Variation in sessile microflora as function of flow velocity on coupons exposed to seawater

Abstract

The variability of several groups of microorganisms on AISI 1020 carbon steel coupons as a function of seawater velocity in a water circulation loop was investigated. The metal probes as well as electrodes were fixed onto ducts connected to a 35l capacity tank, in order to study both biofilm formation and some electrochemical parameters. The experiments were carried out at different seawater velocities. The technique of the most probable number was used to enumerate bacterial aerobes and anaerobes as well as sulphate-reducing bacteria and iron-reducing bacteria. Fungi were quantified by counting the number of colony forming units. At velocities of 3.6 cm/s, which correspond to a laminar flow, the numbers of aerobic and anaerobic bacteria attached to the metal surfaces reached a maximum. Such values were markedly reduced at velocities of 17.4–26.0 and 34.8 cm/s. The corrosion rate at the start of the process was 1.4 mm/year, decaying to levels of about 0.4–0.6 mm/year over the experimental period. Analysis of loss of carbon steel coupons mass after 35 days of the process indicated a mean corrosion rate of approximately 2 mm/year.