Department of Chemical Engineering University of South Carolina SC Novel Process for Deposition of Non-Anomalous Amorphous Ni-Zn-P Alloys for Replacement of Cadmium Coatings Basker Veeraraghavan, Swaminatha Prabu, Bala Haran and Branko Popov Department of Chemical Engineering, University of South Carolina Objectives: To develop corrosion and permeation resistant coatings as a replacement for cadmium plating. Approach: (1) Obtain non-anomalous Ni-Zn-P coatings by Electroless deposition. (2) Increase the amount of Zn in the deposit so that the coatings will exhibit sacrificial properties (3) Obtain deposits with high Ni content to induce barrier properties thereby extending the life of the coatings. (4) Modify the hydrogen evolution, recombination and absorption kinetics at the surface by alloy coatings. Accomplishments: (1) Developed a novel technique that can eliminate the anomalous nature of Zn-Ni electrodeposition process. (3) Developed a mathematical model to evaluate the permeation characteristics under corroding conditions. (4) Ni-Zn-P coatings have been developed that are environmentally benign and can be regarded as replacements for cadmium plating. Permeation Cell Impact & Transition: (1) The ability to control Zn-Ni ratio offers galvanic compatibility with many metals that need protection. (2) Improved corrosion and permeation characteristics would lead to replacement of cadmium coatings.

Department of Chemical Engineering University of South Carolina SC Cadmium Plating Advantages Disadvantages Good barrier against corrosion. Sacrificial coating. Galvanically compatible with steel & aluminum alloys. Toxicity. Electrodeposition from cyanide baths. Hydrogen embrittlement of steel due to hydrogen permeation and due to plating process.

Department of Chemical Engineering University of South Carolina SC Objectives  To deposit a non-anomalous, amorphous Ni-Zn-P (74-wt% Ni) alloy that  Has low dissolution rate in corrosive media  To increase the Zn content in the Electroless Ni- Zn-P alloy so that the alloy  Provides Sacrificial Protection to iron  Compare the corrosion characteristics with Zn, Zn-Ni and Cd

Department of Chemical Engineering University of South Carolina SC Experimental  Electroless deposition was carried out in a bath containing NiSO 4 and NaH 2 PO 2.  ZnSO 4 in the bath was varied to obtain deposits with different Zn content.  pH was varied by adding NaOH in the bath.  Electrochemical characterization studies were carried out in a EG&G 273/273A potentiostat.  A solution of 0.5 M Na 2 SO M H 3 BO 3 at pH 7.0 was used for the characterization studies.

Department of Chemical Engineering University of South Carolina SC EDAX Analysis on Ni-P and Ni-Zn-P Coatings Zn L  P K  Ni K  Ni K  Zn K  Ni L  P K  Ni K  Ni K  a) Ni-P coating b) Ni-Zn-P coating

Involved Reactions Anodic Reaction-Oxidation of Hypophosphite Ni Deposition Possible Mechanism of Zn Inclusion Cathodic Reactions Zn Deposition? H 2 Evolution E 0 =-0.75 V vs. SCE E 0 =-0.53 V vs. SCE E 0 =-0.28 V vs. SCE E 0 =-1.05 V vs. SCE  Underpotential deposition * * B. N. Popov, G. Zheng and R. E. White, Corrosion Science, 36, 2139 (1994)  Ni-rich phase Zn-Ni alloy deposition ** ** F. J. Fabri Miranda, O. E. Barcia, O. R. Mattas, R. Wiart, J. Electrochem. Soc., 144, 3449 (1997) Department of Chemical Engineering University of South Carolina SC 29208

Department of Chemical Engineering University of South Carolina SC Cross-sectional SEM Analysis of Ni-P and Ni-Zn-P Coatings 10.2  m Ni-Zn-P 15.6  m Ni-P Magnification-3000 X Magnification-5000 X

Department of Chemical Engineering University of South Carolina SC Effect of Zn 2+ Ions on Thickness of Coatings

Department of Chemical Engineering University of South Carolina SC Effect of Zn 2+ Ions on Deposit Composition

Department of Chemical Engineering University of South Carolina SC Corrosion Potential As a Function of Zn Content in the Coatings

Department of Chemical Engineering University of South Carolina SC Tafel Plots As a Function of Zn Content in the Coatings

Department of Chemical Engineering University of South Carolina SC Corrosion Characteristics of the Coatings As a Function of Zn Content Zn content of the deposit (wt%) E corr (V vs. SCE) I corr (A/cm 2 ) x x x x x 10 -5

Department of Chemical Engineering University of South Carolina SC Material Balance on Zn Unknowns [Zn 2+ ] [Zn(OH) + ] [Zn 2 (OH) 3+ ] [Zn(OH) 2 ] [Zn (NH 3 ) 4 2+ ] [Ni 2+ ] [Ni(OH) + ] [Ni(OH) 2 ] [Ni (NH 3 ) 6 2+ ] [OH) - ] [SO 4 2- ] [HSO 4 - ] Construction of pH-Concentration Diagram Material Balance on Ni Electroneutrality Condition

Department of Chemical Engineering University of South Carolina SC Effect of pH on the Equilibrium Concentrations

Department of Chemical Engineering University of South Carolina SC Effect of pH on the Morphology of Coatings pH 8.5pH  m4.2  m Magnification 12000XMagnification 6000X

Department of Chemical Engineering University of South Carolina SC pH 10.5pH 11.5 Magnification 12000XMagnification 6000X 2.1  m4.2  m Effect of pH on the Morphology of Coatings

Department of Chemical Engineering University of South Carolina SC Effect of pH on Deposition at 85 o C

Department of Chemical Engineering University of South Carolina SC SEM Analysis of Samples After Immersion in pH 7 Na 2 SO 4 + H 3 BO 3 Solution pH 10.5pH  m Magnification 2000X 12  m

Department of Chemical Engineering University of South Carolina SC Effect of Temperature on Deposition at pH 10.5

Department of Chemical Engineering University of South Carolina SC Comparison of Resistances for Various Sacrificial Coatings

Department of Chemical Engineering University of South Carolina SC Corrosion Rate of Various Sacrificial Coatings Zn Zn-Ni Cd Ni-Zn-P

Department of Chemical Engineering University of South Carolina SC Stability of Various Sacrificial Coatings

Department of Chemical Engineering University of South Carolina SC Conclusions  Developed a novel technique that can control the Zn-Ni ratio and thereby the barrier properties of the deposits.  Increase in ZnSO 4 concentration in the deposition bath resulted in developing Ni-Zn-P coatings with high Ni content (74-wt%).  Increased nickel content decreased the dissolution rate of the sacrificial metal coating in corrosive media, while the presence of 16-wt% Zn ensures sacrificial protection to the substrate.  Ni-Zn-P coatings have been developed that are environmentally benign and can be regarded as replacements to Cd plating.

Department of Chemical Engineering University of South Carolina SC Acknowledgements Financial Support by Vinod Agarwala and A. John Sedricks, Office of Naval Research is gratefully acknowledged.