ABSTRACT
Due to its low weight there has been an increased interest for using aluminum in subsea structures in the oil & gas industry during the last years. In the literature, limited information is published about the behaviour of AA5083 and AA6082 under cathodic protection in real seawater. It is known that the electrochemical potential affect the corrosion behaviour of these alloys in seawater.
This paper presents and discusses results from testing of AA5083 and AA6082 in natural seawater at 10°C. Samples exposed up to 6500 hours under different condition including polarization to -1500 mV vs. Ag/AgCl, - 1050 mV vs. Ag/AgCl and -700 mV Ag/AgCl. In addition, samples were freely exposed to measure the development in open circuit potential (OCP). At defined intervals anodic and cathodic polarization curves were measured on separate samples. After the end of the exposure period, the surfaces were examined and characterized with optical microscopy and scanning electron microscopy (SEM).
The outcome of the test program shows that both alloys had pits on the surface after exposure at all potential levels. At -700 mVAg/AgCl severe crevice corrosion was found under the coating. AA6082 had a stable OCP 100 mV more negative than AA5083.
BACKGROUND
The oil and gas industry is constantly seeking for new cost effective solutions. Due to its low weight the interest for using aluminum alloys in subsea structures has increased in the oil and gas industry during the last few years. So called corrosion resistant aluminum alloys belonging to the AA5000 and AA6000 series have been used for many years in ship hulls. Good experiences have been achieved even if some corrosion failures have occurred.1,2 One main problem is connection to more noble metals like carbon steel, stainless steel and copper alloys will result in galvanic corrosion of the aluminum alloy.3 Used for subsea structures in the oil & gas industry, aluminum alloys will normally be in metallic contact with carbon steel (and sometimes stainless steel and copper alloys). Carbon steel is always cathodically protected system in seawater, which means that aluminum also is connected to the cathodic protection system.
