Effect of Mechanical Treatment on Intergranular Corrosion ...€¦ · intergranular corrosion....
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Experiment Effect of Mechanical Treatment on Intergranular Corrosion of 6064 Alloy Bars Sláma P., Nacházel J. Microstructure and corrosion ABSTRACT E-mail: [email protected] / www.comtesfht.cz Al-Mg-Si-type alloys (6xxx-series alloys) exhibit good mechanical properties, formability, weldability and good corrosion resistance in various environments. They often find use in automotive industry and other applications. Some alloys, however, particularly those with higher copper levels, show increased susceptibility to intergranular corrosion. Intergranular corrosion (IGC) is typically related to the formation of microgalvanic cells between cathodic, more noble phases and depleted (precipitate-free) zones along grain boundaries. It is encountered mainly in AlMgSi alloys containing Cu, where it is thought to be related to the formation Q - phase precipitates (Al 4 Mg 8 Si 7 Cu 2 ) along grain boundaries. The present paper describes the effects of mechanical working (pressing, drawing and straightening) and artificial ageing on intergranular corrosion in bar of the 6064A alloy. Corrosion tests showed that with continuing ageing and overaging, deep IGC changes into pitting corrosion with a smaller depth of attack. However, the corrosion resistance of the bars is impaired by post-quench mechanical working (drawing and straightening). The chemical composition of the EN AW-6064A bars is shown in Table (in wt.%). The bars were made by an industrial hot extrusion process and right after extrusion the bars were water wave-cooled (temper T1). The quenched bars were then drawn to the final diameter of 15 mm (temper T2) and straightened (temper T2S). The final operation is artificial ageing (temper T8). Effects of artificial aging (underaging, peak-aging, overaging) on the intergranular corrosion was investigated on the bars in the T1 and T2 temper. The resistance to intergranular corrosion was mapped using corrosion tests according to EN ISO 11846, method B . Corrosion attack of surface and cross-sections of bars after straightening Hardness HV10 and depth of corrosion attack after artificial aging Hardness HV10 after artificial aging (one-stage and two-stage aging) CONCLUSION The bar surface exhibited three types of corrosion: •IGC in underaged specimens: typically extensive corrosion with a depth of more than 300 µm. •Pitting corrosion in more aged and overaged extruded/drawn bars, where the corrosion depth was approximately 100 µm. •Transgranular pitting corrosion in more aged and overaged bars which had undergone final straightening. Here, the corrosion depth was larger and exceeded 300 µm. With more intensive ageing and overageing (temperature, time), IGC changed into pitting corrosion in extruded/drawn bars. There was an adverse impact of the post-drawing straightening operation on the resistance to surface corrosion in the bars, evidenced by deep transgranular pitting corrosion. In most cases, the transverse cross-sections exhibited very deep pitting corrosion with depths up to 800 µm which followed the bands of coarse cathodic phases. Exceptions were found in severely overaged bars (extruded or extruded and straightened) which showed sporadic pitting corrosion with depths of approximately 100 µm. Si Fe Cu Mn Mg Cr Pb Bi 0.60 0.23 0.27 0.04 1.03 0.05 0.28 0.49 The depth of corrosion attack on the surface of the bar in µm The depth of corrosion attack on the cross-section of the bar in µm Microstructure of the bars in T8 temper Large elongated particles consist of Bi or Bi+Pb. The small ones are alpha-Al 15 (Fe,Mn,Cu,Cr) 3 Si 2 particles. Other small particles are Mg 2 Si particles. The small particles along grain boundaries are cathodic particles Q-phase (Al 4 Mg 8 Si 7 Cu 2 ). Surface – temper T2 Surface – temper T8 Cross-section – temper T8 a) a) b) c) Corrosion tests after experimental heat treatment (artificial ageing) Surface - three types of corrosion c) IGC – underaged condition from T1 and T2 temper Pitting corrosion – peak aged and overaged condition from T1 and T2 temper Transgranular corrosion – peak aged and overaged condition from T2S temper (after straightening) 70 80 90 100 110 120 130 None 160°C/4h 160°C/8h 180°C/4h 180°C/8h 160°C/4h+205°C/4h 160°C/8h+205°C/4h 180°C/4h+205°C/4h 160°C/4h+220°C/4h 160°C/8h+220°C/4h 180°C/4h+220°C/4h Extruded T1 Drawn T2 Straightened T2S Temper T1 extruded + aging 160°C/4h+205°C/4h Temper T2 drawn + aging 180°C/4h+205°C/4h Temper T2S straightened + overaging 160°C/4h+220°C/4h Cross-section - the typical depth of corrosion is in aged condition Underaged Peak aged Overaged
Transcript of Effect of Mechanical Treatment on Intergranular Corrosion ...€¦ · intergranular corrosion....
