Abstract
Diode components used in the electronic relay packages of avionics of a weapon system had been found corroded. Two sets of the complete assembly of the damaged diode parts consisting of diode body, plain washer, spring washer and mounting nuts were analysed for finding out the root causes of failures. Detailed metallographic characterization of the failed materials was carried out using optical microscope, scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) analysis within SEM and microhardness study. Inadequate electroless nickel (EN) plating along with significant amount of porosity and presence of MnS type of stringers affected the corrosion resistance of the diode body tremendously for both the sets. Inadequate electroplated Zn layer was observed on spring washer, plain washer and nut, wherein both uniform and pitting corrosion were detected. Important recommendations included: (i) thicker (more than 25 µm) EN coating with as-specified porosity level for diode body, (ii) replacement of electroplated zinc layers with galvanized zinc (of higher thickness) layer on all the fasteners, (iii) precaution in minimizing formation of galvanic couplings observed in the present system, like that between EN-coated diode body and zinc coated fasteners, between mild steel and stainless steel fasteners etc.
Highlights
Introduction to Surface Engineering forCorrosion and Wear Resistance (#06835G), www.asminternational.org, last accessed on 10.05.2018.9
It is to be noted that the spring washer among all the components of set 1 seems to have experienced least corrosion damage, while plain washer of set 2 assembly did not reveal any sign of corrosion
It is to be noted that the stud part for diode body of set 2 assembly is tapered unlike that for set 1
Summary
Introduction to Surface Engineering forCorrosion and Wear Resistance (#06835G), www.asminternational.org, last accessed on 10.05.2018.9. LITERATURE SURVEY 2.1 Power Diode A power diode has larger PN junction area as compared to its smaller signal diode cousin [1] This results in a high forward current capability of up to several kilo-amps and a reverse blocking voltage of up to several kilo-volts. It is not suitable for high frequency applications above 1 MHz, but special and expensive high frequency high current diodes are available for the said purpose. The failed present diode system is a typical commercially available general purpose standard recovery high current rectifier 1N2131 200V 60A Cathode Stud type [2] It usually comes with mounting hardware like plain washer, spring washer and nut (Figure 1). It is usually used in applications such as dc power supplies, inverters, converters, ultrasonic systems, choppers and as a free-wheeling diode
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