A Study of Performance of Aluminium Anode by Addition of Zinc in Sea Water

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1 A Study of Performance of Aluminium Anode by Addition of Zinc in Sea Water G.M.Pradeep 1, R.M.Ravindran 2, A.Gokulram 3, R.Rajesh 4, B.YogeshEswaran 5, L. Samson Joshua 6 1 Assistant professor, Dept of Mechanical Engineering, Velammal Institute of Technology, T.N, India, 2 Assistant professor, Dept of Chemistry, Velammal Institute of Technology, T.N, India. 3,4,5,6 UG Students, Dept of Mechanical Engineering, Velammal Institute of Technology, T.N, India ABSTRACT Aluminium Anode plays a major role on corrosion resistance in marine vessels. Since the dry docking of the vessels was not being done periodically due to frequent shipping operations, the structures of the vessels were badly affected. In order to overcome the issue we have planned to fix the sacrificial anodes in the hull structures of the any one of the vessel of the port trust with variation of the composition of the anode to reduce the corrosion period and downtime of the vessels. The methods used to analyze the compositions of the aluminium anode is polarization study and salt immersion test. The main objective of our project is to increase the dry docking period of the vessel and also to reduce the cost of the maintenance. The result obtained is to find the efficiency of the most corrosive resistant aluminium anode. Key words: Corrosion, sacrificial anode, polarization study, anode efficiency. Introduction Corrosion is defined as the deterioration of a material, because of its destructive attack of a metal by chemical or electrochemical reaction with its environment. In the most common word corrosion is the loss of the metal by loosing electrons of metals reacting with the water and oxygen [1]. Cathodic protection by sacrificial anode has been a most effective way of preventing metals from corrosion. Cathodic protection by sacrificial anode is used to protect engineering structures from corrosion, such as pipelines, underground storage tanks, locks and ship hulls. It is a method of preventing the corrosion by minimizing the difference in potential between anode and cathode. Iron and steel are the most versatile, least expensive and most widely used of the engineering materials. They are unequalled in the range of mechanical and physical properties with which they are endowed by alloying and heat-treatment. Their main disadvantage is is that iron and steel has a poor resistance to corrosion [2]. In order to protect iron or steel made marine crafts and other areas where the act of corrosion occurs. A sacrificial anode is used. Sacrificial anodes are of aluminium, zinc and magnesium. Aluminium anodes have reliable long-term performance and also have better current and weight characteristic than zinc anode. Aluminium anode can be improved the performance by controlling alloy composition [3]. This paper is only concerned with the behavior of several Al-Zn alloys used as anodes for cathodic protection in sea water. The effectiveness cathodic protection potentials and protective area ratio of steel to sacrificial anode were also determined. 1. Materials: The aluminium alloys used in this research are obtained from the Chennai port trust, where the existing aluminium anode used is 2% of zinc in the aluminium anode. The comparative study has been made in order to find an effective way of sacrificial anode by the addition of zinc to the aluminium anode. The comparative study of 6% and 10% is done. This research is based on the effectiveness of the different aluminium anodes by the addition of zinc. 1. Effect of zinc on aluminium anode: The commercial 7xxx series of ISSN: Page 6

