Alloys aqueous corrosion

Corrosion of metals and alloys—Aqueous corrosion testing of zirconium alloys for use in nuclear power reactors... 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-ceU reactions which control the rate of corrosion. 

Consideration will also be given to attack arising from contact with solids such as refractories, and with molten materials such as salts, glasses, and lower-melting-point metals and alloys. On a fundamental basis, the distinction between some of these latter reactions and normal-temperature aqueous corrosion is not always clear, since galvanic effects may be of significance in both cases, but for practical purposes a distinction can be made on the basis of the temperature involved. 

Test method for sandwich corrosion test Recommended practice for preparing, cleaning, and evaluating corrosion test specimens Practice for aqueous corrosion testing of samples of zirconium and zirconium alloys Test method for corrosion testing of products of zirconium, hafnium and their alloys in water at 633 K or in steam at 673 K [metric] Recommended practice for conventions applicable to electrochemical measurements in corrosion testing... 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

Since nearly all environments that involve burial or deposition on soil-vegetation surfaces involve some water, dry corrosion is usually superseded by aqueous corrosion. However, many metal objects will have undergone dry corrosion prior to deposition. When a freshly polished, bright metal is left exposed to a dry atmosphere, it may become dull and tarnished. For instance, a new copper alloy coin will form a layer largely composed of red-brown copper (I) oxide, cuprite (Cu20). 

It is possible to summarize the general features that can be attributed to the alloying elements in the alloy with respect to aqueous corrosion resistance as given... 

The nominal composition of two cobalt alloys designed to perform well with respect wear and aqueous corrosion is given in Table 4.55. 

Table 4.55 Nominal composition of aqueous corrosion and wear-resistant cobalt alloys...

The low cross-section for absorption of neutrons and high-temperature (330-350°C) aqueous corrosion resistance as well as its good mechanical properties promote the use of zirconium alloys in the nuclear reactors. In the development of zirconium alloys care must be taken that the added minor elements do not posses high cross-sections for the absorption of neutrons and contribute to greater corrosion resistance and improved mechanical properties. The good corrosion resistance of the alloys in acids and bases favors the use of zirconium alloys in chemical plants. 

Aluminum pistons in an engine that bums H2 will be exposed to not only H2 but also H2O at temperatures of 80 to 120°C. Aluminum alloys can be totally immune to H2 embrittlement and H2-induced crack growth if the natural AI2O3 oxide is intact. However, there are processes that can disrupt this film, and it is known that aluminum alloys will absorb H2 when exposed to H2O vapor at 70°C. There will also be periods when the engine is cool and condensed water will be present so that aqueous corrosion could occur, but this is not expected to be any different than with an engine with cast aluminum pistons that bums gasoline. 

Special Metals Corporation. In High-Performance Alloys for Resistance to Aqueous Corrosion Special Metals Corporation New Hartford, NY, 2000. 

Draley, J. E., TID-7587, "Aqueous Corrosion of 1100 Aluminum and of Aluminum-Nickel Alloys,"... 

McWhirter, J. W., and Draley, J. E., ANL-4862 "Aqueous Corrosion of Uranitim and Alloys Survey of Project Literature," Argonne National Laboratory, May 14, 1952. 

The term high-temperature requires definition. In contrast to aqueous corrosion, the temperatures considered in this book will always be high enough that water, when present in the systems, will be present as the vapour rather than the liquid. Moreover, when exposed to oxidizing conditions at temperatures between 100 and 500 °C, most metals and alloys form thin corrosion products that grow very slowly and require transmission electron microscopy for detailed characterizahon. While some principles discussed in this book may be applicable to thin films, high temperature is considered to be 500 °C and above. 

It is possible that CF, SCC, and HE could occur at the same time in some service conditions. The simultaneous operation of SCC and HE can occur in some systems. The interrelationship among stress corrosion, CE, and HE are discussed in the literature. The cross-hatched regions represent the most serious practical situations involving ductile alloy/enviromnent systems. These regions indicate the combination of any two failure mechanisms. In the center, all three phenomena interact, which is probably realistic in ductile alloy/aqueous environment systems (4). 

TABLE 23.1. Alloying Elements and Their Major Effects in Alloys for Aqueous Corrosion Resistance... 

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