Chemistry of Materials PDF

# Chemistry of Materials ## Tema 4: Corrosion - Tratamientos Materials degrade due to fatigue (use, application) and corrosion, or a combination of both. ### Corrosion Corrosion is the interaction of metals with the surrounding environment, producing a chemical change that deteriorates their pro...

# Chemistry of Materials ## Tema 4: Corrosion - Tratamientos Materials degrade due to fatigue (use, application) and corrosion, or a combination of both. ### Corrosion Corrosion is the interaction of metals with the surrounding environment, producing a chemical change that deteriorates their properties. * **Fe + O2 → Oxide of iron (FeO2)**. The corrosiveness of the atmosphere and technological advancements are becoming increasingly severe, causing greater losses due to corrosion. * **Corrosion accelerates due to fluids**, causing ruptures in pipes, for example. * **Corrosion-Cavitation**: Vapor bubbles form in a liquid due to pressure changes and collapse near a metallic surface, causing physical and chemical damage to the material, particularly corrosion of the metal. * **Corrosion Visible/Invisible**: Corrosion can also occur from the inside of a material and be invisible on its surface (pipes, cables, etc.). * All materials corrode, some more than others. The corrosion mechanisms are not the same and depend on the material. ### Corrosion by Material * **Metallic Corrosion**: Different colors depending on the liquid (water, oil). * **Air**: Yellow discoloration (silver), green stains (copper, bronze (Sn+Cu)). * **Water**: Gray/green stains (copper), gray/various colors (lead), black stains (Pb). * **Ceramic Corrosion**: Discoloration, erosion, presence of organisms (bacterias, mold). * **Atmosphere (humidity)**: Discoloration and weakening (cement), discoloration (sandstone). * **Water**: Erosion (limestone), moss, lichens, bacterias (granite). * **Plastic Corrosion**: Takes years, but biodegradable PE corrodes due to the atmosphere. * It also discolors/swells due to cycles of cold-heat, exposure to sunlight, and brake fluids. * **Sunlight**: Decomposes materials (mainly plastics) photochemically. * Temperature also deforms materials, making them less rigid and causing discoloration. ## Why do materiales corrode? * Materials in their natural state are corroded, mixed with other substances. Obtaining pure materials requires energy. When a material corrodes, it reverts to its natural state, releasing energy. * Cancellation of treatment processes and material formation. The more difficult it is to extract a metal, the more susceptible it is to corrosion. ### Examples in Construction Materials * **Chlorides**: CO2 + Oxygen in reinforced concrete. They dissolve in water and spread through the network of pores in the concrete. * **CO2 reacts with a component of the concrete (Ca(OH2)), forming CaCO3 and lowering the pH, which corrodes and alters the properties of the cement.** * **Sulfates (SO2) in concrete**: Mg Ca2+ (substitution). MgSO4 Ca(OH)2 → Mg(OH)2 * **Etringita**: Needle-shaped crystals that increase the concrete's volume and cause cracking. * **Corrosion of metals**: Caused by the atmosphere and rain, some corrode more. * **Polymer degradation**: Caused by heat, solar radiation, and water. ## Protection against Corrosion 1. **Choosing the right material**: Depends on its function and the environment in which it is being used (e.g., titanium instead of iron for a structure). 2. **Designing the material properly**: Prevention of galvanic cells, where two metals with different corrosion properties are present. * Steel oxidizes first in these cells compared to other materials. * Making the area of the metal that oxidizes much larger than the area of the stainless metal. * Avoiding indentations in assembled or joined materials. Welding and adhesive bonding can be better joining techniques compared to rivets/screws. * Mechanical bonding is more prone to corrosion (storage of corrosive materials). * **Mechanical = bolts** * **Welding = welding** * **Adhesive = glue** 3. **Using Protection Methods**: * **Coatings**: Permanent and act as barriers against corrosion. * **Paint**: Polymers * **Nickel plating**: Metallic * **Anodizing**: Metallic oxides * **Cathodic Protection**: The material to be protected is connected to another material that corrodes