Galvanic Corrosion Evaluation of Discarded Military Munition Steel Casing and Copper Driving Bands in Seawater
Authors: Srinivasan Raghu , Hihara Lloyd H. 
Published: February 2020

Discarded military munitions (DMM) were disposed off of the Waianae shoreline on the Island of Oahu, Hawaii at the Ordnance Reef (OR) after World War II. To study the corrosion morphology and to estimate the corrosion rate of the DMMs that were submerged in seawater for more than 60 years, few DMMs were recovered from the seafloor and demilitarized. Breaches in DMMs due to corrosion in seawater present a potentially dangerous situation for the marine environment and to humans. The objectives of this research were 1) to conduct metallographic analysis on the DMMs to identify the alloying elements in the steel DMM casing and copper driving bands, and 2) to estimate the galvanic corrosion between the steel DMM casings and the copper driving bands.

Potentiodynamic polarization and zero-resistant ammeter (ZRA) electrochemical experiments were conducted on electrodes made from the DMM steel and copper driving band in simulated (3.15 wt% sodium chloride solution and ASTM seawater) and real seawaters. Polarization experiments were used to identify the governing corrosion mechanisms, and ZRA experiments were used to determine the steady-state galvanic current (IGalv) between steel and copper. Results from polarization and ZRA experiments conducted in the simulated seawaters were correlated. The IGalv between steel and copper electrodes were measured in real seawater for 14 days. The ZRA experiments were conducted under immersed condition in real seawater for two configurations: specimens in the unburied condition and specimens buried under sand. The IGalv from ZRA experiments were used to calculate the corrosion rate in grams per meter squared per day (gmd) using Faraday’s law. Electrodes from polarization and ZRA experiments were analyzed with a Hitachi S-3400N Scanning Electron Microscopy (SEM) and Oxford Instruments Energy Dispersive X-ray Analysis (EDXA) for elemental analysis.

The DMM casings were identified as high-carbon 1069 steel (UNS G10690), and the driving band were 99.7 % pure copper (UNS C12210). The galvanic corrosion rates predicted by the polarization experiments in aerated 3.15wt% NaCl were in agreement to those in aerated ASTM seawater. The galvanic currents determined from polarization were only in agreement with those measured in the ZRA experiment for the initial period. The galvanic currents measured with the ZRA diminished with time in real and ASTM seawater when compared to those in 3.15 wt% NaCl solutions. The calcareous deposits on the copper cathode in the aerated seawaters stifled oxygen reduction resulting in the decrease in the galvanic current with time compared to that in 3.15 wt% NaCl solution where calcareous deposits do not form. hence, the polarization experiments had utility in identifying the mechanisms of galvanic corrosion (i.e., cathodic control in this case); whereas, ZRA experiments were more useful in determining the evolution of IGalv with exposure period. The seawater ZRA experiments also showed the corrosion rate of steel coupled to copper in the unburied condition (0.163 mm/year) was much higher than those buried under a few inches of sand (0.030 mm/year) due to lack of dissolved oxygen in the buried condition. This implies that the corrosion damage of DMMs can vary significantly depending on whether they are boldly exposed or buried under sand or debris.