I'm listening through the Fellowship audiobook again and the corruption of Boromir is a lot more dramatic and sudden than I remember. He had always made the military and political argument to use the Ring as a weapon of Gondor, but it's not until after Galadriel testing his heart that he really starts to act weird.

Galadriel's Test:

But this I will say to you: your Quest stands upon the edge of a knife. Stray but a little and it will fail, to the ruin of all. Yet hope remains while all the Company is true.’ And with that word she held them with her eyes, and in silence looked searchingly at each of them in turn. None save Legolas and Aragorn could long endure her glance. Sam quickly blushed and hung his head.

Boromir's Temptation:

All of them, it seemed, had fared alike: each had felt that he was offered a choice between a shadow full of fear that lay ahead, and something that he greatly desired: clear before his mind it lay, and to get it he had only to turn aside from the road and leave the Quest and the war against Sauron to others. ‘And it seemed to me, too,’ said Gimli, ‘that my choice would remain secret and known only to myself.’ ‘To me it seemed exceedingly strange,’ said Boromir. ‘Maybe it was only a test, and she thought to read our thoughts for her own good purpose; but almost I should have said that she was tempting us, and offering what she pretended to have the power to give. It need not be said that I refused to listen. The Men of Minas Tirith are true to their word.’ But what he thought that the Lady had offered him Boromir did not tell.

Boromir's First Weirdness:

‘I shall go to Minas Tirith, alone if need be, for it is my duty,’ said Boromir; and after that he was silent for a while, sitting with his eyes fixed on Frodo, as if he was trying to read the Halfling’s thoughts. At length he spoke again, softly, as if he was debating with himself. 'If you wish only to destroy the Ring,’ he said, ‘then there is little use in war and weapons; and the Men of Minas Tirith cannot help. But if you wish to destroy the armed might of the Dark Lord, then it is folly to go without force into his domain; and folly to throw away.’ He paused suddenly, as if he had become aware that he was speaking his thoughts aloud. ‘It would be folly to throw lives away, I mean,’ he ended. ‘It is a choice between defending a strong place and walking openly into the arms of death. At least, that is how I see it.’ Frodo caught something new and strange in Boromir’s glance, and he looked hard at him. Plainly Boromir’s thought was different from his final words. It would be folly to throw away: what? The Ring of Power? He had said something like this at the Council, but then he had accepted the correction of Elrond.

Boromir's Second Weirdness:

Merry and Pippin in the middle boat were ill at ease, for Boromir sat muttering to himself, sometimes biting his nails, as if some restlessness or doubt consumed him, sometimes seizing a paddle and driving the boat close behind Aragorn’s. Then Pippin, who sat in the bow looking back, caught a queer gleam in his eye, as he peered forward gazing at Frodo.

And then of course Boromir's betrayal.

So, do we think that Galadriel's test contributed to that descent? An alternate reading is that they were nearing the decision point between Minas Tirith and Mount Doom and Boromir was feeling that he was running out of time.