Attack Armor is a stat that negates a set percent of Attack Damage. The algorithm for how much Attack armor converts to what percent Attack Damage negated, the percent negated is:

(Armor - Armor Penetration) / (Armor - Armor Penetration + 100)

In other words, with 100 effective armor, your effective health against that type of damage would be twice of your actual health. With 200 effective armor, it is tripled, and so on.

Below is a list of all artifacts that give Attack Armor, the value given at max Tier, and the Credit Cost at Max Tier.

Attack Armor has no limit.