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Anode Backfill

Direct CP is a supplier of impressed current and sacrificial anode chemical backfills.

IMPRESED CURENT BACKFILL
Impressed current anodes buried in soil generally have a low resistance carbonaceous backfill surrounding them. The backfill is usually a modified calcined petroleum coke breeze and presents as a dark grey course powder. Some backfills have surfactants to help them mix better with water and this aids in pumping.  The most common brands of carbonaceous backfill used in Australia are Corrcarb and Loresco, Direct CP is a supplier of both brands. It practical terms both are very similar.

In horizontal anode groundbeds the backfill is poured in the anode trench dry, whereas with vertical anode groundbed boreholes the backfill is mixed to form a slurry and pumped into the space surrounding the anode element. Anode centralisers can be used to keep the anode element central in the borehole, pumping the backfill slurry from the bottom of the borehole helps prevent air pockets.

The carbonaceous backfill becomes part of the anode itself as current is discharged from the outer surface of the backfill effectively enlarging the active surface area of the anode, this reduces the anode resistance to remote earth, thus reducing the voltage gradient around the anode for a given output and extending the life of the anode. Most carbonaceous backfills are consumed at the rate of approximately 1Kg per Amp per Year.

22.7 Kg bag of Loresco.

SACRIFICIAL ANODE BACKFILL
Under some circumstances magnesium and zinc anodes benefit from being buried within a chemical backfill. Sacrificial anodes installed in high resistivity or dry soil, or in sand are sometimes packaged in and/or surrounded in a low resistivity chemical backfill. The backfill typically comprises a mix of gypsum and sodium bentonite and presents as a fine off white powder.

The backfill serves two main purposes; providing a lower resistivity environment immediately around the anode, and helping to retain moisture around the anode. Both of these attributes assist in improving the ability of the anode to discharge protective current.  Australian Standard AS 2239 specifies the components of several sacrificial anode backfill types.

Small sacrificial anode packed in calico bag containing chemical backfill