Anodic protection mechanism

Many protection mechanisms related to ICPs have been proposed in the literature, and in some cases opposing evidence exists. This confusion may be attributed to the wide variations in experimental procedures used (coating type, substrate preparation, corrosive environment, test method). In the following, we just list the main mechanisms that have appeared in the literature. Further details can be found in the review by Spinks et al. [7]. However, the anodic protection mechanism will be discussed at length because it is the most commonly accepted mechanism in previous studies. 

Wessling [41, 42] postulated that ICPs could stabilize the potential of a metal in a passive region, maintaining a protective oxide layer on the metal. In this model, oxygen reduction on the polymer film replenished the polymer charge consumed by metal dissolution, thereby stabilizing the potential of the metal in the passive 

Type of iron or steel Passivation region (V, versus SCE) Passivation state current (mAcm ) 

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Corrosion inhibition of chemically and electrochemically synthesized coatings of PANI, poly (o-methoxyaniline), and their copolymers on stainless steel and on aluminum alloy (6061-T6) was evaluated by immersion in 3% NaCl (steel) and 0.1 M NaCl (A1 alloy). These authors concluded from polarization studies that protection involved an anodic protection mechanism [206]. 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

A special kind of anodic film formation is called passivation and it is itself the best example of anodic protection. So important has passivation been over the years, and so puzzling its mechanism, that the subject (which implicitly presents anodic inhibition) will be treated in a separate section (12.4). 

Figure 17 Fracture mechanics data plotted as log apparent crack velocity vs. stress intensity. Each point represents the exposure of a single specimen. The specimens were immersed in a simulated impregnation zone liquor at 115°C for 151 days. No crack propagation was observed at anodic protection potentials (—730 mVSCE). (From Ref. 10.)...

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

The electrochemistry of amino acids has been studied in strong acid solutions. In general, the compounds are decomposed to carboxylic acids, aldehydes, ammonia, and carbon dioxide. The results are reviewed by Weinberg [35]. The anodic oxidation mechanism has been studied in pH 10 buffer solution. Decarboxylation accompanied by C-N bond cleavage is the main reaction process [182]. The synthetically interesting Hofer-Moest decarboxylations of A/ -protected amino acids and a-amino malonic half esters under the formation of A/ -acyliminium ions is treated in the following section. 

Silicates may be used as an anodic inhibitor in low salinity water in the presence of oxygen. The protection mechanism involves the formation of a silica layer in the presence of some corrosion products of the metal. Bahadur [1993] states that the silica film is self limiting in thickness and self healing when damaged. Continuous treatment is required. It is noteworthy that silica inhibits the further corrosion of steel already having an oxidised surface. Sodium silicate in liquid or dry form, may be used as the additive. A useful property of this treatment is that there are no toxicological problems since silicates are naturally occurring. 

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 sprayed anodes are widely used in the United States for impressed current cathodic protection. Originally, the mesh anodes were mechanically clamped to the pile. In later versions, a GRP jacket containing the zinc mesh is attached to the pile and filled with gront after connecting the zinc to the reinforcement (Figure 7.6). All these systems and their performance in Florida are discussed in Kessler et al. (1995, 2002). 

CPs are redox-active materials having a positive equilibrium potential with respect to those of iron, aluminium and other alloying elements (Table 10.1). This suggests anodic protection as more expected corrosion protection mechanism. For CPs electroactivity, it exists a range of potential values because reduction potential depends on kind and level of doping. 

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