Types of corrosion process

Analysis of the products of corrosion has its historic origin in the light-microscopic studies of metallographic cross sections that started in the 1950s. The.se were supplemented with TEM and SEM investigations in the 1960s of 

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In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

When two dissimilar metals are in electrical contact with one another and both are contacting the same electrolyte, one of the metals will preferentially corrode, a process known as galvanic corrosion (also the principle by which certain types of batteries function). The more active metal will corrode, which is the metal having the more negative open-drcuit (or corrosion) potential, when immersed all by itself in the electrolyte the more noble metal (having the more positive open-circuit potential) will support the reduction reactions. The more active metal is, therefore, the anode and corrodes faster than it would all by itself, whereas the other more noble metal becomes the cathode and corrodes slower than it would alone (or maybe not at all). The electrolyte resistance, important in all corrosion processes, may play a particularly influential role in this type of corrosion process. 

Visual examination of the time record trace may provide indications as to the type of corrosion processes that are occurring. The following example illustrates how a simple examination of EN measurements could reveal the corrosivity of various points of an industrial gas scrubbing system where highly corrosive thin-film electrolytes are known to form [23]. These conditions arise when gas streams are cooled to a temperature below the dewpoint. The resulting thin electrolyte layer (moisture) is often highly concentrated in corrosive species. 

Depending on the type of corrosion process, the corrosion resistance may be measured in the increase/decrease of the weight of the tested materials or in the increase/decrease of the volume. Usually, static, dynamic, cup, and immersion tests are isothermal. Microscopic investigation is highly recommended after all corrosion tests. 

Several Pseudomonas spp. have been documented in the literature as common contaminants and derivatives of petroleum fluids (aromatic and paraffinic hydrocarbons, kerosene fuels, cutting oil emulsions), as well as, asphalt and asphalt-based coatings. Generally, their presence produces an alteration of fuel quality which leads to slime formation and subsequent blockages of filters and injection systems. In addition, several types of corrosion processes have been demonstrated which primarily involve pitting (Genner and Hill, 1981). Occasionally, the presence of marcescens has been reported in association with... 

Liquid sterilants are known to corrode the metal parts of articles and instmments that are to be sterilized, although articles composed exclusively of glass or certain type of corrosion-resistant metal alloys can be safely processed. Because the degree of corrosion is related to length of exposure, many articles are merely disinfected in a shorter exposure time. Disinfection may be suitable for certain appHcations. The safety of using Hquid sterilants must be judged by a qualified microbiologist. 

Fig. 2-1 Flow diagram of corrosion processes and types of damage (schematic).

Electrochemical corrosion is understood to include all corrosion processes that can be influenced electrically. This is the case for all the types of corrosion described in this handbook and means that data on corrosion velocities (e.g., removal rate, penetration rate in pitting corrosion, or rate of pit formation, time to failure of stressed specimens in stress corrosion) are dependent on the potential U [5]. Potential can be altered by chemical action (influence of a redox system) or by electrical factors (electric currents), thereby reducing or enhancing the corrosion. Thus exact knowledge of the dependence of corrosion on potential is the basic hypothesis for the concept of electrochemical corrosion protection processes. 

The principle of electrochemical corrosion protection processes is illustrated in Figs. 2-2 and 2-5. The necessary requirement for the protection process is the existence of a potential range in which corrosion reactions either do not occur or occur only at negligibly low rates. Unfortunately, it cannot be assumed that such a range always exists in electrochemical corrosion, since potential ranges for different types of corrosion overlap and because in addition theoretical protection ranges cannot be attained due to simultaneous disrupting reactions. 

In this type of corrosion, metal ions arising as a result of the process in Eq. (2-21) migrate into the medium. Solid corrosion products formed in subsequent reactions have little effect on the corrosion rate. The anodic partial current-density-potential curve is a constant straight line (see Fig. 2.4). 

If the products of electrolysis favor other types of corrosion, electrochemical protection processes should not be applied or should be used only in a limited form. Hydrogen and OH ions are produced in cathodic protection according to Eq. (2-19). The following possible corrosion danger must be heeded ... 

