Tarnishing film-induced brittle cracking of brass
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Tarnishing film was developed on the brass surface in Mattsson’s solution at room temperature. The filmed brass was removed from the solution, dried, and subjected to a slow strain rate (loading speed = 0.5 mm/min) in air for studying the effect of the film on crack propagation in the brass substrate. It was observed that initial cracks started to emerge in the film and then propagated to the brass matrix in a brittle intergranular manner. However, it changed into a ductile mode after removing the deposited film. The galvanic current between platinum wire and filmed brass sample in Mattson’s solution was investigated. The results showed that periodic current fluctuations were observed when the sample was under a constant applied load. These observations showed that the film rupture-formation occurred at cracks under the stress-corrosion cracking condition.
I. INTRODUCTION
Many studies have shown that surface films such as a passive film and a dealloyed layer can cause the brittle fracture of many normally ductile metallic materials and thus induce stress-corrosion cracking (SCC).1–8 The film-induced cleavage model that has been developed mainly by Sieradzki, Newman, and colleagues1–4,7 proposes to explain both transgranular and intergranular SCC resulting from the surface films. The model was based on the idea that a crack, originating in a surface layer, can obtain a high enough velocity to penetrate into the underlying substrate even if the substrate is a facecentered cubic (fcc) metal. Newman et al.3 found that a film-induced cleavage fracture occurred when a brass foil with a dealloyed layer was strained rapidly in 15 M NH4+ + 0.05 M Cu(NH3)2+ solution or in air at 77 K. Kelly et al.4 reported that thin Ag–20Au foils had undergone a brittle fracture under a positive polarization in 1 M HClO4 without applied stress. Chen et al.5 showed that a Cu–30Au alloy with a dealloyed layer fractured in an intergranular mode under an impact load in 0.6 M NaCl. Devasenapathi et al.6 reported that an Mnsubstituted stainless steel with a layer of thick film showed brittle intergranular SCC under impact load in 1 M HCl, whereas the samples with removed films and subsequently impact loaded in air exhibited ductile fracture. Saito et al.7 showed that film-induced intergranular fracture occurred in Ag–20Au foil after dealloying for 10 s at 1050 mVSCE in 1 M HClO4, whereas a ductile fracture occurred after a pore-coarsening treatment to the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0289 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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dealloyed layer under various potentials. Cassagne et al.8 observed a Cu3Au single crystal with dimensions of 1.5 3 17 mm3 with a 3-mm-thick dealloyed layer. The results showed that, after immersion in 2% FeCl3 for 10 days and bending in air, many cracks formed in the dealloyed layer, some of which developed up to 20 mm into the substrate. After immersion for 30 days, a gold sponge formed bec
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