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Measurement of the corrosion rate of aluminium in sodium hydroxide using holographic interferometry
Optics and Lasers in Engineering
15 (1991) 183-188
Measurement of the Corrosion Rate of Aluminium in Sodium Hydroxide Using Holographic Interferometry M. R. Sajan, T. S. Radha, Instrumentation
and Services Unit, Indian Institute 560 012 &
Department
B. S. Ramprasad India
E. S. R. Gopal
of Physics, Indian Institute
(Received 5 December
of Science, Bangalore,
of Science, Bangalore,
India 560 012
1990; revised version received and accepted 1 February
1991)
ABSTRACT
The application of holographic interferometry to the measurement of the corrosion rate of aluminium in sodium hydroxide is investigated. Details of the fabrication of the corrosion cell and the experimental procedure are given. Thickness loss of aluminium was found for different dissolution times and compared with the conventional weightloss method using a microbalance.
1 INTRODUCTION There are many industrial and domestic uses of aluminium and its alloys, ranging from cooking utensils to aircraft structures. While this material has good resistance, in general, under different environmental conditions, it has poor resistance to attack by alkaline solutions. In the dairy industry, the use of caustic soda to clean milk cans has generated interest in the study of aluminium corrosion in sodium or potassium hydroxide. ‘J Studies have been carried out on the effect of alloying elements on the rate of dissolution of aluminium in alkaline solutions,3 and also on the use of inhibitors to overcome or reduce the corrosive attack.4 The corrosion rate is typically measured by the weight-loss method’ and resistance monitoring.5 Corrosion of aluminium and its alloys continues to be of interest, as evidenced by a recent paper by Macdonald et ~1.~ They measured the corrosion of aluminium alloyswhich contained Zn, Bi and other elements-in potassium hydroxide, to evaluate their usefulness for aluminium-air batteries. 183 optics and Lasers in Engineering 0143-8166/91/$03.50 Ltd, England. Printed in Northern Ireland
0 1991 Elsevier Science Publishers
184
M. R. Sajan et al.
In this paper, we report preliminary results on the measurement of the corrosion rate of aluminium in sodium hydroxide solution using holographic interferometry. The method is simple and gives, directly, the thickness of material removed due to dissolution compared with conventional methods. Accelerated dissolution tests were performed in order to simulate the corrosion process, which may take months in practice. Radha and Ramprasad7 reported that etch depth in aluminium specimens can be measured using speckle photography. This indicates that the microstructure is not so much modified as to make the wavefront decorrelated. If the wavefronts under the original and etched conditions are not decorrelated, in other words they are coherent, there is every reason to believe that holographic interferometry should work. To test this premise, experiments were conducted using holographic interferometry. It is shown that the dissolution of aluminium in sodium hydroxide can be measured by holographic interferometry within certain limitations. To our knowledge, only one attempt has been made to use holographic interferometry’ for corrosion studies: that was to find the corrosion rate of brass in nitric acid. No useful results were reported, however, due to the rapid change in the surface microstructure of the brass by dissolution in the nitric acid. Subsequently, no further work using holographic interferometry for corrosion studies has been reported.
2 EXPERIMENTAL
DETAILS
During the last three decades, holographic interferometry has advanced from the stage of a curiosity to a widely accepted tool for many industrial applications and non-destructive testing.’ Experiments were performed with an off-axis holographic recording system (see Fig. 1) using a 2 mW He-Ne laser. Holograms were recorded with AGFA 10 E 75 plates. Figure 2 shows the cell used for the corrosion tests on the sample. The cell, 5 x 5 X 5 cm3, is made out of glass. The sample aluminium plate is screwed onto a support S. The cell is tilted at an angle of 30” to avoid the reflections of the direct beam from the front cell wall and the sample plate at the recording plane. The mount is fixed on the cell using commercial epoxy adhesive. A glass funnel F is fixed on the cell for pouring in solutions. Drainage tube D is provided at the bottom of the cell for removing the waste solutions from the cell.
Measurement of corrosion rate of aluminium in sodium hydroxide
185
B.S
Fig. 1.
