Introduction in the topic of passivation - Swiss Medtech
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Introduction in the topic of
passivation
Dr. Patrik Schmutz
EMPA, Laboratory for Joining Technologies and Corrosion
(Head: Dr. Lars Jeurgens)
8600 Dübendorf, Switzerland
contact: patrik.schmutz@empa.ch
INSIGHT Fachtagung, Swiss Medtech Veranstaltung
16.02.2021
Outline
◼ Introduction about fundamentals of passivation
◼ Surface analysis of nm-thick “passivated” films
- Principle of X-Ray Photoelectron Spectroscopy
- Passive films on Steel
- Weakest spot
◼ Facts about localized corrosion resistance - Local methods
- Corrosion resistance and alloying elements
◼ Summary - Homogeneous
- Large scale
Durability
Functionality
Electrochemistry at metallic surfaces
Core competences of the group
• Passivation of reactive metal surfaces
• Electrochemical characterizations
• Biocompatibility of functional metallic surfaces Dr. Patrik Schmutz
Micro and Nano Electrochemical surface Surface analytical
Electrochemistry Modification characterizations
N. Ott / J.Kollender M. Khokhlova / R. Han R. Hauert / C. Cancellieri
Platinum
Mg hydroxide
Mg alloy
FIB sections: Ion beam images
- Microcapillary electrochemistry - EPMA, SEM (EDX) - Environmental AFM
- SEN nanoelectrochemistry - Anodizing - XPS /AES spectroscopies
- Synchrotron “electrochemical“ - Electrodeposition - Synchrotron methods
tomography - Photo-electrochemistry (HAXPES / EXAFS)
What is passivation !!!
Material science
◼ Spontaneous formation (milliseconds in
atmospheric / aqueous environment) of a Reactions involved
nm-thick protective layer
1) Metal oxidation
◼ Thickened by Laser Treatments 2) Metallic ion dissolution
3) Water splitting and anion
incorporation
Material Oxide Solution
Microelectronics
Thickness: 2 nm
◼ Hydrogen surface termination
of silicon to avoid formation
of a nm-thick oxide
Chemical passivation
◼ Reinforcement (homogenization) of an
existing nm-thick oxide layer
Iron oxide thermodynamic stability Passivation is linked to the possibility to form stable oxide. Your first reflex should be to consult the relevant Pourbaix diagram (e.g: iron in water): Reaction considered: I: Electrochemical equilibrium (Redox potentials) II: Electrochemical equilibrium involving reaction with water III: Chemical equilibrium
Complex microstructure and surface defects
Steel inclusions - MnS Production related
surface modified layer
Punches
Intergranular
localized attack
FIB section
Deformed
layer
Welding process
Acidic chemical treatment
What can be reached depending from natural passive film stability ?
◼ Etching/cleaning (for low alloyed steel)
◼ Alloying element enrichment in oxide (around the natural
passivation threshold)
◼ Improved corrosion resistance / removal of surface defects
◼ Oxide surface functionalizing
◼ Surface cleaning (Ti alloys)
Classical salt spray testing Nitric acid chemical passivation
1.4305 1.4980 1.4545 1.4534
1.4568 1.4301 1.4541
1.4021 1.4016 1.4125
Citric acid chemical passivation
13-16%Cr + Mo
• Corrosion management
(failure analysis network @ Empa)
1.4305 1.4980 1.4545 1.4534
16-18 %Cr + 7-9%Ni
- Accelerated corrosion tests
condensation-water (EN ISO 6270-2) 1.4568 1.4301 1.4541
ditto + SO2 (EN ISO 6988) 12-18% Cr
salt spray test (EN ISO 9227)
1.4021 1.4016 1.4125
Passivation: important parameters
active
log I
passive
Transpassive
dissolution
Or
Water
dissociation
ER,a Epass Eact Ed E
O2 + 2H2O + 4e- = 4OH- Erev,O2 = 1.23 - 0.059 pH [V]
On an anodic polarization measurement, following domains are important:
Active: very rapid current increase (charge transfer controlled)
Active–passive transition: order of magnitude decrease of current
Passive: current in the microampère/cm2 domain, stable surface
Most used passive metals
10X-Ray photoelectron spectroscopy
Focusing monochromator
Electron gun
Electron b)
energy analyzer
QUANTUM 2000
i-XPS
Al Anode
Probe
Scanning XPS (ESCA):
Focused monochromatized X-Ray source
for point analysis and chemical mapping
Measurement:
- All elements except H
- Measurement depth ca. 2-4 nm (inelastic
mean free path)Surface analysis : XPS and Auger spectroscopies
• AES with focused electron beam (good
lateral resolution)
• Electron energy is element
specific • XPS poorer lateral resolution because
of the X-Rays but easier access to
chemical informationWhere is the chemical information ?
