The Soil Air Interface is the area above and below the natural ground level where tendon corrosion is most likely to occur. It is an area greatly affected by moisture levels, certain bacteria, temperatures, oxygen levels and pH levels. An increase in corrosion can also occur if this area is exposed to agricultural fertilizers, animal excrement and any chemicals with excessive pH levels.
Non aggressive soils with pH levels between 6.5 and 7.5 have little effect on the wire tendon. Further down in the ground the wire tendon is surrounded by compacted soil which reduces its vulnerability to all of these elements and the chance of corrosion.