A further type of chemical process, which is analogous to high-temperature Corrosion, is the reaction of metals with organic sulphur compounds, which follow the equation... 

Pourbaix diagrams (Pourbaix, 1963) indicate graphically the conditions of redox potential (Eh) and pH under which different types of corrosion behaviour may be expected. These plots of potential vs. pH indicate the phase and species in equilibrium with iron under various conditions (see Chap. 8). The solid phases indicated are those that are thermodynamically the most stable owing to kinetic factors other phases may be present during the initial stages of the corrosion process. What the different regions show, however, are the predominant oxidation states to be expected. 

For our purposes, the classification system is not as important as (a) recognizing that many different forms of corrosion exist and (b) understanding the fundamental kinetic processes behind these different types. To this end, we will first look at the important principles of corrosion and then see how they can be applied to some important types of corrosion. 

Specific features of corrosion processes at semiconductors (as against to metals) are caused by the fact that charge carriers of both signs, namely conduction band electrons and valence band holes, take part in charge exchange between a solid and a solution. Therefore, the condition of Eq. (43) is insufficient, so account should be made of charge balance for each type of the carriers because equilibrium between the bands, which is established via generation-recombination processes, may not be reached. 

There are many types of corrosion, as would be expected from its general definition. It has been traditional (4) to divide the study of corrosion into two areas the study of low temperature corrosion by aqueous or other solutions, controlled by electrochemical processes (wet) and the study of gaseous corrosion at high temperatures, controlled by thermodynamics and diffusion processes (dry). In addition to the obvious differences, the two areas have many phenomena in common. 

Corrosion is a chemical attack on metal surfaces, and rust is a specific type of corrosion involving ferrous metals (Fe, Co, Ni). The process of combustion introduces corrosive materials in automotive lubricants, which are composed of... 

The type of conversion process will depend on the substrate, the nature of the oxide layer on its surface, and the type of adhesive or sealant used. The formation of a noncon-ductive coating on a metal surface will also minimize the effect of galvanic corrosion. 

Loss of metal is due to an anodic reaction at the metal surface this may occur locally or be spread equally over the metal surface. The manner in which this anodic process occurs dictates the type of corrosion obtained. For instance, if a brass and steel pipe are coupled together, as in many condensate lines, then the brass behaves as the cathode and causes the steel pipe to corrode close to the contact region of the two pipes. This may proceed to... 

Erosion corrosion is caused by the conjoint action of corrosion and mechanical abrasion by a moving fluid or suspended material in the fluid. Turbulent flow or jets of liquid on a metal surface may lead to erosion corrosion. The mechanical action of the fluid removes the protective corrosion deposit, thus exposing fresh metal to the corrosive. As corrosion products build up, they are removed and so the process continues. The surface of a piece of metal exposed to this type of corrosion has a characteristic structure (Fig. 8). 

It was concluded that nitrate stress corrosion caused the crack in the reactor at Flixborough. The reactor was removed and replaced with a 20 in. pipe that failed. The cracking occurred because cooling water containing nitrite had been sprayed on the reactor (Lees, 1980). Many other types of corrosion can occur with catastrophic consequences. The process designer should be aware of special problems that can result in the process and should obtain specialized help if necessary since corrosion is a specialized and very complicated field. It is impossible for most process designers to keep up with corrosion technology without help. 

Selective leaching is the process whereby a specific element is removed from an alloy because of an electrochemical interaction with the environment. Dezinci-fication of brass alloys is the most familiar example of this type of corrosion. It occurs most commonly when there is exposure to soft waters and can be accelerated by high carbon dioxide concentrations and the presence of chloride ions. 

The most common method of identifying and monitoring corrosion is visual inspection. Evidence of leakage, staining, or a change in surface appearance can be an indication that some type of corrosion is taking place. Experience with certain types of equipment and processes may help dictate inspection intervals and areas on which to focus the inspection. Records of vessel operation, maintenance, and repair can be helpful in establishing a pattern of performance that will improve predictability and minimize down time. 

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