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Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
What is corrosion?
Definition: EN ISO 8044:1999
Corrosion: Physicochemical interaction between a metal and its
environment that results in changes in the properties of the metal, and
wich may lead to significant impairment of the function of the metal,
the environment, or the technical system, of which these form a part
NOTE: This interaction is often of an electrochemical nature
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Some more definitions….
Corrosion effect: change in any part of the corrosion system caused
by corrosion
Definition: EN ISO 8044:1999
Corrosion damage: corrosion effect that causes impairment of the
function of the metal, the environment or the technical system, of
whichthese form a part
Definition: EN ISO 8044:1999
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Another definition…
Corrosion system: System consisting of one or more metals
and those parts of the environment that influence corrosion
NOTE: Parts of the environment may be, for example, coatings
surfaces layers or additional electrodes
Definition: EN ISO 8044:1999
metal
environment
Phase boundary
A coating is part of the corrosion system!
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Costs of corrosion:
(http://www.nace.org/Publications/Cost-of-Corrosion-Study/)
A 2002 federal study, initiated by NACE, about costs and control of corrosion in the U.S.:
• direct costs ~ 3.1 % of gros domain product
• indirect costs are on a similar level
• about half of the costs could be saved by consequently executing existing knowledge about corrosion control
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Corrosion and costs
The expenditure for quality assurance and corrosion prevention increases with proceeding product cycle
cost
s
1
10
100
1000
development construction production utilization
corrosion prevention corrosion damage
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Methods of corrosion protection
planning and design material environment
electrochemical protection
material selection
corrosion prevention by
design
packaging, storge and transport
(temporary protection)
corrosion inhibitors
sacrificial anodes
impressed current
metal coatings
conversion coatings
organic coatings
„inorganic“ coatings
removal of stimulators
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochemisty of metal dissolution in aquaeous environment
Oxidation:
O2OH-
e-ia ik
i
Men+
(anodic reaction)Reduction, e.g.:(cathodic reaction)
Me Men+ + n e- 2H2O + O2 + 4e- 4OH- (oxygen reduction)
2H3O+ + 2e- H2 + 2H2O (hydrogen envolution)
Na+Cl-
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
The electrochemical potential – the driving force
example: corrosion of zinc in acids
Zn + 2H3O+ -> Zn2+ + H2 + 2 H2O
anodic reaction: Zn -> Zn2+ + 2e-
2H3O+ +2e- -> H2 + 2 H2O
for chemical reactions it has to be considered:
G<0 => Eeq,a < Eeq,c
cathodic reaction:
G: free Gibbs enthalpy
Eeq,a: potenial of anodic reaction
Eeq,c: potenial of cathodic reaction
educt
producteq a
aln
zF
RTEE 0
Walther Nernst, 1866 – 1941(www.wikipedia.de)
Nernst equation
Pourbaix-diagram
Take care to explaincorrosion with Pourbaix
diagrams becausecorrosion is a processbeyond the equilibrium
state!!!
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Kinetics of metal dissolution
Me → Men+ + z e-
charge transport => electric current
Faraday law:
I: current [A]
Q:charge [C]
t: time [s]t
QI
Michael Faraday , 1791 – 1867[Wikipedia]
dt
dm*
A
1*
M
F*zi
i: current density [A/cm²]
F: Faraday constant
M: molecular weight [g/Mol]
A: area [cm²]
m: mass [g]
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Kinetics of metal dissolution
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
iaik
assuming the homogenic mixed electrode:
curr
ent
den
sity
i
potential E
overall reaction
ia=ik => i=0 => externally currentless (no equilibrium!!!)
rest potential ER
Eeq,a Eeq,k
icorr
Zn → Zn2+ + 2 e-
O2 + 2 H2O + 4 e- 4 OH-
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
kinetics of metal dissolution
- why depends metal dissolution on the pH value?
Zn → Zn2+ + 2 e-
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
kinetics of metal dissolution – polarisation curves
E /mVH
icorr
linear plot
Ecorr
curr
ent
den
sity
i /
mA
/cm
2
E /mVH
log
I i I
/ m
A/c
m2
icorr
Ecorr
logarithmic plot(Tafel plot)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Localization of corrosion
t1
t2
Corrosion on steel in below a electrolyte drop containing NaCl with addition of phenolphtaleine und sodiumhexacyanoferrat
(Evans drop test)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Localized corrosion
Localized corrosion is characterized by a locally limited but often accelerated corrosion which may quite quickly to failures
Requirements for localized corrosion are local differences, e.g. by:
• different aeration (aeration cell)
• local destruction of scale layer
• heterogeneous micro structure
• different concentrations (concentration cell)
• electrical contact of different metals (galvanic element)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Localized corrosion
erosion corrosion (3)crevice corrosion (4)
galvanic corrosionpitting corrosion
©Fraunhofer IFAM
©Fraunhofer IFAM
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Localized corrosion, area effect
Under certain conditions, e.g.:
• cathodic polarisation resistance is limiting factor
• galvanic current accords approximately to corrosion current
the following equation is valid
large cathodic area and small anodic area -> very dangerous!!