faster (sacrificial anode, typically Mg or Zn). * It is widely used to protect buried pipes (e.g., 3mm steel pipe without protection lasts 2.5 years, with protection 250 years). * **Inhibitors**: Substances added to the medium in small concentrations that slow down corrosion. * They form thin films on the surface of materials and offer temporary protection. ## Polymers Substances (generally organic) of natural/artificial origin with high molecular masses. * They are formed by the union of low molecular mass molecules called monomers. * The number of monomers in a polymer can reach thousands or millions. ### Polymerization The process of converting a monomer into a polymer. (e.g., vinyl chloride → polyvinyl chloride) ### Degree of Polymerization The number of units that repeat in a chain. * In real polymers, the number is not the same for all chains, and an average degree of polymerization is calculated (more complex process). ### Physical Properties * **Low melting points**: Thermal and electrical insulators, high thermal expansion coefficients (change dimensions in cold/heat cycles). * **Mechanical Properties**: More ductile than metals, low tensile strength, low toughness. * **Chemical Properties**: Stable in aggressive environments (more or less unstable in light (UV) and heat). ### Types of Polymers * **Homopolymers**: All monomers are the same. * The same monomer along the chain. * **Copolymers**: Formed by two or more different monomers. * The way monomers are arranged in the chain affects its properties. ## Polymer Structures * **Linear**: The same type of bonding occurs in a linear way (thermoplastics). * They do not have side branches. * **Branched**: Side chains linked to the main chain. * Higher branching leads to stronger and more viscous polymers (thermoplastics). * **Cross-linked**: Links form between neighboring chains (thermosets (highly cross-linked) and elastomers (slightly cross-linked)). * Higher cross-linking results in greater rigidity. ## Mechanical and Thermal Properties * **Thermoplastics**: More or less linear chains (can have a certain degree of branching), flexible, interact through Van der Waals forces. * They melt without decomposition and can withstand repeated cold/heat cycles without degrading the polymer. * **Thermosets**: Highly cross-linked chains joined by covalent bonds. * Strong and brittle; they harden when heated. * **Elastomers**: Highly flexible chains with a certain degree of cross-linking. * They deform elastically without permanent changes in shape when the cause of deformation is removed. * Elastic. ## Spatial Chain Orientation * **Amorphous**: Disordered (thermosets, thermoplastics, elastomers). Common polymers. * **Crystalline**: Ordered (thermoplastics only, to a certain degree). * **Semicrystalline**: With disordered regions and ordered regions (spherulitic structures in an amorphous matrix) ### Degree of Crystallinity and Properties * **Increased crystallinity**: Increased density, stiffness, strength (normal and thermal). * **If a polymer is transparent in an amorphous state, it becomes opaque in a semicrystalline state.** * Values such as crystallinity, density, elastic modulus, and melting point increase when the density of PE is high. ## Glass Transition Temperature (Tg) The temperature at which a polymer hardens like an amorphous solid. * Below Tg, the polymer becomes more brittle. * At Tg, the polymer acquires a rubbery appearance (elastic). ## Polymers in Construction * Insulation, carpentry, floor coverings, roofing, solar panels, acoustic and safety screens, pipes, electrical insulation. ## Ceramics Compounds formed by metallic and non-metallic elements with predominantly ionic bonds and high melting points. * They are often brittle, hard, and have poor ductility and toughness. * They are highly sensitive to sudden temperature changes (thermal shock). * Though crystalline at the atomic scale, they can be amorphous (glasses). * They are thermal and electrical insulators. * Examples include porcelain, bricks, glass, and cement. ### Ceramic Structure Compounds are fully or partially ionic, consisting of metal cations and non-metallic anions. * They are formed by two components and often exhibit complex crystalline structures. * Stable structures occur when the cations (anions) are fully surrounded by anions (cations). ## Traditional Ceramics Bricks, cement, tiles, silicates (silica and alumina). * Silicates are the most important components of rocks and, consequently, of the Earth's crust. They are made of silicon, oxygen, and other metals. * **Silica (silicon dioxide):** A compound of silicon and oxygen, commonly known as silica. * It is spatially ordered forming quartz and its variations (SiO2). * **Alumina:** Aluminum oxide (Al2O3). ## New Ceramics Special type of ceramics made of compounds like Al2O3 (aluminum oxide), SiC (silicon carbide), Si3N4 (silicon nitride). * They are designed to achieve a specific shape and their properties can be modified. * They are expensive and produced in laboratories. * They have excellent mechanical and high melting point properties. ## Amorphous Ceramics (Glass) The basic unit is SiO4 (silicate). They have very high melting points, are very viscous, and contain modifiers (fluxing agents) that lower the viscosity. * Traditional, but formed differently. ## Glass Products and Additives | Product | SiO2 | Na2O | CaO | Al2O3 | MgO | K2O | PbO | B2O3 | Others | |---|---|---|---|---|---|---|---|---|---| | Soda-lime glass | 71 | 14 | 13 | 2 | - | - | - | - | - | | Window glass | 72 | 15 | 8 | 1 | 4 | - | - | - | - | | Glass for containers | 72 | 13 | 10 | 2 | 1 | - | - | - | - | | Glass for light bulbs | 73 | 17 | 5 | 1 | 4 | - | - | - | - | | Laboratory glass | 96 | 1 | 3 | - | - | - | - | - | - | | Vycor | 81 | 4 | 2 | - | - | - | 13 | - | - | | E-glass (fibers) | 54 | 1 | 17 | 15 | 4 | - | - | 3 | - | | S-glass (fibers) | 64 | - | 26 | 10 | - | - | - | 12 | - | | Optical glass | 67 | 8 | - | - | - | 6 | - | 12 | ZnO | | Boron glass | 46 | - | - | 3 | - | - | 45 | - | - | | Lead glass | - | - | - | - | - | - | - | - | - | * These modifiers are specific and depend on the glass application. * Laboratory glasses: Shock-resistant. ## Conventional Concrete A composite material made from a mixture of aggregates, a binder, and water. * **Aggregate**: Granular material with high mechanical and chemical resistance (sand, gravel, crushed stone). * **Binder**: Cement (CaO, SiO2, Al2O3, Fe2O3, and other compounds), which acts as a binder when mixed with water. * **Cement + aggregate + water = setting (reaction) (solid material).** * **Strength**: Cement + Aggregate quantity. * **Product**: Granular material obtained from the calcination of limestone and clay, used in cement production. * **Cement**: A mixture of clinker, gypsum, and additives (change viscosity or adjust setting time), which are dosed according to the intended use of the cement. ## Polymer Concrete Conventional concrete where part or all of the cement is replaced by polymers to improve physical and mechanical behavior. * **Most used polymers**: Polystyrene, epoxy, polyester, and polymethyl methacrylate. ## Properties of Polymer Concrete * **Excellent mechanical resistance** in tension and compression. * **Production of lightweight elements.** * **High resistance** to corrosive agents and UV radiation (low permeability). * **Low water absorption**: Less than 0.5%. * **Resistant** to freeze-thaw cycles; polymer concrete is much less porous than conventional concrete. * **High dimensional stability**: Prevents fermentation and bacterial growth. * **Smooth surface**: No porosity. * **Good vibro-absorption.** * **Easy to color.** * **High impact resistance.** ## Metals and Alloys * **Metallic bonding (metal + metal)** and ordered crystalline structure. * **High electrical and thermal conductivity** due to the mobility of electrons. * **High ductility** (permanent deformation). * **Shiny** (reflect light). * **Excellent ability to form alloys.** ### Iron and Steel * **Transition metal.** * Its crystalline structure changes at different temperatures: * 25°C Fe BCC (ferrite α) → 912°C Fe FCC (austenite) → 1394°C Fe BCC (ferrite δ) → 1538°C Fe liquid. * **Fe-C System (Steels and Cast Irons)**: C is an interstitial solute in Fe. * At low solubility (maximum 0.022% at 727°C), cementite forms when the solubility limit of C in Fe is exceeded. * Cementite is hard, brittle, and increases the strength of some steels. * **Steel**: Iron + Carbon (small amount). * Carbon is in an octahedral interstitial position. * **Fe + C (<2.1%)**. * **Cast Irons**: Fe + C (2.1% - 6.7%). * Carbon dissolves in small amounts. * **C % 6.7%, Fe + C → Fe3C (cementite, brittle and hard).