A New Sacrificial Corrosion Protection Mechanism for High Performance Zinc/Aluminum Flake Coating Systems and Applications

Transcription

1 A New Sacrificial Corrosion Protection Mechanism for High Performance Zinc/Aluminum Flake Coating Systems and Applications Shusaku Ishikawa Senior Technical Manager, Engineering Yuken America Inc. Introduction Zinc/Al Flake Coating with High Corrosion Protection Performance Popular in Automotive and Construction Industries Zinc plating Zinc plating + Trivalent Cr Zinc Nickel Zinc Nickel + Trivalent Cr /Al Flake coating 5 1 SST Time to [Hr] 1

2 Introduction Important to Form Basic Zinc Chloride (Insulator) β(h) 2 Protective, insoluble H 2 4(H) 2 Cl 2 6(H) 2 Cl 2 2+, H - Stop! Protective, insoluble 2 Minimize formation! Less protective, insoluble (H) 6 (C 3 ) 2 Protective, insoluble 2 Introduction Investigation: Corrosion Product on Coating Film Relationship with Corrosion Resistance New & Current Product Comparison Cycles to [cy] Current Type (2C2B) 3 11 SST CCT Current Type (2C2B +YC-T) 8 18 New Type (2C2B) Time to [ hr ] 3

3 Experiment (Sample) Test Samples Sample Table 1. Summary of Zinc/Aluminum Flake Coating Systems Main Components Base Coat Thickness [ μ ] Main Components Top Coat Thickness [ μ ], Al, Si 8-1 No topcoat - + T, Al, Si 8-1 Si, 1 2, Al, Si 8-1 No topcoat - Alloy, Ni (17%) 8-1 No topcoat - 4 Experiment (Process) Coating Process of Base Coat First coat (1C1B) Second coat (2C2B) Degrease Base Blast Bake Coat Base Coat Bake 5 o F (26 o C) 3 min 5 o F (26 o C) 3 min Top Coat Process Top Coat Dry 212 o F (1 o C) 15 min 5

4 Experiment (Evaluation) Corrosion Protection Evaluation 1) CCT: Salt spray (5% NaCl, 5 ) 17 hrs dry (7 ) 3 hrs salt spray (5% NaCl, 5 ) 2 hrs natural dry (25 ) 2 hrs 2) SST: 5% NaCl, 35 3) Salt water immersion test: 5% NaCl, 25, exposed to the atmosphere Corrosion Product Evaluation X-ray diffractometry (XRD) Surface Condition Evaluation SEM-EDS Time-Dependent Changes in Natural Potentials Natural potentials measured in 5% NaCl solution 6 Results and Discussion (Corrosion Resistance) - CCT - Cycles Sample cy. 2cy. 4cy. 8cy. 12cy. + T cy. 1cy. 2cy. Fig. 5. CCT results Corrosion Resistance: > +T > > 7

5 Results and Discussion (Corrosion Resistance) - SST - Hours Sample h 1h h h h + T h 25h 5h Fig. 7. SST results Corrosion Resistance: > +T > > 8 Results and Discussion (Corrosion Resistance) - NaCl Immersion Test - Hours Sample h 24h 24h 48h 72h + T Fig. Salt water immersion test results 9

6 Results and Discussion (XRD NaCl Immersion Test) Intensity [counts] a) b) +T Before After 24-Hour Almost no change Intensity [counts] c) d) Significant decrease 5 (H) 8 Cl 2 H 2 5 (H) 6 (C 3 ) 2 Intensity [counts] 1 Intensity [counts] θ [deg.] Fig. 2. XRD Pattern of Each Film Before and After 24-Hour NaCl Immersion Test Basic Zinc Chloride Was Mainly Formed in, and. 1 Results and Discussion (XRD Intensity Changes) - NaCl Immersion Test - Intensity of 5(H)8Cl2 H2 or 5(H)6(C3)2 [counts] (H) 8 Cl 2 H 2 / 5 (H) 6 (C 3 ) 2 +T 5(H)8Cl2 H2 5(H)6(C3)2 Intensity of [counts] T Immersion Time [H] Immersion Time [H] Fig. 4. XRD Intensity Changes of Corrosion Product and Zinc (NaCl Immersion Test) Basic Zinc Chloride Intensity Change: > > +T > 11

7 Results and Discussion (XRD Intensity Changes) - CCT - Intensity of 5(H)8Cl2 H2 [counts] (H) 8 Cl 2 H 2 +T Intensity of [counts] Consistent +T CCT [Cycle] CCT [Cycle] Fig. 6. XRD Intensity Changes of Corrosion Product and Zinc (CCT) Basic Zinc Chloride Intensity Change: > > +T > 12 Intensity of 5(H)8Cl2 H2 [counts] Results and Discussion (XRD Intensity Changes) - SST (H) 8 Cl 2 H 2 +T SST [Hour] As consistent as in CCT Intensity of [counts] SST [Hour] Fig. 8. XRD Intensity Changes of Corrosion Product and Zinc (SST) +T 13

8 XRD Trends in Salt Water Immersion Test Basic Zinc Chloride Formation on Each Film in Immersion Test Amount +T Speed Results and Discussion (SEM Image) Granular Crystals of Basic Zinc Chloride + T Fig. 9. SEM Image of Each Film After 24-Hour SST Granular Crystal Ratio: > +T > 15

9 Results and Discussion (Natural Potential) Immersion Time [ day ] Potential was stable with basic zinc chloride. Potential [ V ] formed basic zinc chloride at early stage T plating Characteristic in between & plating Potential increased with -1.1 formation of basic zinc chloride. Fig. 1. Time-Dependent Change in Natural Potential of Each Film 16 Results and Discussion (Pattern Diagram) 2H 2 + 2e - 2H - + H 2 Basic Na + Cl - Zinc Chloride H 2 H 2 Na + Cl - Na + H 2 Cl - 2+ Basic Zinc Chloride 2e e - Base Metal (Fe) Base Metal (Fe) Fig. 11. Corrosion Protection Mechanism of /Al Flake Coating Corrosion Reaction Controlled by Basic Zinc Chloride 17

10 Summary Important Factors for Achieving High Corrosion Resistance Control excessive zinc dissolution Formation of basic zinc chloride Film with high corrosion resistance Formation of basic zinc chloride during initial stage of corrosion Finer crystals 18 Thank you for your attention! 19