you guys don't pay attention what you watch/read? tekkai isn't haki, nowhere near that... and come on hit a logia with TEKKAI? wtf?
The Tekkai (鉄塊 Tekkai?, literally meaning "Iron Mass" or "Iron Body") hardens the users' muscles to the level of iron, in order to nullify damage taken from attacks. However, it can be broken by strong enough forces such as Luffy's various Gears, Zoro's Shishi Sonson and Tatsumaki, Sanji's Diable Jambe and Franky's Strong Hammer.
When Tekkai is active, the user is unable to move, with the exception of Jabra's Tekkai Kenpo. Tekkai can also be used to enhance the users attacks by the increased density one gets from using Tekkai, similar to Busoshoku Haki.
The strength of Tekkai can vary for users with different physical conditions, as a person with a higher level of muscular strength would be able to take stronger forces and enhance attacks with his/her use of Tekkai. Furthermore, despite the increased physical density that one gains when activating this move, one should remember that the user's body is still flesh and blood. Thus, its resistance to other than solid attacks remain pretty much the same, making the user as vulnerable as ever to certain attacks. An example was during Sanji's fight with Jabra where he used the burning force of friction to incinerate through the latter's Tekkai or when Franky used his actual iron fist to break through Fukuro's Tekkai. As such, even masters of Tekkai are far from indestructible, though Rob Lucci was able to resist even cannonballs at the age of 13.
In the Viz Manga and FUNimation subs, this is called Iron Body.