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Trivalent Chromium Based Conversion Coatings Containing Cobalt on the Zinc Plated Steel

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Trivalent Chromium Based Conversion Coatings Containing Cobalt on the Zinc Plated Steel Trivalent Chromium Based Conversion Coatings Containing Cobalt on the Zinc Plated Steel Dissertation zur Erlangung des akademischen Grades Doktor der Ingenieurwissenschaften (Dr.-Ing.) Vorgelegt der Fakultät für Elektrotechnik und Informationstechnik der Technischen Universität Ilmenau von M.Sc. Sanaz Hesamedini Herr Univ.-Prof. Dr. rer. nat. habil. Dr. h.c. Andreas Bund Gutachter: Technische Universität Ilmenau Herr Prof. Dr. rer. nat. habil. Uwe Ritter Technische Universität Ilmenau Frau Prof. Dr. Sannakaisa Virtanen Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der Einreichung: 10.12.2019 Tag der wissenschaftlichen Aussprache: 11.08.2020 urn:nbn:de:gbv:ilm1-2020000368 Acknowledgement This work would not have been possible without the advice and support of many people. Firstly, I would like to express my gratitude to Prof. Andreas Bund for his support, patience, and guidance throughout this study. I am especially grateful for his confidence and the freedom he gave me to do this work. I sincerely thank the committee members Prof. Spiess, Prof. Ritter, Prof. Svetlozar Ivanov, and Prof. Müller for their kind attention to this work. In addition, a thank you to Prof. Virtanen for her kind support and being such a role model for all women in science. I wish to express my gratitude to Dr. Gernot Ecke for his kind support with AES measurements. I would like to appreciate Dr. Philip Nickel in Atotech Deutschland GmbH for giving me the opportunity to access the ICP-OES device and his kind support. I extend my sincere thanks to all members of the Department of Electrochemistry and Electroplating and the Institute of Micro- and Nanotechnologies of Technische Universität Ilmenau for their support and kindness. Appreciation is due to Dr. Ralf Peipmann, Mathias Fritz, Dr. Henry Romanus, Dr. Michael Stich, Mario Kurniawan, Steffan Link, Dr. Wassima El Mofid, Diana Rossberg, Karin Keller, Marianne Lerp, Anneliese Täglich for the kind support and helpful discussions throughout this work. Many thanks to Dr. Werner Richtering and Ercan Karapinar in Atotech Deutschland GmbH for their kind support. III Acknowledgment is also given to Atotech Deutschland GmbH and Technische Uni- versität Ilmenau for financial support in the form of two studentships. I would especially like to express my gratitude to Dr. Ali Khajehesamedini and Dr. Thomas Müllerleile for generously sharing their expertise and for their helpful suggestions whenever I had problems during the journey of doing my PhD. Nobody has been more important to me in the pursuit of this goal than the members of my family. I am grateful to my beloved parents for their patience and support throughout my life and especially during this time. I am indebted to my best companions, my beloved siblings, Farnaz and Ali, for their presence and emotional support. IV Zusammenfassung Im Zuge neuer EU-Direktiven wurde die Verwendung von Cr(VI)-Verbindungen stark reglementiert. In der Oberflächentechnik wurde daraufhin sechswertiges Chrom durch sicherere und gleichzeitig effektive Passivierungen auf Cr(III)-Basis ersetzt. Der Kor- rosionsschutz von Cr(VI)-Beschichtungen ohne Wärmebehandlung ist im Allgemeinen besser. In bisher durchgeführten Untersuchungen zeigte sich, dass durch Zusatz von Übergangsmetallionen der Korrosionsschutz der Cr(III)-Passivierungsschicht verbessert werden kann. In dieser Arbeit wird der Einfluss der Zusammensetzung von Cr(III)-Passivierung mit Kobaltanteil auf die Bildung und die Struktur von Konversionsschichten auf verzinkten Substraten untersucht. Auf den verzinkten Stahl wurden Modelllösungen mit zwei verschiedenen Komplexbildnern, nämlich Fluorid und Oxalat, mit und ohne Kobalt aufgetragen. Rasterelektronenmikroskopie (REM) und Rasterkraftmikroskopie zeigten Oberflächenmorphologien mit mikrostrukturellen Defekten. In Anwesenheit von Kobalt wurden die Schichten gleichmäßiger. Die elementare Zusammensetzung der Schichten wurde mit der Augerelektronenspek- troskopie (AES) untersucht. Die Mengen an Cr und Co in den Beschichtungen wurden mithilfe der optischen Plasma-Emissionsspektroskopie (ICP-OES) bestimmt. Sowohl AES als auch ICP-OES zeigten Co-Gehalte in den Schichten. Mithilfe eines thermodynamischen Modells wurde die Konzentration von Cr(III)-, Zn(II)- und Co(II)-Spezies in der Behandlungslösung im pH-Bereich von 0,0 bis 14,0 und auch der minimale pH-Wert für die Abscheidung der Metallionen in der entsprechenden Lösung berechnet. Die Ergebnisse der Korrosionstests (Polarisationsmessung und elektrochemische Impedanzspektroskopie) legen nahe, dass die Bildung einer dichten Schicht für eine gute Korrosionsbeständigkeit entscheidend ist. Außerdem