• Oxidation state can be characterized by energy shifts because of the
different amount of electrons surrounding an atom in ions
• Oxides and hydroxide can be very well distinguished because the influence
of the proton (H+) on O2- energy level is stronger than the influence of the
surrounding metallic ions.
4000 A) O 1s
3000
Intensity [counts/s]
2-
O -
2000 0H
1000
2-
SO4
H2 O
0
528 530 532 534
Binding Energy [eV]XPS spectra of chromium 2p level
Fe25Cr alloy passivated at 0.5V SHE during 5 minutes 2500
A) Cr 2p 3/2
(solution: 0.1M H2SO4 + 0.4M Na2SO4). 2000
Intensity [counts/s]
1500
1000 Cr hyd
3+
2000 Cr
Cr 2p 500
Cr met
Cr other
0
1500 572 576 580
Binding Energy [eV]
Intensity [counts/s]
Detail of the Cr 2p3/2 peak as a function of the
1000 Cr hyd X-Ray source used:
a) Al ka monochromatized, pass energy 5.85 eV
3+
500 Cr
b) Al ka standard, pass energy 5.85 eV
Cr met
Cr other
satellite 3000
0
B) Cr 2p 3/2
575 580 585 590 2500
Binding Energy [eV]
2000
Intensity [counts/s]
1500
Large amount of Chromium 3+ in 1000
Cr
3+
Cr hyd
oxide and hydroxide form is found 500
Cr met Cr other
0
in the passive film 572 576
Binding Energy [eV]
580XPS: passivation of stainless steel
• Chromium is the important element to build a stable passive film
(an amount of 12% is necessary to reach is significant enrichment in the
passive layer in acidic media)
• Often quantified in terms of Crox/(Cr ox+ Fe ox) or Crox/Feox ratio
• Ni and Mo are almost not present in the passive oxide film
9
Crox / (Cr ox+ Fe ox)
4
Cr content in alloy (x100)Chemical passivation: efficiency and industrial use
◼ Material: 1.4021 (X20 Cr13)
Sandblasted surface (1.4021) Laser welding (1.4021)
6 6
without passivation
5 5
Deconex passivation
4 4
Crox/Feox
3 3
2 2
1 1
0 0
◼ Material : 1.4301 (X5 CrNi18-10)
Sandblasted surface (1.4301) Laser welding (1.4301)
6 6
without passivation
5 traditional passivation 5
4
Deconex passivation 4
Crox/Feox
3 3
2 2
1 1
0 0Laser induced surface oxidation ?
Stainless steel
Titanium and Ti-alloys
• Thicker surface oxide requiring
modified chemical treatment to remove • Extreme case due to high oxygen
thicker Fe-oxide (or not if preferential affinity of Ti crystal structures
Cr-oxidation can be obtained) Laser Treatment
Surface treatment Oxide thickness
Machined 8 nm
Laser treated around 100 nm
Passive surface
K. M. Łęcka et al. Journal of Laser
Applications 28, 032009 (2016)• Is an alloying element enrichment in the
passive oxide sufficient ?
- What about corrosion behavior of passivated surfaces ?
Electrochemical characterization of oxide breakdown
behavior is necessary because a good passivation
induces a risk of localized corrosion - Weakest spot
- Local methods
- Homogeneous
- Large scale
Durability
FunctionalityLocalized corrosion
For a passive metal to become susceptible to localized corrosion…
Two conditions have to be fulfilled:
1) Presence of aggressive anions (element: Cl, F, Br, I)
local attack (dissolution) of the passive film
2) The equilibrium potential of the material must be higher than a
characteristic potential
Pitting potential
Pitting of Cr-Ni stainless steel in HClImportant stages of localized attacks
• Pit initiation
Increase
Penetration Island absorption Oxide cracking Crox / Fe ox ratio
• Metastable pitting
• Pit growth
cathodic partial reaction
Increase Passive
oxide
Stainless steel
substrate
alloying
Anodic metal dissolutionTypical pitting potentials
Metal Pitting potential
The base material influence: (SHE)
Aluminum
• stability of the passive film /
Nickel
or Zirconium
18/8 CrNi steel
• presence of defects (DIN 1.4301)
12% Cr steel
30% Cr Steel
Chromium
Pitting potential of different Titanium
metallic materials in
0.1M NaCl, T= 25°C Really good localized corrosion
resistance, if
O2 + 2H2O + 4e- = 4OH-
Epit > Erev,O2 = 1.23 - 0.059 pH[V]Conclusions
◼ Corrosion resistance of steel (stainless steel) is obtained by chromium
cation enrichment in the passive oxide with an alloy threshold value of 12%.
