Accelerated corrosion test and field test of steel and zinc coupons at buffer areas and cross passages in a mass rapid transit tunnel

A study has been conducted to measure the distribution of temperature, relative humidity and corrosivity due to uniform corrosion along the Mass Rapid Transit (MRT) North East Line (NEL) tunnel in Singapore. The corrosion rates of carbon steel 1010 and zinc coupons at the buffer areas of ten stations and five cross passages along the NEL were monitored over a period of one year. The median temperatures were between 31 and 36°C while the median relative humidity varied between 43 and 53%. At all exposure sites, the values of relative humidity corresponding to the 75th percentile values were below 60%, with very few data points registering above 80%. The cross passages showed a lower degree of fluctuations in the temperature and relative humidity compared to the buffer areas. The corrosivity levels showed some differences between exposure sites, but their values were generally low and could be categorised as C1 according to the ISO 9223:2012 categorisation. A set of procedures for an accelerated laboratory test to approximate the corrosion in the tunnel was also studied. The test procedures involved the exposure to a salt fog containing 0.12% Na2SO4 + 0.05% NaCl, followed by a drying phase and subsequent exposure to a constant temperature of 40°C and a constant relative humidity of 60%. It was found that after exposure for 80 h, the accelerated tests were able to generate the degree of corrosion observed in the most severe conditions in the field.