Evaluation of coated steel under weathering test using Under-film Corrosion Tester based on Current Interrupter Technique

Transcription

1 (AETOC April 2013) Evaluation of coated steel under weathering test using Under-film Corrosion Tester based on Current Interrupter Technique Hiroyuki Tanabe, Ryuichi Aoki, Shinichiro Tame Dai Nippon Toryo Co., Ltd.

2 Introduction One of DC techniques, Current Interrupter method was adopted as ISO standard which was published as ISO in October, This paper present some applications of C.I. focusing on outdoor exposure test under collaboration with Central Research Institute of Electric Power Industry. Industry requires non-destructive inspection of coatings.

3 Outline of the presentation 1. What is the C.I. technique for coatings 2. Evaluation of polyurethane single coat exposed outdoor. 3. Evaluation of coating system exposed outdoor. 4. Degradation analysis(ft-ir) of coating cross-section for exposed panels 5. Summary

4 What is C.I. technique Principle of UFCT

5 Cf Cdl R τf : 1 msec τe : 10 sec Rf Re Cf : 10-9 F/cm 2 Cdl : 10-5 F/cm 2 Rf : 10 6 W cm 2 Re : 10 6 W cm 2 Fig.2 Equivalent circuit of coated mild steel. Generally coated steel is composed of two elements that the paint film bearing the time Constance of msec or so and that of metal coated bearing the time Constance of sec or so. The remarkable difference of the two figures makes possible to separate each other.

6 Principle of UFCT (Carrent interrupter method) Slope : 1/2.3τ Small current was applied to coated steel, switched off, Potential decay of coated steel was measured.

7 off τ f =C f R f τ e =Ce Re log V (mv) time Decay curve of coated steel when put off

8 ir log V -1/2.3τ t (sec) Fig.3 Relation between potential and time. The time constance τ was obtained from slope. Further film resistance, film capacitance, Polarization resistance and double layer capacitance were obtained.

9 Fig. Photograph of under film corrosion tester (HL201) This equipment was compact and not heavy. The measurement cell was unique.

10 Figure. Schematic drawing of the test cell for electrochemical measurement N.Kawai, J.Tani, M.nagai, H.Tanabe, Study on evaluation methods for degradation of coatings for power transmission and distribution systems degradation process of coated film during atmospheric exposure tests -,Central Research Institute of Electric Power Industry (CRIEPI) Report, No.Q08031(August 2009):P3

11 Measurement of dummy circuit

12 Table comparison of nominal values and measured values of fixed elements. Model circuits Nominal Values Measured Values Rf Cf Re Cdl Rf Cf Re Cdl No.1 95MΩ 330pF 105MΩ 0.01µF 95.9MΩ 320pF 105.9MΩ 0.01µF No.2 100MΩ 1000pF 100MΩ 0.01µF 102MΩ 980pF 98.4MΩ 0.012µF Two electric circuit composed of different dummies were examined. Each value obtained accorded with nominal value even when it had high resistance.

13 Application example 1 Evaluation of polyurethane single coat Blasted steel Single coat : Polyurethane coating 60µm Outdoor Exposure test : 27 months

14 Figure. Film impedance and film capacitance as a function of exposure time for blasted carbon steel, measured after 27 months of atmospheric exposure by the AC impedance method at 200 Hz (blasted carbon steel; polyurethane resin paints; exposure angle of 45 ). N.Kawai, J.Tani, M.nagai, H.Tanabe, Study on evaluation methods for degradation of coatings for power transmission and distribution systems degradation process of coated film during atmospheric exposure tests -,Central Research Institute of Electric Power Industry (CRIEPI) Report, No.Q08031(August 2009):Pⅲ, 9

15 Film Resistance(Ω cm 2 ) 1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 1.E-03 1.E-04 1.E-05 1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 Film Capacitance(F/cm 2 ) 1.E+03 1.E Exposure Time (months) Figure. Film resistance and film capacitance as a function of exposure time for blasted carbon steel, measured after 27 months of atmospheric exposure by the CI method (blasted carbon steel; polyurethane resin paints; exposed angle of 45 ). N.Kawai, J.Tani, M.nagai, H.Tanabe, Study on evaluation methods for degradation of coatings for power transmission and distribution systems degradation process of coated film during atmospheric exposure tests -,Central Research Institute of Electric Power Industry (CRIEPI) Report, No.Q08031(August 2009):P11