This process happens naturally in acidic media
◼ Below this composition threshold value, only surface etching/cleaning
effects can be obtained
◼ In this «transition» composition domain (around 12% Cr), additional
strengthening/ functionalizing of the oxide can be obtained by a targeted
acidic chemical passivation (increase Crox/Feox ratio)
◼ A really stable oxide on stainless steel in very acidic environment can only
be obtained with high chromium alloy concentration. In this case, additional
chemical passivation can only bring some surface functionalities
◼ Localized corrosion resistance in presence of chlorides (Epit > Erev,O2) is
difficult to obtain by passive film engineering and is enhanced by adding
slowly dissolving element like MolybdenumThank you for you attention
• Alloying element, corrosion resistance and field of application of various stainless steels
Advanced passivation characterization
Oxide stability (EQCN) Oxide lateral homogeneity
(Microcapillary cell)
platinum wire
electrolyte bridge
holder
(SCE)
Silicone
sealing
rubber
sample
d = 300 nm - 1000 µmType of Corrosion Abbreviated name Material PREn**) Applications
steel resistance no.
Type of steel Corrosion Abbreviated Material no. PREn Applications
class*)
resistance name
CrNi I X5CrNi18-10 1.4301 18 humid areas
steels X4CrNi18-12 1.4303 18
X6CrNiTi18-10 1.4541 18
X2CrNi19-10 1.4306 19
CrNiMo II X5CrNiMo17-12-2 1.4401 24 mild outdoor climate,
steels X6CrNiMoTi17-12-2 1.4571 24 weathered
X2CrNiMo17-12-2 1.4404 24
X3CrNiMo17-13-3 1.4436 26
X2CrNiMo18-14-3 1.4435 27
X2CrNiMoN17-11-2 1.4406 30
X2CrNiMoN17-13-3 1.4429 32
III X2NiCrMoCu25-20-5 1.4539 35 outdoor climate, unweathered
X2CrNiMoN17-13-5 1.4439 37 industrial atmosphere,
weathered
X2CrNiMoN22-5-3 1.4462** 37
*)
IV X1NiCrMoCuN25-20-7 1.4529 47 aggressive media
X1CrNiMoCuN20-18-7 1.4547 48 indoor swimming pools,
X2CrNiMnMoNbN25- 1.4565 50 tunnels, sewage treatment
18-5-4 plants
Special NiCr21Mo14W 2.4602 (66) Combination of aggressive
materials NiMo16Cr15W 2.4819 (68) media
NiMo16Cr16Ti 2.4610 (69) chemicals industry• Online “chemical passivation” characterization electrochemical method
EQCN: “Passive” oxide film stability
Ferritic Stainless Steel: Fe25Cr Metal Oxide Solution
in 0.1M H2SO4 + 0.4M Na2SO4
Thickness: 2 nm
• Mass decrease (oxide dissolution in the
passive domain) on the electrode
0
current density |i| [mA/cm
4
]
2
Current density (mA/cm2)
mass change
2)
-2000 current density 2
•
(ng/cm
mA/cm2 current densities
²m [ng/cm
1 measured still represent
-4000 8
6 mm/year dissolution
change
4
change
-6000
2
• Larger amount of chromium
are necessary in the alloy to
Mass
-8000
mass
8
0.1 stabilize the oxide
6
2
-10000
]
-0.5 0.0 0.5 1.0 1.5
Potential E (vs. SHE) [V]• Laterally resolved electrochemical defect identification / characterization
Lateral defect distribution on Steel
1.4301 Stainless Steel with 0.017% S SEM before
• MnS inclusions form and are
preferential sites for localized corrosion
MnS
attack interface
bulk
• Size dependent analysis: defect
distribution
12 µm
3
Pitting potential as function of measured area 10
1 M NaCl MnS
-2
10
10
2 interface
]
2
Current density [µA/cm
-3
10
1
10
Current [nA]
bulk
-4
10
0
10
-5
10
-1
10
-6
d = 2.5 µm 10
-2 area
10
-500 0 500 1000
Potential [m V] (SCE)You can also read
