The Effect of Chloride Concentration on the CorrosionFatigue Behavior of Custom 465-H950
J. Ryan Donahue and Prof. James Burns
Center for Electrochemical Science and Engineering
Department of Materials Science and Engineering
S-N Characterization
Motivation
PPC hardened, ultra-high strength stainless steels (UHSSS) are
being considered as replacements for legacy high strength steels
that require environmentally and personally hazardous coatings
to ensure corrosion resistance. However, aqueous chloride
environments can significantly decrease the fatigue endurance
limit (Figure 1)[1].
The compounding
influences of mechanical
loading and electrochemical
interactions that govern this
deleterious effect are not
understood. Such insight is
Hata, 2008
required to inform next
Figure 1) Fatigue life decrease for 13-Cr
generation life prediction
stainless steel in NaCl solution[1].
models.
Research Goals
This research will:
Custom 465 Typical Mechanical Properties
K IC
σ YS
UTS R.A.
E
Hardness
(HRC)
(MPa) (MPa) (%) (GPa) (MPa m)
1648 1765
57
193
95
49.5
Goals:
1. Quantify how MSC growth rate depends on [Cl-]
2. Determine how corrosion damage and [Cl-] affects Nf
- Study threshold pit sizes and governing factors for pit-tocrack transition
Experimental Method
Quantify the dependence of MSC growth rate (i.e. Np-small) on
Load induced fracture surface markings
enable quantification of MSC growth rates.
Long Crack Growth
Quantify the dependence of long crack growth rate (i.e. Np) on [Cl-]
SEN specimens fatigued at:
- Constant σmax = 300 MPa (R=0.5; f =2)
- Environment:
- 0.0006-3.0M NaCl (-200mVSCE )
- Humid N2 Gas (RH>90%).
- dcPD-based Crack Monitoring
2)
6)
Figures:
6) Marker band
7) Marked fracture
surface
8) da/dN vs. a results
for a range of [Cl-]
Initiation site a=0
100μm
50μm
10)
7)
Figure 10) Cross hatch indicates
initiation at an inclusion.
* Growth rates plotted against distance from initiation site. Similar ΔK vs.
a relationships are assumed as crack initiation locations and growth
vectors are similar. Complete ΔK analysis will be performed in the future.
Results:
1. Little [Cl-] dependence observed for MSC
growth rates at 0.06-3M.
2. Fracture morphology is typically
transgranular, though an intergranular
morphology is observed for 3M for a>250 μm.
The effect of [Cl-] on the MSC growth behavior likely has
a secondary influence on fatigue life (Nf)
S-N Data Trends/Analysis
Figure 9) Graph showing S-N data for
all exposures. Micrograph inlays
show crack initiation sites for each.
20 μm 20 μm
Exposure 3
20 μm
5 μm
Results:
1. Generally there is a single nucleation point for each pit.
2. [Cl-] of 0.6M or greater consistently initiate on the pit walls close
to the surface and occasionally on Ti/Cr-O inclusions (crossed).
- Some 0.06M tests initiated from homogenous microstructure
at the very base of the pit.
3. Corrosion product develops in the pit during fatigue testing;
potentially suggesting further dissolution during loading
- Pre-test vs. post test characterization is ongoing to investigate
the degree of pit growth during testing.
When present second phase particles along the pit front
dominate the macro-pit location and pit micro-topography .
Cracking generally nucleates at the pit mouth consistent with
smooth pit-to-crack literature[3].
Conclusions
2 μm
3)
4)
Exposure 1 & Exposure 2
Pristine
5 μm
1. [Cl-] has little influence on MSC and long crack growth, indicating
that these regimes do not strongly affect Nf .
2. [Cl-] is secondary to initiation site morphology in controlling S-N
behavior.
3. Isolated data suggest that a small TiC particle (≈10μm) with rough
local topography, produces a similar reduction in life to a ≈50μm
pit.
4. The presence of inclusions on the periphery of a relatively large,
smooth pit surface dictates crack initiation site.
20 μm
Future Work
Results:
1. Pristine in humid N2: Short crack growth is transgranular,
transitioning at 5 μm < a < 100 μm to intergranular cracking.
Results:
No particular initiation feature is observed.
