Papers by Author: Kemal Nisancioglu

Abstract: The properties of oxide formed on pure aluminium in acetate buffers under controlled potential, temperature, and pH are characterized by in situ electrochemical techniques and spectroscopic visual ellipsometry (VISSE). Monitoring the current density (cd) during transition between different applied potentials gives direct information about Faradaic changes in the oxide properties, while the ellipsometer monitors the changes in the actual thickness and refractive index of the film. Electrochemical impedance spectroscopy (EIS) reveals, in addition to film properties, information about the charge transfer processes and adsorption at steady-state conditions. VISSE and ex situ transmission electron microscopy (TEM) indicate the presence of a two layered oxide structure, consisting of the barrier and a porous hydrated layer. The thickness and density of these two layers are monitored in situ during growth and dissolution. The thickness of the barrier layer depends reversibly on the applied potential, while the thickness of the hydrated layer is less affected by the potential. The absorption sensitive parameter is correlated with the buffer concentration and therefore believed to be related to the acetate.

Abstract: High temperature heat treatment of aluminium alloys causes surface enrichment of the trace elements in Group IIIA - VA, specifically the low melting point elements Pb, Bi, In and Sn. The phenomenon has practical significance in promoting certain types of localised corrosion, such as galvanic and filiform corrosion, while mitigating other types, such as pitting corrosion of the bare surface. The purpose of this paper is to investigate the surface enrichment and microstructure of indium relative to the available data for Pb. Model binary AlIn alloys, containing 20-1000 ppm of In, were used after heat treatment at various temperatures. In addition to electrochemical investigations, the microstructures were characterised by field emission scanning electron microscopy (FEG SEM) and field emission transmission electron microscopy (FEG TEM). Heat treatment at temperatures as low as 300°C gave significant segregation of In as opposed to 600°C for Pb. As a result of this and yet unresolved oxide film breakdown mechanism on aluminium, In was significantly more effective than Pb in anodically activating aluminium. These results suggest the possibility that significant activation earlier observed on certain commercial alloys as a result of low temperature heat treatment may be due to the trace elements In.

Abstract: Intergranular corrosion (IGC) of model alloys in the 6000-series, with and without 0.2 wt% Cu, was studied using an accelerated corrosion test (BS ISO 11846 B), FE-SEM and FE-TEM. Low Cu alloys (0.02wt%) did not exhibit IGC even though they contained excess Si. The high-Cu, naturally aged material (T4) was susceptible to severe superficial etching. In the underaged state (below peak strength), the Cu-containing material was highly susceptible to IGC. Materials aged to peak strength (T6) or overaged were only slightly susceptible to IGC, with localized, shallow attacks. FE-TEM investigation of the underaged material revealed scattered, small AlMgSiCu-type precipitates, as well as a Cu-enriched film along the grain boundaries. The overaged material showed more extensive, coarse grain boundary precipitation. However, the Cu-enriched film was still present at localized sites. The reduced susceptibility to IGC upon artificial ageing was attributed to breaking of the continuity of the grain boundary film. The possible role of matrix precipitation is also discussed.

Abstract: Applicability of the wedge test and the accompanying fracture mechanical data analysis, commonly used for testing of adhesively bonded joints, was inestigated in an attempt to develop a quantitative laboratory test for evaluating paint adhesion on aluminium. The test was further used, along with a relatively better accepted laboratory test for filiform corrosion of painted aluminium to study the mutual effects of corrosion and adhesion of various pretreatments and paint systems on extruded AA 6082 T6 aluminium alloy. Pretreatments used were commercial processes for deoxidising, chromating, hot AC anodising and Ti/Zr conversion coating. The paints were polyester TGIC and epoxy DICY, both pigmented commercial products. In particular, the compatibility of pretreatment-paint combinations were investigated The wedge test methodology and sample preparation developed proved to be a satisfactory approach for quantitative evaluation of pretreatment-paint combinations for aluminium. Ti/Zr treatment gave exceptionally good adhesion and corrosion results when coated with polyester. Epoxy coated Ti/Zr bars failed completely. However, hot AC anodising, although in general significantly more robust than Ti/Zr, was more compatible with the polyester than polyester coating. With the advent of chromate-free pre-treatments, the need to establish the compatibility of the modified aluminium surface and applied organic coating was demonstrated.