Influence of Microstructural Constituents and Applied Thermomechanical Processes on Corrosion Behaviour of Aluminum Alloys Produced with Twin Roll Casting (TRC) Technique

Abstract

Aluminum alloys produced with Twin Roll Casting (TRC) technology still necessitate to be thoroughly investigated in some areas. Corrosion mechanisms operating under special conditions with the contribution of unique features of the microstructure are among those. Materials produced with TRC have unique features inherently generated due to the solidification path of the material during casting. Contrary to the very fine particles at the locations close to the free surface, centerline segregation (CLS) occurring at the mid-plane of the thickness have different morphological and compositional features than rest of the microstructure. While, unless directly exposed, it has almost no contribution to the general corrosion behavior of the alloy, some manufacturing processes generation new free surfaces in the material for CLS to be exposed to the corrosive media can be harmful for overall general corrosion performance of the alloy. It differentiates itself in corrosion behaviour with its compositional and morphological features than rest of the microstructure. Hence, influence of CLS on corrosion behaviour of two different alloys in 3000 and 8000 series employed for heat exchangers and packaging applications, respectively, are studied in the present study. Since CLS does not disappear with rolling passes but only changes its morphology, samples were taken at appropriate thickness of the downstream process that allow corrosion tests to be conducted at the cross section of the samples. Metallographical preparation techniques were used to reveal the CLS. Samples were dipped in to the HCl-NaCl containing test solution to observe the progress of corrosion in the matrix and heavily populated CLS areas during the course of test. Open-circuit potential (OCP) measurements were carried out on the rolled free surface, quarter plane and mid plane of the samples after milling the surface of the samples. Different samples produced with compositional and thermomechanical processing route variants were used. Results show that composition of the alloy and applied thermomechanical processes influence the corrosion characteristics of CLS and accordingly the overall corrosion performance. Other important finding is the contribution of manufacturing method to corrosion mechanism whether if it reveals the CLS by creating new free cross sectional surfaces.