Corrosion and High Temperature Oxidation Behavior of 316L Stainless Steel Joined with Cu-Ag Based Braze Alloys

Brazing is used as a materials joining technique when conventional methods may not provide adequate performance. Many industrial processes achieve greater efficiency at increased temperatures and hence materials with higher working temperatures such as ceramics are attractive candidates. In addition to metal-to-metal brazes, robust metal-to-ceramic joints are sought after to support higher temperature applications. Although many different braze alloys exist for specific materials systems, the effect of brazing on corrosion behavior has not been widely investigated. In this study, commercially available Cu-Ag-Ti and Cu-Ag-In braze alloys were obtained and used to join 316L stainless steel samples. These brazes are suitable for creating hermetic seals with ferrous based metals as well as joining ferrous metals to ceramics. The impact of using such brazes on the corrosion behavior of the system is investigated in this research in progress.

         316L samples of various geometries were joined under air and Ar atmosphere using both braze alloys. Following joining, elemental and phase composition across the braze and parent metal interface was characterized with optical microscopy and SEM with EDS. Elemental and phase information provides likely initiation spots for both galvanic attack at the metal/braze interface or dealloying within the braze material due to formation of multiple stable phases upon cooling after the brazing furnace profile. Baseline electrochemical testing was performed on each of the braze alloys in the fired and unfired condition. Additionally, metal-to-metal braze specimens were prepared in order to expose the braze interface to 0.6 M NaCl electrolyte where the free corrosion potential was monitored. Following exposure to the aggressive solution, the corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. The critical potential for localized corrosion initiation was also investigated for the braze alloys when connected galvanically to 316L samples to determine the impact of brazing on localized corrosion. Moreover, bulk metallic braze samples were provided to collaborators at CMRDI to perform additional corrosion and high temperature oxidation testing. Initial results indicate dissimilar metal driven corrosion attack at the braze metal interface into the parent 316L as well as preferential dissolution of the Cu rich phase within the braze alloy when exposed to 0.6 M NaCl. Moreover, oxidation studies indicate that the joints will remain stable up to 600 C, depending on the environment in which the joints were formed.