The copper coating must be maintained after driving the rod into the ground, protecting the electrode against corrosion
IEC 60364 is the international Standard for low-voltage electrical installations. Section 5-54 of this standard deals with earthing, detailing among others the materials and dimensions of the conductors forming it. This standard specifies that the thickness of the copper coat that covers steel rods must be at least 250 microns thick, as required in lightning protection Standards.
The Standard IEC 60364 allows an exception: the thickness may be reduced to no less than 100μm where special precautions to avoid mechanical damage of copper during the installation process are taken. That is, if the copper coat is thinner than 250 microns, then the earth rod must be installed inside a previously dug hole, avoiding the copper coat to damage while driving the rod. Then the hole should be filled with some clay-like powder to assure a good contact between the electrode and the soil. All this process raises the cost of the installation and also increases the possibilities of bad contact with the soil as well as the electrode corrosion.
Recommended references: AT-077H, AT-041H & AT-098H