At the end of this lecture you should be able to

Transcription

1 LECTURE 5 FRACTURE AND OXIDATION OBJECTIVES At the end of this lecture you should be able to 1. Define fracture, and distinguish between different types of fracture 2. Describe measures that can be used to prevent brittle fracture 3. Differentiate between oxidation and reduction 4. Define corrosion and explain its costs to human society 5. Mention the various types of corrosion 6. Relate corrosion to fracture 7. Cite method of corrosion prevention 5.1 FRACTURE Fracture is a phenomenon you are quite familiar with in your daily experiences. Very often you break a wooden stick or piece of chalk into two or many pieces, crush a stone/boulder into small pieces to form ballast, etc. Each time you break a single object once you create two new surfaces. We can thus define fracture as the separation of a surface into two surfaces Types of Fracture Generally there two types of fracture: (i) ductile fracture, and (ii) brittle fracture. The ductile fracture is generally characterized by the cup-and-cone shapes of the two new surfaces created. Materials, (basically ductile materials such as steel, copper, or most metal) which exhibit ductile fracture also show extensive necking prior to failure (fracture). Brittle fracture, on the other hand is accompanied by very little, if any, necking. It occurs quite suddenly without warning and in the case of tensile loading; the fracture path is nearly perpendicular to the axis of the specimen. Examples of material whose fracture is brittle include ceramics (e.g., chinaware, clay pots etc), glass products, and metals that have been subjected to very cold temperatures (liquid nitrogen temperature ~ C or 77 K). Figure 5.1 shows schematically these two types of fractures.

2 A B A B (a) Necking before fracture (b) Minimal necking Fig Schematics depicting (a) ductile (b) brittle fractures. Shown in figure 5.2 are photographs of components that have failed in either ductile or brittle manner. (Insert Figure 5.2 here) Prevention of Brittle Fracture Experience has shown that brittle materials always fail by brittle fracture. The origins of the brittle fracture are mainly sharp microscopic (invisible to the naked eye) present either on the exterior surface or in the interior of the materials. These sharp cracks act as stress concentrators (i.e., points around which the there is already very high stress) such that when an external stress is applied to the material, the strength of the material is easily exceeded even for small values of external stress, which otherwise would be inadequate to cause failure. When the material strength is exceeded in the vicinity of the crack, the crack begins to grow, leading to failure. Since the stress at the tip of the crack is inversely proportional to the square root of the crack-tip radius, it is now obvious that one way of preventing brittle failure is to blunt the crack tips, particularly those of the exterior microcracks. This can be achieved by etching using appropriate chemicals. c Fig. 5.3 A plate with a surface microcrack of length c and tip radius

3 The other procedure of removing surface microcracks is by polishing using very fine abrasives such as diamond lapping (extremely fine particles of diamond in liquid suspension). Introduction of ductile fibres during the fabrication of certain brittle components is another way of ensuring that failure through brittle fracture is minimized, as the ductile fibres will prevent crack growth CORROSION Before we proceed we shall define some terminologies that are going to recur frequently in this section. Anion Cation Ion Oxidation Reduction Valency Free energy An ion with a negative charge An ion with a positive charge An atom that has a net electric charge A reaction that generates electrons A reaction that consumes electrons The characteristic number of electrons lost when a metal atom becomes an ion This the portion of the substance s internal energy which is available to do work You are very familiar with rusting of iron or steel items. This is just an example of corrosion. Corrosion can be defined as the degradation of metals by an electrochemical reaction with the environment. Metals are very important natural resources to human beings. It is therefore pertinent that we understand corrosion and how to control it in order to sustain our technological development and diminish the financial or material waste or inconvenience cost associated with. Activity List and explain a number of incidences you are aware of that are associated with or attributed to corrosion to illustrate the financial, material waste, and inconveniences to human society. Question: What, in your opinion causes corrosion to proceed?

4 You will learn in thermodynamics that there is a strong tendency for high-energy states to transform into low energy states. It is this tendency that for metals, they combine with other elements found in the environment, which leads to corrosion. For example, an iron oxide is a low-energy state compared to pure iron. We know state that the driving force for corrosion emanates from chemical energy. All interactions between elements are compounds are regulated by the free energy changes available to them. If we denote the net energy change by G, and together with the fact that natural changes involve transitions from high to low energy levels, then by convention, we denote energy absorbed by +ve sign, and energy given by ve sign. For spontaneous reaction to occur then G must be ve. Question Why do gold, platinum and other metals not corrode in wet aerated environment? If G is +ve then corrosion is unlikely, and further if the free energy of activation (energy required to overcome a barrier) is too large, the corrosion rate may be too slow or corrosion may not proceed. When a metal atom corrodes, that is, undergoes a corrosion reaction, it is converted into an ion by a reaction species available in the environment. If we denote the metal by letter M, then the corrosion can be represented by the equation M M Z Ze (5.1) where Z = 1, 2, or 3, and is called the valency. High values of Z are quite rare. Equation (5.1) is the general form of a corrosion reaction. In the above reaction, th emetal has been oxidized. In the reaction represented by equation (5.1) the principal of electroneutrality must be conserved. That is, the electrons created must be consumed somewhere, a process accomplished by generating a negative ion (anion) to balance the +ve ion (M Z+ ). This is the reverse process of oxidation, and is known as reduction. For example, Example: Na Cl Na Cl (5.2) When nickel (Z = 2) is heated in oxygen (Z= 2) to form nickel oxide, the reaction can be written as

5 2 Ni Ni 2e 2 O 2e O 2 Ni O Ni O 2 (oxidation) The factors Affecting Corrosion include: (i) Temperature of the surrounding, (ii) The diffusion rates of the reaction products. There different types of corrosion and these are i) Direct corrosion by dry gases 1 M O2 MO G 2 (where G is the change in the free energy). In this type of corrosion if G increases then corrosion is not possible. ii) iii) Ionization of water (electrochemical corrosion) This corrosion occurs only in the presence of water or aqueous solutions and is dependent on the concentration of ions present in the solution. Dissimilar metals corrosion As the name suggests this type of corrosion occurs when two different metals are joined or coupled together to form a basic wet corrosion cell. A schematic of such a cell is shown below. Electrical connection Anode Cathode Electrolyte Fig. 5.4 Basic wet corrosion cell