Corrosion and Stress Corrosion Cracking of Magnesium Alloys

206 Pages·2015·30.84 MB·English

by  Fuyong Cao

Corrosion and Stress Corrosion Cracking of Magnesium Alloys
Fuyong Cao
B. Eng

A thesis submitted for the degree of Doctor of Philosophy at
The University of Queensland in 2015
School of Mechanical & Mining Engineering

Abstract
Magnesium (Mg) alloys are attractive for light-weight applications such as in the aerospace
and automobile industries, due to their high strength-to-weight ratio. The widespread
application of Mg alloys in automobiles can decrease fuel consumption through light-
weighting, which benefits our environment. Mg alloys are also regarded as promising
biodegradable implants for use in the human body. However, the poor resistance of
corrosion and stress corrosion cracking (SCC) limits their more wide-spread application in
both industry and medical application. It is therefore necessary to better understand the
mechanisms and the important factors, which control Mg corrosion and SCC, and to find
better ways to improve their corrosion and SCC performance. In this doctoral dissertation,
an effort was made to understanding the following issues regarding the corrosion and SCC
mechanisms, and behaviour of pure magnesium and magnesium alloys:
(1) The corrosion behaviour of ultra-high-purity Mg in 3.5% NaCl solution saturated
with Mg(OH)
2
(2) The corrosion behaviour of as-cast and solution-heat-treated binary Mg-X alloys in
salt spray and 3.5% NaCl solution saturated with Mg(OH)
2
(3) Influence of hot rolling on the corrosion behaviour of several Mg-X alloys
(4) The influence of casting porosity on the corrosion behaviour of Mg0.1Si
(5) Stress corrosion cracking of several solution heat-treated Mg-X alloys
(6) Stress corrosion cracking of several hot-rolled Mg-X alloys
A range of advanced techniques were employed such as optical microscopy (OM),
scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS),
electrochemical polarization, electrochemical impedance spectroscopy (EIS), gas
collection of hydrogen evolution from corroding samples, and linearly increasing stress
testing (LIST).
For the ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH) , the intrinsic
2
corrosion rate measured with weight loss, P = 0.25 ± 0.07 mm y-1. The average corrosion
W
rate measured from hydrogen evolution, P , was lower than that measured with weight
AH
loss, P , attributed to dissolution of some hydrogen in the Mg specimen. The amount of
W
dissolution under electrochemical control was a small amount of the total dissolution. A
new hydride dissolution mechanism was suggested.
For solution-heat-treated Mg–X alloys (X = Mn, Sn, Ca, Zn, Al, Zr, Si, Sr), corrosion rates
did not meet the expectation that they should be equal to or lower than those of high-purity
I
Mg. There was circumstantial evidence that the higher corrosion rates were caused by the
particles in the microstructure; the second phases had been dissolved.
For the hot-rolled Mg–X alloys (X = Gd, Ca, Al, Mn, Sn, Sr, Nd, La, Ce, Zr or Si) in 3.5%
NaCl solution saturated with Mg(OH) , the corrosion rate for all Mg–X alloys (except
2
Mg0.1Zr and Mg0.3Si) decreased after hot rolling, attributed to fine-grained alloys having a
more homogeneous microstructure, and fewer, smaller second-phase particles. For
Mg0.1Zr and Mg0.3Si, the corrosion rate increased after hot rolling. There were a number
of possible reasons, one of which was a greater sensitivity to the precipitation of
deleterious Fe-rich particles.
The influence of casting porosity on the corrosion behaviour of Mg0.1Si was studied.
Specimens with porosity had higher corrosion rates attributed to the corrosion associated
with the pores activating significant corrosion over the whole specimen surface, wherein
important aspects were (i) the breakdown of a partly protective surface film, and (ii) micro-
galvanic acceleration of the corrosion by Fe-rich particles.
For SCC behaviour of solution-heat-treated Mg0.1Zr, Mg1Mn, Mg0.1Sr, Mg0.3Si, Mg5Sn,
Mg5Zn and Mg0.3Ca in distilled water (DW), SCC susceptibility was related to the stress
rate for all the Mg-X alloys except for Mg0.1Sr. In DW, Mg5Zn and Mg0.3Ca suffered the
most serious trans-granular SCC. Some specimens tested in DW preferred to crack at
solute atoms, second phase particles, grain boundaries and defects, attributed to H
trapping. It was confirmed that hydrogen atom played a significant role in the SCC
behaviour of Mg alloys.
For SCC behaviour of hot rolled Mg0.1Zr, Mg0.1Sr, Mg1Mn, Mg0.3Si, Mg5Sn, Mg0.7La,
Mg0.9Ce, Mg0.6Nd, Mg6Al, Mg5Gd and Mg0.3Ca, all the alloys (except for Mg1Mn and
Mg0.7La) had good SCC resistance in DW, and the hot-rolled Mg1Mn and Mg0.7La had
acceptable SCC resistance. The increase of SCC resistance by hot rolling was related to
improvement of the microstructure. There was no obvious difference of the fractography
between the specimens tested in air and in DW for each hot rolled Mg-X. They all cracked
through trans-granular cracking, presenting either smooth trans-granular feature or rough
trans-granular feature, as a combination result of tensile stress and shear stress.
II
Declaration by author

