Due to his base armor and armor increase per level, Rammus effectively receives a bonus to his base attack damage (~5) and additional attack damage per level (~1) up to ~21 bonus attack damage at level 18.
Any buff or debuff that affects Rammus' armor will increase or reduce the bonus damage respectively as well. (e.g. Bulwark or Spirit Fire)
Similarly to Runic SkinGalio, Spiked Shell will cause a decrease in attack damage if Rammus' armor is reduced below 0.