Atmospheric Corrosion and Galvanic Corrosion Processes - Case Study Example

Name Course Tutor Date Table of Contents Abstract 2 Introduction and Objectives 2 Experiment A: Corrosion of metals 3 Apparatus 3 Procedure 3 Corrosion of nails results 3 Experiment B: Corrosion of dissimilar metals 4 Introduction and objectives 4 Apparatus 5 Procedure 5 Results 6 Discussion 8 Galvanic corrosion 8 Oxidation 9 Reduction 9 Calculation of electrical potential Difference 10 Conclusion 10 References 12 Abstract This experiment will focus on atmospheric corrosion and galvanic corrosion processes. Atmospheric corrosion is electrochemical process that occurs naturally on the surface of a metal. Galvanic corrosion occurred between two metals in an electrolyte. The polarity of each metal is determined by galvanic series where a more negative metal it the series becomes anode. A current produced is due to the difference in reactivity between the two metals that gives the potential of the cell (Hummel 2004). Introduction and Objectives Metals that are found in pure form are uncreative. Reactive metals like iron are found in compound form like oxides or sulphides and therefore metals have to be reduced using a specific process. Construction metals like iron are liable to corrosion through electrolytic process or chemical attack (Baboian 2004). Sulphuric acid produced by some bacteria attack iron used for reinforcement in building. Iron may also corrode in the presence of oxygen and water. The main objective of this experiment was to find out if oxygen and water are required for corrosion to occur. Three tests tube are set up containing water, oxygen or both. Experiment A: Corrosion of metals Apparatus In this experiment the apparatus used include: 3 x Iron nails, 1 x 250ml glass beaker, 3 x Glass Jars with lids, Hotplate, Sand paper, Water, Silica gel and washing up liquid Procedure The glass beaker was half filled with water and then boiled for 5-10 minutes on the hot plate. The 3 iron nails were cleaned with sand paper then one nail was placed in jar containing silica gel and covered with a lid. The second nail was placed in a second jar which was filled with water. This time the jar left open. After the boiled water has been left to cool slightly, it is poured into the third glass jar to approximately ¾ full. The third nail is placed in water and a few drops of washing up liquid is dropped on the surface. The tubes were then labelled with group name and left for 1 week before the observations were made. Corrosion of nails results Nail in silica gel – no corrosion Nail in tap water – corrosion Nail in boiled water - no corrosion. There formations of rust on nail 2 owing to the presence of both water and oxygen. Boiled water in jar 3 had no oxygen and therefore there were no rust. Jar 1 had no moisture and therefore there were no rust. Experiment B: Corrosion of dissimilar metals Introduction and objectives Corrosion is an electrochemical process that occurs in two ways. A) It occur when a metal react with its environment to form metal oxide or other compounds. B) It occurs when two different metals are connected electrically at one end, the other end being immersed in an electrolyte. This is called galvanic corrosion. The cell produced in this way is called composite cell or galvanic cell. Two conditions are necessary for a galvanic cell to be formed: 1) there must be an electrolyte where the two metals are immersed. 2) The two different metals are electrically in contact. Electrochemical corrosion involves two half-cell reactions; an oxidation reaction at the anode and a reduction reaction at the cathode. For example, half cell reactions of iron corroding in water at near neutral pH are represented by the following cathode and anode reactions. When two different metals are placed in an electrolyte a circuit is produced. Metal ions and electrons migrate from a negatively charged anode to a positively charged cathode. This process occurs in corrosion and used by battery. The electro-chemical potential determines whether a metal will undergo corrosion when it is in contact with other metal or gas. Metals that are more positive potential are less reactive as compared to the more negative. The more the two metals are separated from one another in the electrochemical series, the more powerful is the electric current produced by their contact in the presence of an electrolyte (Hummel 2004). It is the aim of this experiment to derive an electro-chemical potential for a range of metals, used in construction, by measuring the potential difference exhibited during the establishment of a corrosion cell. Apparatus The apparatus used in the experiment include: 3 x 250ml plastic beakers Multimeter and leads and clips Stands and clamps Tap Water Acidic solution like hydrochloric acid Alkali solution for example Sodium hydroxide solution Metal samples which include: Aluminium, Brass, Copper, Lead, Mild Steel, Stainless steel and Zinc. Sand paper Indicator paper (pH Paper) Procedure The electrolyte in the container 1 is first tested whether it is an alkali, an acid or a neutral solution using pH and the values are recorded in table 2. The same test is done in container 2 and container 3 and the results are recorded in table 3 and table 4 respectively. All metals samples are cleaned using sand paper and the equipments are arranged as shown below. Current Cathode anode Electrons Figure 1: Apparatus set up. Two metals strips are selected and attached to the clips. Metal 1 was attached to the black terminal while metal 2 was attached to the red terminal, placed in container 1 and the voltage was recorded. The metal was removed and replaced with the next metal in sequential order, placed in the electrolyte and the voltage was recorded. This was done for the remaining metals and the voltages results were recorded in the appropriate tables. Each metal was place on the terminal