The Effect of Intermetallic Precipitation on the Corrosion Resistance of Ultra High Strength Stainless Aerospace Steels

Abstract

Abstract In the present work, the microstructure, precipitation behaviour and corrosion resistance of a novel maraging stainless steel (Alloy P, Fe20Co15Cr5Mo0.016C) under a range of ageing conditions (as quenched, aged at 460°C, 500°C and 530°C) are investigated. The relationship between the microstructural evolution of the alloy matrix and ageing temperature was then evaluated, with a focus on subsequent effect on passivation and susceptibility to localised corrosion. The alloy possessed a lath martensitic microstructure with an average prior austenite grain size of 228 ± 2μm. A number of thin (100 hours) would result in the segregation of the matrix into Fe-rich and Cr-rich martensites, partially contributing to the strengthening of the alloy by solid solution and phase boundary effects. The passive film formed on the steel consisted of a bi-layer structure, including a Cr-rich compact inner layer made up of oxides and hydroxides of Cr3+ and a non-uniform porous outer layer made up of oxides and hydroxides of Fe and Co in their Fe/Co2+ and Fe/Co3+ oxidation states and Mo in its Mo4+ and Mo6+ oxidation states. Film thickness remained consistent with increasing ageing temperature at 4-5nm thick based on oxygen EDX mapping. This was due to the lack of availability of Cr from the alloy matrix, caused by clustering and segregation lowering the concentration of Cr in the adjacent alloy matrix below that necessary for the steel to passivate. This was observable as a change in the uniformity of the film from a compact, continuous structure in the as quenched state to a rough and discontinuous structure with cavities where passivation was unable to take place due to insufficient Cr being available. This is consistent with the thickness of the outer Fe-rich layer of the film remaining unchanged. Electrochemical analysis found the alloy became more susceptible to pitting and general corrosion with increasing ageing temperatures. Pitting occurred in the 530â�°C aged condition on the application of any anodic overpotential relative to OCP, while the as quenched state had a pitting potential of 450 mV vs Ag/AgCl ref and no observable surface corrosion. This was due to the passivation of the alloy being compromised as a result of the lack of available Cr in the alloy matrix. A comparative alloy based upon custom 465® did not exhibit a change in susceptibility to pitting as a result of ageing.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Skeldon (Supervisor) & Xiaorong Zhou (Supervisor)