Lab. of Energy Conversion & Storage Materials · 2010-02-10 · Organic Coated Metal Protection of active metals by covering their surface with organic coating •Mechanical properties

대기대기대기대기대기대기대기대기 환경에환경에환경에환경에환경에환경에환경에환경에 의한의한의한의한의한의한의한의한 금속금속금속금속금속금속금속금속 소재소재소재소재소재소재소재소재 (organic film coated steel)(organic film coated steel)(organic film coated steel)(organic film coated steel)(organic film coated steel)(organic film coated steel)(organic film coated steel)(organic film coated steel)의의의의의의의의

퇴화퇴화퇴화퇴화퇴화퇴화퇴화퇴화 현상현상현상현상현상현상현상현상 평가평가평가평가평가평가평가평가 연구연구연구연구연구연구연구연구

CorrosionCorrosion

Lab. of Energy Conversion & Storage MaterialsLab. of Energy Conversion & Storage Materials

Produced by K. B. KimProduced by K. B. KimProduced by K. B. KimProduced by K. B. Kim

• Introduction

• AC Impedance Spectroscopy

• Application of AC Impedance to Corrosion of Organic Film

Coated Steel

• Experimental

• Results and Discussion

• Conclusions

Organic Coated Metal

Protection of active metals by covering their surface with organic coating

• Mechanical properties of metals

• Preventing metals from corrosion

• Introduction of functional surface properties

Organic Coated Steel

• Steel covered with metallic coatings of Zn and Zn alloys

• Inorganic conversion layer deposited on top of the metallic coating to generate

corrosion resistant interface and to provide link to the organic primer

• Cathodic overcoat as a corrosion resistant organic primer

• Topcoat to give appearance and a barrier between corrosion medium and inner

layers

Degradation of Organic Coated Metal

Coating degradation

• Penetration of water molecules, ions and oxygen to the polymer/metal interface

• Diffusion of these species through small pores or pathways within the polymer that

facilitates this transport

Corrosion of metal underneath the coating

• Corrosion environment formed at the polymer/metal interface

• Corrosion along the polymer/metal interface : electrochemical reaction

• Blister formation due to corrosion underneath the coating

Motivation

• Early detection and quantification of coating degradation

• Early detection and quantification of corrosion

• Estimation of corrosion during the initial stage of corrosion when it

may not be visible

• Prediction of useful life of the coating

• Prediction of useful life of the coated metal

AC Impedance Spectroscopy

(Electrochemical Impedance Spectroscopy, EIS)