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Methods of corrosion protection
planning and design material environment
electrochemical protection
material selection
corrosion prevention by
design
packaging, storge and transport
(temporary protection)
corrosion inhibitors
sacrificial anodes
impressed current
metal coatings
conversion coatings
organic coatings
„inorganic“ coatings
removal of stimulators
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Usage of paints in Germany 2007
26
21
7046 59
95
2531050
Bautenfarben
Industrielacke
Autoserienlacke
Holzlacke
Autoreperaturlacke
Schiffsfarben
Korrosionsschutz
Sonstige
building laquers
industrial laquers
automotive series coating
wood varnish
automotive repair coating
ship coatings
corrosion protection
others
(VDL, 2008)indicated amounts in in 1000 tons
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Examples of organic coatings for corrosion protection
base coat
filler
cataphoretic dip coat
zinc phosphatized coat galvanized steel
clear coat
automobile ca. 120 µm ship ca. 1000 µm
2 pack PUR topcoat
2 pack EP primer
anodizationAl-alloy
aircraft fuselage, inside ca. 80 µm
grid blasted steel
zinc dust primer
EP-intermediate coatings
top coat with anti fouling
©Fraunhofer IFAM ©Fraunhofer IFAM ©Fraunhofer IFAM
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Content and film forming mechanism of organic coatings
polymer pigments
titanium dioxideiron oxidezinc dustzinc phosphatestrontium chromate
additives
levelling agentdesiccantantifrothingdispersing additive
solvents
waterbenzinebutylacetatebutylglycolmethylethylketone
polyaddition
epoxy resins polyurethanes ...
polycondensation
phenolic resinssaturated polyestermelamine resinsethylsilicates
polymerisation
unsaturated polyester
radiation curing acrylate resins
extender
barion sulphatetalcmicaceous iron oxide (MIO)
chemical curing oxidative curing
alkyd resins
physical drying
cellulosepolyvinylchloride polyacrylates chlorinated rubber
coagulation
dispersionsstyrene acrylatespure acrylatespolyurethane
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Examples of film forming processes
physical drying of a solvent based coating
polymer molecules
solventmolecules
dry film (2)
wet film
drying
interdiffusion,crosslinking
structuration
coagulation of a dispersion coating
(2)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Interphase: from substrate to coating
Interphase differs in properties from “bulk” coating by:
•contaminations
•oxides
•component segregation
•degree of cross linking
=> higher diffusion rates at interphase
coating
substrate
components
contaminations
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Some remarks to organic coatings
•organic coatings are composite materials
•Polymers of organic coatings are mostly thermoset materials
• diffusion coefficient: Dinterphase > D Bulk
•heterogenic structure with domains of varying er physical and chemical
properties:
-degree of cross linking
-hydrophilic properties
-hardness
-......
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Some protection effects of organic coatings
protection effects of organic coatings
barrier properties adhesion active protection
inhibition cathodic protection …
providing pigments*)
• aluminium flakes
• zinc flakes
• micaceous ironoxide (MIO)
providing pigments*)
• chromates
• phosphates
providing pigments*)• zinc• magnesium• MgZn
*) „good“ pigments may provide multiple protection effects
• MIO
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Alternating climate impact
“microfogging”
molecular dispers distributed water excess water, formation of micro caverns
T1 > T2
durability to temperatur cycling by micaceous iron ore pigments
micaceous iron ore pigments as condensation nuclei
T1 > T2
(C.H. Hansen, Prog. Org. Coat. 26 (1995) 113)
(H. Ochs, J. Vogelsang, Electrochimica Acta, 2004)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Chromates – the mother of all corrosion protective pigments
•effective on many substrates (not on carbon fibre composites!)
•inhibition of electrochemical reactions, anodic and/or cathodic
•“leaching” in case of coating injury
• solubility and interaction with polymer is important! (preferently strontium chromate)
•had / has to be replaced in nearly all technical applications and industrial fields
Primer
Metall
PrimerPrimer
MetallMetall
coating
substrate
“Leaching”
AA 2024 in 0.1 NaCl
no inhibitor
40 ppm chromate
potential (mVSCE)
curr
ent
den
sity
(A
/cm
²)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Degradation of organic coatings
- chemical species
- UV radiation
- water (with or without ions)
- temperature
(- electrochemical potential)
©F
rau
nh
ofe
r IF
AM
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Degradation by UV radiation
chalking
remedial actions:
•aliphatic polyurethans
•UV-absorbent pigments
•radical catcher
•......
