Sauce from ĥametz food occasionally spills from pots onto the warming tray (“plata”), and since the plata is a heat source, the ĥametz is absorbed with the intensity of a klirishon on the flame. On other occasions, dry ĥametz dishes (such as porridges, kugels, and quiches) fall onto the plata, which constitutes absorption through fire and must be koshered through libun.
A blekh (a metal sheet that is placed atop a gas range on Shabbat) can be koshered through libun if there is no alternative, even though it will warp and become damaged. But when it comes to an electric warming tray, libun is liable to melt the electrical wires and ruin it. Therefore, one must clean it, heat it for an hour, and then cover it with aluminum foil to separate the plata from the Pesaĥ pots.
. Sidur Pesaĥ Ke-hilkhato 8:5 states that one should heat up the plata for an hour and then pour boiling water on it, since it has the status of a kli rishon on the flame. It is also good to cover it with aluminum foil. Hagalat Kelim 13:381 states that one may heat it up or cover it with aluminum foil. I believe that the primary solution is to cover the plata with aluminum foil, since it absorbs like a kli rishon on a flame and possibly even like a direct flame, whereas it is doubtful that the standard heat of a plata would reach the boiling point. R. Mordechai Eliyahu also writes that a plata must be covered with aluminum foil.