From Phenomenology to Model

On the other hand the localized corrosion and more specially the role of galvanic coupling is largely discussed for aluminium alloys. In these alloys, microstmctural corrosion e.g. pitting or intergranular corrosion (IGC) can be initiated at the interface between constiment irrtermetallic (IM) particles and the matrix. Most of the constituent particles contained in stractural alloys have a simple cathodic behavior towards the matrix and support reduc-tioa This reduction can be corrsidered, in a simple way, as a fotu-electron process  

The morphological characteristic of a simple cathodic IM/anodic matrix system after corrosion is the formation of a groove or trench around the almost unattacked particle due to the matrix dissolution. This has been shown on iron rich particles in 

In the work of Alodan and Smyrl, the pH profiles over the particle were simulated by assuming a diffusion-controlled oxygen reduction reaction at the cathode and by setting a limiting diffusion current density of 0.1 itiA cm, calculated for a diffusion layer thickness of 20 pm and O2 concentration of 0.26 mol m.  

Hydrolysis of A1 into AIOH3 at the anode was taken into account. The proton gradient at the anode is controlled by the oxygen reduction limit current density. For an initial pH equal to 7, the maximum pH was around 10 (Fig. 23a). In the studies, modelhng was based on the resolution of second Pick s law but the boundary current densities were set by experimental macroscopic passive dissolution current density measurements. No hydrolysis was considered at the anode. The pH profiles show that alcahniza-tion up to pH = 9.5 is predicted for an initial pH of 6. 

Even if the discussion on the role of alcalinization is always open, the question of the possible convolution of this pH profile with the effect of pH on dissolution of aluminium seems never asked even if siuroimding trench around cathodic IM has been reported as shown in Fig. 24. It is obvious that the alkahnization clearly concerns the smrounding matrix and not only the IM (Fig. 23a), of which the size is fixed to 10 pm, whereas from the calculation the pH is higher than eight for a critical distance of about 25 micrometers which could correspond to the grooving observed sometimes. 

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