C8 CORROSION DEGRADATION OF MATERIALS.pptx

CORROSION & DEGRADATION OF METALS SCB 24503 ENGINEERING MATERIALS Learning Objectives Distinguish between oxidation and reduction electrochemical reactions. Name the eight forms of corrosion and explain each one of it. List five measures that are commonly used to prevent...

CORROSION & DEGRADATION OF METALS SCB 24503 ENGINEERING MATERIALS Learning Objectives Distinguish between oxidation and reduction electrochemical reactions. Name the eight forms of corrosion and explain each one of it. List five measures that are commonly used to prevent corrosion. Explain why ceramic materials are, in general, very resistant to corrosion. For polymeric materials, discuss (a) two degradation processes that occur when they exposed to liquid solvents (b) the causes and consequences of molecular chain bond rupture.  CORROSION Figure 1 is a view of the failure origin showing severe areas of surface corrosion. The corrosive environment is the typical salt/water mixture resulting from the deicing of roads during the winter.  DEFINITION OF CORROSION “Slow and progressive destruction of a metal brought about by the action of an external agent and various causes of a mechanical nature. Corrosive Environments pH (Remember! pH 0 to 6: acidic, pH 8 to 14: alkaline) Lower pH  Higher the amount of free acid present  more corrosive the solution Aggressive species Aggressive species are corrosive materials that can penetrate and brake down protective surface films or modify the shape of the cathodic polarization curve. Ex: chloride ion, sulphur ion Oxidizing species Oxidizing species exhibit mild to severe effects on corrosiveness. Oxidizing species depolarize corrosion reactions. To forms of metal deterioration can be stated: DIRECT OXIDATION: Where the metal’s atoms combine with the aggressive substance: 2Fe +O2 ------2FeO. ELECTROCHEMICAL CORROSION For metals, corrosion process is an electrochemical process where metal will loose electron(s) oxidation M → Mn+ + ne- Fe → Fe2+ + 2e- or Al → Al3+ + 3e - anode The side which oxidation occur Electron(s) generated from the metal (during oxidation) must be transferred to form a chemical reduction species. Process of gaining electron(s) 2H+ + 2e- → H2 cathode Reduction occurs at  Oxidation: Zn → Zn2+ + 2e- Acid solution Reduction 2H+ + 2e- → H2 (gas)  Standard EMF Series more cathodic Au EMF series is gathered Cu Pb by Sn coupling a standard half Ni cell Co of a metal to standard Cd hydrogen electrode Fe EMF series give Cr corrosion Zn tendencies of metals Al Those on top Mg Na …………………> K difficult to oxidise more anodic Those on bottom  FORMS OF CORROSION Stress corrosion Stress & corrosion Uniform Attack work together Erosion-corrosion Oxidation & reduction at crack tips. Break down of passivating occur uniformly over layer by erosion (pipe surface. elbows). Selective Leaching Pitting Preferred corrosion of Forms Downward propagation one element/constituent of small pits & holes. of Fig. 17.17, Callister 7e. (e.g., Zn from brass (Cu-Zn)). corrosion (Fig. 17.17 from M.G. Fontana, Corrosion Intergranular Engineering, 3rd ed., Corrosion along McGraw-Hill Book Company, 1986.) grain boundaries, Galvanic often where special Dissimilar metals are Crevice Between two phases exist. pieces of the same metal. physically joined. The Rivet holes g.b. more anodic one prec. corrodes. attacked Fig. 17.15, Callister 7e. (Fig. 17.15 is zones courtesy LaQue Center for Corrosion Technology, Inc.) Fig. 17.18, Callister 7e.  Erosion-corrosion Erosion corrosion: acceleration in rate of corrosion due to relative motion between corrosive fluid & surface Found mostly in piping; especially at bends, elbows or abrupt change in pipe diameter where fluids become turbulence or fluid change direction suddenly Pitting corrosion Localized corrosion attack Small pits/holes form Oxidation occurs within the pits itself A pit must form from detects like scratches or slight variation in combination Undetected and sudden unexpected failure  Crevice corrosion Corrosion occurs because of different in concentration electrolyte around metal parts Occurs under valve, washers, insulation material, fastener heads, surface deposits, coatings, threads, lap joints & clamps  Galvanic corrosion Two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte Electrochemical reaction leads to corrosion of metal GALVANIC CORROSION 2 dissimilar metal electrodes immersed in solution of their own ions. Electrode that has more negative oxidation potential will be oxidized. oxidize 2+ Zn → Zn d reduced Cu2+ → Cu GALVANIC SERIES ranks of the reactivity of metals/alloys in seawater Platinum Gold Graphite Galvanic series Titanium gives the Silver 316 Stainless Steel cathodic, anodic Nickel (passive) relationship Copper between metals Nickel (active) – E.g in seawater, Tin Zinc is more Lead 316 Stainless Steel active than Iron / Steel aluminum Aluminum alloys Series is Cadmium determined