Metals deterioration

Erosion Corrosion. Erosion corrosion is the increase in the rate of metal deterioration from abrasive effects. It can be identified by grooves and rounded holes, which usually are smooth and have a directional pattern. Erosion corrosion is increased by high water velocities and suspended soHds. 

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is ... 

Metal dusting is a form of metal deterioration that occurs in carbonaceous gas streams containing carbon monoxide and/or hydrocarbons at elevated temperatures. 

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

Hoyt, W. B., and Caughey, R. H., High Temperature Metal Deterioration in Atmospheres Containing Carbon Monoxide and Hydrogen, Corrosion, 15 (12) 627t-30t (1959). 

Corrosion is the process whereby a metal deteriorates. Corrosion caused by atmospheric oxygen is a widespread and costly problem. About one-quarter of the steel produced in the United States, for example, goes into replacing corroded iron at a cost of billions of dollars annually. Iron corrodes when it reacts with atmospheric oxygen and water to form iron oxide trihydrate, which is the naturally occurring reddish-brown substance you know as rust, shown in Figure 11.17 ... 

Carbon dioxide Sulphur oxides Building stone e.g. limestone Metals Deterioration... 

Corrosive water must be avoided. If the column is made of stainless steel, the water should be low in chlorides (131). The author is familiar with one plant where steam-water operations were discontinued in stainless steel columns after it was found that chlorides in the water caused metal deterioration. Ideally, the water should contain a corrosion inhibitor. The corrosion inhibitor should be carefully selected, because several commercial inhibitors promote foaming in subsequent operation. 

Modification of environment Since corrosion is caused by chemical interactions between metal and ionic species in the surrounding environment, removing metal from, or changing, the type of environment the metal deterioration can be reduced. For example, the contact of metal with rain or seawater can be limited or the content of sulphur, chloride or oxygen can be reduced. 

This introductory section includes basic definitions related to chemical and electrochemical reactions in the forward (f) and reverse (r) directions. The word Corrosion stands for material or metal deterioration or surface damage in an aggressive environment. Corrosion is a chemical or electrochemical oxidation process, in which the metal transfers electrons to the environment and undergoes a valence change from zero to a positive value z. The environment may be a liquid, gas or hybrid soil-liquid. These environments are called electrolytes since they have their own conductivity for electron transfer. 

Anodic reactions represent metal deterioration since a corroding metal losses electrons. This is a surface electrochemical phenomenon that may cause drastic effects on metallic structures. Therefore, the driving force for corrosion is the overpotential (17) defined by... 

A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually, fretting is accompanied by corrosion, especially of the very fine wear debris. Fretting refers to metal deterioration caused by repetitive slip at the interface between two surfaces. When metal loss is increased by corrosion, the term fretting corrosion should be used. 

Stray current corrosion differs from other forms in that the source of the current causing the corrosion is external to the affected equipment. This cause of metal deterioration is frequently misdiagnosed. Stray-current corrosion can cause local metal loss in huried or submerged metal structures, but it occurs much less frequently in underwater transporting equipment than in underground structures. Stray-current corrosion is almost always associated with direct current. At the anodic areas, metal goes into solution and the electrolyte tends to become acidic. It is most commonly encountered in soils containing water. 

Change in material properties during cyclic loading (e.g., hardening of metal, deterioration of frictional surfaces, effect of thermal expansion, presence of debris on frictional surfaces, progressive emshing of parent material)... 

The initial research on superconducting devices was done using soft and fragile metals such as Pb and Sn. The superconducting devices made with soft metals deteriorated easily with a thermal cycle between room temperature and liquid He temperature. 

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