A Comparison of Restoration of Obliterated Markings on Aluminium Engine Block by Chemical Etching Techniques

Abstract

The use of aluminium alloys as the material of manufacturing has increased tremendously such as manufacturing of Nissan, Audi, Honda and BMW cars. However, there is lack of complete data on the best technique for restoration of obliterated serial numbers on such alloy used in the engine block. This research study was carried out to identify the most efficient and sensitive reagent among the five different etchants for restoration of obliterated markings on aluminium alloy surfaces. The aluminium alloy engine block of Nissan car was cut into small plates, and each plate was engraved with some alphanumerical characters using a computer-controlled “Gravograph” machine. Then, the marks were erased using abrasive papers to different levels (0.03 mm, 0.05 mm, 0.08 mm and 0.1 mm), below the bottom part of the engraving marks. The erased plates were etched by swabbing or immersion in each tested reagent. Etching with etchant that contained 10 g NaOH and 90 mL of water was useful to reveal the obliterated marks. Marks that were removed from 0.03 to 0.1 mm below the bottom of engraving were successfully restored with good contrast and in less time (minimum time of 23 min). Another reagent 10% aq. H₃PO₄ was also effective to restore the obliterated marks when the erased areas were immersed in it for long hours (minimum time of 21 h). On the other hand, reagent of 10% NaOH and 10% NHO₃ had also achieved fair contrast and reproducible restoration of marks from 0.03 to 0.1 mm below the depth of engraving. All the restored alphanumerical characters on the plates were visualised using fluorescent powder irradiated with UV light. In contrast, other two tested reagents, i.e. 60% HCl and 40% NaOH, 1 N NaOH and 0.1 M HgCl₂ in 0.1 N HCI, showed poor contrast restoration and hence were not recommended for practical application. The findings from this research would help the investigator particularly in stolen car cases to choose the best etching reagent for restoration of obliterated serial numbers as well as to improve etching procedures on the aluminium alloy surfaces used in the car engines.