16 Application Example 2 Evaluation of system coated panels exposed outdoor ( four years)

17 Coating Systems Used for Outdoor Exposure Test Topcoat System Substrate Fluoropolymer Polyurethane Chlorinated rubber Alkyd 3.2 x 100 x 100 mm Sand-blasted steel panel Primer Inorganic zinc-rich primer (75µm) Wash primer (15µm) Undercoat Epoxy resin (100µm) Epoxy resin (100µm) Chlorinated rubber (2 x 35µm) Anti corrosive Undercoating (2 x 35µm) Intermediate Coat Epoxy resin (50µm) Epoxy resin (50µm) Chlorinated rubber (2 x 35µm) Alkyd (2 x 30µm) Top coat Fluoropolymer (30µm) Polyurethane (30µm) Chlorinated rubber (30µm) Alkyd (25µm)

18 Polarization Resistance <log(ω cm 2 )> Exposure Time (years) Fluoropolymer Coating System Polyurethane Coating System Chlorinated Rubber Coating System Fig. Change in polarization resistance after outdoor exposure test in Miyakojima M.Nagai, H.Matsuno, H.Tanabe Shikizai( Color Materials) 66, (1993)

19 Film Resistance after Exposure Test (Miyakojima) Film Resistance (Ω cm 2 ) Exposure Time (Years) : Fluoropolymer System : Polyurethane System : Chlorinated Rubber System

20 Adhesive Strength Retention after Exposure Test (Miyakojima) Adhesive Strength Retention (%) Exposure Time (Years) : Fluoropolymer System : Polyurethane System : Chlorinated Rubber System

21 Example 3 IR analysis of outdoor exposed panels

22 Figure 9 Outdoor Exposure Test1 of Marine test station in service since

23 Figure 10,11 Gloss retention and Chalking Rate of Outdoor Exposure Test 1 Gloss Retention (%) Exposure Time(years) Fluoro-polymer Coating Polyurethane Coating Chalking Exposure Time(year) Fluoro-polymer Coating Polyurethane Coating 23

24 Measurements of film consumption (by observing the section of 15y exposure samples at Hiroshima) Light proof Liht exposure Parts Cutting process Observation of the cross section Exposure specimen 10X30 cm

25 Figure 6 Consumption of topcoat in film thickness after15 year exposure (Cross section) a) Fluoro-polymer Light sealed label b) Polyurethane Film consumption: Film consumption: A B 22 28µm 0 1.1µm /15 years C D /15 years 1/20 shrinking A, B, C, D area were for Imaging IR measurement as below (Figure 7) 25

26 Figure 7 Isocyanates retention of each cross section of coats about light seal or irradiated surface area Light sealed area Light irradiated area A: Fluoro-polymer B: Fluoro-polyme Line of Top coat / air surface Red Color indicates good retention of isocyanate 40µm C: Polyurethane D: Polyurethane Line of Top coat / air surface Yellow Color indicates poor retention of isocyanate 26

27 Issue Summary of degradation phenomena for Fluoro-polymer vs. Polyurethane Unit Exposure Fluoro-polymer Polyurethane Mol. weight 3 years Consumption of DFT µm years cross-linking no change large degradation Retention of crosslinking % 5 years 75% 25% adhesiveness % Okinawa 93% 83% Miyako 4 years SWM 2000 hrs Impedance Ω/cm 2 EXP10*7 EXP10*6 Impedance slopes Gloss retention % 20 Chalking Rate Years at marine environment 1 1/2.5 > 20% / 15years < 20% / 2years number 10 / 20years 0 / 10years 27

28 Summary It was shown C.I. could apply to outdoor exposure test panels C.I. is non-destructive so acceptable to industry s requirement. C.I. will be applied to 20 year exposure test panels. C.I. has a plan to be tried to inspect coatings of on-site steel structures.

29 Acknowledgement Dr Noboru Kawai (Central Research Institute of Electric Power Industry) Mr. Takasi Takayanagi ( Asahi-glass Co. Ltd)

30 Thank you for attention