- The data exhibits only a 2-3x variation in da/dN across all [Cl-]
2. Exposure 1 &2 : Initiation occurs at TiC particles (≈10μm); as
- Fracture morphology is generally transgranular; some SCC
[Cl-] increases 1-2 μm scale corrosion is seen about the
was observed at [Cl-]= 3M.
particles. Fracture morphology is transgranular. Localized
- Linear elastic fracture mechanics modeling (AFGROW: 100 μm
corrosion (<5μm pits) at other locations did not initiate a crack.
surface crack, R=0.5, σmax= 50% σys) shows a maximum 1.5x
3. Exposure 3: Initiation occurs within pre-corrosion pits. Fatigue
change in predicted Nf using bounding da/dN vs ΔK data.
lives are lower than in other exposures at all [Cl-].
The effect of [Cl-] on the long crack growth behavior is
not sufficient to explain the effect of [Cl-] on fatigue life
shown in literature
100μm
1 μm
9)
Figures: 2) SEN specimen with
dcPD wires, 3) Environmental
test cell, and 4) Da/dN vs. ΔK
results for a range of
environmental conditions
9)
Figure 9) Fracture
and corroded
surface images are
used to determine
initiation site
location.
10μm
Where
3. Provide insights into the role of bulk chemistry on the
controlling damage mechanism
[Cl-]
8)*
Nf = (Npit) + Ni + Np-small + Np
2. Establish how each regime will depend on [Cl-]
- Which regime is responsible for large dependence
observed in literature (Figure 1).
What features govern crack formation from corrosion damage?
Goals:
Couple SEM fractograpy of the corroded surface, fracture surface,
with 3D pit profiles produced via white light interferometry to:
1. Precisely identify the initiation location within the pit
2. Identify the micro-scale morphology of the feature
3. Investigate the microstructure proximate to the initiation site
Method:
Flat wishbone fatigue specimens: 5)
- Constant σmax = 70%σYS (R=0.5)
- Marker band σmax=56% σYS (R=0.1)[2]
Corroded
50μm
Surface
- Loading Environment:
- 0.0006-3.0M NaCl (-200mVSCE )
2mm Fracture Surface
- Humid N2 Gas (RH>90%)
Figure 5) Schematic and optical image of a
precorroded specimen.
- Corrosion exposures:
- (1) Concurrent with loading – Various conditions
- Pre-corroded
(2) Area=2cm2 , 0.6M NaCl – Anodic Pol. (150mVSCE) - 15 min, 36-48h
Fracture
50μm
Surface
(3) Four coplanar designer pits averaging; a=45 μm, 2c=170 μm
1. Quantify the fatigue life (Nf) of Custom 465-H950.
Npit = pit formation life
Ni = cycles to form a crack from a corrosion pit
Np-small = microstructurally/chemically small crack propagation
Np = long crack propagation
Pit-to-Crack Transition
The crack formation feature dominates [Cl-] as the factor
controlling the S-N behavior. Reduction in fatigue life due to
TiC inclusions may rival reduction caused by a ≈50μm pit
1. Complete testing of pre-corroded (exposure #3) at low [Cl-] ,
humid air and DI water and perform additional testing if needed.
2. Quantify crack initiation life via marker band analysis.
3. Perform EBSD around initiation sites to study the influence of
local microstructure on MSC growth.
4. Precisely model stresses at the initiation site inside pits to acquire
da/dN vs. ΔK data.
References/Acknowledgements
This work is funded by the US Office of Naval Research (Vasudevan/Mullins).
1.
2.
3.
Hata, S., et al., Investigation of corrosion fatigue phenomena in transient zone and preventive coating and blade design against fouling and
corrosion environment for mechanical drive turbines, in Proceedings of the Thirty-Seventh Turbomachinery Symposium. 2008, Texas A&M
University Turbomachinery Lab. p. 25-34.
Burns, J.T., J.M. Larsen, and R.P. Gangloff, Effect of initiation feature on microstructure-scale fatigue crack propagation in Al–Zn–Mg–Cu.
International Journal of Fatigue, 2012. 42(0): p. 104-121.
Horner, D., et al., Novel images of the evolution of stress corrosion cracks from corrosion pits. Corrosion Science, 2011. 53(11): p. 3466-3485.