Testing company Intertek will provide monitoring equipment and software to prevent rust taking hold in the 2.7km cable-stayed structure.

Operators of the Forth Road Bridge discovered corrosion in the main cables, which hold up the structure, in 2006 and at one stage it was feared the loss of strength in the cables would result in a ban on all vehicles. A spokesman for Transport Scotland said, “The design of the Queensferry Crossing takes advantage of technological advances in the last 50 years to ensure the bridge remains a reliable crossing for many years to come.

“Cables can be replaced with more ease on a cable stayed bridge as opposed to a suspension bridge such as the Forth Road Bridge and can be done as part of normal maintenance works without restricting the bridge to traffic.

“Dehumidification systems inside the box girders and towers will reduce moisture and corrosion, helping to maximise the lifespan of the bridge. Also, wind-shielding will be used to almost entirely eliminate the need for closures during the frequent periods of high winds in the Forth estuary.” Intertek, which has centres all over the world, will work with supplier Strainstall to deliver ‘health monitoring technology’ for the new Queensferry Crossing, which is due to open in December 2016.

Intertek will supply 120 units which will provide real-time monitoring and give readings on corrosion potential, rate and resistivity, as well as temperature and humidity of the concrete. Strainstall will use this information to enable engineers to assess when maintenance is required.

Intertek’s Tom Gooderham said, “We are pleased to be working with Strainstall on such an exciting project to assure the structural health of the new bridge. The Queensferry Crossing will be an iconic landmark and it is an honour for my team to contribute to the safety and integrity of the new bridge for years to come.” The issue of corrosion threatened to be a major problem for the Forth Road Bridge and the Forth Estuary Transport Authority, who operated the bridge at the time the rust was discovered in 2006, identified measures to monitor and minimise any further deterioration.

Dehumidification was the best option and a bespoke system designed specifically for the bridge.

The following year, a dire warning was given that if the process didn’t work, and the corrosion led to a further drop in the strength of the cables, then the bridge would have to close to HGVs by 2013 and possibly all vehicles by 2018.

It was the nightmare scenario and, at that time, there was no new commitment to what is now the Queensferry Crossing.

A study added that if the corrosion couldn’t be stopped and the cables had to be replaced before a new bridge was built, the ‘real’ cost to the economy could have been as high as £2.5 billion, up to seven years of roadworks and the loss of 3200 jobs. The process of treatment for the main cables involved wrapping them in an airtight, waterproof sheath and injecting very dry air into the spaces between the wires with the aim of reducing the relative humidity to a level where corrosion cannot occur.

Happily, in 2013, inspection findings showed that work to tackle this problem were proving successful and a ban on vehicles would not be necessary.