Super High Shininess without lighting = black, more often than not.
Your structural steel red material was super shiny to the point of turning into a mirror.
Turn down the shininess and you start seeing red.
Your plastic is going to render black... because it's black. Sorry to say but you have a black background and a black object. It's like that famous picture we all made in art class of the ghost looking for a piece of blank paper in a snow storm.
Change the BG color or make the plastic a bit greyer, imo.
Other than that, you could use a diffuse map that may add some contrast to the black bg
Another solution is to make that pline their sitting on into a region, make it a contrasting color like light-grey or brown or something, and have the part sit on that, so it shows up. I used to use a nicely material-mapped wooden table when I did parts rendering. The table was pointless and superfluous but it gave a nice place for your designed parts to sit on, and also gave a sense of scale, if enough of the table was in the rendering.