Analysis of Local Defects in Surface Films on Commercial Alloys Using Conductive Atomic Force Microscopy (C-AFM)

Abstract Localized corrosion is still a hotbed for research. Much is still unknown about the initiation of local corrosion and the subsequent evolution of stable pit growth. It is a general belief that discrete heterogeneities in the passive film act as nucleation sites for the initiation of local corrosion, either through anion penetration through the film or electron transfer across the film. In this work, conducting atomic force microscopy (C-AFM) is demonstrated as a technique for qualitative analysis of electrical conductivity at localized sites on surfaces. Electrical conductivity may arise in regions where the passive film is thin enough as to allow electron tunneling across a passive film. Application of a voltage bias between the AFM tip and the conductive specimen generate currents in the sub nA range that can be simultaneously measured with the surface topography. In this paper, emphasis will be given to both Type 304 stainless and Al 6061-T6 alloys. The goal of this research is to correlate conductivity with local defects at the surface of a particular alloy and follow the evolution of the conductivity map as a specimen interacts with its environment. Experiments reported here have probed passivating environments, as well as corroding environments.