Marvel Studios Head of Visual Development Charlie Wen has shared a piece of Ultron (voiced by James Spader) concept art on his Instagram, showing the Avengers: Age of Ultron villain with an extra coat of metal over his external body.
Wen notes in his description that this was not just experimenting with a different look, but the result of a scene which didn't end up being used. Towards the end of the film, Ultron was set to pour Vibranium on himself in order to "upgrade."
"Here’s a #conceptart for crazy Ultron," Wen wrote. "This was a design to show him going off his rocker toward the end of Avengers: Age of Ultron. He pours #vibranium on himself to 'upgrade.'"
Age of Ultron was the second Marvel Cinematic Universe (MCU) film to bring the full team of Avengers together. When Tony Stark (Robert Downey Jr.) and Bruce Banner (Mark Ruffalo) use their scientific expertise to create a powerful new form of artificial intelligence, the project backfires and results in a malicious robot, Ultron, with a mind of its own.
Vibranium is a fictional metal in the MCU that is extremely rare, durable and powerful. The nation of Wakanda uses it to power its entire country and all of its technology. It it was also used to build Captain America's shield.