Innovative processing using ultrafine particulation

Anodic sites like grain-boundaries, micro-voids, micro-inclusions and other stress concentration points such as dislocations, vacancies, etc., take an active part in the galvanic interactions occurring on the surface of metals and alloys. This paper discusses the role of low concentration reinforcement of metals like Zn, Pb and Sn, alloy matrices like Cu-Zn, Cu-Mn, Zn-Al and Al-Zn and non-metals like polyaniline, along with the amorphous Ni-P-B class of current-assisted electroless deposits. It is assumed that ultrafine ceramic particulates in low concentration ranges (1–1.5 wt%) may cover the anodic defect sites such that these anodic boundaries start behaving cathodically as a network, within the already existing cathodic grain matrices, because they are predominantly more cathodic, compared to the grain boundaries, voids, etc. As such, there is a considerable reduction of surface dissolution and in the anodic current of the matrix. It has been shown that surface dissolution is a minimum for a critical threshold concentration of the particulates, above which there is drastic dissolution of the surface. It appears that particulates above that critical concentration cannot be accommodated within the available anodic sites, such that they are dispersed on the grain proper in a random fashion, creating stress spikes and subsequent enhanced matrix dissolution. It further appears that it will thus be possible either to decrease or increase the dissolution of the composite matrices for selective tailor-made applications, by changing the concentration of these ultrafine particulates around the threshold concentration. It has been shown that such a technique may eventually reduce the leaching of copper from conventional brass matrices and also reduce the dissolution of tin in the packaging industries. Grain boundary structures of the metallic matrices have also been correlated with the particulate trapping capacity and their corresponding galvanic stress factors. It has also been applied to amorphous Ni-P-B types of metal-metalloid coatings, for enhancement of surface corrosion resistance. Particulation of epoxy and epoxy-silicone classes of barrier with ultrafine SiC has revealed a more positive potential and lower galvanic currents. Reinforcement of the permalloy type Ni-Fe-Mo class barrier with fine Al2O3 particulates has indicated considerable improvement of the polarization resistance values.