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Niamh Hosking + Mark McMillen

Ford Motor Company & PPG

Scribe Creep of Coated Aluminum Alloy Panels: Exploring Substrate Pre-Treatment Influences & Interactions

Abstract

Scribe creep tests are frequently used to assess the corrosion performance of the E-coated and painted metals used in the vehicle body structure. Scribe creep measurements on coated aluminum alloys can be challenging for several reasons, including their low corrosion rates in cyclic corrosion tests and the localized nature of the corrosion mode, which is not well quantified by the creep length measurement. Nonetheless, important findings for aluminum alloy corrosion have been established using the scribe creep method, such as that grinding the aluminum substrate and higher copper content in the alloy composition have a negative impact on scribe creep performance of the coated panel. However, these general findings are not sufficient for a mechanistic understanding of the coated panel scribe creep performance.

In this work, we undertook a systematic exploration of the interaction effects of the alloy microstructure and pre-treatment conditions on the scribe creep performance by analysis of a single AA6111 substrate with controlled variations in thermal treatments, cleaning conditions, and pre-treatment parameters. A combination of surface analysis and electrochemical techniques was used to characterize the alloy surface condition at each stage of the pretreatment process. A clear dependence of scribe creep resistance under cyclic corrosion conditions on the thermal history (i.e., microstructure) of the AA6111 alloy was established. Specifically, a strong influence of the temper, T4 versus T82, was observed, with higher corrosion activity measured for the former. Cleaning time and copper ion additions to the pretreatment bath also had a significant impact on the coated panel corrosion resistance, although these effects were minimized when the substrate was in the T82 condition. Copper enrichment of the AA6111 surface observed under certain cleaning and pre-treatment conditions was not itself a root cause for reduced corrosion performance. The efficacy of thin-film pre-treatments in the scribe creep performance of automotive panels depends on the system interactions between the pretreatment and the substrate microstructure, and not on cleaning and pretreatment parameters alone.

Biography – Niamh Hosking

Niamh Hosking is a Technical Expert in Coatings & Corrosion Research at Ford Motor Company. She specializes in corrosion mechanisms and protection for lightweight automotive materials, novel coatings, and emerging vehicle architectures. She has over 20 years automotive experience, having worked for Ford in the UK, Germany and Australia, before moving to the US in 2013. Niamh graduated from the University of Nottingham with a PhD in Materials and Mechanical Engineering and has a BEng in Mechanical Engineering from the University of Limerick in her native Ireland.

Biography – Mark McMillen

Mark McMillen is a Senior Development Chemist at PPG Industries. Mark graduated from DeSales University in Center Valley, PA, with a degree in Chemistry, and then entered the pretreatment industry, where he has been working for the last 40 years, including 33 at PPG Industries. Mark originally started at the PPG Chemfil facility in Troy, MI, and is now engaged in research at PPG’s Coatings Innovation Center in Allison Park, PA.