This thesis is composed of my original work, and contains no material previously published
or written by another person except where due reference has been made in the text. I
have clearly stated the contribution by others to jointly-authored works that I have included
in my thesis.

I have clearly stated the contribution of others to my thesis as a whole, including statistical
assistance, survey design, data analysis, significant technical procedures, professional
editorial advice, and any other original research work used or reported in my thesis. The
content of my thesis is the result of work I have carried out since the commencement of
my research higher degree candidature and does not include a substantial part of work
that has been submitted to qualify for the award of any other degree or diploma in any
university or other tertiary institution. I have clearly stated which parts of my thesis, if any,
have been submitted to qualify for another award.

I acknowledge that an electronic copy of my thesis must be lodged with the University
Library and, subject to the policy and procedures of The University of Queensland, the
thesis be made available for research and study in accordance with the Copyright Act
1968 unless a period of embargo has been approved by the Dean of the Graduate School.

I acknowledge that copyright of all material contained in my thesis resides with the
copyright holder(s) of that material. Where appropriate I have obtained copyright
permission from the copyright holder to reproduce material in this thesis.

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Publications during candidature
Peer-reviewed papers:
1. F. Cao, Z. Shi, J. Hofstetter, P.J. Uggowitzer, G. Song, M. Liu, A. Atrens, Corrosion
of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH) , Corros. Sci.,
2
75 (2013) 78-99.
2. F. Cao, Z. Shi, G.-L. Song, M. Liu, A. Atrens, Corrosion behaviour in salt spray and
in 3.5% NaCl solution saturated with Mg(OH) of as-cast and solution heat-treated
2
binary Mg–X alloys: X = Mn, Sn, Ca, Zn, Al, Zr, Si, Sr, Corros. Sci., 76 (2013) 60-97.
3. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Influence of hot rolling
on the corrosion behaviour of several Mg–X alloys, Corros. Sci., 90 (2015) 176-191.
4. F. Cao, Z. Shi, G. Song, M. Liu, M.S. Dargusch, A. Atrens, Influence of casting
porosity on the corrosion behaviour of Mg0.1Si, Corros. Sci., 94 (2015) 255-269.
5. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Influence of specimen
orientation on the corrosion behaviour of high-purity magnesium, submitted for
publication
6. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Stress corrosion
cracking of several solution heat-treated Mg-X alloys, accepted by Corros. Sci..
7. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Stress corrosion
cracking of several hot rolled Mg-X alloys, submitted for publication.
8. Z. Shi, F. Cao, G.-L. Song, M. Liu, A. Atrens, Corrosion behaviour in salt spray and
in 3.5% NaCl solution saturated with Mg(OH) of as-cast and solution heat-treated
2
binary Mg–RE alloys: RE = Ce, La, Nd, Y, Gd, Corros. Sci., 76 (2013) 98-118.
9. Z. Shi, F. Cao, G.-L. Song, A. Atrens, Low apparent valence of Mg during corrosion,
Corros. Sci., 88 (2014) 434-443.
10. A. Atrens, G.-L. Song, F. Cao, Z. Shi, P.K. Bowen, Advances in Mg corrosion and
research suggestions, Journal of magnesium and alloys, 1 (2013) 177-200.
11. K. Schlüter, Z. Shi, C. Zamponi, F. Cao, E. Quandt, A. Atrens, Corrosion
performance and mechanical properties of sputter-deposited MgY and MgGd alloys,
Corros. Sci., 78 (2014) 43-54.
12. Z. Shi, J. Hofstetter, F. Cao, P.J. Uggowitzer, M.S. Dargusch, A. Atrens, Corrosion
and stress corrosion cracking of ultra-high-purity Mg5Zn, 93 (2015) 330-335.
13. A. Atrens, G.L. Song, M. Liu, Z. Shi, F. Cao, M.S. Dargusch, Review of Recent
Developments in the Field of Magnesium Corrosion, Adv. Eng. Mater., 2015,
http://onlinelibrary.wiley.com/doi/10.1002/adem.201400434/full

IV
Conference papers:
1. F. Cao, Z. Shi, G.-L. Song, M. Liu, A. Atrens, Effect of alloying elements on
corrosion behaviour of binary magnesium alloys, Corrosion & Prevention 2013,
Brisbane, 2013.
2. F. Cao, Z. Shi, G.-L. Song, M. Liu, A. Atrens, A study on stress corrosion cracking
of solution heat-treated Mg0.1Zr, Mg5Sn and Mg5Zn, NACE SINOCORR
International Corrosion Conference 2014, Beijing, 2014.

