Mechanical measurements of passive film fracture on an austenitic stainless steel
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Mechanical Measurements of Passive Film Fracture on an Austenitic Stainless Steel D. RODRIGUEZ-MAREK, M. PANG, and D.F. BAHR The initiation of fracture in passive films formed on a 304 stainless steel has been measured using a nanoindentation technique as well as bulk circumferentially notched tensile bars (CNTBs). The nanoindentation method was coupled with scanning probe microscopy to isolate individual grains that were free of any observable inclusions, so as to probe only the properties of the film upon the base alloy. The mechanical response of the film was measured while being anodically polarized in 0.1 M sulfuric acid with various halide concentrations, as well as with respect to the applied potential. The passive film strengthened as the applied potential increased in the passive regime, possibly due to film-thickness changes. In both the bulk and nanoscale tests, the passive film-fracture strength was found to decrease with increasing salt concentration in solution, which cannot be attributed to the uniform thinning of the passive film. The correlation between the bulk and nanoscale tests demonstrates that both methods are viable options of measuring the fracture of passive films on metals. Nanoindentation results are used to estimate the applied tensile stress at film fracture between 1 and 2 GPa for an anodically grown passive film on 304 stainless steel at 0 V vs that on Ag/AgCl in 0.1 pct NaCl–0.1 M sulfuric acid.
I. INTRODUCTION
PASSIVE films on metals, such as stainless steels, aluminum, and titanium, provide resistance to corrosion in a variety of environments. The rupture of the passive film will expose a bare metal surface to the surrounding environment, which leads to a transient increase in the corrosion rate as the metal oxidizes. One methodology adapted in the study of passive films has been to examine the repassivation of a scratched surface, which exposes bare metal to the corrosive solution.[1,2,3] Another method that rapidly exposes bare metal to solution is the fracture of thin metal films on inert substrates.[4,5,6] By exposing a fixed area of bare metal to solution while monitoring the current, it is possible to determine the repassivation characteristics. Both of these methods do not aim to monitor the strength of the film, but, rather, examine the behavior of the repassivation of the fractured area. Mechanical contact damage to the surface of a passive film is only one way in which the mechanical integrity of the film can be compromised. Stresses that develop in the passive film due to lattice mismatch between the oxide and metal and electrostriction,[7] or stresses that are generated by mechanical loading of the bulk part,[8] can also cause passive film failure. In addition, it is possible to break the passive film during loading by the emergence of dislocations from the bulk metal to the free surface.[9] In all these cases, the passive film may fail via mechanical means.[10,11] The stress at which the film breaks is dependent on the potential at which it is tested.[12,13] Studies
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