Passive pitting

Hitzig et al. have produced a simplified model of the aluminium oxide layer(s) to explain impedance data of specimens prepared under different layer formation and sealing conditionsThe model also gives consideration to the formation of active and passive pits in the oxide layer. Shaw et al. have shown that it is possible to electrochemically incorporate molybdenum into the passive film which, as previously noted, improves the pitting resistance. 

NiS04 7H20. 330g/1 Nickel Passive Pitting 0 + 0-08... 

Figure 32 (a) The equivalent circuit model and physical model for a passive pit in a... 

One frequently asked question concerning cathodic protection systems is what happens at the anode edge Is there a risk of accelerated corrosion This is a valid question and the risk is supported by the Pourbaix diagram which shows areas of imperfect passivity, pitting and corrosion around the immune and passive regions (Figure 7.2). However, the author knows of no atmospherically exposed reinforced concrete structure that is totally protected by cathodic protection. Most have anode zones that end before the reinforced concrete does. No cases of accelerated corrosion have been reported between zones or at the end of zones. 

Fig. 1. Schematic polarization diagram explaining the action of the effective cathodic coatings on the steel corrosion ip, icp., ipit, it- respectively currents of initial passivation, complete passivation, pitting formation and corrosion in transpassive state.

However, most pits will stop after a few days. Polarisation studies have demonstrated that when pits stop growing, they will be repassivated. When the metal is polarised once again, these passivated pits will not be reinitiated again, but pitting will start on fresh sites. 

Potential monitoring Potential change of monitored metal or alloy (preferably plant) with respect to a reference electrode Measures directly state of corrosion of plant, e.g. active, passive, pitting, stress corrosion cracking, via use of a voltmeter and reference electrode Moderate... 

Pitting arises when localized/aggressive environments break down the nominally passive and corrosion-resistant film. Pits may form at scratches, mechanical defects, second phase particles, or stochastic local discontinuities in the oxide film. Since by definition pitting refers to local loss passivity, pitting only occurs in the near-neutral pH range for Al. 

Measures directly state of cotrosion of plant Moderate (active, passive, pitting, stress-corrosion cracking via use of a voltmeter and reference electrode. 

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