The SOCORRO team at the University of Brighton, UK have undertaken several experiments to measure corrosion rates in the sea, namely in Newhaven and Shoreham. The team identified that in Shoreham, corrosion is microbially influenced (MIC) and accelerated low water corrosion (ALWC) related. Therefore, a program of environmental baseline measurements has been undertaken at the 3 sites to clarify corrosion mechanisms and identify environmental factors that influence corrosion rate and the presence of MIC. In Shoreham and Newhaven, the corrosion mechanism identified in ALWC blisters involves sulphate reducing bacteria forming hydrogen sulphide on the steel surface, which reacts to form iron sulphides. These are then oxidised by sulphur oxidising bacteria, producing acid which enhances corrosion rates.
Martin explains that environmental measurements show the bacteria come from the Port bed sediments. Measurements of sediment and pore water composition show that the bed sediment harbours sulphate reducing bacteria, which are forming iron sulphides naturally below the sea bed. It is these bacteria which are colonising the steel surface. Bed sediment anoxia may be a factor in the development of ALWC. The same process is not occurring at Southend, probably because of a different flow regime in the outer Thames estuary resulting in active erosion and transport of sediment.
In the SOCORRO project, the team at the University of Brighton have implemented a monitoring system incorporating continuous measure of water quality parameters using Aquaread sondes, and monitoring of corrosion rate using the C-Cubed system in Shoreham. These are accompanied by coupons of S235, S316 and S355 grade steel which are recovered monthly for mass loss measurements and characterisation of corrosion products.
Martin reports that since October 2022, there has been a dramatic increase in corrosion rate which corresponds to the development of a polysaccharide biofilm, presumably produced by algae, on steel coupons installed to monitor corrosion by mass loss measurements. Such biofilms are known sites for bacterial colonisation of surfaces. Continuing measurements will show if this develops into microbial corrosion, and what effect that has on corrosion rate.
X-ray photoelectron spectroscopy depth profiles of intertidal corrosion blisters at Newhaven, Shoreham and Southend showing the main sulphur compounds present. At Newhaven and Shoreham the blister has iron sulphates on the outer surface, and iron sulphides inside. This is typical for the products of steel interacting with hydrogen sulphide produced by sulphate reducing bacteria, which are then oxidised by iron and sulphur oxidising bacteria. At Southend no sulphur compounds are detected at all.
Profiles of sulphate in the bed sediment porewater at the 3 UK study sites. The rapid declien sulphate concentration at Newhaven and Shoreham indicates the presence of sulphate reducuing bacteria.
It was also discovered that at Newhaven and Shoreham the sulphate content reduces with depth, indicating the activity of sulphate reducing bacteria in the bed sediment. At Southend the sulphate content is constant with depth indicative sulphate reducing bacteria are not active. The single low sulphate analysis at Southend is from bedrock clay, not from recently deposited sediment.
Data from the Aquaread water parameter and corrosion rate monitoring at Shoreham Harbour from July to October 2022 demonstrate a high corrosion rate in August which corresponds to a period of high water temperature. However, the overall increase in corrosion rate corresponds to decreasing water T and increase dissolved oxygen as the temperature drops into autumn.