Corrosion Inhibitors for Waterborne Alkyd Technology 28 th Biennial Western Coatings Societies Symposium & Show Las Vegas, Nevada October 2007
Outline: Corrosion Inhibitors Background & Mechanisms Applications & Formulating Strategies Background & Mechanisms
Corrosion Inhibitors
What is Corrosion: The electrochemical deterioration of a metal due to the reaction with its environment involving the oxidation of a metal and the reduction of another material.
Corrosion Simplified: ELECTRON CONDUCTOR IONIC CONDUCTOR ELECTRON ACCEPTOR + - + - + - + - + - + - + - + - + - + - + - Ionic Conductance Electron Conductance
The Result: FACTORS INFLUENCING: Presence of Water Presence of Oxygen Presence of Ions Temperature e ph Grain Structure of Metal Stresses from Fabrication
Corrosion Cell Diagram: Electron Conductor: Metal Ionic Conductor: Water Electron Acceptor: Oxygen 3/2 O 2 n H 20 2HO 2 2 Fe 2+ Fe 2 O 3 H 2 0 4H + O 2 2 Fe 4 e Anodic Site Cathodic Site
How Can We Use Coatings to Protect Metal from Corrosion?:
PASSIVE Protect through the barrier properties of resin or pigment having low H 2 O transmission. (i.e. epoxies, chlorinated rubbers) ACTIVE Inorganic Inhibitive Pigments Organic Corrosion Inhibitors Ion Exchange Vapor Phase SACRIFICIAL Zinc-rich primers protect through the preferential oxidation of zinc metal
Passivation Mechanism (Active-Direct Inorganic Inhibitor) Specific Example: Zinc phosphate - + - + - + + - - + + - + Zn 3 (PO 4 ) 2 - Hydrolysis ys s + - Insoluble FePO 4 Fe 2+ H 2 O Precipitate M x (OH) x Substrate Fe Anode e - Cathode
Applications & Formulating Strategies
Water Based Alkyds
Uses: Lower cost corrosion protection Light duty industrial applications Moderate performance on machinery, tanks, etc. Lower VOC compared to S/B Fast Drying Good adhesion to steel, plastic, & aluminum
Types: Water Reducible Alkyds (WRA) COO - COO - COO - COO- COO COO - - COO - COO - COO- COO - COO - COO - COO - Water Dispersible ibl Alkyds (WDA) ACRYLIC POLYMER SHELL ALKYD CORE
Properties: Feature Water Reducible Alkyd (WRA) Water Dispersible Alkyd (WDA) Ability to achieve low VOC + ++ High gloss + ++ Hydrolysis resistance + ++ Corrosion resistance ++ + Shear stability - ++ Gloss retention ti ++ + Dry time (wet edge/open time) - - Alkaline resistance - + Moisture resistance - - Gelling with basic pigments - + Drier kick-out resistance ++ - Yellowing - -
Hydrolysis of Alkyds: Reversible Reaction in the Presence of Water System ph is critical Amine selection is critical
Typical Formulation (Water Reducible Epoxy Ester) INGREDIENTS % WEIGHT FUNCTION Resydrol AX237w/70BG 33.16 Resin Ammonium hydroxide 0.62 Amine Triethylamine 1.86 Amine Additol VXW-6206 0.38 Drier Troymax Antiskin B 0.18 Antiskin Patcote 577 0.09 Defoamer De-Ionized Water 9.22 RO-4097 Kroma Red 8.32 Pigment Anticorrosive Pigment* 6.02 Corrosion Inhibitor Micro Talc AT Extra 5.23 Filler Millicarb 6.52 Filler Bartex 65 6.62 Filler Aerosil 200 037 0.37 Rheology modifier De-Ionized Water 21.41 TOTAL 100.0 Fast (25%) Amine Package Slow (75%)
Formulating with WRA s: Recommended system ph of 8.00 8.50 Use 100% triethylamine (TEA) for excellent stability & good salt spray resistance. Use a blend of TEA and ammonium hydroxide for good stability & excellent salt spray resistance.
Changes to the Amine Package: Water Epoxy Ester - Cold Rolled Steel 2.0 mils - 336 hrs SS Best Stability Best Performance 100% Triethylamine 75:25 TEA and Ammonium Hydroxide
Stability vs. Amine Package: 4 Heat Aged Stability Weeks @ 50o C 3 2 1 0 100 75/25 50/50 25/75 TEA to Ammonium Hydroxide Ratio
Base Strengths via pka & ph: Amine pka @ Amine ph 20 C TEA 10.78 10.6 (0.005 N aq. Solution) AMP 9.82 10.3 (2-amino-2-methyl-1-propanol) Ammonia 9.24 10.0
Formulating with WRA s: Effect of Corrosion Inhibitor Zinc phosphate with low levels of zinc oxide showed better corrosion resistance (Salt Spray Testing). Better solvent resistance with Zn than Ca (MEK Double Rubs). Higher electrochemical impedance with Zn than Ca (EIS Testing). Synergy witnessed with organic inhibitor.
Formulating with WRA s: Role of zinc oxide Improves the cross link density of film It is an active cathodic inhibitor which works synergistically with zinc phosphate. Can react with mono- & di- carboxylic acid binder breakdown products
Corrosion Performance: ZnO 0% 0% 2% 17% MEK 6 8 7 12
Impedance Spectra (Bode Plots): Strontium zinc phosphosilicate p Calcium phosphosilicate Zinc phosphate Blank
Inorganic-Organic Corrosion Inhibitor Synergy: Water Reducible Alkyd - Cold Rolled Steel 1.0 mil - 500 hrs SS Blank 6% Mod ZnPO4 6% Mod ZnPO4 2% Organic CI
Summary: Balancing the amine package is key to formulating hydrolytically stable WR alkyd primers. Modified (i.e., containing ZnO) zinc phosphate pigments provide excellent e corrosion os o resistance sta compared to non-zinc or traditional zinc phosphate corrosion inhibitors. Salt spray and electrochemical impedance corrosion testing correlated well!
Acknowledgements Hexion Specialty Chemicals Melissa DeGroot, R&D Chemist Guy Lopez Senior Technical Specialist Natalie Wilson Technical Service Representative High Solids, Waterborne and Latex: Three Approaches to Compliance
Contact Information: Andrew Thorn HALOX Technical Service Manager Phone: (219) 933-1560 X 243 Fax: (219) 933-1575 Email: Athorn@halox.com