(1)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Appearance of coating degradation
blister formation (1)
subsurface rusting (1)
filiform corrosion
delamination
©F
rau
nh
ofe
r IF
AM
©Fraunhofer IFAM
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Water uptake
1 2 3
wat
er u
pta
ke
immersion time
•water uptake of an intact coating is a thermodynamic property
•water uptake decreases glas transition temperature Tg signifcantly (1 % H2O ~ 10°-20°)
•above Tg ion mobility is significantly increased
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Degradation – water in organic coatings
8 µm13 µm
TEM: 1K-EP adhesive in 70°C H2O
t = 0 h t = 600 h
the state of the water depends on:
•materials (film former, pigments, substrate)
• temperature / temperature gradient
• salt content (osmosis)
©Fraunhofer IFAM ©Fraunhofer IFAM
1) molecular dispersed2) condensed, excess water3) arranged at functional groups4) condensed at interphases
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Blister formation I
possible causes:
•thermo diffusion (Tsubstrate < Tcoating)
•osmosis (salt residuals)
•polarisation (anodic, cathodic)
(1)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Blister formation II
H2O O2 Na+ Cl- electrolyte
coating
substrate
1
•water uptake, swelling•oxygen diffusion
H2O O2
Cl-Na+2
•decrease of Tg
• creation of conductive pathways•diffusion of ions
H2OO2
Cl- Na+
Fe2+e-
OH-
Cl-Na+3
•metal dissolution• interphase diffusion•polarisation, migration
e-
Cl- Na+
H2O O2
OH-Fe2+ Na+Cl-
4
•alkaline dehesion•blister formation
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Cathodic delamination
©F
rau
nh
ofe
r IF
AM
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Remark on corrosion testing of organic coatings
Why have coated test specimens to be scribed?
- it is required by most standards
- the protection performance in the case of a scribe is an important feature of a coating
- be aware of the area effect!!!
randomly occuringsmall defects
(allowanodic reaction)
properly coated area(allows cathodic reaction)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Comment on corrosion testing of organic coatings
Why have coated test specimens to be scribed?
- it is required by most standards
- the protection performance in the case of a scribe is an important feature of a coating
- be aware of the area effect!!!
defined scribe
(allowsanodic reaction)
properly coated area(allows cathodic reaction)
randomly occuringsmall defect (will not significantly influence the area ratio)
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochmical investigation of organic coatings
Electrochemical investigation methods for organic coatings
• electrochemical impedance spectroscopy (EIS)
• relaxation voltammetry (RV)
• „electrochemical noise analysis“ (ENA)
• scanning techniques:
-scanning reference electrode technique (SRET)
-scanning vibrating electrode technique (SVET)
-scanning kelvin probe (SKP)
…
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochmical impedance spectroscopy - EIS
onset cell
reference electrode
counter electrode (Pt)
substrate (working electrode)
electrolyte
onset cell
coating
pertubation response
• for EIS on organic coatings high ohmic potentiostat needed
• pertubation: amplitude < 100mV, 100 kHz to 10 mHz
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochemical impedance spectroscopy EIS
10-2 10-1 100 101 102 103 104 105
101
102
103
104
105
Impe
dan
z /Z
/ /
cm2
Frequenz f / Hz
0
-20
-40
-60
-80
WE: AA 2024 Primer ohne Chromat in 3% NaCl belüftetE
pol=-0,85V
SCE
typical impedance spectrum (with 2 time constants)
RCoat CCoat
RPore
CPECoat CDL
RP
RCoat CPECoat
fitting of equivalent circuits:
phase shift angle
fitting of equivalent circuits and parameter quantification “may” give time depending information about:
•defect status
•barrier properties
•water uptake
A fitting model is not
necessarily a valid
model!!!
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochemical impedance spectroscopy - EIS
simple model with defect
simple model without defect
ohmic resistance
capacity
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Electrochemical impedance spectroscopy - EIS
electrochemical impedance spectroscopy (EIS) ist a sophisticated method to investigate metal/electrolyte interfaces
it is non destructive
for EIS on organic coatings a suitable high ohmic potentiostat is needed
EIS may give considerable information of the actual (electrochemical) state of the coating if interpreted properly
for proper interpretation you need a model like description of the coating material behaviour (e. g. a valid eqivalent circuit)
a fitting equivalent circuit is not necessarily a valid equivalent circuit !!
for a proper performance of EIS measurement and interpretation on organic coatings, refer to DIN EN ISO 16773:1-4
Short course, Dr.–Ing. Peter Plagemann, Bremen/D
12-13 September 2013 Düsseldorf, Germany Protective coatings
Picture credits
(1) W. D. Kaiser, A. Schütz, „Schäden an Korrosionsschutzbeschichtungen“, Vincentz-Verlag, 2000(2) Goldschmidt, Streitberger, „BASF-Handbuch Lackiertechnik“ Vincentz-Verlag 2002(3) E. Wendler-Kalsch, H.Gräfen, „Korrosionsschadenkunde“, Springer-Verlag, 1998 (4) www.korrosion-online.de
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