Corrosion may be active and unseen underneath fire protection if water has penetrated, over a period of time, the corrosion damage can threaten the structural integrity, with potentially disastrous consequences. Several spheres have collapsed as a result.

Impact-Resonance testing identifies if the bond between the concrete and the steel is still intact, a good bond prevents water ingress, and hence prevents corrosion. When corrosion is active the expansion of corrosion products causes the un-bonded area to expand in size over time.

To identify areas with lack of bond, the "impact-resonance" method is used. The test is usually carried out on a 200mm grid, each test point taking a few seconds. An “unwrapped” picture of each leg is produced showing the dis-bonded areas. Immediate remedial action may be taken, or the map retained and compared with a subsequent test to determine the rate of deterioration.

All the legs on a typical sphere may be 100% tested in one day by a 2-man crew, provided good access at 2m levels is available.