2 aluminium alloy offer some of the highest strength aluminium alloy on the market. This series of aluminium alloy is alloyed with zinc to give a good corrosive resistivity. The 7xxx series of aluminium alloy ranging from 0.8 to 12% of zinc addition which comprises some of the highest strength aluminium alloys.[4] Investigate the effect of Zn content on tensile and electrochemical properties of 3003 Al alloy. The effect of Zn addition on the microstructure, tensile properties and electrochemical properties of as-annealed 3003 Al alloy was investigated. It was found that High density precipitates are observed in the Zn-containing alloys and the alloy with 1.8% Zn addition also has rod-like precipitates.. The alloy with 1.5% Zn addition has the highest ultimate tensile strength. M.C. Carroll, P.I. Gouma et al[5] Studied effect of Zn addition on the grain boundry precipitation and corrosion of Al. Stress corrosion cracking (SCC) concerns in aluminum alloys containing Mg levels greater than 3.5%have been largely Attributed to the formation of the beta-phase (Al3Mg2) at grain boundaries. It has been demonstrated that the beta-phase need not be continuous in order to provide a path for crack propagation, but aging treatments, exposure to intermediate to high temperatures, and excessively corrosive environments can all contribute to early failure of Al-Mg alloys due to SCC. Proof of the presence of a corrosion-prone secondary phase can be demonstrated easily through exfoliation testing and the associated lining of grain boundaries, which can be confirmed optically. Additions of Zn to these Al-Mg alloys in levels of 1 2wt% have been shown to be more SCC resistant due to the formation of a stable ternary Al-Mg-Zn phase, the pie phase. Recent studies have shown that Al-5083 variants which contain even minor levels of Zn ( wt %) perform much better during exfoliation testing. 1. Composition of the existing aluminium anode: Metals used for existing aluminium anode Percentage of Metals Cu % Si 0.030% Mg % Mn 0.001% Fe 0.073% Ti <0.001% Ni <0.0002% Zn 3.13% Pb <0.002% Sn <0.0001% Cr < % Sr <0.0005% Zr <0.001% Ca % Na % P <0.0002% Cd % Be <0.0001% Bi < % Sb <0.0007% Ag % Al 96.7% ISSN: Page 7

3 S.F 0.075% Table 1. Elemental Composition of existing sacrificial aluminium anode alloy Table 1 shows the composition of the existing aluminium anode used for the cathodic protection which is being used for the protection of ship hulls from corrosion. On the general basis of anode capacity and corrosion rate, a comparative study has been made to find the most corrosive resistive anode, 1. Composition of aluminium anode for the comparative study: SAMPLE 1 SAMPLE 2 Cu % Cu % Si % Si % Mg % Mg % Mn % Mn % Fe 0.122% Fe 0.129% Ti 0.011% Ti % Ni - < % Ni - < % Zn 5.73% Zn 9.75% Pb - < % Pb - < % Sn - < % Sn - < % Cr - < % Cr - < % Sr - < % Sr - < % Zr - < % Zr - < % Ca % Ca % Na % Na % P - < % P - < % Cd % Cd % Be - < % Be - < % Bi - < % Bi - < % Sb - <0.0030% Sb - <0.0030% Ag % Ag % Al 94% Al 90% S.F 0.126% S.F 0.134% Table 2. Elemental Composition of proposed study of sacrificial aluminium anode alloy (sample 1 and sample 2) Table 2 shows the composition of proposed study of this research study, in which two new samples have been analyzed and studied for the outcome of the anode efficiency which is more corrosive resistant than the existing aluminium anode. 5. Methods: The aluminium zinc alloy used in this study where made of permanent mould casting. The crucible furnace is used for the aluminium anode smealting. After melting of the pure aluminium proportionate amounts of zn was added to produce alloys of composition 5-6% Zn and 9-10% Zn in ISSN: Page 8

