Electrochemical Corrosion Test Methods for Rapid Assessment of Aerospace Bearing Steel Performance

Abstract

High-performance mechanical systems (bearings and gears) in advanced gas turbine engines are required to operate at ever-increasing operating speeds, temperatures, and loads. Premature failure by corrosion pitting is a major concern in aerospace systems operating in marine environments. To effectively assess the corrosion resistance of candidate heat treatments, a rapid screening test is needed. Electrochemical corrosion testing was performed to rank the relative performance of conventional bearing steels including 440C, American Iron and Steel Institute vacuum induction melting vacuum arc remelting 52100, M50, and M50NiL.Other steels in this study include Pyrowear 675, T15, CSS-42L, Cronidur 30, XD15N, and steels with various heat treatments. Existing corrosion testing methods such as the ASTM standards ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM G31, Standard Guide for Laboratory Immersion Corrosion Testing of Metals, and ASTM G5, Standard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements, lack sensitivity or have too much variability to adequately determine differences in corrosion performance among bearing steels with elevated chromium content. Testing for this study utilized anodic polarization scans and electrochemical impedance spectroscopy scans in simulated synthetic seawater to provide a way to rapidly screen the corrosion resistance. All testing was conducted in aqueous solutions at the free corrosion potential. Electrochemical testing in aviation lubricants is ineffective due to the very high solution resistivity. However, an aqueous solution provided a method to accelerate corrosion initiation that is similar to the processes that occur in-engine and was determined to be an effective method to rapidly obtain a relative ranking of corrosion resistance of the bearing steel variants considered. A ranking of bearing steels was developed, with CSS-42L and 52100 having the highest corrosion rate while the steels 440C, CR30, XD15N, and Pyrowear 675 (CN-A) had the lowest corrosion rates.