Effect of trace element lead on the electrochemistry and corrosion of aluminium alloys
MetadataShow full item record
Electrochemical activation of aluminium in chloride solution as a result of heat treatment at temperatures above 350°C is attributed to enrichment of trace element lead at the oxide-metal interface. Activation is manifested by significant negative shift in the corrosion potential and a significant increase in the anodic current at potentials where aluminium normally is expected to be passive. The phenomenon has been observed on several hot-rolled or extruded standard aluminium alloys studied the past ten years, first without realizing the role of lead, since the Pb bulk concentration was only a few ppm. After lead was detected by use of glow discharge optical emission spectroscopy, an increased effort to find out the role of lead in the activation mechanism was initiated because of its importance in galvanic and filiform corrosion. Consequently, AlPb model alloys in the range of 5 to 50 ppm Pb were prepared and investigated by use of electrochemical polarization, electron optical techniques and glow discharge optical emission spectroscopy. Lead was enriched at the surface during annealing by diffusion along the grain boundaries. The amount of enriched Pb increased with increasing time and temperature of annealing in the range 1-24 h and 300-600°C in air. Most of the annealing experiments were conducted at 600ºC. A maximum of about 1 wt% of Pb was enriched in super-saturated solid solution with aluminium in a surface sublayer of maximum 1 μm thickness by annealing at 600ºC. For samples quenched after up to 1 h annealing at 600ºC, nearly all enriched Pb was in solid solution in the surface sublayer. Additional Pb, which diffused to the surface along the grain boundaries by prolonged annealing longer than 1 h, was segregated at the metal-oxide interface and in the oxide as metallic Pb particles. Slow cooling after heat treatment caused easier surface segregation of Pb particles. Segregated Pb particles did not have any appreciably effect on activation. Electrochemical activation was attributed to Pb enriched in solid solution in the metallic subsurface layer. Controlled corrosion of activated specimens by potensiostatic polarization revealed segregated Pb particles as a result of selective dissolution of aluminium in areas where Pb was originally in solid solution. The particles were probably formed by coalescence of a segregated Pb monolayer or adsorbed Pb atoms by surface diffusion. Activation of the surface was attributed to such adsorbed Pb atoms or Pb monolayer formed by selective corrosion of the aluminium component of the surface sublayer. Heat treatment in low oxygen partial pressure environment provided a new parameter in studying the surface activation by Pb. Annealing at 600ºC in pure argon did not prevent oxide growth in the form of crystalline γ-Al2O3, similar to surfaces annealed in air. Annealing in oxygen-free environment, obtained by the additional use of pure aluminium-foil filter in pure argon atmosphere, prevented oxide growth and formation of crystalline oxide. Pb enriched and segregated on surfaces annealed in the oxygen-free environment were evaporated or extracted by the Al filter, resulting in reduced activation because of reduced solid solution Pb in the metallic subsurface layer. While the effect of adding Fe, Si, Mn or Zn to the binary AlPb alloy was insignificant, alloying with 0.5wt% Cu significantly reduced the anodic activation caused by Pb on air-annealed samples. Annealing in low oxygen environments further reduced and eliminated the activation of the copper containing sample. Copper, which also was enriched as a result of selective oxidation of aluminium, probably reduced the mobility of surface Pb atoms as a result of the affinity of Pb to adsorb in the form of a monolayer on Cu. In addition, copper in solid solution was believed to contract the aluminium lattice, resulting in reduced Pb in solid solution in the near surface sublayer. In addition to the foregoing major part of the thesis on model alloys, welldocumented activation of alloy AA8006 by low temperature annealing was investigated further. Based on earlier information on similar lowtemperature activation of certain standard aluminium alloys and new surface-analytical evidence, maximum activation obtained by annealing alloy 8006 at 450°C was attributed to enhanced surface enrichment of Pb by the presence of magnesium in the material. Reduced activation observed with further increase in the annealing temperature was attributed to the formation of a dense thermal oxide consisting of the crystalline MgAl2O4 spinel embedded in an amorphous aluminium oxide. TEM observation of oxide cross sections verified that the chromicphosphoric stripping process failed to remove the thermally formed oxide layer. GD-OES measurements on alloy AA 3102, which was heat-treated at 600° C, indicated that the oxygen depth profile was not affected by the stripping process. A compact oxide layer, consisting mainly of elongated γ - Al2O3 crystals adjacent to the metal surface, was not affected, while the amorphous part of the oxide was effectively stripped. Present work thus provided new evidence supporting the mechanisms discussed in earlier studies insofar as Pb enrichment and segregation at the surface of aluminium alloys by heat treatment and the importance of solid solution Pb in the activation process. In addition, roles of other alloying elements, in particular Cu and Mg, respectively, in reducing and enhancing activation were assessed. Possible role of melting point depression of thin Pb film segregated by the selective dissolution of aluminium and the possibility of formation of a thin liquid film AlPb alloy in the activation process was discussed.