User blog comment:DancePowderer/Logia Chemistry/@comment-2031609-20100927215053/@comment-1595565-20100929034630

It wasn't because of Haki, Akainu even said that the reason he was able to strike Ace was because his magma was hotter than Ace's fire so hot lost to hotter. If he did use Haki, then it didn't have any additional effect.