Corrosion of brass in a marine environment: mineral products and their relationship to variable oxidation and reduction conditions

Minerals coating brass ammunition shells that rested at the bottom of Halifax Harbour, Nova Scotia, for 52 a have been identified by X-ray diffraction and analytical scanning electron microscopy. The admiralty brass shells, partially buried in anoxic muds, straddle a strong Eh gradient ranging from 0 mV to values characteristic of oxygenated seawater. Whereas the brass surface in contact with the sediment has been preserved, parts of the shells exposed to seawater have corroded throughout their thickness. The corrosion products identified include metallic Cu, djurleite (Cu1.96S), cuprite (Cu2O), atacamite (Cu2Cl(OH)3), spertiniite (Cu(OH)2) and hydrozincite (Zn5(CO3)2(OH)6). These products are those predicted thermodynamically on the basis of ambient Eh and pH. However, this study also revealed the presence of a mineral not previously known to exist and tentatively identified as Cu14Zn14Cl5(SO4)5(OH)41.H2O. This “new” mineral seems to have a stability field in Eh–pH diagrams similar to that of connellite (Cu19Cl4SO4(OH)32.2H2O).