Innovative processing using ultrafine particulation

Anodic sites like grain-boundaries, microvoids, 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 reinforceme nt of metals like Zn, Pb and Sn, alloy matrices like Cu-Zn, Cu-Mn, Zn-Al an d Al-Zn and non-metals like polyaniline, along with the amorphous Ni-P-B cl ass 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 cathod ically as a network, within the already existing cathodic grain matrices, b ecause they are predominantly more cathodic, compared to the grain boundari es, voids, etc. As such, there is a considerable reduction of surface disso lution and in the anodic current of the matrix. It has been shown that surf ace dissolution is a minimum for a critical threshold concentration of the particulates, above which there is drastic dissolution of the surface. It a ppears that particulates above that critical concentration cannot be accomm odated within the available anodic sites, such that they are dispersed on t he grain proper in a random fashion, creating stress spikes and subsequent enhanced matrix dissolution. It further appears that it will thus be possib le either to decrease or increase the dissolution of the composite matrices for selective tailor-made applications, by changing the concentration of t hese ultrafine particulates around the threshold concentration. It has been shown that such a technique may eventually reduce the leaching of copper f rom conventional brass matrices and also reduce the dissolution of tin in t he 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 amorpho us NiP-B types of metal-metalloid coatings, for enhancement of surface corr osion resistance. Particulation of epoxy and epoxy-silicone classes of barr ier with ultrafine SiC has revealed a more positive potential and lower gal vanic currents. Reinforcement of the permalloy type NiFe-Mo class barrier w ith fine Al2O3 particulates has indicated considerable improvement of the p olarization resistance values.