Objection 1. It would seem that the angels are not incorruptible; for Damascene, speaking of the angel, says (De Fide Orth. ii, 3) that he is "an intellectual substance, partaking of immortality by favor, and not by nature."
Objection 2. Further, Plato says in the Timaeus: "O gods of gods, whose maker and father am I: You are indeed my works, dissoluble by nature, yet indissoluble because I so will it." But gods such as these can only be understood to be the angels. Therefore the angels are corruptible by their nature
Objection 3. Further, according to Gregory (Moral. xvi), "all things would tend towards nothing, unless the hand of the Almighty preserved them." But what can be brought to nothing is corruptible. Therefore, since the angels were made by God, it would appear that they are corruptible of their own nature.
On the contrary, Dionysius says (Div. Nom. iv) that the intellectual substances "have unfailing life, being free from all corruption, death, matter, and generation."
I answer that, It must necessarily be maintained that the angels are incorruptible of their own nature. The reason for this is, that nothing is corrupted except by its form being separated from the matter. Hence, since an angel is a subsisting form, as is clear from what was said above (Article 2), it is impossible for its substance to be corruptible. For what belongs to anything considered in itself can never be separated from it; but what belongs to a thing, considered in relation to something else, can be separated, when that something else is taken away, in view of which it belonged to it. Roundness can never be taken from the circle, because it belongs to it of itself; but a bronze circle can lose roundness, if the bronze be deprived of its circular shape. Now to be belongs to a form considered in itself; for everything is an actual being according to its form: whereas matter is an actual being by the form. Consequently a subject composed of matter and form ceases to be actually when the form is separated from the matter. But if the form subsists in its own being, as happens in the angels, as was said above (Article 2), it cannot lose its being. Therefore, the angel's immateriality is the cause why it is incorruptible by its own nature.
A token of this incorruptibility can be gathered from its intellectual operation; for since everything acts according as it is actual, the operation of a thing indicates its mode of being. Now the species and nature of the operation is understood from the object. But an intelligible object, being above time, is everlasting. Hence every intellectual substance is incorruptible of its own nature.
Reply to Objection 1. Damascene is dealing with perfect immortality, which includes complete immutability; since "every change is a kind of death," as Augustine says (Contra Maxim. iii). The angels obtain perfect immutability only by favor, as will appear later (62).
Reply to Objection 2. By the expression 'gods' Plato understands the heavenly bodies, which he supposed to be made up of elements, and therefore dissoluble of their own nature; yet they are for ever preserved in existence by the Divine will.
Reply to Objection 3. As was observed above (Question 44, Article 1) there is a kind of necessary thing which has a cause of its necessity. Hence it is not repugnant to a necessary or incorruptible being to depend for its existence on another as its cause. Therefore, when it is said that all things, even the angels, would lapse into nothing, unless preserved by God, it is not to be gathered therefrom that there is any principle of corruption in the angels; but that the nature of the angels is dependent upon God as its cause. For a thing is said to be corruptible not merely because God can reduce it to non-existence, by withdrawing His act of preservation; but also because it has some principle of corruption within itself, or some contrariety, or at least the potentiality of matter.
Source: Summa Theologica (New Advent)