Publications included in this thesis

1. F. Cao, Z. Shi, J. Hofstetter, P.J. Uggowitzer, G. Song, M. Liu, A. Atrens, Corrosion of
ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH) , Corros. Sci., 75 (2013)
2
78-99. – incorporated as Paper 1 in Chapter 2.
Contributor Statement of contribution
Fuyong Cao Conception design (60%)
Experimental (90%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Conception design (10%)
Experimental (10%)
Data analysis and interpretation (15%)
J. Hofstetter Paper writing and editing (5%)
P.J. Uggowitzer Paper writing and editing (5%)
G. Song Conception design (5%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
A. Atrens Conception design (25%)
Data analysis and interpretation (15%)
Paper writing and editing (15%)

2. F. Cao, Z. Shi, G.-L. Song, M. Liu, A. Atrens, Corrosion behaviour in salt spray and in
3.5% NaCl solution saturated with Mg(OH) of as-cast and solution heat-treated binary
2
Mg–X alloys: X = Mn, Sn, Ca, Zn, Al, Zr, Si, Sr, Corros. Sci., 76 (2013) 60-97.-incorporated
as Paper 2 in Chapter 3.
V
Contributor Statement of contribution
Fuyong Cao Conception design (60%)
Experimental (90%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Conception design (10%)
Experimental (10%)
Data analysis and interpretation (15%)
G. Song Conception design (5%)
Data analysis and interpretation (5%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
A. Atrens Conception design (25%)
Data analysis and interpretation (10%)
Paper writing and editing (25%)

3. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Influence of hot rolling on
the corrosion behaviour of several Mg–X alloys, Corros. Sci., 90 (2015) 176-191. -
incorporated as Paper 3 in Chapter 4.
Contributor Statement of contribution
Fuyong Cao Conception design (60%)
Experimental (95%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Conception design (5%)
Experimental (5%)
Data analysis and interpretation (5%)
G. Song Data analysis and interpretation (15%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
M.S. Dargusch Paper writing and editing (5%)
A. Atrens Conception design (35%)
Data analysis and interpretation (10%)
Paper writing and editing (20%)
VI
4. F. Cao, Z. Shi, G. Song, M. Liu, M.S. Dargusch, A. Atrens, Influence of casting porosity
on the corrosion behaviour of Mg0.1Si, 94 (2015) 255-269. - incorporated as Paper 4 in
Chapter 5.
Contributor Statement of contribution
Fuyong Cao Conception design (85%)
Experimental (100%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Data analysis and interpretation (5%)
G. Song Conception design (5%)
Data analysis and interpretation (15%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
M.S. Dargusch Paper writing and editing (5%)
A. Atrens Conception design (10%)
Data analysis and interpretation (10%)
Paper writing and editing (20%)

5. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Stress corrosion cracking
of several solution heat-treated Mg-X alloys, accepted by Corrosion Science. -incorporated
as Paper 5 in Chapter 6.
Contributor Statement of contribution
Fuyong Cao Conception design (60%)
Experimental (95%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Conception design (10%)
Experimental (5%)
Data analysis and interpretation (5%)
G. Song Conception design (5%)
Data analysis and interpretation (10%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
M.S. Dargusch Paper writing and editing (5%)
VII
A. Atrens Conception design (25%)
Data analysis and interpretation (15%)
Paper writing and editing (20%)

6. F. Cao, Z. Shi, G.-L. Song, M. Liu, M.S. Dargusch, A. Atrens, Stress corrosion cracking
of several hot rolled Mg-X alloys, submitted to Corrosion Science. -incorporated as Paper
6 in Chapter 7.
Contributor Statement of contribution
Fuyong Cao Conception design (60%)
Experimental (100%)
Data analysis and interpretation (70%)
Paper writing and editing (60%)
Z. Shi Conception design (10%)
Data analysis and interpretation (5%)
G. Song Conception design (5%)
Data analysis and interpretation (5%)
Paper writing and editing (10%)
M. Liu Paper writing and editing (5%)
M.S. Dargusch Paper writing and editing (5%)
A. Atrens Conception design (25%)
Data analysis and interpretation (20%)
Paper writing and editing (20%)
VIII