Corrosion monitoring of aluminum alloys in TRIGA IPR-R1 research reactor

Aluminium alloys and stainless steel have been used as cladding materials for nuclear fuel in research reactors, as TRIGA IPR-R1 reactor, located at CDTN, a nuclear research institute in Brazil. Aluminium owes its good corrosion resistance in most environments to the protective barrier oxide film formed and strongly bonded to its surface. When the film is damaged under conditions that normal self-healing does not occur, localized corrosion in the form of pitting or intergranular attack can occur. Another possible type of corrosion is galvanic corrosion that occurs when a metal or alloy is electrically coupled to another metal, usually dissimilar, in the same electrolyte. In order to develop a fundamental understanding of the corrosion problems with aluminium- and stainless steel-clad in TRIGA IPR-R1 reactor, a monitoring program has been initiated, as part of an IAEA sponsored Regional Technical Co-operation Project for Latin America. The program consists of in-pool tests using corrosion surveillance coupons made of aluminium alloys and stainless steel and laboratory electrochemical tests. Corrosion coupons included 1050, 6061 and 5052 aluminium alloys and SS 304 stainless steel. They are 100 and 70 mm circular disks of 3 mm thickness with a central hole designed to fit over the insulated stainless steel rod rack. Surface preparation and treatment of the aluminium alloy coupons were identical to that given to Al-clad fuel plates. One machined and polished Al 1050 coupon per rack was rinsed, degreased and passivated in water at 95 deg. C for 24 hours. One of the surfaces of this coupon was scratched with a 0.5 mm scriber to simulate a damaged fuel element surface. Ceramic disks of non-porous alumina separate the individual coupons and the coupled coupons, one from the other. A 150 mm acrylic disc was added to each corrosion rack to avoid contact with the reactor walls and internals. The coupons were assembled in a corrosion rack in the following order, from top to bottom: Al 1050; Al 6061; Al 1050 (pre-oxidized and scratched); Al 1050 - Al 1050 (couple); Al 1050 - Al 6061 (couple); Al 6061 - Al 6061 (couple); Al 1050 - SS 304 (couple); Al 6061 - SS 304 (couple); Al 5052 - SS 304 (couple). Three corrosion racks were immersed in TRIGA reactor pool in July 2002. They are expected to be removed after one, two and three years of exposure. Analyses of corroded surfaces will then be made to quantify the extent of surface pitting as a function of pool water parameters. This paper presents the monitoring program developed for TRIGA IPR-R1 reactor and the evaluation of the first corrosion rack to be removed from the reactor pool in July 2003, along with results of complimentary laboratory electrochemical tests, that include linear and cyclic potentiodynamic polarization