EARLY DETECTION OF CORROSION
Too frequently, the first sign of corrosion is visual detection by the technician or even the customer. This unfortunate happenstance occurs across the board, from consumer products to critical components. By the time corrosion products are visible to the human eye, considerable surface modification has occurred.

Observational instruments are now available that go well beyond the capability of the human eye, allowing early detection of corrosive products. High resolution scanning techniques can be helpful during process development, for example, to select critical cleaning processes that minimize corrosive processes. For ongoing processing of critical product, early detection can effectively provide failure analysis of manufacturing processes.

One recently developed instrument that can observe the early effects of corrosion is the Atomic Force Microscope (AFM).¹ AFM has the resolution to detect surface topography changes during the early stages of corrosive action
(Figure 1).

AFM can be combined with Surface Potential Imaging to provide greater diagnostic information than the high resolution of surface topography. Surface Potential Imaging, sometimes referred to as Kelvin Probe Force Microscopy, detects differences in electric potential between the AFM-sensing cantilever tip and the surface directly below it.² Since different metals have different potentials, a surface potential image can detect small regions of dissimilar metals (down to sub-micron size) that, because of Galvanic effects with the surrounding matrix, can lead to enhanced corrosion. Coupled with the topographic image, a determination can be made if corrosive pitting has already occurred.

AVOIDING OR FORESTALLING CORROSION
As we discussed in Part I of this series,³ the key to avoiding or forestalling corrosion is to “break” the square (Figure 2) of facets involvedwith corrosive processes.

Books have been written on the control of corrosion. Table 1 indicates a few approaches to breaking this corrosion “square.”

THE GOOD SIDE OF CORROSION — BENEFICIAL CONTAMINATION
Can corrosion actually be good for you? The overwhelming response of industry would be “No!” However, just as there are instances of beneficial contamination,⁴ in a manufacturing world where micro-and nano- are becoming common prefixes to describe the size of components, there are actually caseswhere corrosion processes can be beneficial.