Stainless steel has, in the past, been specified in potentially corrosive environments but an investigation carried out by TRL in Scotland has shown that under certain conditions, severe corrosion of stainless steel elements can occur.
Potentially corrosive environments include natural fills, waste materials and industrial byproducts. TRL advises that situations where the possibility of corrosion exists should be identified and checked as a matter of urgency.
The investigation is focused around a retaining wall built between December 1972 and May 1973 as part of the A901 improvements at Granton in Edinburgh. The wall carries the improved Lindsay Road while Marine Boulevard runs below (Figure 1).
The wall was built from cruciform reinforced concrete panels with stainless steel reinforcing elements and Burnt oil shale, locally known as Blaes, as backfill. The curved wall is up to 6.9m high (7.2m above foundation level) with a 0.6m parapet beam.
In February this year, excavations for service installations along Lindsay Road revealed severe corrosion of stainless steel reinforcing along a 50m length of the wall between 4.5m and 5m from the face, often leading to severing of the elements.
Owner City of Edinburgh Council immediately closed the footpath beneath the wall along Marine Boulevard and commissioned TRL to investigate.
Investigations included an ongoing detailed line and level survey to monitor wall movement. Three trial pits were put down to a maximum of 1.8m along the affected length of the wall, between 2.2m and 5.5m from the face, to ascertain the nature and extent of the corrosion. Samples were taken of the fill, stainless steel and the corrosion products (Figure 2) and testing is being carried out to determine their physical and chemical properties in an effort to establish the cause and mechanisms of the corrosion.
To date, the line and level survey has detected no significant movement of the wall but the investigation showed that corrosion was highly localised and severe (Figure 3), so the council decided to close the affected part of Lindsay Road and restrict traffic on Marine Boulevard to one lane, furthest away from the wall.
TRL says that the apparently random distribution of corrosion appears to rule out the ingress of road salts or other agents as the cause. Infrastructure owners should be aware of the possibility of corrosion, it says, and carry out urgent checks where there is potential risk.
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