Composition Corrosion Cells - Research Paper Example

Running Head: COMPOSITION CORROSION CELLS Study of Corrosive Cells Student’s Name: Course Code: Lecture’s Name: Date of presentation: Table of Contents Table of Contents 3 Abstract 8 Introduction 12 Materials and apparatus 17 Procedure 19 Results for Experiment A 22 Table 1: Experiment A results 23 Discussion 25 Conclusion 28 Experiment B 30 Corrosion of Dissimilar metals 30 Introduction 30 Table 2: Electrochemical Voltage Series 36 List Apparatus and Materials 46 Methodology 49 Results of Experiment B 54 Table 3: Water Electrolyte 54 Table 4: Acid Electrolyte 56 Table 5: Alkaline Electrolyte 59 Discussion 62 Conclusion 66 Recommendation 67 Set of Questions 68 Table 6: Published Results 70 Abstract Iron and other reactive metals are found in ores as oxides or sulphide compounds form. In order to achieve the pure iron element from these complexes, chemical processes are necessary. Iron and other metal widely used in as construction material are very susceptible to corrosion or rusting. Corrosion can occur through either electrolytic process or chemical action (oxygen). The aim of the first experiment is to determine if oxygen and water are both necessary for corrosion to happen. The second experiment is aimed at understanding the corrosion of dissimilar materials. From the first tests; it is evident that oxygen and water are important for corrosion to occur. It is also evident the higher the electric potential exhibited by a material, the lower the chance of it being corroded. Introduction Rarely do we find elements in the pure metal state on the earth’s crust before mining. Only gold and some platinum elements can be found as dust or lumps, lobes or alluvial deposits. Those metal found in these categories are generally not reactive to both oxygen and fallow with other metals. Iron and other reactive metals are found in ores as oxides or sulphide compounds form. In order to achieve the pure iron element from these complexes, chemical processes are necessary. Some of these purification processes require a lot of energy for instance extraction of aluminum metal from bauxite ore. Iron and other metal widely used in as construction material are very susceptible to corrosion or rusting. As indicated above corrosion can occur through either electrolytic process or chemical action (oxygen). In other cases, a sulphate reducing bacteria may produce sulphuric acid and in most conditions, corrosion can occur. The aim of the experiment A is to determine if oxygen and water are both necessary for corrosion to happen. In finding out, three tubes containing different medium were set up and the objective was to determine under which condition corrosion occurs. Hypotheses for Experiment A The experiment seeks to ascertain either of the following hypotheses Ho: Water and oxygen are not necessary for corrosion to occur H1: water and oxygen is necessary for corrosion to occurs Materials and apparatus 1. 3 Iron nails 2. A 250ml glass beaker 3. Hotplate 4. Sand paper 5. Water 6. Silica gel Procedure The experiment generally involved having three different medium and dipping the nails in these separate medium. All the three nails were first cleaned with sand paper to get rid of any deposit layers that could interfere with the results. Silica gel was put in a jar and one nail was placed inside. The jar was completely covered with lid. Care was taken to make sure that the silica gel did not turn pink; a sign of absorption and hence contamination with water. The assembly was labeled as 1. The second assembly involved filling the second jar with tap water and placing the second nail inside; however this jar was left open. In the third assembly, water was boiled in a beaker for about 10 minutes. It was cooled slightly and poured in the third jar to about ¾ full. The third nail was finally placed inside and the stopper replaced. The jars were checked after one week. Results for Experiment A After one week the results were as the table shown below. Table 1: Experiment A results label Medium Results 1 Silica gel No corrosion on the nail 2 Tap Water Corrosion present on the nail (Reddish brown Deposits) 3 Boiled water No corrosion on the nail Discussion For iron to corrode, water (moisture) and oxygen must be present. This means that if you boil water, oxygen dissolved in the water is naturally expelled, and thus corrosion occurs at a very slow rate which cannot be detected over a period of one week. Iron reacts with air to form Fe3+ as shown in the following half equation; 2 Fe  2 Fe3+ +6e- (eq.1) The electrodes released from the above reaction reduce the oxygen in the air to its oxides ions, as shown in the following half oxidizing equation; 1.5O2 + 6e-  3O2- (eq.2) Fe2O3 is thereof formed from the Fe3+ and O2- released from eq.1 and 2 respectively. The ions react with water to form hydroxides ions under wet conditions. The overall rusting equation can be summarized as follows 4Fe(s) +3O2 (g) + XH2O (l) 2Fe2O3.Xh20(s) The equation cannot be balanced since the quantity of X in water may vary from relatively dry to soggy conditions. Conclusion Based from the above explanation, we can conclude that perfect conditions necessary for corrosion were only met by experiment setup labeled 2 i.e. under tap water environment. In silica gel there was no