Investigation of Corrosion Behavior of Wrought Co-Cr-W Super Alloys

摘要

The corrosion behavior of two wrought Co-Cr-W superalloys is studied under both polarization immersion tests. The corrosive media, Green Death solution, is used in both tests. Potentiodynamic polarization and cyclic polarization testes are performed to investigate general and localized corrosion resistance of these alloys. Immersion tests of the two alloys are conducted in Green Death solution to determine Critical Pitting Temperature (CPT), mass loss, thickness change and the Extreme Value (minimum thickness) using Extreme Value Analysis (EVA) model derived from the Gumbel Distribution. A Scanning Electron Microscope (SEM) with Energy Dispersive X-ray (EDX) spectrum is utilized to analyze the chemical composition of the corrosion products (pits). The presence of carbides generates potential in the electrochemical reaction, causing corrosion of the alloys in the solution. The larger the carbide volume fraction, the more the pits are formed in the alloy. Carbide size also affects maximum pit depths; the larger the carbide size, the bigger and deeper the pits. The EDX analysis results of pits show large amount of oxygen in the carbide phase and a small amount of oxygen in the solid solution phase. The Cr-rich carbides react with oxygen forming Cr-rich carbonates which are easily brittle, loose and broken.