Electrochemical and corrosion behaviour of laser modified aluminium surfaces

Abstract

Little data are available on corrosion resistance and electrochemical behaviour of amorphous metals and alloys. In some instances a higher corrosion rate is shown (eg with chemically deposited Nichel[1, 2]) ; in other cases an improvement was displayed[3] . In this paper some preliminaryy results are presented concerning the electrochemical behaviour and pitting corrosion resistance of 99.99 Al specimens after laser irradiations at various energy densities . The Al specimens were disks, 1 .2 cm dia, the surface of which was polished to a mirror finish, then irradiated in air at room temperature with Q-switched ruby laser pulses of about l5ns duration. The energy density ranged between 1.0 and 5.0 cm -2 , special care being given to uniform energy distribution (± 10%). Above 1.0 J cm-2 threshold a liquid phase was observed[4] The fused layer is the thicker, the higher the energy density . Cooling rates ranging near IO°K/s have been calculated. SEM analysis reveals a noticeable increase of the surface area. Quantitative analysis of both N and 0 restrained by the specimens before and after the re-solidification were carried out with nuclear techniques (160(d, p)' 70and ' 4N(d, a) 12C reactions) at INFN National Laboratories of Legnaro. Nitrogen concentration is energy dependent and reaches a maximum value of about 3 x 10 16 atoms/cm 2 ; oxygen is in both cases at the same concentration of about 2 .1016 atoms/cm 2 , ie a few tens of A, the same as natural oxide. Powder X-rays diffraction showed structural modifications of polycrystals, whose dimensions were reduced to a size smaller than a few hundreds of A, that is below the sensitivity of the apparatus. The electrochemical and corrosion behaviour of irradiated specimens was investigated following different techniques : (1) Anodic oxide layers were obtained in ammonium tartrate solution, the current being delivered by a potentiostat under a polarizing potential of 6V, supplied as a potentiostatic 60s long square pulse[5] . (2) Natural and anodic oxide layer coated specimens were submitted to potentiostatic anodic polarization in chloride ions containing solutions. Stepwise anodic polarization curves were recorded by means of an AMEL Mod. 551 potentiostat