The Development And Performance Study Of Mg-Al Composite Sacrificial Anode

Al-based sacrificial anodes and Zn-based sacrificial anodes are usually used toprotect the steel structure in seawater immersed zone. In the initial period, theprotection current that steel structure need is much more than the current needed inthe later period. The potential of traditional Al anode and Zn anode is limited, so it ishard to provide enough currents in the initial period unless oversizing the quantity ofanodes. So the quantity of anodes installed on large steel structure in the beginning isstupendous. As marine exploitation goes deeper and deeper, the demands of sacrificialanode will increase constantly. This means a large amount of energy will be consumedfor the smelting of nonferrous metals, such as zinc, aluminum, etc. Nowadays,resources and energy are in short day-by-day, so this is very unreasonable.By covering a thin layer of Mg anode onto the surface of traditional Zn anode orAl anode, a new kind of sacrificial anode—composite sacrificial anode was developed.In the initial period, the surface layer of Mg anodes will provide enough initialcurrents without oversizing. When Mg anodes were dissolved completely, the currentsthat the steel structure needed were decreased to a very level, then the inner layer ofAl anode or Zn anode begins to dissolve and offers long-term and steady protection tothe steel structure。A new kind of composite sacrificial anode was developed in this thesis. Theouter layer is Mg-Al-Zn-Mn, and the inner layer is Al-Zn-In-Cd. Galvanostatic testand short-circuit test were performed for the outer and inner anode. Ac-impedancemethod and Polarization-curve method were used to study the performances of theinterface between two anodes. SEM was also used to study the characteristic of theinterface. Conclusions are as follows:Compared with Al anode, the Mg anode has much more negative potential. Withthe same surface ratio between anode and cathode, Mg anode can provide muchgreater protection currents than that Al anode can provide due to its high drivepotential. But the cathodic potential of protected structure is usually too negativeunder the protection of Mg anode. The formation of hydrogen was induced in such anegative potential;which made the calcareous deposits fall off from the cathode easily,so the surface ratios between Mg anode and cathode should not be too big in practicaldesigns.The combination position between Al anode and Mg anode bring on thecomposition change because of the casting of the liquid magnesium onto the surfaceof aluminum. The infiltration of magnesium indicates the higher activity of theinterface than the aluminum inside, so the discharging performance of the compositesacrificial anode will not be influenced.The outer layer of Mg anode and inner layer of Al anode were combined tightlyunder the observation of 2000 multiple by SEM. The interface seams no crackle orslot.Theoretical calculation indicated that about 30% traditional Al anode can besaved if this kind of composite sacrificial anode were used in practical project, so thisnew kind of composite sacrificial anode has great foreground in marine engineering.