Brunel's Other Bridge

Photographs of the Brunel Swivel Bridge North Beam base

Since not everybody will have peered under the edge beams, it may be worth noting that there were not many corroded holes through the base plates which could be seen from below although there was generalised corrosion visible on on exposed surfaces. However, specific deep pitting corrosion was noted adjacent to the turntable rail. This appears to be a case of bi-metallic corrosion in the immediate area around the turntable. As it happens, a thin layer of metal (discovered to be lead) is sandwiched between the turntable rail and the base plate of the edge beams. It appears that this lead was introduced to level up the bridge (probably when the bridge was re-set in its current location). BOB checked the corrosion tables and see that lead is widely separated from iron in the galvanic series for seawater (lead more cathodic, iron more anodic). As a result, wrought iron is likely to corrode in preference to lead. This means that for any repairs apart from considering how we replace the lost iron section, we will need to consider whether the lead should stay or how we can protect the wrought iron from corrosion in the future. In this location the use of a normal paint protection system will not be effective in the long term. I don’t pretend to have the answer yet. The lead may be quite difficult to remove because this would mean removing all the fixings between the turntable rail and the superstructure and then replacing them but on the other hand we may need to do this to re-level the turntable to meet all the turntable wheels. The alternative may be some form of cathodic protection?? This is one of quite a few issues that we need to chew over.

Plate 1	Plate 2
Plate 3	Plate 4
Plate 5	Plate 6
Plate 7	Plate 8
Plate 9	Plate 10
Plate 11	Plate 12
Plate 13	Plate 14
Plate 15	Plate 16
Plate 17	Plate 18
Plate 19	Plate 20