Schematic of the holographic set-up: BS, beam splitter; M, and MZ, mirrors; SF, and SF,, pinhole spatial filters.
available Sample plates, 6 x 4 cm2, were cut from commercially 1.2 mm thick aluminium sheet. The plates were lapped well using cerium oxide powder over a spinning felt wheel. The plate was cleaned with a suitable detergent and then screwed onto the support. At this time, the lapped plate is partially specularly reflecting. To obtain a matt surface, the lapped plate was allowed to dissolve in sodium hydroxide solution until the surface was uniform in appearance. The plate was cleaned well and the cell filled with water. All the experiments were performed with 0.25 N sodium hydroxide. The double-exposure hologram was made as follows. First, exposure was made with the aluminium plate in water. After covering the laser beam, the water was drained off and sodium hydroxide solution poured in. The plate was then allowed to dissolve in the sodium hydroxide solution for a fixed time interval. The sodium hydroxide solution was drained and the cell flushed with water a couple of times to remove any
FQ. 2.
Schematic of the cell: C, cell; P, sample plate; S, support; F, funnel; D, pipe.
M. R. Sajan et al.
186
remaining alkaline solution. The cell was refilled with water. Now the second exposure was given. The hologram was developed in Kodak D-19 high-resolution developer and fixed with a standard fixer.
3 RESULTS
OF THE CORROSION
TEST
Figure 3 shows typical interferograms obtained using the set-up. The thickness loss At of the plate is found from the equation
where II is the number of fringes and A is the illuminating wavelength (632.8 nm). To confirm the results, the weight loss of the sample plates were determined using a microbalance after each experiment and the thickness change was computed. If AW is the weight loss due to dissolution, the corresponding thickness loss At is found from At=-
AW Ad
where A is the total area of the sample plate exposed to the solvent and d is the density of the sample. Results obtained by both the methods are plotted in Fig. 4.
(4 Fig. 3.
Real-time aluminium
holographic interferograms for two different dissolution times in 0.25 N sodium hydroxide solution: (a) 10 min and (b) 20 min.
of
Measurement of corrosion rate of aluminium in sodium hydroxide 6-
o - By Holographic
187
lnterferometry
x- By weight loss method -z
5-
-;
4-
4 0
3-
ui v) E Y .uE
2-
I-
I 0
5
10
I5
Dissolution
20
1
I
1
25
30
35
time -(minutes)
Thickness of aluminium plate dissolved in 0.25 N sodium hydroxide solution for different dissolution times using (0) holographic interferometry and ( x ) the weightloss method.
Fig. 4.
4 CONCLUSION The use of holographic interferometry to measure the corrosion rate of aluminium in sodium hydroxide solution has been demonstrated. Fringes of good contrast representing thickness loss of aluminium were obtained. However, for large dissolution times (greater than 35 min), it was found that the contrast of the interference fringes decreased drastically. As pointed out by Ashton et al. ,8 the major limitation of the holographic method is the change in the microstructure of the plate surface that causes deterioration in the fringe contrast.
ACKNOWLEDGEMENT One of the authors, M. R. Sajan, would like to thank the Council of Scientific and Industrial Research for giving him a Senior Research Fellowship which enabled him to pursue this research work.
REFERENCES 1. Streicher, solutions.
M. A., The J. Electrochem.
dissolution of aluminium Sot., 96 (1949) 170-94.
in sodium
hydroxide
188
M. R. Sajan et al.
2. Straumanis, M. E. & Brakss, N., The rate of solution of highest purity aluminium in sodium hydroxide solution. J. Electrochem. Sot., 95 (1949) 98-106. 3. Straumanis, M. E. & Brakss, N., The effect of minor alloying elements on the rate of dissolution of aluminium in bases. J. Electrochem. Sot., 96 (1949) 310-17. 4. Streicher, M. A., Dissolution of aluminium in sodium hydroxide solutions: Effect of gelatin and potassium permanganate. Znd. Engng Chem., 41 (1949) 818-9. 5. Troscinski, E. S., Couper, A. S. & Dravnieks, A., Continuous corrosion monitoring. Mech. Engng, 83 (1961) 47-51. 6. Macdonald, D. D., Lee, K. H., Moccari, A. & Harrington, U., Evaluation of alloy anodes for aluminium-air batteries: Corrosion studies. Corrosion, 44 (1988) 652-7. 7. Radha, T. S. & Ramprasad, B. S., A simple method for measuring the etch depth in metallic specimens using speckle photography. J. Electrochem. Sot. India, 34 (1985) 149-50. 8. Ashton, R. A., Solvin, D. & Gerricsen, H. J., Interferometric holography applied to elastic stress and surface corrosion. Appl. Optics, 10 (1971) 440-l. 9. Jones, R. & Wykes, C., Holographic and Speckle Interferometry, 2nd edition. Cambridge University Press, Cambridge, 1989.
