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

The variability of several groups of microorganisms on AISI 1020 carbon ste el coupons as a function of seawater velocity in a water circulation loop w as investigated. The metal probes as well as electrodes were fixed onto duc ts connected to a 35 1 capacity tank, in order to study both biofilm format ion and some electrochemical parameters. The experiments were carried out a t different seawater velocities. The technique of the most probable number was used to enumerate bacterial aerobes and anaerobes as well as sulphate-r educing bacteria and iron-reducing bacteria. Fungi were quantified by count ing the number of colony forming units. At velocities of 3.6 cm/s, which co rrespond to a laminar flow, the numbers of aerobic and anaerobic bacteria a ttached 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 coup ons mass after 35 days of the process indicated a mean corrosion rate of ap proximately 2 mm/year.