What is an Half Cell Corrosion test?

What do these numbers mean?  If the rebar in the concrete is intact and un-corroded, then electrical voltage traveling through it will encounter with less resistance.  The more corroded the material the higher the resistance, and higher voltage is necessary to carry the current.  Hence larger (negative) numbers are worse, and lower (negative) numbers are better leading to the color code system above with green being good to excellent, yellow being poor to marginal, and red meaning very poor.