wurde der Bildungsmechanismus von Cr(VI) in den Schichten untersucht. Die Anwesenheit von Cr(VI) wurde mittels Spektrophotometrie nachgewiesen. Die Morphologie und V Zusammenfassung Struktur der Filme wurden per REM beobachtet. Die Gesamtwassermenge in den Schichten wurde mittels Karl-Fischer-Titration gemessen. Es zeigte sich, dass die Morphologie des fluoridhaltigen Films mit einer hohen Dichte an Mikroporen die Wahrscheinlichkeit eines Wassereinschlusses erhöht. Dies führte zu einer Oxida- tion von Cr(III) zu Cr(VI) durch Sauerstoff in Gegenwart von Wasser bei erhöhten Temperaturen. VI Abstract Since hexavalent chromium has been recognized as toxic and carcinogenic, its usage has been restricted. Thereafter, Cr(VI) was substituted by a safer, yet effective, trivalent chromium-based treatment solution. The addition of transition metal ions into the Cr(III)-based treatment solution was proposed to improve the corrosion resistance of the produced passivation film. The present study intends to elucidate the effect of treatment solution composition on the formation and structure of Cr(III)- based conversion coatings containing cobalt. Model solutions with two different complexing agents, viz. fluoride and oxalate, with and without cobalt were applied to the zinc-plated steel. The scanning electron microscopy (SEM) and atomic force microscopy images revealed a morphology with microstructural defects that can be improved to a more uniform and adherent structure by adding cobalt to the passivating bath. The elemental composition of the layer was investigated by Auger electron spectroscopy (AES). Furthermore, the amounts of Cr and Co in the coatings were measured with the aid of inductively coupled plasma optical emission spectroscopy (ICP-OES). AES and ICP-OES both detected cobalt in the layers. Using a thermodynamic model, the concentration of Cr(III), Zn(II), and Co(II) species in the pH ranges of 0.0 to 14.0 and the minimum pH for the deposition of each metal ion species in the relevant treatment solution were calculated. The results of accelerated corrosion tests (polarization measurement and electrochemical impedance spectroscopy) suggested that the formation of a dense layer is crucial for good corrosion resistance of the coating. Furthermore, the formation mechanism of Cr(VI) in the layers formed in Cr(III)-based treatment solutions containing Co, was also studied. The presence of Cr(VI) was detected by means of spectrophotometry. The morphology and structure of the films were observed with SEM. Besides, Karl Fischer titration was used to measure the total amount of water in the layers. It VII Abstract was shown that the morphology of the fluoride-containing film with a high density of micropores increased the probability of water entrapment. This resulted in the oxidation of Cr(III) to Cr(VI) by oxygen in the presence of water at elevated temperatures. VIII Contents ZusammenfassungV Abstract VII List of Figures XIII List of TablesXV List of Abbreviations XVII 1 Introduction1 1.1 Motivation and problem definition . .1 1.2 Research aims and objectives . .4 1.3 Structure of the thesis . .5 2 State of Art7 2.1 Introduction . .7 2.1.1 Zinc cyanide baths . .8 2.1.2 Alkaline non-cyanide baths . .9 2.1.3 Acid chloride baths . .9 2.2 Conversion coatings . 10 2.3 Method of literature review . 11 2.4 Research questions . 11 2.5 Search process of literature review . 12 2.6 Discussion . 12 2.7 Conclusions . 45 IX Contents 3 Material and Methods 47 3.1 Specimen preparation . 47 3.1.1 Substrate preparation . 47 3.1.2 Zinc plating . 47 3.1.3 Treatment solution . 48 3.2 Experimental techniques . 50 3.2.1 Morphology characterization . 50 3.2.2 Characterization and analytical techniques . 53 3.2.3 Corrosion characterization . 62 3.2.4 Surface characterization . 64 4 Characterization of Zn layer 67 4.1 Morphology of the Zn layer . 67 4.2 Elemental composition of the Zn layer . 70 4.3 X-Ray Diffraction analysis of the Zn layer . 73 4.4 Atomic force microscope analysis for the Zn layer . 74 5 Physical and Chemical Characterization of Trivalent Chromium- based Conversion Coatings 77 5.1 Morphology and physical characteristics . 77 5.1.1 Focused ion beam scanning electron microscopy . 77 5.1.2 Atomic force microscopy . 80 5.1.3 Influence of chromium amount on the formed layers . 82 5.2 Influence of immersion time and process temperature . 85 5.3 Chemical characteristics . 86 5.3.1 Inductively coupled plasma optical emission spectrometry . 86 5.3.2 Auger electron spectroscopy . 88 5.4 Discussion . 92 6 Characterization of the Corrosion Behaviour of Trivalent Chromium- based Conversion Coatings 107 6.1 Neutral salt spray test . 107 6.2 Potentiodynamic polarization measurements . 108 6.3 Electrochemical impedance spectroscopy analysis . 114 6.4 Composition of corrosion products . 122 X Contents 6.5 Contact angle . 122 7 Formation of Cr(VI) in the
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