What is corrosion? - European Coatings Journal · Protective coatings Corrosion and costs The expenditure for quality assurance and corrosion prevention increases with proceeding

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



Some more definitions….

Corrosion effect: change in any part of the corrosion system caused

by corrosion


Corrosion damage: corrosion effect that causes impairment of the

function of the metal, the environment or the technical system, of

whichthese form a part




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


metal

environment

Phase boundary

A coating is part of the corrosion system!



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



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



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



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-



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!!!



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]



Kinetics of metal dissolution


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-



kinetics of metal dissolution

- why depends metal dissolution on the pH value?

Zn → Zn2+ + 2 e-



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)



Localization of corrosion

t1

t2

Corrosion on steel in below a electrolyte drop containing NaCl with addition of phenolphtaleine und sodiumhexacyanoferrat

(Evans drop test)



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)



Localized corrosion

erosion corrosion (3)crevice corrosion (4)

galvanic corrosionpitting corrosion

©Fraunhofer IFAM

©Fraunhofer IFAM



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!!



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



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



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



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



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)



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



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

-......



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



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)



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)



Degradation of organic coatings

- chemical species

- UV radiation

- water (with or without ions)

- temperature

(- electrochemical potential)

©F

rau

nh

ofe

r IF

AM



Degradation by UV radiation

chalking

remedial actions:

•aliphatic polyurethans

•UV-absorbent pigments

•radical catcher

•......

(1)



Appearance of coating degradation

blister formation (1)

subsurface rusting (1)

filiform corrosion

delamination

©F

rau

nh

ofe

r IF

AM

©Fraunhofer IFAM



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



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



Blister formation I

possible causes:

•thermo diffusion (Tsubstrate < Tcoating)

•osmosis (salt residuals)

•polarisation (anodic, cathodic)

(1)



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



Cathodic delamination

©F

rau

nh

ofe

r IF

AM



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)



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)



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)

…



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



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!!!



Electrochemical impedance spectroscopy - EIS

simple model with defect

simple model without defect

ohmic resistance

capacity



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



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

Documents

What is corrosion? - European Coatings Journal · Protective coatings Corrosion and costs The expenditure for quality assurance and corrosion prevention increases with proceeding