupont, I. et al., Int. Biodeterior. Biodegrad., 41,13,1998.)... 

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