Enhanced Surface Properties Of AZ91Alloy By Eco-friendly Conversion Coatings And Electroless Nickel Deposit

The high stiffness to weight ratio makes magnesium-aluminium-zinc alloy AZ91(9%Al and1%Zn) very desirable for various applications. Nevertheless, the relatively poorcorrosion resistance of this alloy limits its extensive applications. Therefore, it is necessaryto do efficient surface treatment properly onto the surface of the AZ91magnesium alloy toimprove the surface properties and corrosion resistance. This research has motivated toimprove the surface properties of a die-cast AZ91magnesium alloy.(1) Enhancing the corrosion resistance by chrome-free treatments for electroless nickeldeposits on AZ91magnesium alloy: The Phosphate-manganese-molybdate conversion(PMMC), Vanadium-based conversion (VBC), and Tannic-based conversion (TBC)chromium-free coatings were developed successfully onto magnesium substrate. Thecorrosion properties of these chromium-free pre-treatments with electroless nickel (EN)deposits were investigated by electrochemical measurements. The main conclusions of thisresearch investigation are: The shift of zero current potential towards positive direction withascending order (VBC/EN <PMMC/EN <TBC/EN) has been related to modification incathodic reaction processes. The breakdown potential for all electroless Ni-P was more orless similar in nature. Both VBC/EN and PMMC/EN exhibited good Tafel linear region andtheir corrosion current densities (Icorr) were calculated to be103.87μA/cm²(1.135mm/y)and88.07μA/cm²(0.9626mm/y), respectively. Furthermore, Linear polarization resistance(Rp) value gets double for PMMC/EN (5800cm²) than VBC/EN (2929cm²) sample.However, TBC/EN curve not exhibited the expected linear Tafel behaviour in cathodiccurvature region to measure the corrosion rate. From the Tafel plots, the corrosion resistancewas good for TBC/EN compare to other pre-treated EN deposits. The atomic forcemicroscope (AFM) results show that the atomic level smoothness was found in the TBC/ENsamples. Metal cations are more readily dissolved from surface roughness (Ra) than a flatface. The average Ravalue is about3.053and2.986μm for VBC/EN and PMMC/EN,respectively and TBC/EN appears to be relatively smooth and the Ravalue is1.477μm. Thiswas the evident reason for the increase in corrosion resistance for the TBC/EN than other pre-treated in this research. On the whole, the chromium-free treatments has comparablebetter corrosion resistance, preponderant environment friendship and easy for processingthan chromium treated. Furthermore, the electroless deposition with these conversionscoating is an appropriate way to enhance more eco-friendly coating applications onmagnesium alloy.(2) Enhancing the surface properties of electroless nickel deposits by surfactants onAZ91magnesium alloy: Electroless nickel coatings on magnesium alloy was successfullycarried out with the addition of surfactants CTAB (cetyl-trimethyl ammonium bromide) andSLS (sodium lauryl sulphate). The main conclusions of this research investigation are: Theaddition of small concentration of surfactants to the coating baths could increase thedeposition rates by26%compared to that from the surfactant-free bath. X-ray diffractionresult shows addition of CTAB and SLS promotes formation of amorphous plusnano-crystalline phases in the deposits than as-deposited Ni-P. Surface morphology andsurface topography reveals a better smooth surface. The smoothness increased by52%compared to that from the surfactant-free bath, which resulting in good surface finish on theelectroless Ni-P deposited magnesium alloy. The average surface roughness value Rais1.412μm for CTAB and1.789μm for SLS with the optimal quantity of surfactant, which aremuch smaller than the value of2.98μm measured from the coating surface topographywithout surfactants. In addition, the phosphorus content in the coating improved significantlywith addition of CTAB from5.9wt%to9wt%resulted in better corrosion resistance andwith addition of SLS the improvement was only to6.9wt%. Microhardness was improvedfrom400HV₃₀₀to565HV₃₀₀for CTAB and to490HV₃₀₀for SLS addition which is goodfor better wear resistance. Both electrochemical polarization and immersion test on thedeposits with surfactants evidently proved that the corrosion resistance was enhanced. Onthe overall, the study concluded that the surfactants enhanced surface properties of theelectroless Ni-P deposit on magnesium alloy and furthermore, improved the corrosionresistance greater.