** ### Heat Treatment * Heating and cooling a material/alloy for a specific time at a specific heating/cooling rate (FCC). * **Annealing**: Heating to 800-950°C and slow cooling in an oven. * The steel loses hardness and becomes more flexible, removing stresses. * **Normalizing**: Heating to 800-950°C and air cooling (faster than annealing). * This cooling produces a steel with higher mechanical resistance. * **Tempering (Martensite Formation)**: Heating steel above 900-950°C to achieve an austenitic structure. * Rapidly cool with water, oil gases, etc., to a temperature close to ambient temperature to achieve a martensitic structure. * This produces very hard steels (BCC stretched, BCT). ### Temperability * A term used to describe the ability of an alloy to harden to a specific depth by the formation of martensite. * **Jominy Test**: Determines the temperability of steel. ### Surface Thermo-Chemical Treatments Modifications to the surface of an alloy to increase its hardness. * **Carburizing and Nitriding**: * Start with a steel with low carbon content. * The material is exposed to an atmosphere rich in carbon, enriching the surface layer and creating a hard and resistant material. * The surface layer thickness increases with increasing temperature and carburizing time. * **EIN**: Similar hardening effect. * The steel is exposed to a nitrogen-containing gaseous atmosphere. * **Carburizing: MxCy, generates carbides.** * **Nitriding: MxNy, generates nitrides.** * **At temperatures below the melting point, the surface reacts with ammonia to form carbides and nitrides (hard).** ## Alloys ### Ferrous * **Steels**: * Low carbon content: Soft and less resistant, ductile and tough. * Medium carbon content: More resistant and less ductile and tough. * High carbon content: Most resistant and least ductile and tough. * **Cast Irons**: * Gray: Ductile, white: Brittle, malleable (more than steel). ### Non-ferrous * **Copper alloys:** * Good thermal and electrical conductor (conducts heat and electricity well). * Moderately resistant to corrosion (less degradation than Fe in the presence of O2 and humidity). * Ductile (easy plastic deformation). * **Brass**: Cu + Zn (<40%). * **Bronze**: Cu + Sn (<12%). * **Monel**: Cu + Ni (complete solid solubility. High resistance and excellent mechanical properties). * **Aluminum alloys**: * High thermal and electrical conductivity (conducts heat and electricity well). * Highly ductile (very plastic). * Highly resistant to corrosion (self-passivates), * Disadvantages: Low melting point (600°C) and low elastic modulus (poor mechanical resistance). ### Main Alloys * **Al + Mg, Ti, Li (very light alloys).** * High specific strength. * Low weight but high strength (good mechanical resistance). * Competes with steel in many structural applications (increasingly used). ### Advantages * **Low density**: 2.7 g/cm^3 compared to 7.8 g/cm^3 of steel. * **High corrosion resistance** (self-passivation) (steel + Cr is expensive). * **They resist breaking** in application conditions even when the structure is damaged. * **Fatigue resistance**. ### Superalloys (Ni, Co, Ti) * Exceptional mechanical properties at high temperatures. * They can operate at temperatures close to their melting point (strength, Tf ≥ 0.6). * High corrosion resistance in extreme environments. * Due to their high cost, they are mainly used in very demanding applications. * **Turbine blades** made of superalloys of Ti. ### Widely Used Alloys in Construction * **Steel**: Used in concrete reinforcement due to its excellent mechanical properties. * **Corrugated steel**: Capable of plastic deformation (less brittle). * **Aluminum**: Windows, roofs, walls, and coverings (good for energy efficiency in buildings, good thermal insulation, and environmentally friendly). * Recyclable (sustainability). * **Copper**: Resistant, malleable, and 100% recyclable. * Used in water supply lines due to its corrosion resistance, air conditioning, and electrical wiring due to its excellent conductivity. ### Titanium * **Light**, extraordinarily corrosion-resistant, and has a very low thermal expansion coefficient (half that of stainless steel). * Used in heating and cooling systems, pipes, and interior and exterior cladding. ==End of OCR for page 15==