I had a very similar build to you, i had naturally slim arms but my arms went from 14.5 to 16.5 within a year, for arms mate it gets work alot with other body groups - so i stuck with 3 excercises for 2 sets each, barbell curls, hammers, preachers, for triceps i did close grip, skull crushers then 2 sets of extensions ( rope, bar, etc) try increasing the weight every other week even by a 1kg