Zinc experimentally Magnesium for every corrosive  Intergranular corrosion Corrosion occurs along grain boundaries for alloys in specific conditions Specimen disintegrates along the grain boundaries and result in failure  Hydrogen embrittlement Various metal alloys, specifically steels, experience reduction in ductility and tensile strength when atomic hydrogen penetrates into the material. Atomic form of H (H2 in molecular) diffuses interstitially and lead to cracking. Corrosion Prevention – Materials Selections & Design Choose a right metal once the working environment has been detected. Use less active metal (galvanic series and EMF) Avoid dissimilar metals that can cause galvanic corrosion Provide allowance for corrosion of thickness Allow complete drainage Make sure parts can be replaced Weld rather than rivet  Corrosion Prevention – Environment Metal oxide Self-protecting metals! Metal (e.g., Al, -- Metal ions combine with O stainless steel) to form a thin, adhering oxide layer that slows corrosion. Change the environment e.g. lowering the fluid temperature, pressure, concentration and velocity - Reduce T (slows kinetics of oxidation and reduction) Add inhibitors -- Slow oxidation/reduction reactions by removing reactants (e.g., remove O2 gas by reacting it w/an inhibitor). -- Slow oxidation reaction by attaching species to the surface (e.g., paint it!). Cathodic (or sacrificial) protection -- Attach a more anodic material to the one to be protected.  Corrosion Prevention – Anticorrosion Treatments METAL coatings: via ELECTROLYSIS: Protective films of metal using the coating as the anode and the part being coated as the cathode, employing sulphates or cyanides as an electrolyte. The substance is broken down by applying an electrical voltage over a controlled period at a controlled current, giving us the desired layer. Steel parts should be covered with 3-4 alternating layers of Cu, Ni, Cr. – NICKEL PLATING. – CHROME PLATING. – COPPER PLATING. via IMMERSION IN MOLTEN METAL: Immersing the piece in a bath of molten protective metal. – GALVANISING. Pipes/tubes. – TINNING. Tinplate.  via METALLISING: Using a legalising spray gun to coat the piece in molten metal. – Zn,P, Al, Stainless steel. via CASE HARDENING: Methods in which the piece to be coated is covered in powdered Zn and Naphthalene / Cr, alumina / Aluminium / Si , heated up to set temperature for a specific time to create a coating on the piece. – SHERARDIZING. – CHROMIZING. – CALORIZING. – SILICATION. via PLATING: Superimposing plates of metal on one or both sides of the metal to be protected.. – Cu, brass, Ni, Cupro-nickel, Stainless steel. NON METALLIC: inorganic: – Enamel. – Via thermo chemical treatment. PHOSPHATING (immersing the steel pieces in an aqueous solution of metal phosphate at 100ºC). ANODISING (forcing aluminium to oxidise electrolytic ally in order to strengthen the layer of oxide, giving it greater protection), BLUING (treatment applied to steels to protect them against oxidation, producing a dark protective layer of magnetite. Heated up to 200ºC and then tempered in oil). organic: – Paints. CATHODIC PROTECTION: Steel pipes using Mg or Zn. Ships propellers which are made of Cu using electrodes made of Zn & Mg. Ceramic corrosion Corrosion of ceramic materials generally involves dissolution; in contrast electrochemical processes found in metals. Ceramic materials are frequently used because of their resistance to corrosion, for instance: (a)Glasses used to contain liquids (b)Refractory ceramics not only withstand high T and provide thermal insulation but also resist high-temperature attack Polymer degradation Deterioration by means of environmental interactions; physiochemical phenomena. Polymers deteriorate by 1) swelling & dissolution 2) bond rupture Covalent bond rupture as a result of heat energy, chemical reactions and radiation caused reduction in mechanical integrity. Polymer degradation Swelling & dissolution when polymers are exposed to liquids, the small solute molecules fit into and is absorbed within the polymer. the macromolecules are forced apart and caused the material to expand or swells. Dissolution meanwhile is a continuation of swelling. 2) bond rupture Severance or rupture of molecular chain bonds.  Summary Metallic corrosion is electrochemical; oxidation and reduction reactions. Not all metals oxidize with the same degree of ease, which demonstrated with a galvanic couple. Standard of EMF & Galvanic series shows ranking of tendency to corrode of metallic materials when coupled to other metals. Exercises 1. Briefly explain the differences between oxidation and reduction electrochemical reactions. 2. For each form of corrosion, (a) describe why, where, and the conditions under which the corrosion occurs (b) cite three measures that may be taken to prevent or control it. 3. List three differences between the corrosion of metals and (a) the corrosion of ceramics (b) the degradation of polymers