Metallurgical behaviors corrosion

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

A complete discussion of the corrosion behavior of alloy systems and the influence of metallurgical factors on each is available (30). Some of these factors in a few technologically important alloy systems are discussed here. 

Pitting potential increased with increase in chromium contents >20 wt%, and molybdenum of 2-6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5... 

Carbon steels. The corrosion behavior of carbon steel weldments produced by fusion welding can be due to metallurgical effects, such as preferential corrosion of the heat-affected zone (HAZ) or weld metal, or it can be associated with geometric aspects, such as stress concentration at the weld toe, or creation of crevices due to joint design. 

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... 

Jones, R.H. and Danielson, M.J., Role of hydrogen in stress corrosion cracking of low-strength Al-Mg alloys, in Hydrogen Effects on Materials Behavior and Corrosion Deformation Interactions, 861, Warrendale, PA The Metallurgical Society of AIME, 2003. 

French DN, Metallurgical Failures in Fossil Fired Boilers, 2nd ed. John Wiley Sons Inc., NY, USA, 1993. Gleeson B, Cheung WH, Da Costa W and Young DJ, The hot-corrosion behavior of novel co-deposited chromium-modified aluminide coatings. Oxidation of Metals, 1992, 38(3-5) 407-424. 

The development of alloys for controlling corrosion in specific aggressive environments is certainly one of the great metallurgical developments of the twentieth century. The basis upon which alloys resist corrosion and the causes of corrosion susceptibihty of alloys are explored in this and subsequent chapters. In general, the corrosion behavior of alloys depends on the interaction of ... 

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. 

Besides the standards mentioned above, there are several additional ones that have some bearing on the corrosion behavior of implant materials in vivo. A number of standards govern the chemical composition and metallurgical conditions of metallic implant materials. 

The metallurgical characteristics of the aluminum oxide layer also depend on its physical metallurgy, such as defects and metallurgical structure included in the oxide layer. For instance, when intermetallic compound particles as secondary phases are exposed on the surface, a discontinuous oxide film with various defects is often produced at the metal-particle interface. This discontinuous oxide film is weakly or non-protective chemically and physically. Because corrosion is a chemical and electrochemical reaction on the surface, corrosion behavior is readily influenced by surface morphology. The aluminum surface is usually adsorbed or contaminated by water, gases and many kinds of micron-sized substances. Microscopic heterogeneous structures such as vacancies, steps, kinks, and dislocations, and macroscopic heterogeneous structures such as scratches, pits and other superficial blemishes influence the corrosion behavior of aluminum and its alloys to different extents. 

In realistic applications it is necessary as a third step to check the corrosion behavior experimentally under the various electrochemical conditions expected. Environmental changes might include adhesion of corrosive materials metallurgical changes might include selective dissolution or deposition of noble metals. It is essential to inspect the corrosion behavior of aluminum and its alloys in a field test. 

By Materials This section includes a discussion of the nature of each material, such as the effects of composition, alloying, metallurgical treatments, microstructure, surface effects, and natural protective films on the corrosion behavior. 

Secondly, the corrosion of an Mg alloy is the result of anodic dissolution of the matrix phase. The other phases in the alloy are relatively inert. However, their presence can significantly influence the dissolution of the matrix phase. Hence the chemical composition, the amount and the distribution of the other phases all play an important role in the corrosion of the alloy. Following this theory, the corrosion behavior of a Mg alloy is predictable after the electrochemistry of each phase constituent in the alloy is understood. For the same reason, the corrosion performance of an Mg alloy can be improved by modifying its chemical composition and microstructure through a metallurgical approach. Currently, the development of innovative alloys, including metallic glasses, is based on this idea. 

Table 4 Mechanical, metallurgical, and environmental variables that influence corrosion fetigue behavior...

Barrett, C. A., The Effects of Cr, Co, Al, Mo and Ta on the Cyclic Oxidation Behavior of a Prototype Cast Ni-Base SuperaUoy Based on a 26 Composite Statistically Designed Experiment, in Conference on High Temperature Corrosion Energy Systems, Detroit, The Metallurgical Society/AIME, 1984, pp. 667-680. 

The corrosion behavior of carbon steel weldments is dependent on a number of factors. Consideration must be given to the compositional effects of the base metal and welding consumable and to the different welding processes used. Because carbon steels undergo metallurgical transformations across the weld and HAZ, micro-slructures and morphologies become important. 

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