Final Report - Strategic Environmental Research and Development ...

2.4.4 In situ AFM Scratching Results
In situ AFM scratching involves rastering a region in contact-mode with a Si cantilever at
sufficiently high forces (as measured by photodiode setpoint voltage, SP) to destabilize passivity.
This technique provides real time information of the early stages of localized corrosion with submicroscopic
resolution. When analyzing the evolution and kinetics of attack, three things have
to be considered: the force that initiates attack, the time for attack to nucleate, and the corrosion
morphology. Higher forces and longer nucleation times imply greater inhibition performance.
AFM scratching was used to investigate the influence of the various inhibitors in 0.1 M NaCl.
Figure 2.28 shows the chronological sequence of scratching in aerated 0.1 M NaCl solution with
125 mM MoO 4 2- . No attack was observed within or around any IMC, or on the matrix after
scratching for over 10 hours and gradually increasing the scratching force to 525 nN. As the
experiment progressed it is apparent that the IMCs all protrude higher from the surface as
illustrated in the topography images by the increasing brightness of the IMCs. This would
suggest the formation of a protective film over the IMC particles. SEM/EDS analysis after in
situ scratching confirms that no attack occurred within the rastered area (Figure 2.29).
Figure 2.28. In situ AFM scratching in 0.1 M NaCl with 125 mM MoO 4 2- .
Scan size=80µm. Z range=350nm.
Figure 2.29. SEM micrographs before and after in situ AFM scratching in 125 mM MoO 4 2- .
77