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This is the complete lecture note for MME405 Corrosion Science and Engineering. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.
Course credit load: 2 Units Series 1: Fundamentals of Corrosion: Concepts, Causes and Socio-Economic Implications ● Part 1.1: The Concept of Corrosion o Sub Part 1.1.1: Definition and Nature of Corrosion o Sub Part 1.1.2: Why Metals Corrode o Sub Part 1.1.3: Corrosion as an Electrochemical Process ● Part 1.2: Socio-Economic Implications of Corrosion o Sub Part 1.2.1: Economic Costs of Corrosion o Sub Part 1.2.2: Safety, Health and Environmental Consequences o Sub Part 1.2.3: Conservation of Resources and Sustainability ● Part 1.3: Fundamental Causes of Corrosion o Sub Part 1.3.1: Environmental Causes o Sub Part 1.3.2: Material Causes o Sub Part 1.3.3: Design, Fabrication and Operational Causes ● Part 1.4: Corrosion Problems, Failures and Their Analysis o Sub Part 1.4.1: Types of Corrosion Failures o Sub Part 1.4.2: Failure Analysis of Corrosion o Sub Part 1.4.3: Principles of Corrosion Control Introduction Look closely at an old iron gate, a rusted roof sheet or a crumbling bridge support and you are looking at one of the largest and most persistent problems in engineering. Corrosion quietly
consumes metal, money and safety margins in every industry, and its causes are usually understood only after something has failed. This Series introduces what corrosion is, why it matters and where it comes from. The aim of this Series is to equip you to explain the concept of corrosion and its socio-economic implications, and to describe the fundamental causes of corrosion problems and failures. We shall commence with the concept of corrosion, including why metals corrode and how the process works electrochemically. Next, we will explore the socio-economic implications of corrosion. Afterwards, we will turn to the fundamental causes of corrosion in the environment, the material and the design. We will conclude with corrosion problems, failures and their analysis, and an outline of how corrosion is controlled. Series Learning Outcomes Upon completing this Series, you should be able to: ● SLO 1.1: define corrosion and explain why most metals have a natural tendency to corrode, ● SLO 1.2: describe corrosion as an electrochemical process involving an anode, a cathode, an electrolyte and a metallic path, ● SLO 1.3: explain the direct and indirect economic costs of corrosion, ● SLO 1.4: discuss the safety, health, environmental and resource implications of corrosion, ● SLO 1.5: describe the environmental causes of corrosion, ● SLO 1.6: explain how material, design and fabrication factors cause corrosion problems, ● SLO 1.7: describe the common types of corrosion failure and the steps in analysing them, ● SLO 1.8: outline the principles by which corrosion is controlled. 1.1 The Concept of Corrosion 1.1.1 Definition and Nature of Corrosion Corrosion is the deterioration of a material, usually a metal, through chemical or electrochemical reaction with its environment. It changes the metal from its useful metallic form into oxides, hydroxides, carbonates or other compounds, called corrosion products, and it usually destroys strength, appearance or function. Rusting, which is the formation of reddish-brown hydrated iron oxide on steel, is the most familiar example, although other metals corrode in their own characteristic ways.
