Passivity and pitting corrosion of carbon steel in chloride ... pitting corrosion of the steel in terms

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  • Passivity and pitting corrosion of carbon

    steel in chloride containing borate buffer

    solution

    A thesis submitted in partial fulfilment of the

    requirements for the degree of

    M. Tech Dual degree

    by

    Yogesh Kumar Modi

    (Roll No. – 710MM1095)

    Department of Metallurgical and Materials Engineering

    National Institute of Technology

    Rourkela – 769008, Odisha

    India

    May 2015

  • i

    Department of Metallurgical and Materials

    Engineering, National Institute of Technology,

    Rourkela – 769008, Odisha,

    India

    CERTIFICATE

    This is to certify that the thesis entitled “Passivity and pitting corrosion studies of carbon steel in

    chloride containing borate buffer solution” submitted by Yogesh Kumar Modi to National Institute

    of Technology, Rourkela is a record of bonafide research work under our supervision and is worthy of

    consideration for the award of the degree of Integrated B. Tech and M. Tech of the Institute. The

    candidate has fulfilled all prescribed requirements for the thesis, which is based on candidate’s own

    work, has not been submitted elsewhere for a degree or diploma.

    Prof. Archana Mallik

    Supervisor

    Assistant Professor

  • ii

    Acknowledgment

    This project was carried out at National Institute of Technology, Rourkela under a span of 2

    years, which undertook a lot of effort and help from different persons, without whom this

    research work wouldn’t have been as fruitful as it is, and I would like to extend my deepest

    gratitude to all of them for making me what I am today.

    First of all, my supervisor, Prof. Archana Mallik, as dynamic as is her personality, she is one

    of the kind hearted person I have ever met. Her need to keep me innovative and punctual had

    brought in me vivid changes which I will explore throughout my life, She is what we say the

    teacher of “Modern times”, unravelling in me the science. Madam, I have been a little

    insincere and casual yet you have been patient, you made me realise the importance of little

    things, significance of literature and design in technology and of all to be patient. From the

    core of my heart, I would like to thank you, for your constant support and encouragement and

    I really look forward to work with you in the near future.

    I am grateful to my co-supervisor Prof. Anandiya Basu for his supervision and support in

    carrying out this project.

    I am also thankful to Prof. U. K. Mohanty, for he has been there as a family to me,

    supporting and encouraging my causes.

    I would like to thank the laboratory members of Department of Metallurgical and Materials

    Engineering, NIT Rourkela.

    Now to the members of our lab, RamKumar, Sumant, Prekshya and Rajneesh, guys talking to

    you all was a piece of cake, you were always there, alleviating the pressure of the rigorous

    work. Thank you all!! It was a time well spent.

    I would like to express my deepest gratitude to my parents, Pramod Kumar Modi and Sangita

    Modi, who have always been supporting me in my studies. Thanks for your love and support!

  • iii

    I am grateful to Amrita Gokhale, the light in darkness, for her constant support and your

    sometime annoying need to correct my English. And to my wonderful friends, Punit,

    Ravindra, Aishwarya, Alok and Chinu. Thank you guys for keeping me cheerful and happy.

    All errors and limitations remaining in this thesis are mine alone.

    Yogesh Kumar Modi

    (May 2015)

  • iv

    Abstract

    Passivation is the formation of oxide layer on metals which protects the underlying metal

    from corrosion. The breakdown of this protective layer due to various environmental effects

    leads to rapid corrosion, restricting the metal use in long term purposes, and hence a study of

    passivity breakdown allows for the optimisation of process parameter and environmental

    constraints for extending the lifetime of usefulness of the metal.

    Passivity breakdown of high manganese steel in deaerated borate buffer solution has been

    studied in the following thesis. The steel used here is a special high strength C-Mn steel used

    extensively in pressure vessels. This article describes an attempt to predict and interpret the

    pitting corrosion of the steel in terms of the point defect model (PDM) in deaerated borate

    buffer solution at different pH (6, 8, 9, 10) at different chloride ion concentration ([Cl¯] = 0.01,

    0.1, 0.6, 1M). The objectives of the work is to study the state of passivity and the related

    characteristics, determine the breakdown potential (Vc) as functions of pH, chloride

    concentration and potential sweep rate (ʋ), analyze the relationship between Vc and [Cl¯], pH

    and scan rate (v) in terms of PDM to extract parameter values for passivity breakdown and

    assess the ability of PDM to account for passivity breakdown.

    The passive film was found to contain n-type defect with film thickness depending directly on

    the passivation potential. The near normal distribution in breakdown potential is in

    satisfactory agreement with the analytical prediction of the breakdown potential distribution

    obtained from PDM. The linear dependence of breakdown potential on the square root of

    potential scan rate and polarizability dependence of the barrier layer/solution interface upon

    potential and pH are 0.83 and - 0.01 respectively, as predicted by the PDM, yields an estimate

    of the critical areal concentration of condensed vacancies at the metal/film interface (ξ =

  • v

    4.9×1014 cm-2) that leads to passivity breakdown. The value of the critical areal concentration

    of condensed vacancies falls in well agreement with the value obtained from structural

    argument. These provide convincing evidence for the validity of PDM for modelling passivity

    breakdown on high manganese carbon steel

    Keywords:

    High manganese carbon steel, Breakdown potential, Borate buffer, Point defect model,

    Potentiodynamic curves, Critical areal concentration of vacancies, Normal distributions,

  • vi

    Contents Page No.

    Certificate i

    Acknowledgements ii

    Abstract iv

    Nomenclature viii

    List of figures ix

    List of tables xi

    Chapter 1 Introduction ---------------------------------------------------------------------------- 1-5

    1.1. Background 1

    1.2. Research motivation 3

    1.3. Objectives 4

    1.4. Structure of the thesis 5

    Chapter 2 Literature survey---------------------------------------------------------------------- 6-28

    2.1 Significance of corrosion 6

    2.2 Mechanism of aqueous corrosion 8

    2.2.1 Polarization 10

    2.2.2 Passivity 11

    2.3 Forms of corrosion 12

    2.4 Pitting corrosion 14

    2.4.1 Mechanism of Pitting 14

    2.5 Passivity and passivity breakdown- Modelling 17

    2.5.1 Inception and development 18

    2.5.2 Point defect model 20

    2.5.2.1 Passivity 22

    2.5.2.1 Passivity breakdown 24

    2.6 Summary 28

  • vii

    Chapter 3 Experimental details ---------------------------------------------------------------- 29-33

    3.1 Sample preparation 29

    3.1.1 Grinding and polishing 30

    3.1.2 Cold mounting 30

    3.2 Borate buffer preparation 31

    3.3 Electrochemical setup 32

    3.4 Electrochemical measurements 33

    Chapter 4 Results and discussion -------------------------------------------------------------- 34-50

    4.1 Passivity studies 34

    4.1.1 Mott Schottky analysis 36

    4.1.2 Electrical impedance spectroscopy 38