Adam Clarke on Acts 18:11

He continued there a year and six months - He was now confident that he was under the especial protection of God, and therefore continued teaching the word, τον λογον, the doctrine of God. It is very likely, that it was during his stay here that he wrote his first epistle to the Thessalonians, and the second not long after; and some think that the epistle to the Galatians was written during his stay at Corinth.