Abstract
As part of an extensive research program to study recent, unexpected intergranular corrosion (IGC) on 6xxx series aluminum alloys (AlMgSi), this paper investigates the mechanism of initiation and early propagation of IGC on the extruded AA6005-T5 alloy with small Cu content (0.1 wt%) by use of advanced electron microscopy techniques applied for near surface characterization. Corrosion testing was restricted to the accelerated IGC test according to the standard BS ISO 11846, involving exposure to acidified chloride solution. The effect of modifying the as-received extruded surface by metallographic polishing, argon sputtering, and alkaline etching was investigated. Initiation of IGC was delayed on the as-received surface compared to the modified surface, caused by the presence of an approximately 8 nm thick crystalline oxide layer formed during extrusion. IGC initiated at the primary α-Al(Fe,Cu,Mn)Si particles for all types of surfaces. However, these particles corroded rapidly in the test solution forming a residue of Cu and Si on the exposed particle surface. This phenomenon, as well as enrichment of Cu on the Al matrix surface by dealloying, contributed increasingly to the formation of new effective cathodic sites and continuing propagation of IGC. The AlMgSiCu (Q) phase, present as primary and secondary particles, was relatively inert against both oxidation and reduction.
Highlights
Extruded aluminum 6xxx series (AlMgSi) alloys are characterized by a number of favorable properties, such as strength-to-weight ratio, extrudability, especially for fabrication of profiles with complex geometries, and corrosion resistance.[1]
The intermetallic particles α-Al(Fe,Mn,Cu)Si and AlMgSiCu (Q-phase), which are commonly present in the alloy, were considered important as external cathodes at the initiation phase,[15] losing their significance in relation to the Cu film along the susceptible grain boundaries as intergranular corrosion (IGC) propagated into the metal
While the intermetallic phases were largely embedded flush with the surrounding matrix surface on the polished and Ar-sputtered surfaces, they protruded from the matrix on the alkaline-etched surface.[23]
Summary
Extruded aluminum 6xxx series (AlMgSi) alloys are characterized by a number of favorable properties, such as strength-to-weight ratio, extrudability, especially for fabrication of profiles with complex geometries, and corrosion resistance.[1]. Probably the first available paper regarding the possibility of IGC in acidified NaCl solution sets the Cu limit to 0.1%.4 Both elements may cause IGC susceptibility due to unfavorable thermomechanical treatment.[5,9,10]. A practical issue related to the development of Al alloys with resistance against IGC is the frequent requirement to base the studies on a highly accelerated corrosion test (BS ISO 11846) It is frequently used for the ranking of commercial alloys in research and practice, with claims about the good qualitative correspondence between the accelerated test and atmospheric field exposure.[14] The modification of the surface morphology and structure is significant due to the required pretreatment (alkaline etching) and testing (in acidified chloride solution) conditions for the accelerated test
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