Tekkai:
The Tekkai (鉄塊 Tekkai?, literally meaning "Iron Mass") 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, Roronoa 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, 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 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. However, for Rob Lucci, he seemed to be able to resist even cannonball flames at 13 years old. In the Viz Manga and FUNimation subs, this is called Iron Body.
The defense mechanic of it is similar to Haki, but it is not haki.
When using Tekkai, you harden your muscles, tense em, making your body hard as steel.
Armament haki, you coat your body with your willpower. o_o No connection.
"Tekkai is similar to Busoshoku Haki, as both reinforce the users body in some way, the difference being that Haki is based on the user's willpower and Tekkai is based on the user's physical power.
Tekkai also appears to be based on the Iron Shirt technique in Chinese martial arts."
Well, you can´t really make that claim yet and tekkai isn´t someone training their bodies to be hard as steel, they have to use some force to make their bodies like steel otherwise they can´t withstand more then a normal human can, that is tekkai and haki is where you make your body harder by making an invisible armor, isn´t that the same thing?
Someone who trains tekkai doesn´t have a body hard as steel 24/7, that´s not how it works, they must activate the effect before they can become steel.
Besides, no one has tried to punch luffy while using tekkai, so far it´s been used for defense only and we haven´t seen anyone punch someone logia or not while using tekkai but it may happen, i say they are connected and wait untill we see coby punch luffy while using tekkai=haki, we know coby has used some other cp9 tricks and he has probably full control over the haki that hear voices and know others presence.
Untill it is confirmed, i won´t make any claim but so far, it´s apparent that they are connected cuz they do the same thing, except for hitting logia but we haven´t seen it do that yet.
They basically do the same thing, but are still very different things.
Tekkai condenses ones muscles, achieved through physical training. CONFIRMED in the manga.
Haki is the usage of ones willpower, doesn't have anything to do with physical power. CONFIRMED in the manga.
While tekkai protects one from physics harm like a punch or a sword cut, one can still get burned(for example if Akainu uses his df to attack one with tekkai) or frozen(aokiji) while using tekkai, while haki protects against such things.(Again seen when Marco attacks Akainu and when Shanks stops akainus fist with his sword, without the sword getting melted).