Uniform corrosion prevention

This form of corrosion is not of great concern from the engineering aspect, as the service life can be estimated within reasonable accuracy. Uniform corrosion can be prevented or reduced by using the following methods singly or in combination ... 

Electroless deposition as we know it today has had many applications, e.g., in corrosion prevention [5-8], and electronics [9]. Although it yields a limited number of metals and alloys compared to electrodeposition, materials with unique properties, such as Ni-P (corrosion resistance) and Co-P (magnetic properties), are readily obtained by electroless deposition. It is in principle easier to obtain coatings of uniform thickness and composition using the electroless process, since one does not have the current density uniformity problem of electrodeposition. However, as we shall see, the practitioner of electroless deposition needs to be aware of the actions of solution additives and dissolved O2 gas on deposition kinetics, which affect deposit thickness and composition uniformity. Nevertheless, electroless deposition is experiencing increased interest in microelectronics, in part due to the need to replace expensive vacuum metallization methods with less expensive and selective deposition methods. The need to find creative deposition methods in the emerging field of nanofabrication is generating much interest in electroless deposition, at the present time more so as a useful process however, than as a subject of serious research. 

Prevention— Uniform corrosion can be prevented or reduced by (1) the proper choice of metal for a given environment (2) the use of metal coatings, such as a paint coating or nonoxidizing organic inhibitors (e.g., benzotriazole or organic amines [160]) in solution that adsorb onto and protect the metal surface or (3) cathodic protection (discussed below). 

Prevention—Hydrogen embrittlement is prevented or minimized by (1) reducing the uniform corrosion rate on a metal surface (by coating/painting the metal surface, for example), to decrease the rate of hydrogen evolution on the metal surface (2) baking the metal (hydrogen evolution is an... 

Corrosion can affect an entire surface of a metal or just local spots. Uniform corrosion of the complete surface usually only happens in acidic conditions [2]. This generally results in overall thinning and causes no major damage. On the other hand, a very detrimental form of corrosion is pitting. This type is found at a single location on the surface and creates a pit or cavity which is difficult to prevent and often hard to detect. It can result in structural failure (example a cracked pipe). 

The uniform corrosion rates for zinc are not greatly affected by the purity of zinc 98.5 and 99.99% zincs behave similarly in many conditions. This is especially true in open atmospheres, where sufficient oxygen is present to prevent polarization by hydrogen. Some alloying elements increase the corrosion resistance of zinc significantly. 

Soil as a corrosive environment is probably of greater complexity than any other environment. The corrosion process of buried metal structures is extremely variable and can range from rapid to negligible. In fact, pipes in soil can be perforated within one year, presenting very localized or uniform corrosion attack (Figs. 1-2). With background knowledge of the principal soil specifics and their influence on metal corrosion, the most serious corrosion problems can be prevented. 

The following protective measures can be very effective in preventing uniform corrosion ... 

This attack occurs when pH is either very high or very low. The protective aluminum oxide layer dissolves rapidly under the above conditions. Prevention from uniform corrosion requires control of pH by inhibitor additions, or use of cathodic protection, or replacement with a more corrosion-resistant alloy. 

Uniform corrosion is not observed because it is so minute that it cannot be measmed. Other forms of corrosion of aluminium and its alloys such as exfoliation corrosion, stress corrosion, etc. are of course possible on age-hardenable alloys of the 2000 and 7000 series when exposed to weathering. However, the selection of alloys and their protection, if present, normally prevents these forms of corrosion from occurring in outdoor applications such as buildings or bridge railings. 

Copper-containing lead alloys undergo less corrosion in sulfuric acid or sulfate solutions than pure lead or other lead alloys. The uniformly dispersed copper particles give rise to local cells in which lead forms the anode and copper forms the cathode. Through this anodic corrosion of the lead, an insoluble film of lead sulfate forms on the surface of the lead, passivating it and preventing further corrosion. The film, if damaged, rapidly reforms. 

The selection of materials to be used in design dictates a basic understanding of the behavior of materials and the principles that govern such behavior. If proper design of suitable materials of construction is incorporated, the eqiiipment should deteriorate at a uniform and anticipated gradual rate, which will allow scheduled maintenance or replacement at regular inteivals. If localized forms of corrosion are characteristic of the combination of materials and environment, the materials engineer should still be able to predict the probable life of equipment, or devise an appropriate inspection schedule to preclude unexpected failures. The concepts of predictive, or at least preventive, maintenance are minimum requirements to proper materials selection. This approach to maintenance is certainly intended to minimize the possibility of unscheduled production shutdowns because of corrosion failures, with their attendant possible financial losses, hazard to personnel and equipment, and resultant environmental pollution. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

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