The OU has a long article explaining the possible issues with aluminum foil and aluminum pans. Due to the concerns raised in the article there are those who manufacture aluminum in a way to avoid a possible issue.
My understanding is that on Pesach when people are more careful than usual, even those who rely on the leniencies during the year may be extra careful and use only a product that has a strict supervision in place.
During the manufacture of aluminum foil, molten aluminum alloys
undergo a series of rolling processes between top and bottom rollers.
During this process, release agents or lubricants are applied to
production lines that the foil comes into direct contact with.
However, initially the potential kashrus concerns are somewhat abated,
since during production the foil undergoes a process known as
annealing, which exposes the foil to a heat exceeding 1,000 degrees
Fahrenheit. This process would certainly burn any non-kosher residue
the aluminum foil might have come into contact with, and also qualify
as a kashering through the process of libun chamur. However, toward
the end of the process the temperature does drop somewhat
considerably. Although any foreign residue present on the foil’s
surface would still certainly be burnt out, the process would no
longer achieve kashering temperatures of libun chamur, and ta’am
(taste) from lubricant at that stage would be absorbed by the foil.
However, since the presence of release agents is always very minimal,
any ta’am that the foil could possibly impart would always meet bitul
proportions and become nullified in food. In halacha, this is known as
a keili hassui lehishtamesh beshefah, which is a utensil that absorbed
non-kosher taste in such minimal amounts, that the ta’am imparted by
the utensil will always become botel in the food cooked. The Mechaber
is lenient and allows one to use a keili hassui lehishtamesh beshefah
without hesitation. However, the Taz disagrees and only permits the
use of a keili hassui lehishtamesh beshefah if the issur and ta’am
imparted is unpalatable. There is a debate amongst authorities if a
utensil is permissible after a 24 hour period elapses, since once an
eino ben yomo, the bliyos (taste absorbed by the utensil) would be no
longer be palatable . This leniency would certainly apply to aluminum
foil, which is never available for retail sale on the market until
well after a 24 hour period has passed. Moreover, Rav Moshe Feinstein
zt’l is quoted as being lenient with aluminum foil, with slightly
different reasoning. According to Rav Moshe, since this particular
type of kli (utensil) is never available for purchase until after a 24
hour period has elapsed, the gezeira mederabbanan should not apply
Aluminum pans are manufactured in a similar fashion to foil and the
potential kashrus concerns are similar. However, there is one
additional consideration with pans. During manufacturing, thick foil
is stamped and formed into a pan shape and a very thin non-stick
coating is applied. This thin coating is a possible point of concern
as well. Therefore, some recommend washing the pans before use ,
although it is questionable whether this will effectively remove the
coating from the pan and obviate this concern. Nevertheless, since the
amount of any possible treifos present would be highly minimal, there
is still basis to be lenient.
Although there is a possibility on some level that these questionable
materials could contain non-kosher components, research appears to
indicate that this issue is more likely to be just theoretical.
Another very important piece of the puzzle is that very often a
release agent or aid, even when containing a non-kosher component, is
independently foul tasting and not fit for consumption. Although these
materials will even come into direct contact with food, they are
present in such minute amounts that they will not alter a product’s
quality profile or taste. If the agent is foul tasting and
independently inedible it should be permitted on the basis of being
nifsal meachilah. Nevertheless, there is an opinion that if possible,
these items should lechatchila be purchased with a proper hechsher.