Engineering Materials and Corrosion

Introduction

• Corrosion is a chemical or electrochemical reaction between a material and its environment that deteriorates its properties
• Corrosion can occur on metals, ceramics, or polymers
• The environments can be aqueous, non-aqueous liquids, or gases

Material Selection

• Engineers play a key role in mitigating corrosion and wear-related costs in the mining industry
• The selection of materials is one of the solutions to minimize corrosion and wear-related damages

Engineering Materials

• Engineering materials are solids useful for humankind
• Many of these materials are found in combined forms as alloys, compounds, or mixtures
• The application of pure elements is often limited

Metals and Alloys

• Metals are pure engineering materials that are bonded by metallic bonds
• Pure metals have high density, high electrical and thermal conductivity, bright luster, good ductility, malleability, and opacity
• The application of pure metals is limited due to their inherent properties
• Alloys are created by adding one or more elements to a pure metal, improving the overall properties and increasing applicability

Corrosion

• Corrosion is a reaction of a material with its environment
• This reaction can be chemical, electrochemical, physical, or a combination of these
• The material that is vulnerable can be metals, ceramics, or polymers
• The environment can be aqueous and non-aqueous liquids and gases

Electrochemical Corrosion

• The oxidation reaction occurs at the anode of an electrochemical cell
• The reduction reaction occurs at the cathode
• In the case of steel corroding in hydrochloric acid, it produces ferrous chloride and hydrogen gas

Requirements for Electrochemical Corrosion of Metal

• Metal oxidation must take place at the anode of an electrolytic cell
• The reduction of some ions must occur at the cathode
• There must be a potential applied between the anode and cathode
• An electrolyte must be present to facilitate the movement of charges
• An electrical bridge must be completed

Thermodynamic Aspects of Corrosion

• The susceptibility of a material to corrode is evaluated by the Gibbs free energy (∆G)
• The more negative the value of ∆G, the higher the susceptibility for material to corrode
• The Gibbs energy is only indicative of the tendency of the metal to corrode, not directly related to the corrosion rate

Kinetics Effect of Metals Corrosion

• Thermodynamics provides insight into the susceptibility of a system to corrode and requires an equilibrium condition with no net flow of current
• Corrosion reactions cause current to flow, so the kinetics of corrosion reactions must be considered
• The equilibrium between the anodic and cathodic reactions at an electrode is not static and the charged entities are generated and discharged concurrently at the conductive surface

Forms of Corrosion

• There are eight forms of corrosion:
  • Uniform corrosion (general corrosion)
  • Galvanic corrosion
  • Pitting corrosion
  • Crevice corrosion
  • Selective leaching corrosion (parting corrosion)
  • Inter-granular corrosion
  • Stress corrosion
  • Erosion-corrosion

Economic Cost of Corrosion

• Corrosion costs an estimated $2.5 trillion globally each year
• Implementing corrosion prevention protocols could save an estimated $375-875 billion globally
• South Africa alone saw around R130 billion in physical damage from corrosion in 2013
• Mintek and the Corrosion Institute of South Africa (CorrISA) estimate that half of each ton of steel manufactured is to simply replace rusted steel

Corrosion Impacts

• Corrosion is expensive financially
• Corrosion wastes natural resources
• Corrosion causes inconvenience to human beings

Corrosion Control

• Coatings are the most widely applied products for corrosion prevention
• Painting is used to protect metal substrates from atmospheric corrosivity
• Cathodic protection (CP) is a corrosion controlling technique which makes the surface of the metal the cathode of an electrochemical process
  • This can be achieved by using connected sacrificial anodes to create a galvanic cell or by applying an external power current source
• Anodic protection involves the formation of a protective layer on metals by applying an external anodic current

This quiz covers the fundamental concepts of corrosion and the selection of engineering materials to minimize its impact. It explores various materials, including metals and alloys, and their properties. Understanding these principles is crucial for engineers in the mining industry.