15 (1991) 183-188
Measurement of the Corrosion Rate of Aluminium in Sodium Hydroxide Using Holographic Interferometry M. R. Sajan, T. S. Radha, Instrumentation
and Services Unit, Indian Institute 560 012 &
Department
B. S. Ramprasad India
E. S. R. Gopal
of Physics, Indian Institute
(Received 5 December
of Science, Bangalore,
of Science, Bangalore,
India 560 012
1990; revised version received and accepted 1 February
1991)
ABSTRACT
The application of holographic interferometry to the measurement of the corrosion rate of aluminium in sodium hydroxide is investigated. Details of the fabrication of the corrosion cell and the experimental procedure are given. Thickness loss of aluminium was found for different dissolution times and compared with the conventional weightloss method using a microbalance.
1 INTRODUCTION There are many industrial and domestic uses of aluminium and its alloys, ranging from cooking utensils to aircraft structures. While this material has good resistance, in general, under different environmental conditions, it has poor resistance to attack by alkaline solutions. In the dairy industry, the use of caustic soda to clean milk cans has generated interest in the study of aluminium corrosion in sodium or potassium hydroxide. ‘J Studies have been carried out on the effect of alloying elements on the rate of dissolution of aluminium in alkaline solutions,3 and also on the use of inhibitors to overcome or reduce the corrosive attack.4 The corrosion rate is typically measured by the weight-loss method’ and resistance monitoring.5 Corrosion of aluminium and its alloys continues to be of interest, as evidenced by a recent paper by Macdonald et ~1.~ They measured the corrosion of aluminium alloyswhich contained Zn, Bi and other elements-in potassium hydroxide, to evaluate their usefulness for aluminium-air batteries. 183 optics and Lasers in Engineering 0143-8166/91/$03.50 Ltd, England. Printed in Northern Ireland
0 1991 Elsevier Science Publishers
184
M. R. Sajan et al.
In this paper, we report preliminary results on the measurement of the corrosion rate of aluminium in sodium hydroxide solution using holographic interferometry. The method is simple and gives, directly, the thickness of material removed due to dissolution compared with conventional methods. Accelerated dissolution tests were performed in order to simulate the corrosion process, which may take months in practice. Radha and Ramprasad7 reported that etch depth in aluminium specimens can be measured using speckle photography. This indicates that the microstructure is not so much modified as to make the wavefront decorrelated. If the wavefronts under the original and etched conditions are not decorrelated, in other words they are coherent, there is every reason to believe that holographic interferometry should work. To test this premise, experiments were conducted using holographic interferometry. It is shown that the dissolution of aluminium in sodium hydroxide can be measured by holographic interferometry within certain limitations. To our knowledge, only one attempt has been made to use holographic interferometry’ for corrosion studies: that was to find the corrosion rate of brass in nitric acid. No useful results were reported, however, due to the rapid change in the surface microstructure of the brass by dissolution in the nitric acid. Subsequently, no further work using holographic interferometry for corrosion studies has been reported.
2 EXPERIMENTAL
DETAILS
During the last three decades, holographic interferometry has advanced from the stage of a curiosity to a widely accepted tool for many industrial applications and non-destructive testing.’ Experiments were performed with an off-axis holographic recording system (see Fig. 1) using a 2 mW He-Ne laser. Holograms were recorded with AGFA 10 E 75 plates. Figure 2 shows the cell used for the corrosion tests on the sample. The cell, 5 x 5 X 5 cm3, is made out of glass. The sample aluminium plate is screwed onto a support S. The cell is tilted at an angle of 30” to avoid the reflections of the direct beam from the front cell wall and the sample plate at the recording plane. The mount is fixed on the cell using commercial epoxy adhesive. A glass funnel F is fixed on the cell for pouring in solutions. Drainage tube D is provided at the bottom of the cell for removing the waste solutions from the cell.
Measurement of corrosion rate of aluminium in sodium hydroxide
185
B.S
Fig. 1.
