Kelvin Probe Force Microscopy Potential Research Articles

Application of Kelvin Probe Force Microscopy (KFM) to Evidence Localized Corrosion of Over-Aged Aeronautical 2024 Aluminum Alloy

The 2xxx serie aluminum alloys are characterized by good mechanical performances and low density, however they are susceptible to different forms of localized corrosion: pitting corrosion, intergranular corrosion and stress corrosion cracking. The 2024-T351 aluminum alloy is used in the aircraft industry for numerous applications such as fuselage and door skin. Corrosion damage of the material is also very detrimental for the structural integrity of the aircraft. The presence of coarse intermetallic particles, with a heterogeneous size distribution was found to be responsible for the 2024 susceptibility to localized corrosion. These particles are generally the cause of initiation sites. Presence of micro-defects in the oxide film upon coarse intermetallic particles and the galvanic coupling with the matrix contribute to the development of pitting corrosion. The over-ageing treatment (T7) is supposed to stabilize the microstructure and the mechanical properties to improve the corrosion resistance. The 2024 alloy microstructure after the T7 heat treatment remains very complex. The 2024 alloy corrosion behavior was studied in the over-ageing state for three different temperatures (150, 175 and 190 °C). During the corrosion tests in chloride-containing environment, the behavior of coarse intermetallic particles was found to be different. Thus, the 2024 samples suffer a gradual attack upon S-Al2CuMg particles and finally Al (Cu,Mn,Fe,Si) particles. The corrosion damage was studied by Atomic Force Microscopy (AFM) and Kelvin probe Force Microscopy (KFM). This technique allows simultaneous topographical and electric potential mapping to be obtained. This latest potential was shown to be correlated to the corrosion potential of the 2024 alloy. This study focuses on the variation of the KFM potential of the coarse intermetallic particles and the matrix for the over-ageing conditions (T7). Observations using optical microscope and AFM were also performed to obtain the corrosion rate for each condition. The corrosion rate was correlated to the chemical composition variation of the particles obtained by scanning electron microscope observations and EDS analyses.

Potential Characterization of Interconnect Corrosion by Kelvin Probe and Electrostatic Force Microscopies

The fine-scale potential observation was performed by Kelvin probe force microscopy (KFM) and electrostatic force microscopy (EFM) using tungsten interconnects in which the minimum width was 43 nm. It was confirmed that the line connected to pads and/or a transistor has a peculiar KFM potential by comparing it with the surrounding pattern. Only the plugs attached to this line were corroded, while the other plugs remained intact. By comparing the results of KFM and EFM observations, the peculiar potential of the line can be attributed to the electron charge up in the bulk of the sample. Charged electrons induced corrosion not only in the line itself but also in the connected structure. It was also revealed that the particular potential can be suppressed by a wet treatment.

Cut Edge Corrosion Inhibition by Chromate in Primer of Prepainted 55% Al–Zn Alloy Coated Steel

The effect of chromate in primer layers on the cut edge corrosion of prepainted 55% Al–Zn alloy coated (galvalume) steels has been examined under simulated atmospheric corrosion conditions in a marine environment. The chromate released from the primer layers in effect prolonged and expanded the sacrificial protection of the 55% Al–Zn layers. The corrosion potentials, measured by a Kelvin probe, indicated that the cut edges were maintained at low potentials. The galvanic current measured for the 55% Al–Zn/steel couple specimen with the chromate-containing primer was lower than that of the specimen with the chromate-free primer. This concurs with the results of conventional polarization measurements, indicating that the cathodic oxygen reduction on steel substrates is inhibited by the presence of chromate. The corrosion potential variations across the interface between 55% Al–Zn layers and steel substrates were measured using Kelvin probe force microscopy (KPFM). Almost negligible variations in the KPFM potential were found at the interface in the cut edges with the chromate-containing primer. Our findings indicate that the prolonged and expanded sacrificial protection effect of the 55% Al–Zn layers is likely due to the release of chromate from the primer layers, which makes it possible for the 55% Al–Zn layers to dissolve slowly.

Tip-to-sample distance dependence of an electrostatic force in KFM measurements

In most scanning probe methods like an atomic force microscopy, a cantilever is mechanically vibrated in order to obtain topographies. Therefore, a tip-to-sample distance periodically changes during the scanning. Since the electrostatic force, which is a long-range force, is used for potential feedback in Kelvin probe force microscopy (KFM), such mechanical vibration leads to the fluctuation of the electrostatic force between the tip and the sample. In this study, firstly, we performed two-dimensional simulations of the electric fields between surfaces of the tip and the sample and evaluated the tip-to-sample distance dependence of the electrostatic force. Secondly, we experimentally confirmed the existence of the fluctuation of the electrostatic force and the tip-to-sample distance dependence of the electrostatic force was evaluated. Both the simulations and the experiments on the tip-to-sample distance dependence showed the importance of considering the tip sidewall effect in the KFM potential determination.