Corrosion differs from mechanical damage such as wear and erosion, in which material is removed by physical action, although the two can act together. Corrosion also occurs in materials other than metals, since polymers may degrade and ceramics may dissolve, but this course concentrates on the corrosion of metals and alloys. Corrosion is a natural process that can be slowed and controlled but not eliminated altogether, so engineers must plan for it throughout the life of a product. Worked example: A steel sheet left in damp air forms reddish-brown rust, a corrosion product, and loses thickness, whereas a steel shaft worn thin by sliding contact has lost metal through wear, not corrosion. Application: Distinguishing corrosion from wear and erosion is the first step in identifying the cause of a component failure. Fill in the blank: The deterioration of a metal through reaction with its environment is called __________. 1.1.2 Why Metals Corrode Most metals occur in nature as compounds such as oxides, sulphides and carbonates, called ores, because in these forms they are in a lower energy and more stable state. Energy is put into extracting the metal from its ore, so the pure metal is in a higher energy state and has a natural tendency to return to the compound form. Corrosion can therefore be described as extractive metallurgy in reverse, and a negative free energy change shows that a corrosion reaction can occur spontaneously. Metals differ in this tendency. Gold and platinum, called noble metals, have very little tendency to react and are found uncombined in nature, whereas iron, zinc, aluminium and magnesium have a strong tendency to corrode. A strong tendency does not always mean rapid corrosion, however. Aluminium reacts readily with oxygen but forms a thin, adherent oxide film that slows further attack, and this difference between tendency and rate is examined later in the course. Worked example: Iron is found in nature as oxide ores, energy is used to reduce it to metal in a blast furnace, and when the steel rusts it returns to an oxide, which shows corrosion as extractive metallurgy in reverse. Application: Comparing the tendency of metals to corrode helps engineers see why some metals need protection and others do not. Fill in the blank: Gold and platinum, which have very little tendency to corrode, are called __________ metals. 1.1.3 Corrosion as an Electrochemical Process Most corrosion in water and in moist environments is electrochemical, which means that it involves the transfer of electric charge through reactions at the surface of the metal. A corrosion cell has four parts: the anode, where the metal dissolves, the cathode, where a reduction reaction consumes the electrons released, the electrolyte, a conducting liquid that carries ions between them, and the metallic path that carries electrons from the anode to the cathode. If any one of these is removed, corrosion stops.
At the anode the metal is oxidised and gives up electrons, so that iron, for example, gives iron ions and two electrons. At the cathode in neutral or alkaline water that contains dissolved oxygen, oxygen and water take up the electrons to form hydroxide ions, whereas in acid solutions hydrogen ions take up the electrons and form hydrogen gas. The two reactions must proceed at the same rate, since all the electrons released at the anode are consumed at the cathode. Worked example: On a steel pipe in damp soil, iron dissolves at anodic areas and oxygen is reduced at cathodic areas, while the wet soil and the steel itself complete the circuit, so the pipe corrodes. Application: Removing or blocking any part of the corrosion cell stops corrosion, which is the basis of most methods of protection. Fill in the blank: The part of a corrosion cell where the metal dissolves is called the __________. Figure 1.1: A simple corrosion cell showing the anode, cathode, electrolyte and metallic path. Pilot questions 1.1 1. Define corrosion. (Linked to SLO 1.1) 2. Name the reddish-brown corrosion product of steel. (Linked to SLO 1.1) 3. Why do most metals have a natural tendency to corrode? (Linked to SLO 1.1) 4. Name the four parts of a corrosion cell. (Linked to SLO 1.2)
5. At which part of the corrosion cell does the metal dissolve? (Linked to SLO 1.2) 1.2 Socio-Economic Implications of Corrosion 1.2.1 Economic Costs of Corrosion Corrosion imposes very large economic costs. Studies suggest that it costs most industrialised countries about 3 to 4 per cent of their gross domestic product each year, and a global study published by NACE International in 2016 estimated about 3.4 per cent. Direct costs are those of replacing corroded equipment and structures, maintaining protective coatings and using corrosion-resistant materials. Indirect costs result from corrosion but are less obvious, and they include lost production during shutdowns, loss of product through leaks, contamination and lost efficiency. Designers also add to costs through overdesign, which means making parts thicker than needed by adding a corrosion allowance, and through using expensive alloys where cheaper ones would be enough if corrosion were controlled. Studies estimate that between 15 and 35 per cent of the total cost could be saved by applying corrosion control practices that are already known, which shows that much of the loss is avoidable. Worked example: When a corroded pipeline in a refinery must be shut down, the direct cost is that of replacing the pipe, while the indirect cost is the lost output and product during the shutdown, which can be larger. Application: Counting both direct and indirect costs shows the true economic value of corrosion control. Fill in the blank: Costs such as lost production during shutdowns caused by corrosion are called __________ costs. 1.2.2 Safety, Health and Environmental Consequences Corrosion is a matter of safety as well as of money. The failure of corroded structural members, such as bridges, offshore platforms and storage tanks, can cause collapse, and the corrosion of pressure vessels, boilers and pipes can cause explosions or the
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