Restoration of Obliterated Stamped Marks on Aluminium Surfaces by Metallographic Etching Technique

Introduction

Serial numbers are unique identification marks placed on engines and chassis of vehicles, firearm frames or any other metallic object^{1 – 4}. Casting, Engraving, die stamping, pin stamping and LASER are commonly used to introduce these identification marks on the metallic surfaces ^5,6. On the contrary, perpetrators frequently obliterate or remove these marksby several mechanical means i.e., peening, punching, filing, grinding, drilling, welding over-stamping, chemical corrosion, etc.for unauthorized selling and to avoid any connection with the item^{1, 2, 6 – 8}. Restoration of the obliterated or removed identification marks provides major leads in the identification of stolen objects^2,9. The marking process deforms several layers underneath the actual indentation. The depth of deformation depends on the strength of the metal, i.e., on impact, zinc deforms more than iron¹⁰ (Figure 1.a – 1.c). The efficiency of restoration depends upon the depth of obliteration. Treatment of the surface with different chemicals known as macro etching is the most common restoration technique used by the researchers on metals. Macro etching is the most effective metallographic technique among all¹¹.

Figure 1: Cross – sectional of deformed crystal structure by mechanical force. Click here to View figure

A desirable etching reagent for metallic surfacealways give good contrast, reproducibility and consume less time for restoration. A large number of significant studies have been performed on commonly encountered metals and their alloys. Works by Turley¹², Zaili et al.⁷, Wightman and Matthew¹, yin and Kuppuswamy¹³, Wahab et al.¹¹ and Richa et al.⁴, showed that different composition of ‘Fry’s Reagent’ consists of a metallic halide or salts and a strong acid is an excellent etching reagent for iron and steel surfaces. Recently Shankar et al.³ restored obliterated serial markings on copper by a combination of ferric chloride, glacial acetic acid and distilled water and Fortini et al.¹⁴restored obliterated markings on Ni-Cr-Mo steel surface by a combination of Nitric acid and distilled water.

Aluminium and its alloys are rapidly replacing other metals in the vehicle, firearm and machinery industry in recent decades due to its strength and lightweight^2,15-16. Different alloys (e.g. Duralumin) of aluminium also used frequently in various automobile industries¹⁶.Restoration on aluminium surfaces were previously performed only on obliterated engraved marks^2,9,10,15,17. Studies by Chisum¹⁵, Baharumet al.¹⁰, Peeler et al.¹⁷ Bong and Kuppuswamy⁹ and Uli et al.² revealed that etching reagents used for comparatively strong metals and their alloys like iron, steel, etc. are not suitable for the restoration of obliterated engraved marks on aluminium and its alloy surfaces. Few researchers recommended that an alternate application of a dilute solution of a strong acid and a strong alkali can be a good etching reagent on aluminium surfaces^2,9-10. Present study assessed ten best etching reagents used in previous studies of restoration on different metallic surfaces to find out a fast and effective reagent for the restoration of obliterated stamped marks on aluminium surface.

Methodology

In the present study 12.1 mm thick aluminium bricks were used to restore obliterated serial numbers created by punching method. Metallographic etching or chemical etching techniques is used for the restoration in the present study. The procedure of this method has been adapted from the previous studies of restoration of obliterated engraved marks on aluminium and it alloy surfaces^2,9,10,15,17

Sample Preparation

A stamping die was used to introduce numeric characters from 1 to 9. Each character of the die stamp covers approximately 0.3 square inch area. Finally, all the samples were obliterated by the mechanical grinding machine (Figure 2) up to zero visibility. Samples were photographed before and after each step for permanent record and comparison.

Figure 2: Grinder used to create the obliteration on aluminium surface Click here to View figure

Preliminary Examination and Surface Preparation

Surface preparation before restoration is the compulsory step as it makes the surface uniform which in turn enhance the efficiency of the restoration. All the samples were first examined by a magnifying lens to observe any remaining visible number. Commercially available silicon carbide paper (P60) is used to polish the grinded surface by removing scratches and other marks to give a smooth mirror-like finish for better chemical treatment. Polishing is followed by swabbing the surface with acetone to remove any dirt, grease, paint or any other loosely adherent material from the surface.

Chemical Etching

In the present work ten etching reagents (Table 1) were selected from previous studies of restoration on metallic surfaces. All the selected reagents are best in their respective studies. Reagent 1, 2 and 6 were previously used on the aluminium and its alloy surfaces (Al-Zn-Mg-Cu and Al-Si). Reagents were applied on the obliterated surface by swabbing method. A cotton bud is soaked in the reagent and gently applied over the obliterated aluminium surfaces until the marks restored. Single reagents were swabbed uninterruptedly and multi-reagents were applied alternatively until the numbers restore. Since, the restored numbers were visible for a short period of time,instant photographs were captured to keep a permanent record before and after every step. The application method and composition of each reagent are summarized in the Table 1.

