Otto Lunder

Intergranular Corrosion of Copper-Containing AA6xxx AlMgSi Aluminum Alloys

AlMgSi (AA6xxx-series) aluminum alloys are generally resistant to intergranular corrosion (IGC). However, copper may introduce susceptibility to IGC; its role was investigated by using model alloys with 0.02, 0.18, and 0.7 wt % Cu. The lowest copper-containing alloy was resistant to IGC in accelerated corrosion testing. The 0.18 wt % copper alloy showed superficial etching in the naturally aged condition and was highly susceptible to IGC in the underaged temper, but was only slightly susceptible in the peak aged or overaged condition. High-resolution field emission scanning electron microscopy imaging showed no visible grain boundary precipitation in the T4 and underaged tempers, whereas the T6 and overaged tempers had grain boundaries decorated with Cu-containing precipitates. Field emission transmission electron microscopy investigation of the underaged material showed a copper-enriched grain boundary layer and an adjacent copper-depleted zone. The reduced susceptibility to IGC upon extended artificial aging was attributed to the consumption of the copper-rich grain boundary film by the growth of grain boundary precipitates.

Effect of Excess Silicon and Small Copper Content on Intergranular Corrosion of 6000-Series Aluminum Alloys

The required strength and ductility of heat-treatable AlMgSi (6000-series) alloys are often obtained by alloying with either a small amount of Cu or a large excess of Si compared to the stoichiometric Mg/Si ratio corresponding to the Mg 2 Si phase. Both approaches may cause susceptibility to intergranular corrosion (IGC) as a result of unfavorable heat-treatment. Whether alloying with Cu or excess Si gives the optimal combination of mechanical properties and IGC resistance is a controversial subject. The corrosion behavior of a model alloy containing 0.2% Cu is compared with an essentially Cu-free alloy with an excess Si/Mg composition ratio (i.e., unbalanced) by using accelerated corrosion tests and electron optical characterization. In general, the Cu-containing alloy showed a higher susceptibility to IGC than the Cu-free, excess Si alloy. The Cu-containing alloy was especially susceptible in the underaged condition. The Cu-free, excess Si alloy became completely resistant to IGC by removing the cathodic intermetallic particles from the surface by selective etching or by purging the dissolved oxygen from the solution, whereas the Cu-containing alloy was still susceptible to IGC after the same treatment. The difference in IGC susceptibility between the two alloys was attributed to the presence of a cathodic Cu-rich film and discrete Cu-containing particles along the grain boundaries of the Cu-containing alloy, while the cathodic sites on the unbalanced variant were restricted to the material surface. In addition, the IGC susceptibility of both alloys depended on the presence of solute (Si and Cu)-depleted zones adjacent to the grain boundaries.

Pretreatment of Aluminum Alloy 6060 by Selective Removal of Surface Intermetallics

Abstract Different methods of removing the α-Al(Fe,Mn)Si particles from the surface of aluminum alloy 6060 (AA6060 [UNS A96060]) have been studied, with the purpose of finding out whether such pretreatment reduces the susceptibility to filiform corrosion (FFC) of epoxy-coated surfaces. The methods, which included immersion in nitric-fluoric acid (HNO3-HF) as well as potentiostatic etching in chloride solution and nitric acid, respectively, exhibited a high efficiency according to scanning electron microscopic (SEM) observations and cathodic polarization measurements in 0.1-M sodium chloride (NaCl) solution. Removal of the α-Al(Fe,Mn)Si particles from the surface of AA6060 reduced the cathodic activity to a level similar to that measured on an iron-free AA6060 model analogue alloy (AlMg0.5Si0.4). However, while the latter was completely immune against FFC, superficial particle removal treatment did not significantly reduce the FFC susceptibility of AA6060 because filament growth was supported by cathodic a...

Improving the Corrosion Resistance of Aluminum Alloys by Cathodic Polarization in Aqueous Media

Abstract Corrosion of aluminum alloys is often closely related to the type, amount, and properties of intermetallic phases present in the matrix. The phases that contain iron as a component are especially detrimental to corrosion resistance. These phases can be dissolved preferentially, thereby achieving a relatively particle-free surface, by various chemical and electrochemical means. An efficient method appears to be cathodic polarization within a certain potential range in unbuffered salt solutions of neutral pH. It is shown that the method leads to an appreciable improvement of the corrosion resistance of alloys in the 1000, 3000, and 6000 series. The particles are removed physically as a result of crevice corrosion of the matrix adjacent to their surface. The rate of removal depends on the extent to which the cathodic reaction can be depolarized on the particles relative to the substrate. The mechanism is investigated by electrochemical and microanalytical means.

Significance of Low Copper Content on Grain Boundary Nanostructure and Intergranular Corrosion of AlMgSi(Cu) Model Alloys

Intergranular corrosion (IGC) of model alloys in the 6000-series, with and without 0.2 wt% Cu, was studied using an accelerated corrosion test (BS ISO 11846 B), FE-SEM and FE-TEM. Low Cu alloys (0.02wt%) did not exhibit IGC even though they contained excess Si. The high-Cu, naturally aged material (T4) was susceptible to severe superficial etching. In the underaged state (below peak strength), the Cu-containing material was highly susceptible to IGC. Materials aged to peak strength (T6) or overaged were only slightly susceptible to IGC, with localized, shallow attacks. FE-TEM investigation of the underaged material revealed scattered, small AlMgSiCu-type precipitates, as well as a Cu-enriched film along the grain boundaries. The overaged material showed more extensive, coarse grain boundary precipitation. However, the Cu-enriched film was still present at localized sites. The reduced susceptibility to IGC upon artificial ageing was attributed to breaking of the continuity of the grain boundary film. The possible role of matrix precipitation is also discussed.

In situ measurement of pitted area by diffuse light scattering

In studies of pitting corrosion, there is often a need for monitoring the size of growing pits in situ as well as the need for a rapid assessment of the extent of pitting damage ex situ on a large number of test specimens. The well-known method of determining surface roughness by diffuse light scattering has been adapted to satisfy both needs. In situ measurements with aluminium alloys immersed in NaCl solutions indicate that early stages of pit propagation can be monitored accurately. Furthermore, measurements on pre-pitted aluminium show that the technique is suitable for rapid, quantitative determinations of a pitted area up to a fraction of 40%.

Preferential Grain Etching of AlMgSi(Zn) Model Alloys

Preferential Grain Etching of AlMgSi(Zn) Model Alloys B. Holme, N. Ljones, A. Bakken, O. Lunder,* J. E. Lein, L. Vines, T. Hauge, Ø. Bauger, and K. Nisancioglu* SINTEF Materials and Chemistry, NO-0314 Oslo, Norway Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO7491 Trondheim, Norway SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway Department of Physics, University of Oslo, NO-0316 Oslo, Norway Hydro Aluminium, NO-4265 Håvik, Norway Hydro Aluminium, NO-6601 Sunndalsøra, Norway

The Correlation Between Intergranular Corrosion Resistance and Copper Content in the Precipitate Microstructure in an AA6005A Alloy

A positive correlation is observed between the amount of Cu incorporated in hardening precipitates and intergranular corrosion resistance in an artificially aged Cu-containing 6005A alloy. Three mechanisms have been identified to increase Cu absorption in hardening precipitates: by increasing aging temperature, by pre-deformation, and by slow cooling from solution heat treatment. These findings demonstrate the possibility for development of new processing routes to produce Cu-containing Al-Mg-Si alloys with improved corrosion resistance.