I'd say, first off, that probably isn't pixel art. Most of the details look like an image that was painted at a larger size, and then shrunken down and given a few touchups to make it more pixelly. The "68" is the only thing in the image that I'm sure was pixeled.
The texture of the metal appears to be a pattern applied via a separate layer; probably with low opacity and a 'lighten only' type layer mode (eg. Screen). It may be a prefabricated pattern that is applied wherever needed. If I had to paint the pattern from scratch, I would probably use GIMP, with a rectangular brush, low opacity, randomized size. If I had to use a pixelling program, like GrafX2, it could be similar -- rectangular brush, low opacity (but GraFX2 doesn't support dynamic brushes, so I'd need to manually apply different sized rectangles).
Given that the image was going to be shrunken down, the texture wouldn't need to be very detailed; you could just tweak a few pixels after shrinking.