Diffusion galvanics

Sukhomlin et al. (1986) recommend diffusion galvanizing (or aluminizing) for tubes in the circulating water of refrigeration units. 

It also follows that if the solution is stirred the rate of arrival of oxygen at the cathode will be increased. This will result in a corresponding increase in the rate of bimetallic corrosion as is shown in Fig. 1.63 for the aluminium-mild steel couple in stirred 1 - On NaCl solution . The increase in galvanic corrosion rate will be in the inverse relation to the slope of the anodic polarisation curve of the more negative metal, provided that the cathodic reaction is not totally diffusion controlled. 

Corrosion by liquid metals is usually controlled by diffusion processes in the solid and liquid phases and, unlike aqueous corrosion, does not generally involve galvanic effects, and, even where electrochemical phenomena are known to occur, it has not, in general, been demonstrated that they have been responsible for a significant portion of the corrosion observed . In... 

In the predominantly electronically conducting electrodes it is the chemical diffusion of the ions which controls the electrical current of the galvanic cell. This includes the internal electric field which is built up by the simultaneous motion of ions and electrons to establish charge neutrality [14] ... 

One of the features found at interfaces between two electrolytes (a) and ( 3) is the development of a Galvani potential, between the phases. This potential difference is a component of the total OCV of the galvanic cell [see Eq. (2.13)]. In the case of similar electrolytes, it is called the diffusion potential and can be determined, in contrast to potential differences across interfaces between dissimilar electrolytes. 

Galvanic cells that include at least one electrolyte-electrolyte interface (which may be an interface with a membrane) across which ions can be transported by diffusion are called cells with transference. For the electrolyte-electrolyte interfaces considered in earlier sections, cells with transference can be formulated, for example, as... 

Practically all liquid cells with reversible interfacial equilibria examined can be considered as liquid galvanic cells of the Nernst, Haber, or intermediate type [3]. Usually, a dashed vertical bar ( ) is used to represent the junction between liquids. A double dashed vertical bar ( ) represents a liquid junction in which the diffusion potential has been assumed to be eliminated. 

The electrode reaction can involve the formation of a new phase ( e.g. electro-deposition processes used in galvanizing metals). The formation of a new phase is a multi-stage process since it requires a first nucleation step followed by crystal growth (in which atoms must diffuse through the solid phase to then become located in the appropriate site of the crystal lattice). 

GITT also provides very comprehensive information about the kinetic parameters of the electrode by analysis of the electrical current. The current 1, which is driven through the galvanic cell by an external current or voltage source, determines the number of electroactive species added to (or taken away from) the electrode and discharged at the electrode/ electrolyte interface. A chemical diffusion process occurs within the electrode and the current corresponds to the motion of mobile ionic species within the electrode just inside the phase boundary with the electrolyte (at x = 0)... 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

The defect scheme shown in equation 4.68 was later confirmed by electromotive force measurements with galvanic cells (Simons, 1986) and by diffusivity mea-... 

This arrangement is called a galvanic cell. A typical cell might consist of two pieces of metal, one zinc and the other copper, each immersed each in a solution containing a dissolved salt of the corresponding metal. The two solutions are separated by a porous barrier that prevents them from rapidly mixing but allows ions to diffuse through. 

---