Here is my Corki build. I find that not a lot of people don't actually play Corki and I thought id put up a build for him. This is the first build im actually putting up on this site,so please look at my build rate it and comment if u have any tips for improvements. Thanks

I use 9 armor penetration marks, 9 cool down reduction glyphs, 9 attack speed seals, and finally all armor pen quintessences.

I take armor pen to try and eliminate and starting armor value champions start with for early damage and it also helps with late game with taking down tanks. The cool down reduction is because of the large cool down on Valkyrie and also allows you to use your phosphorus bomb and ultimate more frequently. The attack speed seals are just for a minor attack speed boost for more early game. Corki very much lacks with attack speed and before you get your ult his auto attacks can be very effective with his passive.

I go with 21 0 9. I take the traditional AD offensive mastery tree. And for utility I mostly went for the extra experience and the mana regen.

I build the boots of swiftness because Corki is very slow. He has Valkyrie but the cool down is too long so once used for the first time it becomes useless. Even with the boots of swiftness he only has a movement speed of 390. Next is a manamune. Corki early game and mid game has a very large mana problem. The manamune gives corki mana, mana regen, and attack damage. Also the passive on the manamune and Corki's ultimate work well together the more you fire it the more mana and attack damage you gain. Next item i build is a trinity force. This gives you a little bit of everything. I find it very useful on corki. The attack speed and damage on it are very useful and the ability power makes your phosphorus bomb and Valkyrie even more effective. Next I build a Tiamat. A lot of people under estimate the tiamats effectiveness. Its great for farming and gives you a lot of attack damage. The mana and health regen also very effective for corki helping his mana problem. My final items are a blood thirster and a black cleaver. Both items work very well on corki. Depending on the enemy team composition you can decide the order of which you buy first.