and the remaining metals on the red terminal using the same electrolyte. The process was repeated for the remaining 2 electrolytes. Results Table 2 The table below is the results table for the results obtained using water as the electrolyte. Electrolyte pH -10 Positive (red terminal) Ground (Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium -0.47 -0.43 0.12 -0.37 -0.2 -0.35 Brass 0.47 -0.04 0.29 0 0.54 0.08 Copper 0.43 0.04 0.29 0.04 0.6 0.07 Lead -0.12 -0.29 -0.29 -0.3 0.29 -0.24 Mild steel 0.37 0 -0.04 0.3 0.4 -0.01 Zinc 0.2 -0.54 -0.6 -0.29 -0.4 0.01 Stainless steel 0.35 -0.08 -0.07 0.24 0.01 -0.01 Table 3: The table of results obtained using the alkali as an electrolyte Electrolyte pH - 6 Positive (red terminal) Ground (Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium -0.4 -0.4 -0.09 -0.36 0.2 -0.2 Brass 0.4 -0.03 0.29 0.01 0.65 0.19 Copper 0.4 0.03 0.32 0.05 0.66 0.18 Lead 0.09 -0.29 -0.32 -0.3 0.34 -0.16 Mild steel 0.36 -0.1 -0.05 0.3 0.65 0.2 Zinc -0.2 -0.65 -0.66 -0.34 -0.65 -0.27 Stainless steel 0.2 -0.19 -0.18 0.16 -0.2 -0.27 The table of results obtained using the acid as an electrolyte Electrolyte pH - 2 Positive (red terminal) Ground (Black terminal) Aluminium Brass Copper Lead Mild steel Zinc Stainless steel Aluminium -0.603 -0.657 -0.227 -0.035 0.179 -0.546 Brass 0.595 -0.063 0.349 0.285 0.82 0.018 Copper 0.637 0.081 0.41 0.37 0.9 0.83 Lead 0.215 -0.338 -0.41 -0.045 0.47 -0.352 Mild steel 0.25 -0.304 -0.379 0.03 0.38 0.344 Zinc -0.262 -0.832 -0.92 -0.067 -0.084 -0.555 Stainless steel 0.635 0.085 0.011 0.08 0.12 0.635 Discussion Corrosion degrades the surface of a metal. Both water and oxygen are required for corrosion to take place. Iron metal can be protected from corrosion through painting, greasing or coating to keep oxygen and water away from iron. However, when a more reactive metal is placed next to the iron protects it even if air and water can get to it. Galvanic corrosion When two metals located in different positions in the electrochemical series are immersed in an electrolyte, electrical coupling process occurs. This process is called galvanic corrosion process. By referring the electrochemical series we can determine which metal is the cathode and which one is in the cathode terminal. Metals that are high in the electrochemical series are less reactive and are in the cathode while those which are lower are more reactive and are in the anode (Baboian 2004). More reactive metals are easily corroded. A less reactive metal will be protected from corrosion. The potential difference or voltage between an anode and a cathode was measured using a voltmeter. Each metal has a numerical standard electrode potential as can be seen in the table below. Table: The Electrochemical Series. Metal Chemical Symbol Potential (Volts) Magnesium Mg -2.37 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 0.00 Gold Au +1.50 Platinum Pt +1.20 Silver Ag +0.80 Copper Cu +0.34 The more negative the value the greater the chance that corrosion will occur. Oxidation Oxidation is a metal anode reaction by which metal atoms are ionized. The metal ions enter the electrolytic solution while the electrons leave the anode through the electrical connection: M → Mn+ + ne- Metal ions leave the anode and therefore the anode corrodes. For example: Anode reaction: 2Fe → 2Fe2+ + 4e- Reduction This is also called electroplating reaction. Reduction reaction is the reverse of the anode reactions: Mn+ + ne- → M The metal ions, either intentionally added to the electrolyte or formed by the anode reaction, combine with electrons at the cathode. The metal then plates out and covers the cathode surface. Cathode reaction: O2 + 2H2O + 4e- → 4OH- When dissimilar metals come into contact with each other in an electrolyte one metal will corrode in preference to the other. The larger the electrochemical potential the more readily the metal at the anode will corrode. Calculation of electrical potential Difference The potential difference between the two metals was measured using multimeter. The measured potential difference is theoretically equal to the difference between the half-cell potentials measured for the metals individually. The expected potential difference between the two metals is obtained using the formula: Potential Difference = Ecathode – Eanode Where Ecathode = the half cell potential of the cathode metal while Eanode = the half cell potential of the anode metal. Using the series, the electrochemical potential of the cell is equal to the most positive value minus the more negative value. For example using the electrochemical series above: If a cell was created with copper and zinc: The potential voltage for copper is + 0.34. The potential voltage for zinc is - 0.76. So an electrochemical potential for this cell would be: 0.34 – (- 0.76) = 1.1V It was observed in acidic electrolyte solution, the more current flows. Near neutral electrolyte solution had the least values. Conclusion In summary, it was found that corrosion is truly an electrochemical process that occurs naturally. Corrosion occurs when two metals are in an electrolyte or when a metal has moisture and oxygen on its surface. The metal in the anode terminal is more reactive than the one in the cathode terminal. Corrosion current is produced during the galvanic process. The electrochemical series determine the polarity of each metal. References Hummel R. E., 2004. Understanding materials science: history, properties, and applications, New York, NY [u.a.]: Springer Revie R. W.; Uhlig H. H., 2008. Corrosion and corrosion control: an introduction to corrosion science and engineering, Hoboken, N.J.: Wiley-Interscience Baboian R.; et al, 2004. Corrosion tests and standards: application and interpretation, West Conshohocken, PA: ASTM International Experiment 3: The study of composition corrosion cell lab manual script www/corosio/causes.htm www/corosio/corrosion.php.htm Read More