The easiest effect can be done by lightly sponging on a mixture of scorched brown and black. This is a nice random effect and can be furthered by correcting chips with a fine detail brush... I used this technique on the sentinel bellow which you may have seen.
Another way which is a little harder and more controlled is to highlight the armour in small jagged shapes in areas where you want the chips. Then paint small jagged paint chips with the mix mentioned earlier on these areas leaving some of the original highlighting around the edge. If you want to get a rusty effect use thinned down artists' oils and bend and run them into and around the chips, as seen on the sentinel.
Hope this helps.
Chris