Table 1: Comparative effectiveness and sources of chemical etching reagents for the restoration of obliterated stamped marks on aluminium surfaces.

S. No.	Etching Reagent	Composition of Reagent	Application Method	Consumed Time	Observation	Source
1.	Reagent 1 (Villella’s reagent)	Glycerine                  30 ml Hydrofluoric acid     20 ml Nitric acid                10 ml	Continuously swabs the reagent until the numbers appear.	20 – 25	Both contrast and reproducibility were good and can be easily photographed.	Heard, 2008
2.	Reagent 2 (Hume Rothery solution)	Copper Chloride (CuCl2) 200 g Hydrochloric Acid (HCl) 5 ml Distilled water              1000 ml	The solution was swabbed alternatively on the erased surface. First with Hume Rothery solution and then with distilled water.	20	The number appears in good contrast and also reproducible.	Baharum et al., 2008, Heard, 2008
3.	Reagent 3	Ferric Chloride (FeCl3)     25 g HCl                         25 ml Distilled water       100 ml	The reagent was swabbed continuously until the numbers appear.	4 – 6	Good contrast, easily observed and reproducible. Can be easily photographed.	Richa et al., 2012
4.	Reagent 4	Solution 1 Copper Chloride      45 g HCl                         100 ml Distilled water        180 ml   Solution 2 15% Nitric acid	Both solutions were swabbed alternatively on the erased surface. until the numbers appear.	90	No restoration.	Wahab et. al., 2012
5.	Reagent 5	Conc. HCl                10 ml Glacial acetic acid    25 ml	The solution was applied continuously until the numbers appear.	30	Numbers appeared in fair contrast and slightly observable and also reproducible.	Shankar et al., 2014
6.	Reagent 6	Solution 1 10% Sodium hydroxide   Solution 2 10% Nitric acid	Both the solution was swabbed alternatively on the erased surface. First with solution 1 for 3 minutes. Then solution 2 for 1 minute.	60	Both contrast and reproducibility were good and the sensitivity of numbers was also good.	Uli et al. 2010, Petterd, 2000 Bong and Kuppuswamy, 2010
7.	Reagent 7	Solution 1 10% Sodium hydroxide (NaOH)   Solution 2 25% Nitric acid (HNO3) (Used as highlighter for the above chemical)	Swabs with NaOH and then HNO3	58		Technical Procedure of reagent preparation 2014
8.	Reagent 8	10% Sodium hydroxide Distilled water      90 ml	Continuusly swabbing over the erased surface until the numbers appear.	50	Numbers are reproducible and appeared in good contrast.	Katterwe, 2006
9.	Reagent 9	Nitric acid           25 ml Distilled water    75 ml		90	No restoration.	Petterd, 2000
10.	Reagent 10	Nitric acid           20 ml Potassium permanganet          4 g Distilled water   100 ml		90		Shankar et al., 2014

Results

The present study is focused to find out an effective and rapid chemical reagent to restore obliterated stamped serial numbers on aluminium surfaces. Several researchers explored multiple reagents to restore obliterated serial numbers on different metallic surfaces. 10 best etching reagents from different restoration studies on aluminium and other metallic surfaces were selected for the present experiment. The experimental result shows that 7 among 10 reagents (i.e., Reagent 1: Glycerine 30 ml, Hydrofluoric acid 20 ml, Nitric acid (HNO3) 10 ml, Reagent 2: Copper Chloride 200 g, Hydrochloric Acid 50 ml, Distilled water 1000 ml, Reagent 3: Ferric Chloride 25 g, Hydrochloric Acid 25 ml, Distilled water 100 ml, Reagent 5: Conc. Hydrochloric Acid 10 ml, Glacial acetic acid 25 ml, Reagent 6: 10% Sodium hydroxide (NaOH), 10% HNO3, Reagent 7: 10% NaOH, 25% HNO3 and Reagent 8: 10% NaOH, Distilled water 90 ml.) successfully restored the obliterated marks. But, only three reagents i.e., Reagent 1 (Villella’s reagent), Reagent 2 (Hume Rothary Solution) and Reagent 3 (Ferric Chloride 25 g, Hydrochloric Acid 25 ml, Distilled water 100 ml) showed good restoration efficiency in less time (4-25 min) Figure 3.a-3c, 4a-4.c). Reagent 3 was the standout among all as it took the least time (4 – 6 Minutes) and also produce good visibility (Figure 5.a – 5c) of restored numbers. This experiment also revealed that reagents containing alkali, took comparatively less time (4 – 30 minutes) to restore the obliterated marks than the reagents containing it (50 – 60 minutes). Table 2 shows the comparative representation of the consumed time by reagents including alkali and without alkali. Reagent 4, 9 and 10 showed no restoration on the aluminium surface.

