Commercial magnesium anodes were evaluated using ASTM G97-89 as standard test and attached to the ASTM test arrangement a technique of electrochemical impedance was also used . Cylindrical samples were cut from the as-cast Mg anodes. Several treatments and different cooling rates were carried out. The anodic efficiency determined as a function of the cooling rate, showing an increment around 10 to heat was 1270 above the efficiency showed by the non-treated commercial anodes (as-cast condition). The efficiency increments were related to the microstructural characteristics i.e. particle size and/or grain size. Also some morphological aspects were considered .The appearance of second phase particles influences the different forms that the corrosion process exhibits.
The increasing demand for cathodic protection in the past few years for commercial and domestic uses has given rise to widespread interest in new developments and applications of galavanic anodes. Considerable amount of work has been done in the past in order to improve the most important properties of the anodes. Considering mainly the metal purity, the alloying elements , and backfill material(4). These studies concentrate their efforts towards the improvements in current efficiency, polarization characteristics and the distribution of the corrosion attack in the anodes.
Magnesium anodes are particularly recommended for high-resistivity environments where the anodes inherent negative potential and high current output per unit weight is desirable i.e. their capacity to drain the current. Magnesium anodes generally have a current efficiency below that of other galvanic anodes. In practice, current efficiency rarely exceeds 50%. This figure is unfavorable compared with that of other anodes, such as Zn and Al which have current efficiencies better than 90%.. Several factors have been attributed to the low efficiency of the Mg anodes such as the thermal history related to the structure effects and the alloying elements, changes in anion and cation concentrations, which occur close to the dissolving Mg surface and the anodic electrochemistry.
Some significant local consumers of Mg-based sacrificial anodes, and specifically the Mexican national pipeline system are concerned with respect to the casting procedures and the efficiency of the sacrificial anode material which now ranges between 30% and 35%. Cathodic protection is now well established and required by law for pipeline protection. There is, therefore, current interest in improving efficiency of Mg anodes. Salinas et al. pointed out that the metallurgical features of sacrificial anodes (i,e.castings conditions, heat treatments, etc.) are related to the anode operation potential and also to their own efficiency. In the Mg anodes, contrary to the requirements for anode materials, the corrosion occurs by pitting rather by uniform corrosion, shifting the potential to more electronegative values. However a very recent work reported that pitting was found under a local standard that use artificial sea water for the anodes evaluation. Furthermore, the same authors 0) report that when using the ASTM standard for GALVOMAG (trademark of the Dow Chemical Co.) type anodes evaluation, they 201/2 found uniform corrosion, being this difference due to the solution used.
On the other hand, local consumers of Mg anodes pointed out that at present they have several tons of low-efficiency Mg-anodes already cast in stock and they are interested that these anodes could be rehabilitated instead of rejecting them. The low efficiency is related to the hetreogeneity on their chemical composition obtained from several batch of anodes (i.e.higher ratios Fe/Mn esp
