Corrosion Control in a Tunnel Environment
Summary
The aim of this paper is to describe the factors within a tunnel environment that may contribute toward a corrosive atmosphere, and considering these, to determine the corrosivity of the atmosphere throughout a tunnel environment and present methods to mitigate against corrosive damage.
There are various factors that contribute towards a corrosive atmosphere; the relative humidity within a tunnel, the concentration of SO2 and Cl– pollution and particulate pollution such as brake dust. Working with conditions determined by British Standards, the general tunnel environment is designated a C3 corrosive atmosphere. Where a tunnel has a high risk of water seepage, essentially any section of a tunnel that is bored through anything but London Clay, then the corrosivity of the atmosphere increases to C4. With an estimated salinity of around 0.4%, the water does not necessarily pose a high corrosive risk, but if a leak were to occur within a tunnel environment a higher level of protection is required to protect equipment from failure. Where tunnel seepage is deemed very likely, such as at Connaught tunnel, Thames tunnel, areas where a tunnel has been damaged or areas where a tunnel concrete segments interface with SCL, the corrosion category rises to C5.
The final corrosion risk considered is galvanic or bimetallic corrosion. Where two different metals meet in the presence of an electrolyte, such as the salty tunnel seepage water, galvanic corrosion occurs; the more anodic metal corrodes preferentially and this can lead to dramatically accelerated corrosion rates. Where possible, dissimilar metals should not be placed into contact with one another, which is especially true where there is a high risk of tunnel seepage which would provide the electrolyte required for the process to operate. The anodic and cathodic areas in contact with the electrolyte of the materials being utilised must be considered, and if it is deemed that material selection poses a high risk of failure or damage the materials must be changed or electrically insulated from one another. It is recommended that the use of stainless steel should be avoided in a tunnel environment if the application involves contact with other common structural metals.
Where natural and galvanic corrosion processes are likely to be present, a risk based approach must be taken. Whilst this paper outlines some important rules to apply in terms of categorising risk of corrosive damage, there are many areas within a tunnel environment that will need to be risk assessed and a corrosive category applied.