Localized corrosion environmental conditions

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

There are a series of other factors which can result in altering the local environmental conditions and lead to various forms of corrosion and/or failure of the implant. These are discussed in the next section. 

This book consists of nine chapters. The second chapter provides an overview of the important thermodynamic and kinetic parameters of relevance to corrosion electrochemistry. This foundation is used in the third chapter to focus on what might be viewed as an aberration from normal dissolution kinetics, passivity. This aberration, or peculiar condition as Faraday called it, is critical to the use of stainless steels, aluminum alloys, and all of the so-called corrosion resistant alloys (CRAs). The spatially discrete failure of passivity leads to localized corrosion, one of the most insidious and expensive forms of environmental attack. Chapter 4 explores the use of the electrical nature of corrosion reactions to model the interface as an electrical circuit, allowing measurement methods originating in electrical engineering to be applied to nondestructive corrosion evaluation and... 

Additionally, specific environmental conditions can induce localized corrosion such as temperature, conductivity of the corrosive fluid, or thickness of the liquid corrosive film in contact with the metal. In some cases, both metallurgical and geometric factors will influence behavior, such as in stress-corrosion cracking. Preferential weldment corrosion of carbon steels has been investigated since the 1950s, commencing with the problems on icebreakers, but the problem continues today in different applications. (Bond)5... 

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

Environmental conditions leading to localized corrosion are usually associated with nonuniform concentrations of cathodic reactant species or corrosion product ions. These conditions can be brought about by the following ... 

Stainless steel is corrosion resistant because a protective oxide layer naturally forms on top of the surface in the presence of oxygen and humidity. This protective oxide layer typically has a thickness in the order of nanometers, depending on the present environmental conditions. XPS studies of oxide films formed in air on AISI 316 revealed that not only oxidation of the material takes place, but also chromium and metallic nickel accumulate at the interface between oxide layer and bulk material [1]. The protective film is, of course, not perfect but contains defects like inclusions and grain boundaries. At these defects the film may locally break down and dissolution of the bulk material may start [2]. This kind of corrosion is called pitting corrosion and is estimated to cause a third of all chemical plant failures in the United States [3]. 

In the absence of an oxide layer, it appears that reduction of RX involves primarily ET from Fe (i.e., equations 1 and 4). An oxide free iron metal surface can be achieved in electrochemically-controlled laboratory systems (9), and may apply where localized corrosion occurs at defects in the oxide surface layer (see section on The Oxide as a Physical Barrier). Under environmental conditions, however, the layer of iron oxides that covers the Fe surface will contain Therefore, the... 

This system of atmospheric classification is now being revised to create a new approach based on dose-response functions for steel, copper, and zinc. Because the corrosion of aluminum occurs by a pitting or localized mechanism, the traditional approach of using mass loss to determine severity of attack is often misleading. Atmospheric corrosion problems with aluminum alloys are most frequently a result of metallurgical conditions rather than environmental conditions, and the behavior of aluminum may be excluded in the upcoming revision of the ISO 9223-6 documents. 

The in-service cable plant is tested to establish the presence or absence of stray direct currents, the galvanic corrosion effect of "foreign plants, the corrosivity of local environmental conditions, and the corrosive effect of long cells formed by the changes in the environment. The corrosion of the in-place plant is detected through potential surve5fs, current tests, soil resistivity tests, redox potential tests, and pH tests. 

---