water as well as oxygen. This means water and oxygen are necessary for corrosion to occur. Therefore reject H0 in favor of H1. Experiment B Corrosion of Dissimilar metals Introduction In electrochemical process, corrosion occurs when two different metals get connected electrically on one end and then immersed in an electrolyte on the other end. In this kind of setup electric current is produced and this together with presence of impurities; corrosion of either of the metal involved can be greatly accelerated. This form of cell is commonly referred to as Galvanic of Composition cell. Conditions necessary for the setting up of a galvanic cell include: 1. The two dissimilar metals used must be connected electrically 2. For the resulting electric circuit to be completed; the two dissimilar metals must be dipped in a conduction medium solution or electrolyte. Electrochemical corrosion results from two dependent half reactions that occur on the positive and negative terminals of the formed cell i.e. anode and cathode terminals respectively. Oxidation half reaction occurs at the anode terminal while the reduction half reaction occurs on the cathode end. When iron corrodes in water at close to neutral PH, the following half reactions occur; Reaction at anode: 2Fe 2Fe2++ 4e- Reaction at the Cathode: O2 + 2H2O +4e-  4OH- In a galvanic couple, different metals react differently, but the general rule is that the more active a metal (cathode) is, the higher the rate of corrosion with a more passive metal (the cathode). This therefore depends on the relative position of the metals involved, on the reactivity series and hence different potential difference measured between the terminals. Each metal has its own potential difference as shown on the table below. Table 2: Electrochemical Voltage Series Metal Chemical Symbol Potential (Volts) Reactivity Magnesium Mg -2.37 Most Reactive Least Reactive Aluminium Al -1.66 Zinc Zn -0.76 Iron Fe -0.44 Nickel Ni -0.25 Tin Sn -0.14 Lead Pb -0.13 Hydrogen H 0 Gold Au 1.5 Platinum Pt 1.2 Silver Ag 0.8 Copper Cu 0.34 From the above table we can conclude that; the more negative an element is; the higher the affinity for corrosion. If a two dissimilar metals are connected into a circuit by immersing into an electrolyte, the difference in their potential difference cause electrons and metal ions to move from anode (negatively charged terminal) to the cathode ( positively charged electrode). This setup constitutes a battery and it causes corrosion in metals. The potential difference created determines whether a metal will either be corroded or will induce corrosion on the other metal in the battery. From the table we see that metals with a high positive voltage value are mainly cathodic and unreactive. On the other hand, metals with a lower negative potential difference are anodic, more reactive and are highly susceptible to corrosion. The electrochemical potential difference is derived from subtracting the more negative voltage value from the most positive voltage value of a battery setup. For Example; a cell made up by magnesium and copper. The potential voltage of magnesium for magnesium is -2.37V The potential voltage of copper is +0.34V. Therefore, the electrochemical potential for a cell of magnesium and Zinc would be 0.34- (-2.37) =2.71V The power of the electrical current generated in an electrochemical cell is determined by the position of the metals involved on the table 2 above. The bigger the difference between the metals involved the higher the voltage generated. Metals higher on the table are more likely to be corroded, while those at the bottom are most likely to be protected. It is worth noting that the order given above may change depending on particular corrosive conditions for instance under sea water conditions. Two electrically connected metals scrabble for same electrons. By using the sea water electrolyte as a host for ions moving in one direction, the passive metal will most likely take electrons from the active metal. A hierarchy of materials can be computed from the ensuing electrical current flow in such a “medium of interest”. Such a hierarchy is known as galvanic series, and it’s quite useful in understanding and predicting corrosion. The aim of second experiment was to obtain the electro-chemical potential difference for a range of metals samples provided by measuring the electrical potential difference realized in the corrosion cell setup. List Apparatus and Materials Three 250ml Plastic beakers 1. Multimeter 2. Stands and clamps 3. Tap water 4. Hydrochloric acid 5. Sodium hydroxide solution 6. Metal samples of Brass, Copper, Lead, mild Steel, Stainless Steel, aluminum and Zinc 7. Sand paper 8. PH paper Methodology The PH values of the three electrolytes i.e. hydrochloric acid, Sodium Hydroxide and tap water was taken and recorded. The electrolytes were put on the three beakers, each on its beaker and labeled 1, 2, and 3 respectively. Each of the metal samples were then thoroughly cleaned with sandpaper. Starting with samples of Aluminium and brass the equipment were arranged as shown below. The electric potential difference indicated on the multimeter was recorded. Keeping alluminium on the negative all the other metal samples were used to replace brass and the corresponding electrical potential difference voltage