Corrosion Research Group

University of Cádiz. Department of Material Science, Metallurgical Engineering and Inorganic Chemistry

Study of the influence of Surface finishing

ENM STUDY OF THE INFLUENCE OF SURFACE POLISHING ON Al-Mg ALLOY CORROSION BEHAVIOUR (1)

Introduction

The anodic processes that take place in AA5083 alloy (passive film formation, pit attack, etc.) are supported by oxygen reduction occurring mainly on its Al(Mn,Fe,Cr) precipitates (2 ). This reaction causes part of the matrix around these particles to dissolve as the oxygen reduction has made the pH around the particles decrease (3 ). This process is called alkaline pitting and causes the appearance of micropits (4 ). Some of these micropits become macropits, characterised for accumulating a great amount of corrosion products over it. This alloy also develops crystallographic pits (5 ) if the pitting potential is reached.

Experimental

WE area: 1 cm2

WE polishing: 80 or 1200 grits

RE: Ag/AgCl

Solution: 3.5% NaCl (30ºC)

Electrochemical interface: 1287SI

Measure rate: 2.15 points/s

Time record lenght: 2048 points

Nº of record averaged: 6

Optical Microscopy Results

Three kinds of attack were found:

crystallography pits

macropits

micropits


	Fig. 1: Metallographic images of AA5083 Al-Mg samples abraded down to 80 (a) or 1200 grits (b & c) and exposed to 3.5% NaCl solution for 91 hours. Magnification: x100 (a & b) ; x200 (c)

Electrochemical Noise Results


Fig. 2: PSD averaged (5 10-2-2 10-1 Hz) of AA5083 Al-Mg WEs abraded down to 80 or 1200 grits in 3.5% NaCl solution

Fig. 3: Potential of AA5083 Al-Mg WEs abraded down to 80 or 1200 grits in 3.5% NaCl solution	Fig. 4: Spectral noise resistance of AA5083 Al-Mg WEs abraded down to 80 or 1200 grits in 3.5% NaCl solution

Discussion

For both systems, it is observed, specially at the beginning, that: – V_m increases – Z_n⁰ increases		Formation of a passive film is observed in both systems, specially at the beginnning of the experiment
The 1200-grit WEs show: – a decrease of PSD(I)m with time – a decrease of Vm with time – a increase of Zn0 with time		The activity decrease with time due to lost of contact between the precipitates and the matrix as the matrix around the precipitates disappears
The 80-grit WEs show: – lower Vm values at the beginning of the experiment – a slower passive film formation – no significant change in the corrosion activity		Polishing with 80-grit SiC papers make the samples lose a high number of the Al(Mn,Fe,Cr) precipitates from their surfaces since those particles are harder than the matrix
– Crystallography pits were only found microscopically in 1200-grit WEs – Free potencial close to the pitting potential of the alloy (-0.720 V) were measured only for the first day of immersion for the 1200-grit WEs		Crystallography pits appear for the first day of immersion and only on the 1200-grit WEs
– Big pits spread following the lines of the finishing of 80-grits WEs – The current noise, become much higher for the 80-grit WEs than for the 1200-grit ones, at the end of the experiment, when both systems are supposed to have almost the same amount of Al(Mn,Fe,Cr) precipitates		80-grit WEs have a higher tendency to develop big pits than 1200-grit WEs

References

1.- Aballe A, Bethencourt M, Botana FJ, Cano MJ, Marcos M, Eurocorr 2000, London (2000)
2.- Bethencourt M, Botana FJ, Calvino JJ, Marcos M, Pérez J,Rodríguez MA, Mater. Sci. Forum 289-292 (1998) 567
3.- Park JO, Paik CH, Alkire RC, in "Critical Factors in Localized Corrosion II", The Electrochem. Soc. Pennington (New Jersey), 1996
4.- Aballe A, Bethencourt M, Botana FJ, Cano J, Marcos M, Corrosion Reviews 18(1) (2000) 1
5.- Giménez Ph, Rameau JJ, Rebioul MC, Corrosion 37(12) (1981) 673

Acknowledgements

This work has received finacial support from Junta de Andalucía, Comisión Interministerial de Ciencia y Tecnología (CICYT), projects MAT99-0625-C02-01 and 1FD97-0333-C03-02