The external surface of all platinized titanium anodes is essentially pure platinum which exhibits all the normal characteristics of the wrought metal as far as corrosion behaviour is concerned. The secret of the quality of a platinized anode lies in the nature of the interface between the platinum and the titanium. It is well known that normal untreated titanium presents a very inert and intractable surface when it comes to applying precious metal coatings. That this problem was overcome to an adequate extent is demonstrated by the wide acceptance of platinized titanium anodes in different industries throughout the world. Although there are still many platinum electroplated anodes produced without an adequate interlayer, they are used for less demanding applications.
The shortcomings of such anodes are that the coating has pores and cracks which allow access of the surrounding electrolyte to the titanium substrate. This porosity increases during the life of the anode as the platinum coating wears away, thus allowing increased exposure of the substrate, formation of internal crevices under the coating and progressive loss of points of mechanical attachment between the coating and the substrate.
Under mild conditions of operation this degradation is not serious, but it may be aggravated by adverse factors. It must be remembered that all operating anode surfaces generate acidity, the level of which is related to current density and the external environment.
If the acid generated at the surface is not removed quickly enough (particularly from the internal crevices mentioned above) then the interface between platinum and titanium can be attacked, leading to rapid and progressive coating loss by undermining.
This situation can arise under conditions of stagnation or more quickly where deposits are allowed to form on anodes (e.g. manganese dioxide in sea water). An extreme case is where an anode is inadvertently buried or allowed to become silted. Although not allowed for at the design stage these conditions can occur accidentally in many Cathodic Protection situations.
Conditions are sometimes met in metal plating (e.g. gold plating of electronic components) where the composition of the electrolyte will activate the normally passive surface of the titanium and allow deposition of foreign metals within the pores of the coating and at the coating/titanium interface.