CPC Course - Fundamentals Module 1_Lesson 2
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This lesson broaches the question: What is corrosion? This lesson takes the student back to basics discussing anode and cathode and electrolyte path. A general discussion with questions at the end.
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CPC Course - Fundamentals Module 1_Lesson 2
- 3. Corrosion Control and Prevention
- 4. Corrosion Control and Prevention Corrosion Description
- 5. Corrosion Control and Prevention What is Corrosion? Weakening of material due to: Loss of cross-sectional area Shattering of metal due to hydrogen embrittlement Cracking of polymer due to heat or sunlight exposure
- 6. Corrosion Control and Prevention What is Corrosion?
- 7. Corrosion Control and Prevention What is Corrosion?
- 8. Corrosion Control and Prevention What is Corrosion? A battery is an engineered form of controlled corrosion By releasing electrical energy Corroding the internal metal
- 9. Corrosion Control and Prevention
- 10. Corrosion Control and Prevention
- 11. Corrosion Control and Prevention Corrosion Description Result of a material with its Process are Occurs because of the tendency of metals to to their natural states
- 12. Corrosion Control and Prevention Corrosion Description
- 13. Corrosion Control and Prevention Basic Chemical Definitions Basic Elements Involved in the Corrosion Process
- 14. Corrosion Control and Prevention Conditions for Corrosion A corroding metal A metal driving corrosion Electrical contact between the anode and cathode Water, salt water, battery acid, etc
- 15. Corrosion Control and Prevention Conditions for Corrosion Electrons flow to cathode Negative ions from the electrolyte combine with metal ions from the anode Ion flow Electron flow
- 16. Corrosion Control and Prevention Diagram of Corrosion in a Car Battery A car battery contains three of the four required elements for corrosion: The Anode, Cathode, and the Electrolyte Anode + Cathode - Electrolyte
- 17. Corrosion Control and Prevention Diagram of Corrosion in a Car Battery Powering the light, or any other appliance, adds the fourth element - the Conductive Path - thus starting the corrosion process Conductive Path Anode + Cathode - Electrolyte
- 18. Corrosion Control and Prevention Diagram of Corrosion in a Car Battery Pitting Plating Now, electrons flow from cathode to anode and ions will flow from the anode to the cathode resulting in a pitting of the anode and a plating of the cathode Electron flow Ion Flow Anode + Cathode - Electrolyte Conductive Path
- 19. Corrosion Control and Prevention Diagram of Corrosion in a Car Battery Electrolyte Re-plated Deposits removed A car battery is designed to reverse this corrosive action once a reverse voltage is applied from the car alternator Alternator Anode + Cathode - Conductive Path Electron flow
- 20. Corrosion Control and Prevention
- 21. Corrosion Control and Prevention Corrosion Description Acidity or alkalinity of the medium Stability of the corrosion product Biological organisms Variation in composition of the corrosive medium High Temperatures OTHER FACTORS WHICH AFFECT A METAL’S TENDENCY TO CORRODE:
- 22. Corrosion Control and Prevention Corrosion Description Elimination of any of the four conditions stops corrosion
- 23. Corrosion Control and Prevention Corrosion Description Elimination of any of the four conditions stops corrosion
- 24. Corrosion Control and Prevention Electrochemical Process
- 25. Corrosion Control and Prevention Elimination
- 26. Corrosion Control and Prevention REVIEW Q. What are the four requirements for electrochemical corrosion? A. Anode, cathode, electrolyte, conductive path Q. Why is a separator commonly used between the anodic and cathodic cells of a battery? A. Eliminates the conductive path & prevents the corrosion process. It interrupts the electron flow between the anode and cathode.
- 27. Corrosion Control and Prevention Excellent References
Editor's Notes
- cor·ro·sion kəˈrōZHən/ noun noun: corrosion The process of corroding metal, stone, or other materials. "each aircraft part is sprayed with oil to prevent corrosion" damage caused by corrosion. "engineers found the corrosion when checking the bridge"
- What is Corrosion?
- Metallic corrosion is an “electrochemical reaction” which involves : a transfer of electrons oxidation - loss of electrons (corrosion) reduction - gain of electrons (protection) migration of ions
- WHAT IS CORROSION? In a simple statement, corrosion is nature’s way of returning a metal to its natural energy state. Virtually all metals are found in nature in the form of an ore. This ore is mined and refined. The refining process uses heat and electricity to take the ore from a low energy state to the useful energy state of a metal. The more energy used during refining, the more likely the metal is to corrode. Magnesium takes an extreme amount of energy to produce a metal from it’s natural occurring state, and it therefore, corrodes easily. Conversely, gold is found in nature in an almost pure form and little energy is needed to refine it, therefore, it is rarely found to corrode except under the most extreme environments.
- When a metal corrodes, the excess energy is released in the form of electricity. Batteries are a form of controlled corrosion that harnesses the released energy for useful purposes. On an aircraft, the energy is lost through the structure and eventually to the ground.
- Corrosion in a battery and corrosion on an aircraft happen in the same way.
