Uniform corrosion mechanism

Different behaviors and mechanisms were clearly recognized between these resins. Epoxy resin cured with amine showed no degradation during immersion because of its stable crosslinks. Epoxy resin cured with anhydride showed the uniform corrosion with the softening and dissolution of the surface and also behaved similar to the oxidation corrosion of the metal at high temperature obeying linear law. 

For this study by Thompson et al (42), ion implantation and RBS were combined with more traditional electrochemical measurements to help establish the corrosion mechanisms of alloys in which a noble metal (Pt) was combined with an active/passive base metal (Ti). The alloys were created by ion implantation of Pt into pure Ti and were not of uniform bulk composition. Such surface alloys offer the possibilities of using a very small amount of a noble material to create a corrosion resistant coating on an otherwise chemically unstable but inexpensive metal or alloy. 

Types of Corrosion Phenomena. The major categories of phenomena ( include uniform, localized, and pitting corrosion selective dissolution and corrosion acting together with a mechanical phenomenon. In uniform corrosion, all areas corrode at the same rate. Examples of uniform corrosion include tarnishing and active dissolution of metals in acids. In localized corrosion some areas corrode more readily than others crevice corrosion and filiform... 

There are two implications associated with these two concepts of the mechanism of cathodic protection in respect of underprotection. If the older theory applies then it is to be expected that there will be more or less uniform corrosion over the surface. The later theory of LaQue implies that pitting... 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

It is of importance to predict the lifetime of a metallic component in a specific corrosive system. For uniform corrosion (also called homogeneous corrosion), the lifetime can be calculated if the kinetics of the reactions are known and if no localized attack or corrosion cracks could appear. In this chapter, the kinetic aspects of the homogenous corrosion process will be considered. Only electrochemical corrosion mechanisms are regarded, not chemical and physical ones. 

The mechanism of iron corrosion is depicted in the literature (Fontana and Greene, (17)). It represents a section of the pit and a growing pit inside the metal. The pitting factor P/d consists of the deepest pit in comparison with uniform corrosion loss as shown in Figure 1.10. However, pitting corrosion is generally reported as the average of ten deepest pit depths, as recommended in ASTM G48. 

In addition to ductile iron and PVC, copper and lead are used in pipes, and brass in fixtures and connections. Lead is released because of uniform corrosion. Copper is also released because of uniform corrosion, localized-attack cold water pitting, hot water pitting, MIC, corrosion fatigue, and erosion-corrosion. Lead pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes erosion-corrosion, impingement corrosion, dezincification, and SCC. The direct health impacts are because of increased copper, lead, and zinc concentrations in the drinking water. Mechanical problems because of corrosion include leaks from perforated pipes, rupture of pipes, and the loss of water pressure because of blockage of pipes by corrosion products. 

The first chapter constitutes an introduction to corrosion and various forms of corrosion such as general or uniform or quasi-uniform corrosion, galvanic corrosion, stray current corrosion, localized corrosion, such as pitting and crevice corrosion, metallurgically influenced and microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking. 

Fig. 4.42b) even at high temperatures (60 °C), high mechanical properties, uniform corrosion at a very low rate and much longer battery cycle life at high temperatures (Fig. 4.43) than any other battery type [81]. 

Immersion tests provide no information about reaction mechanisms and often they require relatively long exposure times. Electrochemical tests do not have these drawbacks and they are therefore widely used in practice. In the following electrochemical polarization methods are presented that provide information on the rate of uniform corrosion under conditions where the rate is controlled by charge-transfer. Other electrochemical test methods will be presented in subsequent chapters. 

Corrosion of copper shielding conforms to the mechanism of uniform corrosion. It is inherently resistant to differential aeration and only uniform corrosion of exposed areas occurs. 

The chassis and undervehicle is most susceptible to mechanical damage because of its proximity to external road conditions [24] (Fig. 4). Usual corrosion mechanisms apply such as those due to the effect of stone impingement and crevice and uniform corrosion. Coatings failure can enhance deterioration as can poultice built up in entrapment areas. 

The mechanism of degradation due to the deposition of airborne particles has recently received attention [3i]. When the system is exposed to moderately elevated relative humidity (typically 60 %), the critical humidity of the deposited particle is exceeded and the surface absorbs sufficient moisture to form an electrolyte solution. Various corrosion mechanisms including uniform and localized types can therefore occur, leading to failures. 

Corrosion is not always a uniform wearing away of material. Nonuniform corrosion phenomena can be classified as macroscopic or microscopic [1-6]. Macroscopic corrosion mechanisms include ... 

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