The passivation and localized corrosion of a-,

The composition of corrosion layers formed on a-, /3-and (a + &brass anodized in the passive region in borate-boric acid buffer and 0.5 M NaCl + borate-boric acid buffer (pH 9) was studied comparatively by X-ray photo-electron and Auger electron spectroscopy. Passivation of brass in both solutions involves the formation of a complex passive layer consisting of ZnO and CyO. In both solutions, the ZnO electroformation results in a dezincification so that a thin Cu rich layer is formed at the alloy/metal oxide interface. Passive layer composition and dezincification of the alloy surface explain the localized corrosion resistance of brass as compared to polycrystalline Cu and Zn.

The passivation and localized corrosion of a-brass and B-brass in an aqueous borateboric acid buffer (pH 9) containing different concentrations of NaCl (c~,~, ) in the temperature range Y'CI T<45"C were studied comparatively by potential sweeping at 2 x 10-2Vs-' and 2 x 10-4Vs-' and de-alloying measurements. The passivation of a-brass and /?-brass was related to the electroformation of a complex ZnO xH,O/CutO layer, its thickness and compactness presumably increasing with temperature. For both a-brass and j-brass immersed in an aqueous NaCl-containing buffer, pitting corrosion was observed as the value of the breakdown potential (&) was exceeded. At constant temperature, the value of & shifted negatively as either cNaCl or the zinc content in the alloy was increased. For j-brass, the value of & decreased slightly with increasing temperature in the range 5"C< T145"C. In the range ~"CI T<25"C, for a-brass the value of Et, was close to that reported for copper, whereas for T>25"C it approached those values measured for b-brass. De-alloying measurements in aqueous 0.5 M NaCl indicate that zinc surface enrichment of a-brass was responsible for the decrease in pitting corrosion resistance at T>45"C.

The dealloying of -brass in 0.5 M aqueous NaCl was studied by electrochemical techniques at different temperatures in the range 278 K e T e 318 K, complemented with in situ scanning tunneling microscopy (STM) imaging. In the potential region where the electrodissolution of zinc and the formation of vacancies and copper islands take place, two different roughness regimes were distinguished. When dealloying involves only a few monolayers (ML) the process approaches a quasiuniform alloy electrodissolution, whereas after electrodissolution of more than 20 ML, void formation takes place. In both regimes the interface evolution was analyzed by applying the dynamic scaling method to in situ STM imaging data. The first roughness regime exhibits a stable interface consisting of copper-rich islands that coarsen with time according to a surface diffusion controlled process. The second roughness regime exhibits an unstable interface due to a curvature dependent corrosion rate enhancing zinc electrodissolution at cavities. The overall interface evolution is well-described by a differential stochastic equation containing an electrodissolution term and surface rearrangement terms related to surface diffusion and negative surface tension effect.