Figure 3: Serial mark(a) pre obliteration (b) post obliteration (c) restored by Villella’s Reagent Click here to View figure

Figure 4: Serial mark(a) pre obliteration (b) post obliteration (c) restored by Hume Rothary solution Click here to View figure

Figure 5: Serial mark (a) pre obliteration (b) post obliteration (c) restored by Reagent 3 Click here to View figure

Table 2: The comparative representation of the consumed time by reagents including alkali and without alkali.

Reagent (Without Alkali)	Composition	Consumed Time (Min)	Reagent (Including Alkali)	Composition	Consumed Time (Min)
Reagent 1 (Villella’s reagent)	Glycerine                  30 ml Hydrofluoric acid     20 ml Nitric acid(HNO3)   10 ml	20 – 25	Reagent 6	Solution 1 10% Sodium hydroxide   Solution 2 10% Nitric acid	60
Reagent 2 (Hume Rothery solution)	Copper Chloride (CuCl2) 200 g Hydrochloric Acid (HCl) 5 ml Distilled water              1000 ml	20	Reagent 7	Solution 1 10% Sodium hydroxide (NaOH)   Solution 2 25% Nitric acid (HNO3) (Used as highlighter for the above chemical)	58
Reagent 3 (Fry’s Reagent)	Ferric Chloride (FeCl3)     25 g HCl                         25 ml Distilled water       100 ml	4 – 6	Reagent 8	10% Sodium hydroxide Distilled water      90 ml	50
Reagent 5	Conc. HCl                10 ml Glacial acetic acid    25 ml	30

Discussion

Richaet al.⁴ performed a similar experiment on steel surface by using multiple combinations of Fry’s reagent and found effective results. Reagent3 produced the best visibility in minimum time (4 – 6 min.) with better reproducibility among all the ten reagents used. In the study by Chisum¹⁵, a similar combination of metallic chloride (Mercuric Chloride) and a strong acid (HCl) showed good restoration on aluminium and its alloy surfaces that is similar with the results of the present study. An alternative swabbing of 60% Hydrochloric Acid (HCl) and 40% NaOHproved to be a good etching reagent for obliterated engraved marks on aluminium and its alloys in the experiments by Baharum et al.,¹⁰ Bong and Kuppuswamy⁹ and Peeler et al.,¹⁷. Uli et al. used an alternate swabbing of 10% Nitric Acid and 10% NaOHin their study which showed good results on aluminium-silicon alloy surfaces². Previous studies revealed that the effect of etching reagent on the metal surface is affected by the marking procedure. Engraved aluminium surfaces did not restore obliterated marks by the etching reagents used for strong metals because it corrodes most of the underlying shallow deformations^2,9-10. Stamping method potentially causes greater deformation on the metal surface that make it persistentto the exposure of strong etching reagents as observed in the present study (Reagent 3). Previous studies suggested that aluminium and its alloys show gradual corrosion to the exposure of caustic solutions like an alkali². NaOH(Alkali) is present in most of the best-suited reagents previously used for aluminium and its alloys^2,9,17. The absence of alkali probably accelerates the caustic effect on the aluminium surface that reduces the restoration time. The reagents without NaOH (i.e., Reagent 2, 3 and 5) restored the obliterated serial numbers faster than the reagents consisting NaOH.A comparison between the results of the previous and the present study on aluminium surfaces is summarized in Table 3.

Table  3: Summarized table of previous and present study on aluminium surfaces.

Comparative Study
S.No.	Study	Composition Of Desirable Reagent	Restoration Time (Minutes)
1.	Chisum WJ, 1963	0.1M Mercuric Chloride in 0.1N Hydrochloric Acid	20
2.	Baharumet al. 2008	Solution 1 60% hydrochloric acid Solution 2 40% sodium hydroxide	3 – 60
3.	Uliet al. 2010	Solution 1 10% Sodium hydroxide Solution 2 10% Nitric acid	8 – 40
4.	Bong and Kuppuswamy, 2010	(i) 10% Aqueous Phosphoric Acid (ii) Solution 1 60% hydrochloric acid Solution 2 40% sodium hydroxide	2 – 960 2 – 20
5.	Present Study	Ferric Chloride              25 g Hydrochloric acid          25 ml Distilled water             100 ml	4 – 6

Conclusion

The present study is the first attempt to restore obliterated stamped serial numbers on the aluminium surface in contrast to all the previous studies on obliterated engraved markings.Among 10 chemical reagents used, reagent 3 (Ferric Chloride 25 g, HCl 25 ml and Distilled water 100 ml) produce desired results in minimum time (4 -6 min). The study also revealed that the absence of the alkaline compound potentially enhances the caustic effect to minimize the restoration time. Reagent 2, 3 and 5 including HCl showed a similar result as produced by reagent 6, 7 and 8 including NaOH. But acid-containing reagents took lesser time to restore the obliterated numbers.