E = I Z, Z = Z‘ - jZ“

Z : impedance,

Z‘, Z“ : real part and imaginary part of impedance

~~~~ ~~~~

AC Impedance Spectroscopy-circuit element

Electrochemical system :

equivalent to electrical circuit composed of parallel or series combination

of R,L, and C

Black BoxAC excitation

Electrochemical system

under test

AC response

Electrical equivalent circuits

• Uncoated metal under corrosionUncoated metal under corrosionUncoated metal under corrosionUncoated metal under corrosion• Organic film coated metal withOrganic film coated metal withOrganic film coated metal withOrganic film coated metal with

sound coating qualitysound coating qualitysound coating qualitysound coating quality

• Organic film coated metal withOrganic film coated metal withOrganic film coated metal withOrganic film coated metal withunder corrosionunder corrosionunder corrosionunder corrosion


Xc=1/ (2πf C)At low frequency : infinite capacitive reactance

At high frequency : infinitesimally small capacitive reactance



Evaluation of electrical parameters

• Graphical analysis

• Circuit analysis using simulation


5. It is possible to study the influence of

gaseous phases on the corrosion rate under

thin electrolyte

5. It is difficult to study the influence of

gaseous phases on the corrosion rate under

thin electrolyte layers

4. It is possible to determine the corrosion rate

under thin electrolyte layers

4. It is difficult to determine the corrosion rate

under thin electrolyte layers

3. It is easy to determine the mass loss

without removing the sample

3. It is difficult to determine the mass loss

without removing the sample and weighing to

determine the mass change

2. The minimum time required for

determination the mass changes is in the

order of hours

2. The minimum time required for

determination the mass changes more than

one week

1. The corrosion rate can be determined

through a short time exposure

1. The corrosion rate cannot be determined

through a short time exposure

AC Impedance MethodGravimetric and salt spray method

Application of AC Impedance to Corrosion of

Organic Film Coated Steel

Degradation of Organic Coated Metal

Coating degradation

• Penetration of water molecules, ions and oxygen to the polymer/metal interface

• Fast diffusion of these species through pathways within the polymer and local

defects formed during production or during the lifetime of the coated materials

Corrosion of metal underneath the coating

• Corrosion environment formed at the polymer/metal interface

• Corrosion along the polymer/metal interface : electrochemical reaction

• Blister formation due to corrosion underneath the coating

Water Uptake and Corrosion

No electrolytic solution within the metal/organic coating interface,

� neither electrochemical double layer formation nor faradaic reaction occurs.

Once corrosion reactants reach the metallic substrate, corrosion process may start.

Rate of permeation of corrosion reactants (water, ions, oxygen) through the polymer

to the metal surface depends on

� Thickness of the coating

� Diffusivity and solubility within the homogeneous polymer :

chemical nature of the polymeric layer, effective cross linking during curing

� Presence of macro/micro defects in the coating formed during production and life time :

significantly increase the apparent diffusion coefficient of water

� Temperature effect : thermal cycling


• High ohmic resistance of organic coatings :

� impedes the use of DC type electrochemical measurements

• Electrochemical Impedance Spectroscopy (EIS) :

- dielectric properties of film

- processes of coating degradation

- corrosion underneath the coating


• Organic film coated metal with

sound coating quality before

corrosion reactants reach the

metallic substrate

• Organic film coated metal with

under corrosion after corrosion

reactants reach the metallic substrate

Electrical equivalent circuit

Ionic resistance of the coating : inversely proportional to average cross section of the

conductive pathway within the coating layer

• Higher than 107 ohmcm2 : high anti corrosion resistance

• 5*105 to 106 ohmcm2 : area of corrosion increased to 0.3 to 1 %

• Lower than 103 ohm cm2 : no protection due to weathered coating

Impedance of metal-organic coating system

• Solution resistance : Rs

• Capacitance of coating : Cc

• Resistance of coating : Rpo

• Charge transfer resistance of metal substrate : Rct

• Double layer capacitance of polymer/ metal interface : Cdl

Water uptake : Fick’s 2nd law

Finite diffusion of water into organic film

Governing equation

Initial condition

Boundary conditions

Solution

Water uptake : Water sorption test

Integration of water concentration over organic film thickness

� amount of water uptake

Mt / M∞ = 4 (D½/δπ½) t½

Mt : amount of absorbed water at time t

M∞ : amount of absorbed water at equilibrium

δ : thickness coating

Plot of Mt / M∞ against t½/δ

Diffusion coefficient of water (D)

� calculated from the initial slope of the linear region

Water uptake : Water sorption test

Z. Z. Lazarevic, Corrosion Science 47 (2005) 823-834

Water uptake : EIS

Cc = εεoA/l

ε :G Dielectric constant of organic film (4-5 for organic coatings)

εo: Electric permittivity of free space (8.854 * 10-12 F/m)

A : true surface area of electrode

l : thickness of coating

