Most of the moisture that ends up in the service panel is from rain water. A poor seal at the top of the electric meter and/or a frayed main service cable are the majority of the culprits for the moisture problems. Water can travel down the interior of the coated electrical cable and then enters the electrical panel box. If the meter is not properly sealed where the main service wire enters it the service cable will act like a sealed pipe delivering moisture into the electrical panel box. Often times wind driven rain can enter a frayed main service wire and travel down to the electrical panel box. A bottom service entrance cable will also contribute or can be the cause of moisture problems as well.
During the home inspection we will look for this type of water infiltration into the service panel. Often the terminals and the breakers will become rusted and corroded. May times the bottom of the service panel will be rusted. On more severe cases water is sometimes found in the bottom of the panel box. I had one instance where water could be seen dripping from the bottom of the box before the cover was removed. This cover never was removed because of the unsafe condition.
Other less common caused of panel box corrosion are from seepage through the basement walls where the panel is attached to it. Also interior condensation caused by high moisture in the home can contribute to panel box corrosion. These high levels of humidity can be cause by ground water entering the basement or through other leakage in the home.
A licensed electrical contractor should be called in to evaluate and correct the condition. Often the electrical panel box is too far gone and will need to be replaced. Sometimes the box can be saved, in any event the cause of the moisture must be determined in order to avoid causing more damage. Electric panel boxes and water do not mix well.