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AA5083 and AA6082 Exposed to Seawater – Effect of Temperature and Potential on Corrosion Behaviour

This paper presents and discusses results from testing of AA5083 and AA6082 in natural seawater at 100C. 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.

Product Number: 51317--8942-SG
ISBN: 8942 2017 CP
Author: Roy Johnsen
Publication Date: 2017
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Due to its low weight the interest for using aluminum in subsea structures has increased in the oil & gas industry during the last years. So called corrosion resistant aluminium alloys belonging to the 5000- and 6000 series have been used for many years in ship hulls. Good experiences have been achieved even if some corrosion failures have been observed. The main problem is connection to more noble metals like e.g. carbon steel stainless steel and copper alloys that will give galvanic corrosion on aluminium. Used for subsea structures aluminium will normally be in metallic contact with carbon steel (and even with more noble alloys). Carbon steel is always connected to a cathodic protection system in seawater which means that the aluminium alloy also will be connected to the cathodic protection system.AA5083 and AA6082 are the most frequently used aluminium alloys for seawater applications. In the literature limited information is published about the behavior of AA5083 and AA6082 under cathodic protection in seawater. It is known that both the temperature and the electrochemical potential affect the corrosion behavior of these alloys in seawater.This paper presents and discusses results from testing of AA5083 and AA6082 in natural seawater at 10C and 32C. Samples were exposed for up to 1500 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. At defined intervals anodic and cathodic polarization curves were measured on separate samples. After ended exposure period the surfaces were examined and characterized with optical microscopy and scanning electron microscopy (SEM). Electron diffraction spectroscopy (EDS) was also used to define chemical elements in selected areas on the surfaces.

Key words; aluminum alloys, AA5083, AA6082, seawater, corrosion, cathodic polarisation, pitting, crevice

Due to its low weight the interest for using aluminum in subsea structures has increased in the oil & gas industry during the last years. So called corrosion resistant aluminium alloys belonging to the 5000- and 6000 series have been used for many years in ship hulls. Good experiences have been achieved even if some corrosion failures have been observed. The main problem is connection to more noble metals like e.g. carbon steel stainless steel and copper alloys that will give galvanic corrosion on aluminium. Used for subsea structures aluminium will normally be in metallic contact with carbon steel (and even with more noble alloys). Carbon steel is always connected to a cathodic protection system in seawater which means that the aluminium alloy also will be connected to the cathodic protection system.AA5083 and AA6082 are the most frequently used aluminium alloys for seawater applications. In the literature limited information is published about the behavior of AA5083 and AA6082 under cathodic protection in seawater. It is known that both the temperature and the electrochemical potential affect the corrosion behavior of these alloys in seawater.This paper presents and discusses results from testing of AA5083 and AA6082 in natural seawater at 10C and 32C. Samples were exposed for up to 1500 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. At defined intervals anodic and cathodic polarization curves were measured on separate samples. After ended exposure period the surfaces were examined and characterized with optical microscopy and scanning electron microscopy (SEM). Electron diffraction spectroscopy (EDS) was also used to define chemical elements in selected areas on the surfaces.

Key words; aluminum alloys, AA5083, AA6082, seawater, corrosion, cathodic polarisation, pitting, crevice

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