Electrochemical Corrosion Behavior of Explosively Welded Ti/Type 304L Stainless Steel in Nitric Acid

Two metallurgically incompatible materials, titanium grade 2 and Type 304L stainless steel (SS), were joined through explosive welding for applications in nitric acid service. The morphology and microstructure of the bonded interface were examined using a scanning electron microscope (SEM) and an optical microscope. Typical wavy interfaces along with solidified melted zones at wave vortexes were observed. Various intermetallic phases, having high hardness, were shown to be formed in these solidified melted zones. Elemental distribution in these intermetallic phases was established using SEM coupled with energy dispersive spectroscopy (EDS). Low corrosion rates were observed after exposure of the weld specimens in boiling 14.5 M and 4 M HNO₃ solutions. In addition, potentiodynamic polarization tests in 4 M HNO₃ solution have been performed at different surfaces at the plane of the welded interface by successive material removal by grinding from 304L SS through weld to titanium. Despite the presence of different intermetallic phases at the test surfaces, it was found that the current densities in the passive regime of potentials were low. The morphological examination of the tested surfaces revealed that corrosion attack was mainly along the boundaries between SS/intermetallic phases, Ti/SS, and along the pre-existing cracks in the intermetallic phases. No corrosion attack was observed along the boundaries between Ti and intermetallic phases.