References

1. Wightman, G.; Matthew, J. Restoration of stamp marks on steel components. Forensic Science International.,2008, 180, 32–36. DOI: 10.1016/j.forsciint.2008.06.017.
   CrossRef
2. Uli, N.;Kuppuswamy, R.;Amran, M.F.C. A survey of some metallographic etching reagents for restoration of erased engraved marks on aluminium–silicon alloy surfaces. Forensic science International.,2010, 208, 66-73. DOI: 10.1016/j.forsciint.2010.11.005.
   CrossRef
3. Shankar, N.;Kesharwania, L.; Mishra, M.K.;Chattree, A.; Kumar, R.; Sharma, R.; Gupta.A.K.New Restoration Reagent for Development of Erased Serial Number on Copper Metal Surface. Malaysian Journal of Forensic Sciences.,2014, 5(2), 53-66.
4. Richa.;Kesharwani, L.; Gupta, A.K.; Mishra, M.K. Development of new reagent for restoration of erased serial number on metal plate. Egyptian journal of Forensic Science.,2013, 3(1), 26-34. DOI: dx.doi.org/10.1016/j.ejfs.2012.11.004.
   CrossRef
5. Nickols, L.C.The Scientific Investigation of Crime.London:Butterworth & Co., 1956.
   CrossRef
6. Heard, B.J. Handbook of firearms and ballistics: Examining and interpreting forensic science evidence. 2nd ed.Hoboken NJ: Wiley John& Sons.,2008.
   CrossRef
7. Zaili, M.A.M.;Kuppuswamy, R.;Harun, H. Restoration of engraved marks on steel surfaces by etching technique. Forensic science International.,2006, 171, 27-32. DOI: 10.1016/j.forsciint.2006.09.010.
   CrossRef
8. Kuppuswamy, R.;Senthilkumar, M. Restoration of vehicle identification numbers. Journal of forensic Identification.,2004, 54, 13–21.
9. Bong, Y.U.;Kuppuswamy, R. Revealing obliterated engraved marks on high strength aluminium alloy (AA7010) surfaces by etching technique. Forensic Science International.,2010, 195, 86–92. DOI: 10.1016/j.forsciint.2009.11.018.
   CrossRef
10. Baharum, M.I.M.;Kuppuswamy, R.; Rahman, A.A. Recovering erased engraved marks on aluminium surfaces by etching technique. Forensic Science International.,2008, 177, 221-227. DOI: 10.1016/j.forsciint.2008.01.004.
    CrossRef
11. Wahab, M.F.A.;Izwani, N.;Ghani.M.;Kuppuswamy, R. An investigation into the suitability of some etching reagents to restoring obliterated stamped numbers on cast iron engine blocks of cars. Forensic Science International.,2012, 223, 53–63.DOI:dx.doi.org/10.1016/ j.forsciint.2012. 07.02012.
    CrossRef
12. Turley, D. Restoration of Stamp Marks on Steel Components by Etching and Magnetic Techniques. Journal of Forensic Sciences., 1987, 32(3), 640-649. DOI: doi.org/10.1520/JFS12368J13.
    CrossRef
13. Yin, S.H.;Kuppuswamy, R. On the sensitivity of some common metallographic reagents to restoring obliterated marks on medium carbon (0.31% C) steel surfaces. Forensic Science International.2009, 183, 50–53. DOI: 10.1016/j.forsciint.2008.10.009.
    CrossRef
14. Fortini, A.; Merlin, M.;Soffritti, C.;Garagnani, G.L. Restoration of Obliterated Numbers on 40NiCrMo4 Steel by Etching Method: Metallurgical and Statistical Approaches. Journal of Forensic Science.2016, 61(1), 160 – 69. DOI: 10.1111/1556-4029.12849.
    CrossRef
15. Chisum, W.J. A catalytic process for restoration of serial numbers on aluminium. Journal of Forensic Science Society.,1966, 6(2), 89.
    CrossRef
16. Okayasu, M.; Sato, K.; Mizuno, M. A new etching technique for revealing the plastic deformation zone in an Al–Cu–Mg alloy. Journal of Material Science.,2008, 43, 2792–98. DOI: 10.1007/s10853-008-2544-y.
    CrossRef
17. Peeler, G.;Gutowski, S.;Wrobel, H.; Dower, G. The restoration of impressed characters on aluminium alloy motorcycle frames. Journal of Forensic Identification.2008, 58(1), 27–32.

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