Abstract
The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed.
Highlights
Failure, whether chemical or mechanical in nature, often occurs where dissimilar materials are joined together
Metallographic samples of the post-braze joint cross-section were examined via Scanning Electron Microscope (SEM) in order to Metallographic samples of the post‐braze joint cross‐section were examined via SEM in order to characterize theStainless resulting joint (Figure 1)
scanning Kelvin probe force microscopy (SKPFM) results showed the stainless steel surface had an average Volta potential known and observed corrosion behavior, in that corrosion preferentially occurs within the braze alloy, with theMicrogalvanic coupled stainless acting as a cathode in the∆Volta potential differences (VPD)
Summary
Whether chemical or mechanical in nature, often occurs where dissimilar materials are joined together. Brazing is an alternative joining technique in which a filler material is heated so as to form metallurgical bonds with the surfaces of the parts (parent materials) being joined. Good compatibility is achieved when the braze alloy wets the target material well. This ensures excellent surface coverage and any diffusion that occurs does not result in poor mechanical behavior. Titanium is often added to silver based braze alloys in order to enhance their ability to wet the parent material(s) being joined, which is vital when joining dissimilar materials, including ceramics [1]. Indium additions in a titanium-containing braze alloy increase activity of titanium and wettability [8,9]
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