Clad aluminium alloys, corrosion

This chapter presents a discussion of the fundamentals of aluminium alloy corrosion applicable to the wet storage of spent nuclear fuel throughout the world. It examines the effects of variables on corrosion in the storage environment and presents the results of corrosion surveillance testing activities at SRS, as well as discussions of corrosion at fuel m storage basins at other production sites of the USDOE. Aspects related to the corrosion of aluminium clad fuel at SRS apply to research and test reactor fuel worldwide. 

Nevertheless, there are instances where corrosion mechanisms may play the primary role in affecting the service performance of bonded joints. For example, clad aluminium alloys may present a particular problem and the... 

However, whilst this mechanism of corrosion inhibition may be effective for exposed aluminium alloy structures, if one considers the mechanisms whereby clad aluminium alloy achieves its corrosion resistance then the clad layer is actually undesirable in the context of adhesive bonding i.e. once the clad layer... 

Figure 8.20 Progressive pitting of bare and clad aluminium alloys in a corrosive environment [114].

Figure 8.21 Schematic diagram of corrosion as a primary mechanism of environmental attack on clad aluminium alloy joints [114].

In the USA the trend has been to move away from adhesive bonding to clad aluminium alloys, but where unclad alloys are bonded and used in areas exposed to corrosive environments any non-bonded, exterior surfaces must be protected by appropriate means, e.g. special primers and paint schemes, in order to limit surface corrosion. In Europe, clad aluminium alloys are still being widely used. 

Composites of aluminium alloy with a thin cladding on one or both surfaces of a more anodic aluminium alloy or pure aluminium, enable sheet, plate and tube to be produced with special combinations of strength and corrosion resistance appropriate to service conditions. Although originally applied to high strength aircraft alloys, this principle of cladding is now utilised in several important industrial applications. 

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

A common method of maintaining high corrosion resistance of aluminium alloys is to clad the alloy with pure aluminium. Subsequent to cladding, the alloy cannot be heat treated as diffusion of alloying metals into the pure aluminium cladding will again reduce corrosion resistance. 

A substrate metal may be clad to improve corrosion resistance, in which case the cladding is applied by hot rolling of the substrate and cladding metal. Aluminium alloys are often clad with high-purity aluminium, the latter having improved corrosion resistance over its alloys. 

The scientific investigations undertaken during the CRP involved ten institutes in nine countries. The IAEA furnished corrosion surveillance racks with aluminium alloys generally used in the manufacture of nuclear fuel cladding. The individual countries supplemented these racks with additional racks and coupons specific to materials in their storage basins. 

A thorough state of the art literature review on the corrosion of aluminium alloys was compiled by the IAEA in 1998. This review was published in IAEA-TECDOC-1012, Durability of Spent Nuclear Fuels and Facility Components in Wet Storage. It covered a wide range of quantitative and semi-quantitative data on cladding alloys used in nuclear fuel elements and assemblies, and included separate sections on corrosion of aluminium, zirconium, stainless steel, carbon steels and copper alloys in a wet storage... 

Some understanding of the corrosion of aluminium alloys used as cladding on research and test reactor fuel has been obtained from the CRP. Aluminium corrosion is extremely complex and the variables affecting localized corrosion (pitting and crevice corrosion) act both independently and synergistically. Additional information about the effects of deposited particle composition on the corrosion behaviour of aluminium alloys is needed. Surface finish affects the corrosion of aluminium alloys, and more information is required with respect to this parameter. Additional data on the effects of certain impurity ions in basin water on localized corrosion behaviour are necessary to better identify the ions that cause corrosion. A goal would be to develop an equation for pitting as a function of water chemistry parameters. 

In wet storage of aluminium clad spent nuclear fuel, different types of corrosion can occur. A short discussion of the more important types of corrosion as they pertain to the aluminium alloys is provided below. 

The corrosion of aluminium alloys in high purity water is complex and many of the factors responsible for this corrosion are interrelated. In high purity, deionized water, general thinning of the cladding caused by uniform corrosion is very low. The fuel enters the basin with, in some cases, several millimetres of protective oxide coating formed at high temperatures. When corrosion by water... 

The factors promoting corrosion of aluminium alloys are complex and interrelated. They often operate synergistically, making prediction of corrosion difficult. In wet storage of aluminium clad spent fuel, there are a number of corrosion mechanisms involved. The most important mechanisms as related to spent nuclear fuel are briefly discussed here. Other details and definitions related to aluminium corrosion can be found in the normative publications and ISO standards provided therein. 

These guidelines are for corrosion protection of the aluminium cladding, to prevent breach of this cladding and subsequent corrosion of the fuel core. Most research reactor fuel is fabricated from uranium-aluminium alloys, and this type of fuel exhibits corrosion behaviour similar to that of aluminium. Therefore implementation of these guidelines should also minimize corrosion of the fuel core. The corrosion of a metallic uranium core is much more rapid than that of a uranium-aluminium alloy core. Implementing the guidelines to protect aluminium cladding will also reduce the corrosion of this type of fuel core. 

In 1996 the IAEA initiated a CRP on the corrosion of aluminium clad spent research reactor fuels to help evaluate the state of the spent fuel assemblies and to inform pool/basin operators regarding maintenance and housekeeping procedures to extend the lives of the FAs. The main activities of this programme are related to exposing racks of aluminium alloy specimens (coupons) in different spent fuel basins around the world. Five racks were suspended in the 1EA-R1 reactor pool and were subsequently withdrawn after different time spans to evaluate the extent of corrosion of the coupons as a function of alloy composition, crevices, bimetallic effects and water chemistry. During this period the pool water was monitored for pH, conductivity, chloride ion content and radiometry (Table 6.3). The IAEA CRP racks are denoted as racks 1,2A, 2B, 3A and 3B. 

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