- The natural deterioration or destruction of a material as a result of its interaction with its environment Corrosion is the electrochemical deterioration of a material or its properties due to its chemical reaction with the surrounding environment Generally, the processes are chemical or electrochemical, although physical and mechanical factors contribute to corrosion This process occurs because of the tendency of metals to return to their natural states This reaction occurs because of the tendency of metals to return to their naturally occurring state, usually oxide or sulfide ores. For example, iron in the presence of moisture and air will return to its natural state, iron oxide or rust. Aluminum and magnesium form corrosion products that are white oxides or hydroxides. When a water solution containing soluble salts is present, corrosion of many alloys can occur easily at ambient temperatures. This type of corrosion can be effectively treated by maintenance personnel as discussed in this manual. Corrosion can also occur in the absence of water but only at high temperatures, such as those found in gas turbine engines.
- At normal atmospheric temperatures, metals do not corrode appreciably without moisture All metals will corrode to some extent in a natural environment. When a metal corrodes, the atoms lose electrons and become positively charged. In solution, the positively charged metal ions can combine with negatively charged ions to form corrosion products, such as metallic chlorides, oxides, hydroxides, and sulfides.
- Basic Chemical Definitions 3-5.1. ATOM. The smallest unit of an element, made up of a positively charged nucleus surrounded by a system of negatively charged electrons. There are over 100 elements, including metals (such as aluminum, magnesium, gold, platinum, iron, nickel, titanium, cadmium, chromium, copper, silver, lead, beryllium, zinc), and non-metals (such as carbon, boron, sulfur, chlorine, hydrogen, oxygen, nitrogen, and helium). 3-5.2. ELECTRON. A negatively charged subatomic particle. An electric current occurs when electrons are forced to move through metal conductors. Electrons flow through liquid solutions only in the presence of ions. 3-5.3. ION. An atom or group of atoms or molecules which has acquired a net electric charge by gaining (negative ion) or losing (positive ion) electrons. When ions are forced to move through liquid solutions, an electric current can occur. Ions cannot move through metal conductors. 3-5.4. ELECTROLYTE. A liquid (usually water) solution containing ions. Sea water is an electrolyte: an aqueous (water-based) solution whose major components are sodium and chloride ions. Electrochemistry is the branch of science concerned with chemical reactions at surfaces in contact with electrolytes.
- All metals will corrode to some extent in a natural environment. When a metal corrodes, the atoms lose electrons and become positively charged. In solution, the positively charged metal ions can combine with negatively charged ions to form corrosion products, such as metallic chlorides, oxides, hydroxides, and sulfides. CONDITIONS FOR CORROSION Four elements are required for corrosion to occur. These are: An Anode – presence of a corroding element (anode), this is the metal that corrodes; A Cathode – presence of a cathode, this is the metal that causes the anode to corrode; A Conductive Path - presence of a continuous liquid path (electrolyte and salt or other contaminate), between the anode and the cathode allowing electrical energy to flow and; An Electrolyte - presence of a conductor to carry flow of electrons from anode to cathode, allows the metal ions to flow from the anode to the cathode Corrosion is considered as an electrochemical process. Four conditions must exist before electrochemical corrosion can proceed:
- CONDITIONS FOR CORROSION Note that the Anode and Cathode may be the same piece of metal and are microscopically close to each other, or they may be two different metals in contact with each other. The conductive path must be an electrical path between the Anode and Cathode. The electrolyte must be conductive, such as water, salt water, battery acid, etc. A nonconductor such as fuel or oil will not cause corrosion. If oxygen is present, the anode ions form a metal oxide corrosion product instead of plating on the cathode.
- When you pull a car battery off the shelf, you already have 3 of the 4 components present. You have a positive and negative post – these are your anode and cathode. You also have an electrolyte – sometimes called the battery acid – inside. Now – All we have to do is add the fourth component: the conductive path.
- All you have to do is flip the switch to light the light bulb or turn the key to start the car and you have added the fourth element – the conductive path.
- When you flip the switch to light the light or turn the key to start the car, electrons will flow from the Cathode (lead) to the Anode (lead dioxide) through the conductive path. This is the usable electricity. At the same time, the ions will flow from the Anode to the Cathode through the Electrolyte. This results in a pitting on the Anode and a plating (lead sulfate) on the Cathode.
- With a car battery, this pitting and plating is designed to be reversed by applying a reverse current from the alternator. However, the deposits that are removed from the Cathode don’t always go back to the Anode in the same place that the pitting occurred so eventually the Anode is pitting to the point where the battery fails. This usually occurs at about the 5 year and 1 day point – just after the warranty runs out. In theory, we might be able to do this same thing to our aircraft. However, since we cannot tell it that we want a little re-plating in one place, a lot in another, and, by the way, don’t mess with any of our avionics while doing it, there is no practical way to hook a big alternator to our aircraft to reverse it’s corrosion action.
- Other factors which affect a metal’s tendency to corrode are the following: acidity or alkalinity of the medium, stability of the corrosion products, biological organisms, and variation in composition of the corrosive medium. The elimination of any one of the four conditions will slow or stop corrosion. For example, a paint film on a metal surface will prevent the electrolyte from connecting the anode and cathode, thereby stopping the electric current. A change in the electrolyte can also affect the rate of corrosion. Two connected dissimilar metal parts placed in distilled water corrode very slowly due to a lack of ions in solution to conduct the electric current; in sea water the corrosion reaction is accelerated by a factor of 1000 or more.
- Elimination of any of the four conditions stops corrosion; for example, an organic film on the surface of the metal will prevent the electrolyte from connecting the anode and cathode
- Elimination of any of the four conditions stops corrosion; for example, an organic film on the surface of the metal will prevent the electrolyte from connecting the anode and cathode
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