Cleaning of aluminium alloy statuary material with a Q-switched Nd:YAG laser

Abstract

Corroded aluminium alloy material from Liverpool’s Gilbert Eros statue was laser cleaned using a Q-switched Nd:YAG laser. Elements present in the corrosion products were identified by XRD and EPMA as S, Cl, Ca, P and K in addition to Al, Cu, Si, Fe and Zn arising from the base material. Lighter elements such as O, N and H which were assumed to be present were undetected by these techniques. Laser parameters affecting the removal of the corrosion layer were investigated using process control features (laser power, working distance, number of pulses per position) which are typical of practical cleaning in a conservation environment. Surfaces of the material after laser cleaning were characterised by SEM, XRD and EPMA.The threshold for the onset of cleaning occurred at a laser applied voltage of 75 mJ (which corresponded to a fluence of approximately 107W/cm2). Cleaning took place by an ablation mechanism which is thought to be connected with the effect of a laser induced “plasma” which causes spallation of the corrosion layer. As a result of laser cleaning at this power density, a majority of the additional elements present as a result of corrosion were removed from the corrosion layer. At applied voltage greater than 120 mJ (which corresponded to a fluence of approximately 108W/cm2) damage to the cleaned surface occurred with surface melting of the underlying aluminium alloy. SEM observations suggest that this involved the selective removal of a second phase Al-Si grain boundary precipitate.Corroded aluminium alloy material from Liverpool’s Gilbert Eros statue was laser cleaned using a Q-switched Nd:YAG laser. Elements present in the corrosion products were identified by XRD and EPMA as S, Cl, Ca, P and K in addition to Al, Cu, Si, Fe and Zn arising from the base material. Lighter elements such as O, N and H which were assumed to be present were undetected by these techniques. Laser parameters affecting the removal of the corrosion layer were investigated using process control features (laser power, working distance, number of pulses per position) which are typical of practical cleaning in a conservation environment. Surfaces of the material after laser cleaning were characterised by SEM, XRD and EPMA.The threshold for the onset of cleaning occurred at a laser applied voltage of 75 mJ (which corresponded to a fluence of approximately 107W/cm2). Cleaning took place by an ablation mechanism which is thought to be connected with the effect of a laser induced “plasma” which causes spallation of ...