The corrosion rate of the copper wire is given as 5.8 microAmp/cm. This value is already the corrosion current density. Corrosion current density i_corr is the measure of the rate of corrosion in terms of current per unit area. It is usually expressed in microAmp/cm or mA/cm. In this case, the corrosion current density of the copper wire in the given environment is 5.8 microAmp/cm.