Tribocorrosion of Passive Metals and Coatings

D Landolt, S Mischler

• Publisher: Woodhead Publishing
• Year: 2011
• ISBN:
  9780857093738
• Topic: Technology & Engineering
• Subtopic: Mining Engineering
• Index: Technology & Engineering

Tribocorrosion causes the degradation or alteration of materials through the combined action of corrosion and wear. It limits the performance and life-time of installations, machines and devices with moving parts, and controls certain manufacturing processes such as chemical–mechanical polishing. The effects of tribocorrosion are most pronounced on passive metals which owe their corrosion resistance to a thin protecting oxide film. Most corrosion-resistant engineering alloys belong to this category.This book provides an introduction to the developing field of tribocorrosion and an overview of the latest research. Part one reviews basic notions of corrosion and tribology, before presenting the most recent results on the growth and structure of passive oxide films. Tribocorrosion mechanisms under fretting, sliding and erosion conditions, respectively, are then discussed. Part two focuses on methods for measuring and preventing tribocorrosion. It includes chapters on electrochemical techniques, the design of tribocorrosion test equipment, data evaluation and the optimisation of materials' properties for tribocorrosion systems. Part three presents a selection of tribocorrosion problems in engineering and medicine. Three chapters address the tribocorrosion of medical implants including test methods and clinical implications. Other chapters examine tribocorrosion issues in nuclear power plants, marine environments, automotive cooling circuits, elevated-temperature metal working and chemical–mechanical polishing.With its distinguished editors and international team of expert contributors Tribocorrosion of passive metals and coatings is an invaluable reference tool for engineers and researchers in industry and academia confronted with tribocorrosion problems.

• Comprehensively reviews current research on the tribocorrosion of passive metals and coatings, with particular reference to the design of tribocorrosion test equipment, data evaluation and the optimisation of materials' properties for tribocorrosion systems
• Chapters discuss tribocorrosion mechanisms under fretting, sliding and erosion conditions before focussing on methods for measuring and preventing tribocorrosion
• Includes a comprehensive selection of tribocorrosion problems in engineering and medicine, such as the tribocorrosion of medical implants, and tribocorrosion issues in nuclear power plants, marine environments, automotive cooling circuits and elevated-temperature metal working

Part I

Fundamentals of tribocorrosion

1

Corrosion and passivity of metals and coatings

S. Virtanen, University of Erlangen-Nuremberg, Germany

Abstract:

Physicochemical basics of electrochemical corrosion reactions of metallic materials are presented in this chapter, starting with a description of the thermodynamics and kinetics of electrochemical reactions. Passivity, localized breakdown and specific modes of corrosion of passive metals and alloys are described. The most important principles of corrosion protection measures are presented.

Key words

corrosion

thermodynamics

kinetics

passivity

passive films

localized breakdown of passivity

1.1 Introduction

All materials are prone to aging and degradation when in use in different types of applications. These processes lead to time-dependent deterioration of their functionality. The negative effects of materials degradation are wide-ranging, from economic considerations due to repair and material loss through to the safety of human beings, for instance when materials are employed as structural elements in civil engineering or in aviation. Materials degradation can have a direct influence on health when metals and alloys are employed in the human body for healing; here the concern is for the possible toxicity of the degradation products.

Materials degradation in their surroundings can take place via different mechanisms such as thermal destruction, chemical dissolution, electrochemical corrosion processes, or mechanical wear. The dominant type of failure mechanism depends on the materials type, on the environment, and of course on the loads given by the application. Considering chemical attack, polymers, for example, can degrade by swelling or chemical dissolution. Ceramics, even though mostly highly chemically stable, can chemically dissolve in sufficiently aggressive environments. Metals mostly need to become oxidized in order to be soluble. In order for the oxidation of the metal to take place, a reaction partner in the environment needs to be reduced; hence metal corrosion takes place by electrochemical reactions. This chapter summarizes the physicochemical basics of electrochemical corrosion reactions of metals and alloys. First, the thermodynamics and kinetics of electrochemical reactions are described. This is followed by a section dedicated to metal passivity, as this special kinetic feature is of utmost importance for the successful application of many technically important metals and alloys. The breakdown of passivity and specific modes of corrosion of passive metals and alloys are briefly described. From the understanding of the electrochemical origin of corrosion reactions, principles of corrosion protection measures are derived and described. For the interested reader, a large number of books on corrosion science and engineering are available, e.g. [1–6]. In addition, a more detailed description of fundamentals of thermodynamics and kinetics of electrochemical reactions can be found in many textbooks of physical chemistry or electrochemistry.

All materials classes can also suffer from different types of mechanical failures, and a combination of mechanical and chemical attack can lead to accelerated failure, due to synergetic effects. These aspects, however, are the topics of other chapters in this book and are not discussed in this chapter.

1.2 Thermodynamics of electrochemical corrosion reactions

The corrosion reaction of metals can be divided into the oxidation reaction of the metal (anodic reaction) and the reduction reaction of a reaction partner in the surroundings (cathodic reaction). In many natural environments, the cathodic reaction is either the reduction of protons (dominant in acidic environments), or the reduction of dissolved oxygen gas. In the simplest manner, these reactions can be written as in the following:

Metal oxidation:

[1.1]

Hydrogen ion reduction:

[1.2]

Oxygen reduction:

[1.3]

The electrode potential of a metal electrode in an electrolyte results from the requirement of the chemical potentials of a species i to be identical for two phases in contact. Hence, when a metal is in contact with an electrolyte, the tendency of a metal atom to leave the metal lattice as a cation (i.e., metal oxidation reaction) may be higher or lower than the tendency of the metal cations to become part of the metal lattice (i.e., after reduction of metal cations). Depending on the nature of the metal and the electrolyte, this leads to a formation of a negative or positive space charge region on the surface of the metal; this surface charge is compensated by the orientation of polar molecules or accumulation of ions with a counter-charge in the vicinity of the metal surface in the electrolyte, leading to a potential difference across the metal/electrolyte phase boundary. This potential difference is of utmost importance for the kinetics of electrochemical reactions and will be discussed in the next section.

The standard electrode potential of a metal (E°) therefore describes the tendency of the metal to be oxidized or reduced. A high positive electrode potential indicates a metal, for which the thermodynamic tendency to become oxidized is low (noble metals such as Pt, Au, Ag), whereas a large negative electrode potential indicates a high tendency to oxidation (e.g., Mg, Al, Zn,…). According to definition, the potential of the hydrogen electrode (equation [1.2]) under standard conditions (

, a^{H +} = 1, T = 25 °C) is zero, and other standard electrode potentials are given in reference to the standard ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Introduction
• Part I: Fundamentals of tribocorrosion
• Part II: Methods for measurement and prevention of tribocorrosion
• Part III: Tribocorrosion in engineering and medicine
• Index