values were recorded. Alluminium was then replaced with the other metal with being paired sequentially with the other samples. The steps were then repeated by replacing tap water with Hydrochloric acid and sodium hydroxide as the electrolyte medium. Results of Experiment B Table 3: Water Electrolyte Electrolyte PH Value: 7 POSITIVE (red terminal) GROUND(Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium 0.43 0.36 -0.01 0.22 -0.27 0.18 Brass -0.46 -0.03 -0.33 -0.12 -0.58 -0.11 Copper -0.29 0.02 -0.25 -0.17 -0.54 -0.08 Lead 0.02 0.27 0.25 0.21 -0.29 0.16 Mild steel -0.17 0.05 0.05 -0.21 -0.47 -0.04 Zinc 0.34 0.54 0.53 0.30 0.52 0.49 Stainless steel -0.14 0.05 0.06 -0.18 0.04 -0.44 Table 4: Acid Electrolyte Electrolyte PH Value: 3 POSITIVE (red terminal) GROUND(Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium 0.64 0.61 0.23 0.23 -0.32 0.25 Brass -0.53 0.11 -0.33 -0.36 -0.91 -0.36 Copper -0.61 -0.22 -0.36 -0.35 -0.84 -0.32 Lead -0.21 0.17 0.32 0.01 -0.50 0.02 Mild steel -0.23 0.17 0.31 0.02 -0.51 0.05 Zinc 0.31 0.65 0.75 0.54 0.54 0.57 Stainless steel -0.23 0.11 0.25 -0.02 -0.02 -0.53 Table 5: Alkaline Electrolyte Electrolyte PH Value: 12 POSITIVE (red terminal) GROUND( Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium 1.25 1.22 0.90 1.23 0.99 1.05 Brass -1.40 -0.04 -0.47 -0.14 -0.40 -0.38 Copper -1.39 -0.47 -0.15 -0.42 -0.34 -0.32 Lead 0.16 0.09 0.34 0.47 0.46 -0.92 Mild steel -1.19 0.14 0.13 -0.35 -0.26 -0.11 Zinc -1.10 0.36 0.36 -0.13 0.23 0.09 Stainless steel -1.10 0.31 0.32 -0.17 0.18 -0.02 Discussion From the data obtained from the experiments above, we can deduce relative electrode potential of different metals at acidic, alkaline and neutral environments. It is evident that zinc metal connected on positive terminal gave negative electrode readings with all other metal samples. Also aluminium metal gave positive readings with all metal samples when it was connected on negative (black) terminal. Copper on the other hand gave positive readings when connected on the positive terminal and negative readings, when connected on negative terminal. Metals which give positive electrode readings when connected on cathode terminal are generally unreactive and hence not susceptible to corrosion and vice versa. Therefore we can conclude copper is least reactive and zinc most reactive. Based on the relative positive and negative readings of the rest; we can arrange them as aluminium, Zinc, mild steel, lead, Stainless Steel, brass and copper being the least reactive. Higher potential values were recorded on alkaline and acidic solutions but slightly lower in water electrolyte. This can be attributed to presence of ions necessary for transportation of released electrons in both alkaline and acidic conditions. Conclusion Corrosion is dependent on environmental conditions such as PH. The higher the positive potential values recorded the lower the chances of a metal being corroded. The effect of PH on corrosion is affected by temperatures. Recommendation More realistic figures can be obtained if a series of similar tests area done at various temperatures. This will help in determining the optimum temperatures at which corrosion occurs. Set of Questions 1. Corrosion refers to the gradual degradation of metal through chemical action with the environment. The chemical action can either be caused by a chemical electrochemical action and hence electrochemical oxidation or by the exposure of the metal to oxygen in the atmosphere and hence at atmospheric oxidation. For a better understanding of corrosion, it’s crucial for one to look at the process involved in coming up with these metal components. 2. Sample of published potential difference values Table 6: Published Results Metal Electrode Potential alluminium -1.66 magnesium -1.55 zinc -0.76 iron -0.44 lead -0.13 Nickel +0.2 3. An electrolyte refers to any substance having free ions that conduct electricity. An electrolyte may either be in molten or in solution form. When metal samples or electrodes are immersed in an electrolyte and a voltage is applied, current flows through the electrolyte. A chemical action happen at the cathode taking up electrodes released at the anode. Another chemical reaction occurs at the anode, giving out electrons that travel to the cathode. The ions present in the electrolyte neutralize the produced electrons; thus ensuring a continuous electrons flow and hence reactions 4. a) Zinc is mainly used for galvanizing other metal especially iron. This improves the latter’s ability to withstand corrosive environment. It is used for making anode electrodes for batteries. Brass an alloy of copper and zinc is mainly used for manufacture of machine bushes. Zinc oxide is utilized as a white pigment in the manufacture of paints. b) Mild steel is used mainly in making reinforcement bars and structural steel beams and columns. This is mainly due to its superior strength attributes and low cost. c) Due its low density, alluminium is widely used in aerospace industry for manufacture of airplanes fuselage, aluminum vessels and doors and frames. Powdered aluminum is also used in the making of paints and electronic appliances d) Due to its high density, lead metal is used in as ballast keel in vessel to counterbalance the rowing effect of wind on sails. Lead is also utilized to form glazing bar used in