when he took off his mask, he was actually trying to introduce himself to sasuke by showing his face. he even said that, as for izanagi, we are not sure yet, it wasn't confirmed by the manga, so it's only speculation as of now.
You are right, I just reread it and he was already starting to take off his mask. However, I doubt that it was kamui considering that in this panel he has on only his 3-tomoe, not his MS. And we know that kamui is an MS tech, whereas Izanagi is not.
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Then he was engulfed in flames and screaming. So we know that some damage was done.
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And then he comes out with no injuries.
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Last time I checked, kamui doesn't have the ability to heal you. :/
Also, I know that Obito was showing half of his face to Sasuke. He wanted to show Sasuke that he had a sharingan in order to prove that he was "Madara". I just mentioned it because the panel where he is taking off his mask is drawn alot like the one where he was taking off his mask in his fight against Konan.