4 Aluminium alloys. After casting the samples where cut into the sample dimensions of 20mmx20mm as shown in figure 1. The samples were polished, degreased in acetone, dried, weighed and stored in a desiccators. The samples initial weights were taken using a weighing machine. Corrosion test: The pitting corrosion behavior of the specimen in Table 2 and table 1 was carried out by two examination methods: 5.1 Immersion test: Immersion was applied on the specimens of table 2 and table 1 composition. The medium used for the immersion test was the natural sea water. Specimens were measured before the immersion test and the weighted before the test. The immersion test conducted was for 5 days and 10 days containing natural sea water. Refer figure 1 & 2. After surface preparation, (i.e. surface polishing, degreased in acetone and dried) the specimens were placed in the beaker which contains sea water for immersion test (figure 1). Since the area enters the formula for calculating the corrosion rate, the original areas 20mmx20mm were used to calculate the corrosion rate. Specimens are electrically insulated and isolated from contact with another metal to avoid any galvanic effects. All original the areas of 25mm x 25mm are completely immersed in the tested solution [6]. 5.2 Electrochemical test: The prepared specimen of the table 1 was fixed in the holder and similarly the specimen of the table 2 was also fixed in the holder for the electrochemical test as shown in figure 3. The reference electrode was fixed about 1mm away from the surface of the specimen to be tested. The reference electrode used in this study was Saturated Calomel Electrode (SCE). The auxiliary electrode used in the electrochemical cell was platinum type. The specimen holder (working electrode), together with the reference and auxiliary electrodes were inserted in their respective positions in the electrochemical cell used for this purpose so that they fit all these electrodes. The cell used was made of glass. Constant potentials (anodic or cathodic) can be impressed on the specimen, by using the potentiostat. This potentiostat is able to induce a constant potential ranging from 1V to +1V. They are the potentials of the standard reference electrode used in this study. The potential difference between the working electrode (WE) and the reference electrode (RE) and any current passing in the circuit of the working electrode where the auxiliary electrode can be measured.[6] Any potential difference between the working and reference electrodes as well as any current in the working electrode circuit were be automatically recorded. The results and plots were recorded using Windows. The scan rate can be selected also. Polarization resistance tests were used to obtain the micro-cell corrosion rates. In the tests, cell current reading was taken during a short, slow sweep of the potential. The sweep was taken from 100mV to +100mV relative to OPC. The scan rate defines the speed of potential sweep in mv/sec. In this range, the curve of current density versus voltage is almost nearly linear. A linear data fitting of the standard model gives an estimate of the polarization resistance in order to calculate the corrosion current density and corrosion rate. ISSN: Page 9

5 Fig.1. Specimens for testing Fig.2. 5 days samples Fig days samples 6. Experimental work: 6.1. Materials and requirements: A varying composition of aluminium anode of dimensions 520x80 mm of 40 mm thickness was prepared. Table 1 and table 2 illustrate the compositions of the desired aluminium alloy. An analysis of the metal composition was cracked by the Sargam metals Pvt, Ltd Preparation of the specimen: Specimens of (20x20) mm were cut 6.3. Preparation of the surface specimen: The surface specimen is grounded with abrasive paper of grades 400, 800, 1000 and 1200 respectively.the grinding process was continued until the clay layer was removed. Another way of polishing the surface is by using a grinding machine to polish the Fig.4. Electrochemical testing from the metal plate. The dimension of the surface to be tested is 15cm x 15mm, a 0.5mm upper edge was left in order to suspend the specimen in the solution from a 0.2mm diameter hole located in the center upper edge of the 0.5mm. For the electrochemical testing a hole is made on the top of the specimen and a mild steel rod is inserted in order to supply voltage to the specimen. surface of the specimen. The specimen was then cleaned with acetone to remove any dirt or grease from the polished surface. 7. Classification of the specimen: The specimens were classified into three groups as specified in the Table 1 & 2. Program procedures: ISSN: Page 10

6 Fig. 5. Working procedure 8. Observation: 8.1. Salt Immersion Test: Weight difference for five days SAMPLE SAMPLE SAMPLES INITIAL WEIGHT(g) WEIGHT DIFFERENCE FOR FIVE DAYS(g) SAMPLE Table Weight difference of different samples for five days ISSN: Page 11

7 Weight difference for ten days SAMPLES INITIAL WEIGHT (g) WEIGHT DIFFERENCE FOR TEN DAYS(g) SAMPLE SAMPLE SAMPLE Table Weight Difference of different samples for ten days 8.2. Polarization test: Determination of Potential and Current for Existing Sample POTENTIAL CURRENT Table Determination of potential vs current for existing sample Determination of Potential and Current for Sample 1 POTENTIAL CURRENT Table Determination of potential vs current for sample Determination of Potential and Current for Sample 2 POTENTIAL CURRENT Table Determination of potential vs current for sample 2 ISSN: Page 12