multi-lit windows such as stained glass. In acoustic construction, the metal is useful in making of sound proofing layers on building walls such as sound studios. Sheet lead is used as cladding, flushing and gutter joints and also in decorative motifs. e) Copper is ductile and possesses high electrical and thermal conductivity properties. It is mostly exploited in the manufacture of copper conductors wires and electromagnets, also in printed electronics circuit boards. Due to high heat conductivity levels, copper is used in making heat sinks and piping of heat exchangers, air conditions and refrigerators. In buildings, cooper is used as lightning terminal rods, conductor plates as well as underground earth plates. Copper alloys such as brass and bronze have various uses also such as plumbing works on buildings. f) Brass is an alloy of zinc and copper with varying proportions of the two ingredients determining the resulting properties. Due to its low friction properties, brass is useful in making bushes, gears, ammunition and valves for plumbing and electrical uses. g) Stainless steel refers to a steel alloy with minimum chromium content of not less than 10.5 or 11% by mass. It does not corrode, stain or rust in water like other forms of steel. It is used aerospace and automotive structural alloys. Its is widely used in food industries for making tanks and containers due to its antibacterial properties and also due to the fact that it does not require painting or other synthetic finishes. It is also used in construction of bridges due to its superior structural properties. 5. Composition or galvanic cells are created from electrochemical process where; corrosion occurs when two different met get connected electrically on one end and then immersed in an electrolyte on the other end. In this kind of setup electric current is produced and this together with presence of impurities, corrosion of either of the metal involved can be greatly accelerated. The accelerated corrosion therefore occurs where is in electrical or physical contact with a less reactive or noble metal (cathode). 6. Minimizing corrosion on construction materials a) Active corrosion protection: the aim of this type of protection is to inhibit reactions which normally lead to corrosion; this is done by controlling the contents and corrosive agent in such way that corrosion is prevented. For instance the use of corrosive- resistant alloys and the inclusion of inhibitors to the aggressive metal material b) Passive corrosion protection: This involves mechanically isolating the aggressive active material from the package contents, for instance by using protective layer, paints and films. This protection does not however aggressiveness of active material nor the susceptibility nature of the packaged contents c) Permanent corrosive control: These processes offer a near definite solution to corrosion problem on materials. These methods include Copper plating, galvanizing, coating, enameling and tin plating. 7. Brass is an alloy of copper and tin. On the electrochemical series copper has a lower chance of corrosion than tin. This means that copper has a higher positive potential and thus more cathodic and unreactive than tin; thus predominant. 8. Cathodic Protection This technique is used to control corrosion of metal surface by making the protected surface to be the cathode end of the electrochemical cell. The technique is mostly applied in the protection of ship hull, offshore oil platforms, fuel and water pipelines and submerged steel piles. Sacrificial Anode protection This technique seeks to ensure that the surface protection is polarized negatively until a uniform potential is realized. The potential difference becomes zero and thus the force necessary for corrosion is halted. 9. Effect of PH on corrosion PH refers potential of Hydrogen, i.e. the negative logarithm of the hydrogen ions concentration. In acidic range, oxygen reacts with the atomic hydrogen thus depolarizing the metal surface and subsequently the reduction reaction progresses. At PH level of 4, ferrous oxide dissolves as it is progressively formed rather than being deposited as a film on the surface. Without this protection film the metal is in direct contact with acidic solution and this hastens corrosion process. A higher PH value makes oxygen to react with the oxide layer to give Fe2O3 thus more protection. 10. If a two dissimilar metals are connected into a circuit by immersing into an electrolyte, the difference in their potential difference cause electrons and metal ions to move from anode (negatively charged terminal) to the cathode ( positively charged electrode). 11. Copper has a potential of +0.34V and Zinc has a potential of -0.76V. If connected together zinc will corrode as it has a higher affinity to be reduced from its metal to its oxide. 12. Alluminium has a potential of -1.66 and zinc is -0.76. The potential difference is -0.76 – (-1.66) =+0.9. Zinc will most likely corrode since it has higher potential than alluminium. Bibliography Breakell, M . et al (2003). Management of Accelerated Low Water Corrosion in Steel Maritime Structures. Sheffield, Oxford publishers Jones, D. (1996). Principles and Prevention of Corrosion (2nd edition ed.). Upper Saddle River, New Jersey: Prentice Hall. Laboratory manual Lecture notes Read More