This paper reports the behavior of galvanic anodes for cathodic protection in natural sea water under anaerobic and aerobic conditions. This work is a part of a research on the study of cathodic protection conditions and anodes behavior in a simulated deep-water environment. A series of tests, in aerobic and anaerobic conditions, were performed on carbon and stainless steel in natural sea water. It has been confirmed that the protection potential in anaerobic condition has to be lower than ?800 mV SCE; the protection current density is not highly influenced by the anaerobic environment; the anodic efficiency vary with the kind of alloy and their impurity concentration.
Cathodic protection (CP) is widely applied for corrosion prevention on steel structures in contact with sea water; one of its most important applications is the protection of platforms and submerged structures. Nowadays, the gas and oil production is being developed in deeper waters (between 500 m and 2,000 m). At such depths, maintenance is difficult to carry out, so that reliability of materials and protection systems are very important, as well as the choice of the optimal protections conditions. The environmental conditions in deep water are severe, characterized by the absence of oxygen (then possibly promoting the growth of sulphate-reducing bacteria), the presence of sulphides and neutral to slightly acid pH. All these factors influence the chemical equilibrium (in particular carbonatebicarbonate), modifying the protection conditions and affecting the electrochemical behaviour of the anodic material.
Aims of this research, co-funded by MIUR (Italian Ministry of Education, University and Research), were the evaluation of the effect of a typical deep sea water environment on CP conditions, such as: evaluation of CP parameters, in particular current density and protection potential; calcareous deposit formation: chemical and microstructural characterization; anodic alloys behaviour.
The research consists in three phases: laboratory tests, field tests at atmospheric pressure and autoclave tests. Laboratory tests in synthetic sea water, at environmental temperature, were presented in a previous work. This paper deals with field tests made in the CNR IENI facilities in Bonassola, Italy, using natural sea water (Ligurian Sea) in aerobic and anaerobic conditions. The third phase will be done in the future.
The literature on CP in sea water is very wide, however, there are few studies of the behaviour in anaerobic conditions or at high depth.
In anaerobic conditions and in absence of sulphate-reducing bacteria (SRB) or sulphide ions, protection current density should be lower as confirmed in CP applications in sea mud. In presence of suphide, protection potential is more negative (-900 mV vs SCE) and protection current density could increase very much, as reported in literature: about 200 mA/m was measured.
In contact with sea water, the most common stainless steels are subject to localised corrosion due to pitting and crevice corrosion. The protection potential for stainless steels depends on the material and, in general, changes between ?500 and ?400 mV vs SCE; however, in practical applications, the protection potential is usually fixed at ?700 mV vs SCE, because at this potential the protection current and its throwing power are higher and can reach the inside of pits and crevices and increases the surface pH up to three points, extending the passivity range. Moreover, biofilm formed over a stainless steel surface modifies the kinetics of the oxygen reduction reaction and the protection current required is greater than that necessary to maintain immunity, becau