Corrosion. Lab. of Energy Conversion & Storage Materials. Produced by K. B. Kim

Transcription

1 Corrosion 대기환경에의한금속소재 (organic film coated steel) 의퇴화현상평가연구 Lab. of Energy Conversion & Storage Materials Produced by K. B. Kim

2 Introduction AC Impedance Spectroscopy Application of AC Impedance to Corrosion of Organic Film Coated Steel Experimental Results and Discussion Conclusions

3 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

4 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

5 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

6 AC Impedance Spectroscopy (Electrochemical Impedance Spectroscopy, EIS)

7 AC Impedance Spectroscopy ~ ~ E = I Z, Z = Z - jz Z : impedance, Z, Z : real part and imaginary part of impedance

8 AC Impedance Spectroscopy-circuit element Electrochemical system under test AC excitation Black Box AC response Electrochemical system : equivalent to electrical circuit composed of parallel or series combination of R,L, and C

9 Electrical equivalent circuits Uncoated metal under corrosion Organic film coated metal with sound coating quality Organic film coated metal with under corrosion

10 Electrical equivalent circuits X c =1/ (2πf C) At low frequency : infinite capacitive reactance At high frequency : infinitesimally small capacitive reactance

11 Electrical equivalent circuits

12 Evaluation of electrical parameters Graphical analysis Circuit analysis using simulation Electrical equivalent circuits

13 AC Impedance Spectroscopy Gravimetric and salt spray method 1. The corrosion rate cannot be determined through a short time exposure 2. The minimum time required for determination the mass changes more than one week 3. It is difficult to determine the mass loss without removing the sample and weighing to determine the mass change 4. It is difficult to determine the corrosion rate under thin electrolyte layers 5. It is difficult to study the influence of gaseous phases on the corrosion rate under thin electrolyte layers AC Impedance Method 1. The corrosion rate can be determined through a short time exposure 2. The minimum time required for determination the mass changes is in the order of hours 3. It is easy to determine the mass loss without removing the sample 4. It is possible to determine the corrosion rate under thin electrolyte layers 5. It is possible to study the influence of gaseous phases on the corrosion rate under thin electrolyte

14 Application of AC Impedance to Corrosion of Organic Film Coated Steel

15 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

16 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

17 AC Impedance Spectroscopy 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

18 Electrical equivalent circuits 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

19 Electrical equivalent circuit Impedance of metal-organic coating system Solution resistance : R s Capacitance of coating : C c Resistance of coating : R po Charge transfer resistance of metal substrate : R ct Double layer capacitance of polymer/ metal interface : C dl Ionic resistance of the coating : inversely proportional to average cross section of the conductive pathway within the coating layer Higher than 10 7 ohmcm 2 : high anti corrosion resistance 5*10 5 to 10 6 ohmcm 2 : area of corrosion increased to 0.3 to 1 % Lower than 10 3 ohm cm 2 : no protection due to weathered coating

20 Water uptake : Fick s 2 nd law Finite diffusion of water into organic film Governing equation Initial condition Boundary conditions Solution

21 Water uptake : Water sorption test Integration of water concentration over organic film thickness amount of water uptake M t / M = 4 (D ½ /δπ ½ ) t ½ M t M δ : amount of absorbed water at time t : amount of absorbed water at equilibrium : thickness coating Plot of M t / M against t ½ /δ Diffusion coefficient of water (D) calculated from the initial slope of the linear region

22 Water uptake : Water sorption test Z. Z. Lazarevic, Corrosion Science 47 (2005)

23 Water uptake : EIS C c = εε o A/l ε : Dielectric constant of organic film (4-5 for organic coatings) ε o : Electric permittivity of free space (8.854 * F/m) A : true surface area of electrode l : thickness of coating Volume of water fraction = log (C t /C o ) /log ε w ε w : dielectric constant of water (80) C t, C o : coating capacitance at time t = t and t = 0 (log C t - log C o )/ (log C - log C o ) = 4 (D ½ /δπ ½ ) t ½ C : coating capacitance at saturation

24 Water uptake : EIS, Coating resistance and capacitance Time course of pore resistance and coating capacitance, 3% NaCl, Aluminum Z. Z. Lazarevic, Corrosion Science 47 (2005) 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

25 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 Z. Z. Lazarevic, Corrosion Science 47 (2005) ) Development of the conductive macropores through the coating due to diffusion of Cl-, Na+ and oxygen; slower moving ions through macropores

26 Time course of R s, C c, R po, R ct and C dl Correlation among changes in R s, C c, R po, R ct and C dl With coating degradation and corrosion progress Solution resistance : R s, assumed to be constant Capacitance of coating : C c Resistance of coating : R po Charge transfer resistance of metal substrate : R ct Double layer capacitance of polymer/ metal interface : C dl

27 Degradation and Corrosion of Organic Film Coated Steels S. Gonzalez et al, Progress in Organic Coating, 46 (2003) 317

28 Degradation and Corrosion of Organic Film Coated Metal Lacquer coated Cu in 0.5 M NaCl Initial resistance of coating R po for the 2 nd day : in the order of 10 7 ohm Rapid drop of resistance after 6 days No significant further change in resistance for a period of days At 26 th day, adhesion of lacquer totally lost Resistance plateau at low frequencies charge transfer resistance of metal surface plus solution resistance 2 nd plateau in high frequencies solution resistance Constant coating capacitance till 20 th day K.-M. Yin et al, Surface and Coatings Technology, 106 (1998) 167

29 Degradation and Corrosion of Organic Film Coated Metal R po decrease due to the continuous diffusion of ionic species within the free volume of coating R ct decrease with more ions reaching the metal/film interface, causing corrosion rxn Reduction in variation of R po and R ct during a period of days After the day 22, rapid further drop in R po and R ct Water saturation in 8 days (from C c ) C dl plateau in 15 days C dl 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

30 Degradation and Corrosion of Organic Film Coated Metal 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 days. Coating breaks down in 70 days.

31 Degradation and Corrosion of Organic Film Coated Metal 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

32 Degradation and Corrosion of Organic Film Coated Metal 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 40 th 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

33 Degradation of Organic Film Coated Metal Schematic of thermal cycle protocol

34 Degradation of Organic Film Coated Metal Impedance modulus (absolute impedance) during thermal cycling : reversible behavior

35 Degradation of Organic Film Coated Metal Impedance modulus (absolute impedance) during thermal cycling : irreversible behavior

36 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.