Schematic of the holographic set-up: BS, beam splitter; M, and MZ, mirrors; SF, and SF,, pinhole spatial filters.
available Sample plates, 6 x 4 cm2, were cut from commercially 1.2 mm thick aluminium sheet. The plates were lapped well using cerium oxide powder over a spinning felt wheel. The plate was cleaned with a suitable detergent and then screwed onto the support. At this time, the lapped plate is partially specularly reflecting. To obtain a matt surface, the lapped plate was allowed to dissolve in sodium hydroxide solution until the surface was uniform in appearance. The plate was cleaned well and the cell filled with water. All the experiments were performed with 0.25 N sodium hydroxide. The double-exposure hologram was made as follows. First, exposure was made with the aluminium plate in water. After covering the laser beam, the water was drained off and sodium hydroxide solution poured in. The plate was then allowed to dissolve in the sodium hydroxide solution for a fixed time interval. The sodium hydroxide solution was drained and the cell flushed with water a couple of times to remove any
FQ. 2.
Schematic of the cell: C, cell; P, sample plate; S, support; F, funnel; D, pipe.
M. R. Sajan et al.
186
remaining alkaline solution. The cell was refilled with water. Now the second exposure was given. The hologram was developed in Kodak D-19 high-resolution developer and fixed with a standard fixer.
3 RESULTS
OF THE CORROSION
TEST
Figure 3 shows typical interferograms obtained using the set-up. The thickness loss At of the plate is found from the equation
where II is the number of fringes and A is the illuminating wavelength (632.8 nm). To confirm the results, the weight loss of the sample plates were determined using a microbalance after each experiment and the thickness change was computed. If AW is the weight loss due to dissolution, the corresponding thickness loss At is found from At=-
AW Ad
where A is the total area of the sample plate exposed to the solvent and d is the density of the sample. Results obtained by both the methods are plotted in Fig. 4.
(4 Fig. 3.
Real-time aluminium
holographic interferograms for two different dissolution times in 0.25 N sodium hydroxide solution: (a) 10 min and (b) 20 min.
of
Measurement of corrosion rate of aluminium in sodium hydroxide 6-
o - By Holographic
187
lnterferometry
x- By weight loss method -z
5-
-;
4-
4 0
3-
ui v) E Y .uE
2-
I-
I 0
5
10
I5
Dissolution
20
1
I
1
25
30
35
time -(minutes)
Thickness of aluminium plate dissolved in 0.25 N sodium hydroxide solution for different dissolution times using (0) holographic interferometry and ( x ) the weightloss method.
Fig. 4.
4 CONCLUSION The use of holographic interferometry to measure the corrosion rate of aluminium in sodium hydroxide solution has been demonstrated. Fringes of good contrast representing thickness loss of aluminium were obtained. However, for large dissolution times (greater than 35 min), it was found that the contrast of the interference fringes decreased drastically. As pointed out by Ashton et al. ,8 the major limitation of the holographic method is the change in the microstructure of the plate surface that causes deterioration in the fringe contrast.
ACKNOWLEDGEMENT One of the authors, M. R. Sajan, would like to thank the Council of Scientific and Industrial Research for giving him a Senior Research Fellowship which enabled him to pursue this research work.
REFERENCES 1. Streicher, solutions.
M. A., The J. Electrochem.
dissolution of aluminium Sot., 96 (1949) 170-94.
in sodium
hydroxide
188
M. R. Sajan et al.
2. Straumanis, M. E. & Brakss, N., The rate of solution of highest purity aluminium in sodium hydroxide solution. J. Electrochem. Sot., 95 (1949) 98-106. 3. Straumanis, M. E. & Brakss, N., The effect of minor alloying elements on the rate of dissolution of aluminium in bases. J. Electrochem. Sot., 96 (1949) 310-17. 4. Streicher, M. A., Dissolution of aluminium in sodium hydroxide solutions: Effect of gelatin and potassium permanganate. Znd. Engng Chem., 41 (1949) 818-9. 5. Troscinski, E. S., Couper, A. S. & Dravnieks, A., Continuous corrosion monitoring. Mech. Engng, 83 (1961) 47-51. 6. Macdonald, D. D., Lee, K. H., Moccari, A. & Harrington, U., Evaluation of alloy anodes for aluminium-air batteries: Corrosion studies. Corrosion, 44 (1988) 652-7. 7. Radha, T. S. & Ramprasad, B. S., A simple method for measuring the etch depth in metallic specimens using speckle photography. J. Electrochem. Sot. India, 34 (1985) 149-50. 8. Ashton, R. A., Solvin, D. & Gerricsen, H. J., Interferometric holography applied to elastic stress and surface corrosion. Appl. Optics, 10 (1971) 440-l. 9. Jones, R. & Wykes, C., Holographic and Speckle Interferometry, 2nd edition. Cambridge University Press, Cambridge, 1989.