In general, the status of a sink is similar to that of a countertop, though in one respect it is less strict because it usually contains soap, which befouls the tastes of foods, and in another respect, it is more strict (if made of porcelain), because many poskim say that porcelain has the status of earthenware, which does not release tastes through hagala.
There are two accepted practices for koshering sinks. Those who are lenient clean the sink well and then pour boiling water all over it. Before pouring boiling water on a sink or countertop, it must be dried well, so that the boiling water touches it directly and is not cooled by any cold water on its surface. For this reason, one must first pour the boiling water on the sink and then on the countertop, starting with the areas closest to the sink and moving further away.
Those who are stringent, in addition to pouring boiling water on the sink, put a plastic insert in it or line it with thick aluminum foil in order to separate between the sink, which has absorbed ĥametz, and the Pesaĥ utensils. These people are also careful not to use boiling water in the sink during Pesaĥ.
. Its most intense form of absorption is as a kli rishon removed from the flame, since sometimes one places pots of boiling-hot ĥametz in it. One who wants to kosher it based on its most serious usage must place a scalding stone in boiling water and pour the water onto the sink. However, according to many poskim, a porcelain sink has the status of earthenware and cannot be koshered via hagala. In such a case, one must cover the walls of the sink with a plastic insert or aluminum foil. They are also careful not to fill the sink with boiling water, lest the taste absorbed in the walls of the sink become absorbed in the Pesaĥ utensils. Even if the sink is not ben yomo and tastes absorbed in it are thus foul, according to Rema 447:10 even this is forbidden on Pesaĥ.
Those who are lenient rely on SA 451:6 that we follow the main usage, which in the case of a sink is with cold foods. This method is thus effective even if the sink is made of earthenware. Additionally, according to Knesset Ha-gedola, porcelain is akin to glass, which does not absorb at all, and not earthenware. Moreover, since there is usually residual dish soap in the sink, any ĥametz taste absorbed into the sink would have been foul from the outset, so the sink would not need to be koshered. And even if we assume that the sink absorbed flavorful ĥametz, it would turn foul once twenty-four hours had elapsed. Even though according to Rema foul-tasting ĥametz is forbidden on Pesaĥ, the present case involved absorption that is, at worst, third degree (nat bar nat bar nat), and, as we have seen, MB 447:98 states that one may be lenient about foul tastes where there is no existing custom, and there is no existing custom about nat bar nat bar nat. Sidur Pesaĥ Ke-hilkhato 8:1 and Hagalat Kelim 124-127 write more about koshering sinks.