8 9. RESULTS AND DISCUSSIONS From the Salt Immersion test the result noted from the weight loss technique for the This investigation includes the results 5 th day and 10 th day is found to be in a slight and discussion of the fifth day and tenth day variation. For the 5 th day samples the initial samples of salt immersion and impedance weight was Sample 1: g, Sample 2: and polarization study by electrochemical g and Existing Sample: g. The polarization test. The formations of pitting weight loss for the 5 th day is Sample 1: corrosion of the Al alloy were noted in g, Sample 2: g and Existing natural sea water. Sample: g. By the weight loss 9.1. Salt Immersion test: technique the Sample 3 (existing Initial Weight vs Final Weight for composition) is the metal which has the most fifth day dissolution of metal when compared to other 5 th day samples. Fig Initial weight and final weight of three samples (5 th day) Initial Weight vs Final Weight for tenth day For 10 th day samples the initial weight was found to be Sample 1: g, Sample 2: g, and Existing Sample: g. The weight loss for 10 th day is Sample 1: g, Sample 2: g, and Existing Sample: g. By the weight loss technique the Sample 1 has the most weight loss when compared to other 10 th day samples. ISSN: Page 13

9 Fig Initial weight and Final weight of three samples (10 th day) 9.2. POLARIZATION STUDY Existing Sample: The Existing sample has the aluminium composition of 3% zinc. The obtained Polarization graph and Impedance graph are analyzed. Bode graph: Fig Current vs Potential for existing sample ISSN: Page 14

10 Sample 1: Fig Impedance vs Frequency existing sample The sample 1 has the aluminium composition of 6% zinc. The obtained Polarization graph and Impedance graph are analyzed. Polarization graph: Bode graph: Fig Current vs Potential applied for sample 1 ISSN: Page 15

11 Sample 2; Fig Impedance vs Frequency for sample 1 The sample 2 has the aluminium composition of 10% zinc. The obtained Polarization graph and Impedance graph are analyzed. Polarization graph: Bode graph: Fig Current vs Potential applied for sample 2 ISSN: Page 16

12 Fig Impedance vs Frequency sample 2 Fig Electrolyte used (Natural sea water) 10. Conclusion: Based on the study the results has been evidenced the conclusion. By Salt immersion study for five days and ten days samples, the metal dissolution has been found to be more in the existing sample (i.e., Al-3%Zn) and for the tenth day samples, the metal dissolution has been found to be more in sample 1 (i.e., Al-6%Zn). By the polarization study, the Sample 1 and the existing sample has shown the highest corrosive resistivity. Sample 2 (i.e., Al-10%Zn) tends to be corroded more. The sample 1 of 6%Zn shows the highest corrosive resistivity when compared to existing sample of 3%Zn by the polarization study. The Sample 1 of 6%Zn is Fig Aluminium anode samples after polarization study more efficient than the existing sample which has been used by the Chennai port trust. Acknowledgement: The authors would like to thank the Chennai Port Trust authorities for permitting us to access the port and examine the Sacrificial anodes and the Ship hulls. We would also like to thank K.Ranganathan Assistant Engineer (Mechanical and Electrical Engineering department), Chennai port trust for support received in welding our final result samples in one of the marine tug. This research continues to receive support from the Chemistry department of Velammal Institute of Technology. ISSN: Page 17

13 Reference: [1] Shikar agrwal, Sachin kumar, Corrosion: A General review. [2] Muazu, A. And Yaro, S.A., Effects of zinc addition on the Performance of Aluminium. [3] Tai-Ming Tsai, Protection of steel using aluminium sacrificial anodes in artificial seawater. [5] M.C. Carroll, P.I. Gouma Effects of zn additions on the grain boundary precipitation and corrosion of al Scripta mater. 42 (2000) [6] Evaluation of the pitting corrosion for aluminum alloys 7020 in 3.5% NaCl solution with range of temperature ( ) C Abdulkareem Mohammed Ali Alsamuraee, Hani Aziz Ameen and Sami Ibrahim Jafer [4] nowledge/understanding-the-alloys-of-aluminum.cfm. ISSN: Page 18