Disbonding, polymer-metal

The reaction between PANI and the hydroxide ions limits the increase in pH at the polymer-metal interface and creates a buffering effect on the electrolyte pH. This is a very important quality for inhibiting cathodic disbondment, which strongly depends on the concentration of OH" in the interface [72]. 

Two principle mechanisms that are discussed as possible corrosion protection mechanisms on mild steel are discussed in short. ICPs may induce the formation of a passive oxide [206]. The ICP will be reduced as a consequence of passivation and will be reoxidized by oxygen reduction. Consequently, the ICP may promote the cathodic oxygen reduction on the polymer surface rather than at the metal-polymer interface. On the basis of the good corrosion results gained by the combination of a molecular adhesion promoter and the subsequent electrodeposition of the polymethylthiophene film Rammelt and coworkers [207] concluded that the essential aspect of the corrosion protection by ICPs could be the local separation of iron oxidation and oxygen reduction. This would eliminate the local pH increase at the metal surface and subsequent cathodic disbondment. 

EIS examines the response of the corroding system to ac excitations at frequencies from 100 MHz to 100 KHz. The current or voltage is measured when a small amplitude voltage or current is applied to the working electrode. The method was initially used by DoHn and Ershler [50] and developed later by Randles and Somerton [51] and Grahame [52]. In corrosion research, EIS was extensively used to study the passivation of metals [53-57] to determine corrosion rates and to study the inhibitor performance, sacrificial and barrier coating performance, and the disbondment of polymer-coated metals [9,23,58—66]. Macdonald and Me Kubre pubHshed an excellent review on the use of EIS [67]. 

The fact that the reduction of O2 can take place at the polymer surface is also of great importance because this implies that the OH species, responsible for disbondment and delamination [38,39,41,42], has no effect on the adherence of the polymer. Unfortunately, as said previously, a conducting-polymer film is not fully impermeable, and some O2 and H2O will pass through the pores and be reduced at the metal-polymer interface with a release of OH ions and local delamination at the detriment of protection against corrosion. Delamination and disbondment of the coating upon reduction will be discussed later in this chapter, but it is useful to state that this is an important factor in the failure of the coating. 

---