Surface properties self-healing

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. 

Zinc is an example of a metal with an amphoteric oxide layer. The Pourhaix diagram is shown in Figure 10.20. The oxide is stable between pH 8 and pH 12. A natural oxide layer can be formed on the zinc surface but the usual procedure is a surface treatment in an oxidizing solution. An example is the chromatizing described in Section 10.2.5. With this treatment a mixed oxide of the formal composition ZnO CrO(OH) is formed. In the case of chromate treatment the oxide film contains chromate ions. The chromate ions are responsible for the self-healing properties of these films. 

More recently, it has been pointed out that aged gels possess self-healing properties at room temperature, without the need for any stimulus or healing agent [10]. Welding occurs spontaneously via chain diffusion across the interface provided that a sufficient number of PVA free hydroxyl groups survive at the contact between the two cut surfaces [10]. 

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