Formation & Properties of Zn–Ca– Mn Tri – Cation’s Phosphate Coating on St37 Steels

Rasouli, Hassanali | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41422 (07)
  4. University: Sharif University of Technology
  5. Department: Material Science and Engineering
  6. Advisor(s): Afshar, Abdollah; Ghorbani, Mohammad
  7. Abstract:
  8. Addition of heavy metal cations such as manganese and nickel in low zinc phosphating bath increases the corrosion resistance of coatings and provides more suitable substrates for painting (especially electrophoretic painting). In addition, due to accelerating property and low cost availability of heavy metal cations such as Mn, multi– cationic phosphating is cost effective. In the present study, chemical composition and operating parameters of zinc - calcium - manganese Tri- cationic phosphating solution were optimized. Then the effects of various parameters such as phosphating time, bath temperature and pH on properties of coating were investigated. According to the results, about 1 g/L was obtained as optimum concentration of manganese ions in the bath. When manganese ions were added to the solution, the concentration of calcium and zinc ions decreased simultaneously. Tri- cationic coating weight was almost 4.3 g/m2 while the similar amount of Bi- cationic coating was 5.1 g/m2. Also the time to reach maximum thickness in the optimized Tri- cationic bath was less than 6 minutes which it was less than similar value in Bi- cationic coating. Optimum temperature for Tri- cationic phosphating was in the range of 45- 50 °C (5 -10 °C less than Bi- cationic coating). Optimum pH for Tri- cationic coating in 50 °C was between 2.8 to 3. Scanning electron microscopy images showed that optimized Tri- cationic phosphate coating contain cubic and sheet crystalline structure. These structures have better paint adhesion than spherical crystals. Also, in comparison with Bi- cationic coating, atomic force microscopy indicated 70 nm (about 23%) reductions in Tri- cationic coating roughness. Electrochemical parameters obtained from electrochemical impedance spectroscopy and potentiodynamic polarization tests indicated a slight increase in corrosion resistance of Tri- cationic coating compared with Bi- cationic. Such that corrosion current decreased from 3 to 1.9 ?A/cm2, polarization resistance increased from 5770 to 6330 ?.cm2 and double layer capacitance changed from 312 to 110 ?F/cm2. Finally, the amount of Tri- cationic coating porosity was less than 1% despite its low coatings thickness
  9. Keywords:
  10. Phosphate Coating ; Manganese ; Calcium ; ZINC ; Corrosion Resistance ; Tri-Cation Coating

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