Experiment
Effect of Mechanical Treatment on Intergranular Corrosion of 6064 Alloy Bars
Sláma P., Nacházel J.
Microstructure and corrosion
ABSTRACT
E-mail: [email protected] / www.comtesfht.cz
Al-Mg-Si-type alloys (6xxx-series alloys) exhibit goodmechanical properties, formability, weldability and goodcorrosion resistance in various environments. They oftenfind use in automotive industry and other applications.Some alloys, however, particularly those with highercopper levels, show increased susceptibility tointergranular corrosion. Intergranular corrosion (IGC) istypically related to the formation of microgalvanic cellsbetween cathodic, more noble phases and depleted(precipitate-free) zones along grain boundaries. It isencountered mainly in AlMgSi alloys containing Cu, whereit is thought to be related to the formation Q - phaseprecipitates (Al4Mg8Si7Cu2) along grain boundaries.
The present paper describes the effects of mechanical working (pressing,drawing and straightening) and artificial ageing on intergranular corrosion in barof the 6064A alloy.Corrosion tests showed that with continuing ageing and overaging, deep IGCchanges into pitting corrosion with a smaller depth of attack. However, thecorrosion resistance of the bars is impaired by post-quench mechanicalworking (drawing and straightening).
The chemical composition of the EN AW-6064A bars is shown in Table (in wt.%).
The bars were made by an industrial hot extrusion process and right after extrusion the bars were water wave-cooled (temper T1). The quenched bars were then drawn to the final diameter of 15 mm (temper T2) and straightened (temper T2S). The final operation is artificial ageing (temper T8).Effects of artificial aging (underaging, peak-aging, overaging) on the intergranular corrosion was investigated on the bars in the T1 and T2 temper.The resistance to intergranular corrosion was mapped using corrosion tests according to EN ISO 11846, method B.
Corrosion attack of surface and cross-sections of bars after straightening
Hardness HV10 and depth of corrosion attack after artificial aging
Hardness HV10 after artificial aging (one-stage and two-stage aging)
CONCLUSION
The bar surface exhibited three types of corrosion:•IGC in underaged specimens: typically extensive corrosion with a depth ofmore than 300 µm.•Pitting corrosion in more aged and overaged extruded/drawn bars, where thecorrosion depth was approximately 100 µm.•Transgranular pitting corrosion in more aged and overaged bars which hadundergone final straightening. Here, the corrosion depth was larger andexceeded 300 µm.With more intensive ageing and overageing (temperature, time), IGC changedinto pitting corrosion in extruded/drawn bars. There was an adverse impact ofthe post-drawing straightening operation on the resistance to surface corrosionin the bars, evidenced by deep transgranular pitting corrosion.In most cases, the transverse cross-sections exhibited very deep pittingcorrosion with depths up to 800 µm which followed the bands of coarsecathodic phases. Exceptions were found in severely overaged bars (extruded orextruded and straightened) which showed sporadic pitting corrosion withdepths of approximately 100 µm.
Si Fe Cu Mn Mg Cr Pb Bi0.60 0.23 0.27 0.04 1.03 0.05 0.28 0.49
The depth of corrosion attack on the surface of the bar in µm
The depth of corrosion attack on the cross-section of the bar in µm
Microstructure of the bars in T8 temper
Large elongated particles consist of Bi or Bi+Pb. The small ones are alpha-Al15(Fe,Mn,Cu,Cr)3Si2particles. Other small particles are Mg2Si particles. The small particles along grain boundaries are cathodic particles Q-phase (Al4Mg8Si7Cu2).
Surface – temper T2 Surface – temper T8 Cross-section – temper T8
a)
a) b) c)
Corrosion tests after experimental heat treatment (artificial ageing)
Surface - three types of corrosion
c)
IGC – underaged condition from T1 and T2 temper
Pitting corrosion – peak aged and overaged condition from T1 and T2 temper
Transgranular corrosion –peak aged and overaged condition from T2S temper (after straightening)
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None
160°C/4h
160°C/8h
180°C/4h
180°C/8h
160°C/4h+205°C/4h
160°C/8h+205°C/4h
180°C/4h+205°C/4h
160°C/4h+220°C/4h
160°C/8h+220°C/4h
180°C/4h+220°C/4h
Extruded T1
Drawn T2
StraightenedT2S
Temper T1 extruded + aging 160°C/4h+205°C/4h
Temper T2 drawn + aging 180°C/4h+205°C/4h
Temper T2S straightened + overaging 160°C/4h+220°C/4h
Cross-section - the typical depth of corrosion is in aged condition
Underaged Peak aged Overaged
