What causes corrosion of ties?
The important factors influencing corrosion are: what materials make up the tie, the exposure of the mortar and the tie to wetting (affected by environment, design and workmanship), temperature, pollutants in the air (primarily SO2), and additives or contaminants such as calcium chloride in mortar. New mortar provides a protective alkaline environment to the ties similar to that provided to steel reinforcement in concrete. This protection does not last long, however, because the higher porosity of mortar makes it vulnerable to rain, carbon dioxide, and such pollutants as sulfur dioxide, which reduce the alkalinity (a process called carbonation). This explains why corrosion usually occurs on the part of the tie within the mortar joint or under mortar droppings in the cavity – areas likely to absorb and retain moisture.
