Development of Environmentally Friendly Silica-Based Conversion Coatings for Zn-Ni Alloys

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1 Development of Environmentally Friendly Silica-Based Conversion Coatings for Zn-Ni Alloys James F. O Keefe Vanderbilt University Advisor: Dr. Branko N. Popov, Dr. Prabhu Ganesan July 26 th, 2007

2 Corrosion Corrosion in Metals Electrochemical reaction between a metal and its surroundings. Formation of rust is due to oxidation of iron. Electroplating Often used to introduce a sacrificial anode, granting the underlying metal substrate cathodic protection from oxidation.

3 Cadmium Coatings for Corrosion Protection of Steel Good barrier protection but is deposited from toxic cyanide baths. Zinc Good sacrificial protection but coating dissolves fast due to large potential difference between Zn and Fe. Zn-Ni Alloying reduces potential difference but alloy has high Zn content and thus still dissolves quickly V V V V V 0.00 V Zn Cd Fe Ni Sn SCE Conversion coatings are used to prolong the life of the Zn or Zn-Ni coatings.

4 Conversion Coatings Method Advantages Disadvantages Phosphating No single conversion coating compared combines with Cr passivation. acceptable corrosion resistance with desired Hexavalent Robust corrosion Deposited from environmental performance benignity. with highly toxic and Chrome Trivalent Chrome Environmentally friendly self-healing properties. High heat tolerance. Unsatisfactory performance when carcinogenic baths. Use is widely restricted by law. Unsatisfactory performance with no self-healing properties.

5 Objective To develop and characterize the corrosion resistance of an environmentally friendly Sibased conversion coating on Zn-Ni alloys. To determine the viability of silica conversion coatings as a replacement for environmentally harmful or otherwise inefficient commercial conversion coatings.

6 Experimental Sample Preparation Zn-Ni coating was electrodeposited from an alkaline bath. Silica coating was deposited through surface modification and catalyzed precipitation. Plating Bath Composition ZnSO 4 : 125 g/l NiSO 4 : 84 g/l (NH 4 ) 2 SO4: 30 g/l Sodium Citrate: 60 g/l Ammonium Hydroxide - ph : Material Characterization SEM (morphology) EDAX, XRF (composition) Electrochemical Characterization Linear & Tafel Polarization OCP vs. Time, EIS

7 Sample Preparation Schematic Zn-Ni coating deposited on steel from an electrolytic alkaline bath. Heat treatment at 120 C for 2 hours to compact coating. Sample immersed in surface modifier solution at RT for 3 minutes. Surface modified Zn-Ni Sample immersed in 1:3 H 2 0/Na 2 SiO 3 solution at 75 C for 15 minutes.

8 Chemical Reactions Reactions in SM Solution Ni 2+ + Zn Zn 2+ + Ni Zn + H 2 O ZnO H + + 2e - Reactions in Silicate Solution SiO 2 + 2H 2 O Si(OH) 4 Zn OH Zn O OH O + Si(OH) 4 = O Si Zn OH Zn O OH Si OH + Si OH Si O Si + H 2 O

9 Mechanism of Silica Deposition 10 µm Silica Zn-Ni Steel Zn-Ni Zn-Ni/Ni strike Zn-Ni/Ni strike/sio 2

10 Silicate Coating Cross Section Silicon Dioxide Layer ( 500 nm) Metal Silicate ( nm) Zn-Ni Substrate Magnification: 90,000 X

11 Surface Morphology and EDAX Zn-Ni/SM/SiO 2 Zn-Ni Zn-Ni/SM Zn-Ni/SM/SiO 2 Element Element Composition Element Composition Composition (wt. %) (wt. %) (wt. %) Zn Zn Zn Ni Ni Ni O Si 19.2

12 Mechanical Properties Adhesion Test Bend Test for Adhesion Good Performance No flakes/peeling of the coating when bent 180 (ASTM B ) Zn-Ni/SM/SiO 2 Zn-Ni

13 Mechanical Properties Vicker s Hardness Test Material HV 100gf Steel (0.8 mm) Commercial Cd VHN = 2P d 2 α sin 2 Commercial Zn-Ni/Cr USC Zn-Ni/NiSt/SiO * Coating thickness 12 µm

14 Electrochemical Characterization Studies Potentiodynamic Polarization Sample E corr (V) I corr (A/cm 2 ) Corrosion Rate (mpy) C u r r e n t ( A / c m2) Zn-Ni e Zn-Ni-SiO e Zn-Ni Zn-Ni-SiO Potential (V) vs. SCE

15 Electrochemical Characterization Studies Open circuit potential (OCP) vs. Time OCP (V) Zn-Ni Zn-Ni/SM/SiO Time (Days)

16 Electrochemical Characterization Studies Nyquist Plot & Equivalent Circuit 2.00E+03 Z" 1.80E E E E E E E E+02 R p = Polarization resistance E corr = Difference in corrosion potential of steel+coating and steel β a = Anodic Tafel slope of steel 2.00E E E E E E E E E E+03 Z'

17 Electrochemical Characterization Studies Polarization Resistance and Porosity vs. Time R p ( o h m s /c m 2 ) R p P o r o s i t y (%) Porosity Time (Days)

18 ASTM B117 Salt Chamber Test Day 7 O Si Ni Zn 15.35% 14.96% 13.27% 56.42% O Si Ni Zn 4.110% 0.660% 12.36% 82.87% Si coating (~1 micron) Zn-Ni coating (~12 micron) Steel substrate

19 ASTM B117 Salt Chamber Test Day 21 O Si Ni Zn 5.04% 9.24% 17.37% 68.34% O Si Ni Zn 1.93% 0.36% 14.31% 83.39% Si coating Zn-Ni coating Steel substrate

20 ASTM B117 Salt Chamber Test Day 35 O 4.88% Si 6.13% Ni 17.40% Zn 71.59% Si coating Zn-Ni coating O 2.14% Si 0.801% Ni 16.44% Zn 80.62% Steel substrate

21 ASTM B117 Salt Chamber Test Day 48 O 4.60% Si 3.06% Ni 17.61% Zn 74.73% Si coating Zn-Ni coating O 3.14% Si 0.62% Ni 17.19% Zn 79.05% Steel substrate

22 Comparison with Commercial Conversion Coatings Day 7 Day 21 Coating Zn-Ni/Cr3+ Zn-Ni/SM/SiO2 Day 35 Day 48 Time of Time of White Rust Red Rust Formation Formation Phosphate 24 h 72 h Trivalent Cr Hexavalent Cr 24 h 48 h 144 h 312 h 144 h None after 1152 h SiO2

23 Conclusions Eco-friendly Si-based conversion coating performs better than commercially available trivalent or hexavalent passivation coatings. Enhanced performance comes from metalsilicate layer which improves barrier property of Zn-Ni coatings. Quick, cost-efficient, and eco-friendly deposition process- viable alternative for commercial conversion coatings.

24 Acknowledgements Dr. Prabhu Ganesan Dr. Branko N. Popov NSF-REU ONR Award No. N Ashley Hetrick