In Captain America: The Winter Soldier, we can see that Bucky's metal arm can exert a force when needed. We can see and hear that in Cap and Bucky's fight scene.
And as far as I know, Bucky's metal arm can also discharge some EMP.
(This was taken from Wikipedia )
That can disable Iron Man for short amount of time. So is there any MCU explanation why Bucky's arm lost this kind of skill or power?
The Wikipedia page you took that from is mainly about the original comic character.
(Emphasis mine)
Bucky is the name used by several different fictional superheroes appearing in American comic books published by Marvel Comics...
He never had the EMP ability in the films.
A reason hasn't been given, but we can probably assume it was to avoid making him too overpowered.
If the Winter Soldier could easily defeat Iron Man, the movie wouldn't be very interesting.
From the MCU Wiki:
Bionic Arm: HYDRA also removed the remnants of his severed left arm and replaced the missing limb with a cybernetic one. The bionic arm gives him superhuman strength superior to that of Captain America, though not quite on par with that of Spider-Man.
The Winter Soldier was able to rip open a S.H.I.E.L.D. armored SUV, rip apart Falcon's EXO-7 suit, and smash concrete by punching it. With his bionic arm supporting his already enhanced natural strength, the Winter Soldier was shown to overpower Captain America in a fight.
generating an EMP required the Winter Soldier to build up the energy necessary to emit such a pulse. That energy buildup was easily detected by his suit and he powered down vulnerable components in time to prevent any serious damage. Stark comments he has experienced EMPs enough that he has contingencies for them since his suit relies heavily on electronics to function.
Given that Stark was a defense contractor, nuclear weapons and electromagnetic radiation would be a threat to his suit, it makes sense for him to harden his entire suit just like he would for his client's technology.
In the first Avengers movie, he absorbs a burst of lightning from Thor's hammer which should have acted as an EMP. Instead his suit absorbed the surge of energy turning it into power for him. This implies a degree of radiation hardening: already inherent in the suit. Yes, the suit took damage but it maintained sufficient integrity to continue the battle.
Radiation Hardening is the act of making electronic components and systems resistant to damage or malfunctions caused by ionizing radiation (particle radiation and high-energy electromagnetic radiation), such as those encountered in outer space and high-altitude flight, around nuclear reactors and particle accelerators, or during nuclear accidents or nuclear warfare.
Most semiconductor electronic components are susceptible to radiation damage; radiation-hardened components are based on their non-hardened equivalents, with some design and manufacturing variations that reduce the susceptibility to radiation damage. Due to the extensive development and testing required to produce a radiation-tolerant design of a microelectronic chip, radiation-hardened chips tend to lag behind the most recent developments.