Volume of water fraction = log (Ct/Co) /logG εw

εw : dielectric constant of water (80)

Ct, Co : coating capacitance at time t = t and t = 0

(log Ct - log Co)/ (log C∞ - log Co) = 4 (D½/δπ½) t½

C∞ : coating capacitance at saturation

Water uptake : EIS, Coating resistance and capacitance

Pore resistance decrease and coating capacitance increase for the first few days

• Pore resistance : presence of ions in water solvent

• Coating capacitance : presence of water in organic film

Time course of pore resistance and coating capacitance, 3% NaCl, Aluminum



Water uptake : EIS, Coating resistance and capacitance

Time required to saturate the coating with pure water (6hrs)

� much shorter than that for the initial decrease in pore

resistance.

� close to the time for charge transfer resistance decrease

and double layer capacitance increase

Diffusion coefficient of Cl- ions across polymer

� one order of magnitude smaller than diffusion coefficient

of water (faster mobility and smaller size, molecular radius

1.38A) hydrated ions

1) Diffusion of water into the micropores of the polymer

network, coating saturation with water

2) Development of the conductive macropores through the

coating due to diffusion of Cl-, Na+ and oxygen; slower

moving ions through macropores

Time course of Rs, Cc, Rpo, Rct and Cdl

With coating degradation and corrosion progress

• Solution resistance : Rs , assumed to be constant

• Capacitance of coating : Cc

• Resistance of coating : Rpo

• Charge transfer resistance of metal substrate : Rct

• Double layer capacitance of polymer/ metal interface : Cdl

Correlation among changes in Rs, Cc, Rpo, Rct and Cdl

Degradation and Corrosion of Organic Film Coated Steels

S. Gonzalez et al, Progress in Organic Coating, 46 (2003) 317

Degradation and Corrosion of Organic Film Coated Metal

Lacquer coated Cu in 0.5 M NaCl

K.-M. Yin et al, Surface and Coatings Technology, 106 (1998) 167

Initial resistance of coating Rpo for the 2nd day :

� in the order of 107 ohm

Rapid drop of resistance after 6 days

No significant further change in resistance for a

period of 16-20 days

At 26th day, adhesion of lacquer totally lost

Resistance plateau at low frequencies

� charge transfer resistance of metal surface

plus solution resistance

2nd plateau in high frequencies

�solution resistance

Constant coating capacitance till 20th day


Rpo decrease due to the continuous diffusion of ionic

species within the free volume of coating

Rct decrease with more ions reaching the metal/film

interface, causing corrosion rxn

Reduction in variation of Rpo and Rct during a period of

16-20 days

After the day 22, rapid further drop in Rpo and Rct

Water saturation in 8 days (from Cc)

Cdl plateau in 15 days

Cdl increase at a slower rate than Cc

Water first saturates the coating and then ionic species

follow the flaws in the coating and gradually saturate

the metal surface


Gradual decrease in impedance in 40 days

Impedance and phase angle do not vary in the

period of 40 to 50 days.

Degradation of the film intensifies and a total

delamination of the coating occurs in 70 days.

Bode plot reveals different trends before and

after the interval of 40-50 days.

Coating breaks down in 70 days.


The first semi circle in the high frequency region does not

change much in size

� water borne polymer film remains in a stable structure

The second semi circle that corresponds to the metal

surface impedance shrinks very rapidly

� electrolyte ions quickly saturate the interface during this

period of time.

Rapid decline in charge transfer resistance at the copper

surface during a period of 54 to 64 days

impedance of the polymer film declines very rapidly and

finally disappears

� no protection capability after 50 days

Small semi circle in the low frequencies representative of

the electrode charge transfer resistance does not vary in

size


Coating resistance drops rapidly in less than 10 days

Concurrent Increase in coating capacitance

� Water saturation

In 0 – 40 days, coating resistance and charge transfer

resistance decrease gradually

�Slow diffusion of ionic species within the film

Rapid drop in charge transfer resistance and double layer

capacitance around 40th day

� Breakthrough of the ionic species to the metal surface

After 40 days, charge transfer resistance is close to that

of a piece of uncoated metal

After 60 days, entire coating delamination

Degradation of Organic Film Coated Metal

Schematic of thermal cycle protocol


Impedance modulus (absolute impedance) during thermal cycling :

reversible behavior


Impedance modulus (absolute impedance) during thermal cycling :

irreversible behavior

Corrosion of Organic Film Coated Metal

- Anodic blister

- Cathodic blister : no corrosion products

Strong alkaline electrolyte formed during

oxygen reduction,

� stabilization of the oxide on the metal.

Anodic metal dissolution within this zone

� never observed

Delamination of the organic coating

� caused by bond breaking within the adjacent

organic layer through oxidative destruction

of the interface

Instability of the substrate/polymer interface

� linked to the rate of oxygen reduction.

Documents

Lab. of Energy Conversion & Storage Materials · 2010-02-10 · Organic Coated Metal Protection of active metals by covering their surface with organic coating •Mechanical properties