Types of corrosion

Corrosion is characterized by the controlling chemi-physical reaction that promotes each type. Each of the major types is described below. 

Uniform corrosion is the deterioration of a metal surface that occurs uniformly across the material. It occurs primarily when the surface is in contact with an aqueous environment, which results in a chemical reaction between the metal and the service environment. Since this form of corrosion results in a relatively uniform degradation of apparatus material, it can be accounted for most readily at the time the equipment is designed, either by proper material selection, special coatings or linings, or increased wall thicknesses. 

Galvanic corrosion results when two dissimilar metals are in contact, thus forming a path for the transfer of electrons. The contact may be in the form of a direct connection (e.g., a steel union joining two lengths of copper 

Erosion corrosion occurs in an environment where there is flow of the corrosive medium over the apparatus surface. This type of corrosion is greatly accelerated when the flowing medium contains solid particles. The corrosion rate increases with velocity. Erosion corrosion generally manifests as a localized problem due to maldistributions of flow in the apparatus. Corroded regions are often clean, due to the abrasive action of moving particulates, and occur in patterns or waves in the direction of flow. 

Concentration cell corrosion occurs in an environment in which an electrochemical cell is affected by a difference in concentrations in the aqueous medium. The most common form is crevice corrosion. If an oxygen concentration gradient exists (usually at gaskets and lap joints), crevice corrosion often occurs. Larger concentration gradients cause increased corrosion (due to the larger electrical potentials present). 

Corrosion in cooling systems is multifarious. The permutations of all the various forms of corrosion and damage possible, associated with the variety of metals commonly used that may suffer metal wastage, is beyond the scope of the practical intent of this volume, but some observations are  

Adequate and early involvement with water treatment vendors is a necessary prerequisite to be sure of meeting the required standards and is, unfortunately, rarely done in practice. 

Similarly, a failure to consider in advance the monitoring techniques to be used, or the real relevance of the measured corrosion rate to be thus reported, renders the practice virtually meaningless and doomed to failure. 

water treatment vendors do themselves no favors by promoting their chemical products under a blanket coverage of attainable very low corrosion rates without first considering the circumstances of any particular situation. Furthermore, they do the industry a disservice when they accept onerous contract specifications, without mutual agreement with the owners and operators, on what is possible and what is not. 

The achievement of a genuinely low corrosion rate performance in a cooling system usually requires considerably more water management time, effort, and cost than many owners are prepared to tolerate. 

Galvanic corrosion results when two dissimilar metals are in contact, thus forming a path for the transfer of electrons. The contact may be in the form of a direct cormection (e.g., a steel union joining two lengths of copper piping), or the dissimilar metals may be immersed in an electrically conducting medium (e.g., an electro-l5rtic solution). One metal acts as an anode and consequently suffers more corrosion than the other metal, which acts as the cathode. 

The driving force for this type of corrosion is the electrochemical potential existing between two metals. This potential difference represents an approximate indication of the rate at which corrosion will take place that is, corrosion rates will be faster in service environments where electrochemical potential differences between dissimilar metals are high. 

Corrosion is the degradation of a material caused by exposure to an environment. AU material will degrade with time, but the degradation process can be retarded significantly by careful consideration of the following factors  

These three factors must be considered together in order to understand how materials corrode. In most cases, the last factor cannot be changed, the environment being defined by process requirements. However, there are some methods that can be used to mitigate corrosion problems  

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  

Galvanic Corrosion that is electrochemical in nature. Galvanic corrosion is caused by placing two dissimilar metals in contact with each other, directly or ough a conductive fluid. The rate of this type of corrosion increases with the distance between the metals in the galvanic series (see Table 3-2). The further apart the metals, the more rapidly the anodic material will corrode. Variations of galvanic corrosion include 

Muntz metal Yellow brass Admiralty brass Aluminum brass Red brass Copper Silicon bronze 90/10 copper-nickel 70/30 copper-nickel Nickel Inconel Silver 

Since 64 113, corrosion will continue to occur. In strong NaOH solution, rusting is reduced because the Fe203 forms a protective layer over the metal. 

Various forms of corrosion in chemical industry have been categorised into eight areas, as discussed below. 

Different forms of corrosion prevail in the chemical process industries as described in brief below. 

Corrosion in metallic components occurs when pure metals and their alloys form stable compounds with the process fluid by chemical reaction or electrochemical processes resulting in surface wastage. Appreciable corrosion can be permitted for tanks and piping if anticipated and allowed for in design thickness, but essentially no corrosion can be permitted in fine mesh wire screens, orifice plates and other items in which small changes in dimensions are critical. Rates of corrosion can be heavily affected by temperature changes and whilst a material of construction may be suitable at one temperature, it may not be appropriate for use at a higher temperature with the same process fluid. 

The corrosion of non-metallic materials is essentially a physio-chemical process that manifests itself as swelling, cracking or softening of the material of construction. In many instances non-metallic materials will prove to be attractive from an economic and performance point of view. Rubbers especially are interesting materials 

The use of various substances as additives to process streams to inhibit corrosion has found widespread use and is generally most economically attractive in recirculation systems, however, it has also been found to be attractive in some once-through systems such as those encountered in the petroleum industry. Typical inhibitors used to prevent corrosion of iron or steel in aqueous solutions are chromates, phosphates, and silicates. In acid solutions, organic sulphides and amides are effective. 

Many metals suffer from stress corrosion cracking under certain conditions. In piping the most frequent failures from stress corrosion cracking occur with stainless steels in contact with solutions containing chloride. Even trace quantities of chlorides can cause problems at temperatures above 60°C. 

In wet storage of aluminium clad spent nuclear fuel, different types of corrosion can occur. A short discussion of the more important types of corrosion as they pertain to the aluminium alloys is provided below. 

Some of the other factors affecting galvanic corrosion are area ratios, distance between electrically connected materials, and geometric shapes. Galvanic corrosion of the anodic metal takes the form of general or localized corrosion, depending on the configuration of the couple, the nature of the protective films formed and the nature of the metals. 

Galvanic corrosion of spent nuclear fuels in storage basins is active and can be reduced considerably by removing the couple whenever possible and by lowering the basin water conductivity. At low conductivity, in the range 1-3 pS/cm, the galvanic effect should be minimized. Basin water deionization will remove the corrosion-causing anions and cations from the water and will increase the resistance to current flow. 

Crevice corrosion. Crevice corrosion is a highly localized form of corrosion and occurs on closely fitted surfaces upon entry of water into the crevice [2.6]. Recent work has shown that the mechanism is complex. Chloride ions are drawn into the crevice as metal dissolution occurs and the conditions inside the crevice become acidic. Metals like aluminium that depend on oxide films or passive layers for corrosion resistance are particularly susceptible to crevice corrosion. 

Stress corrosion cracking (SCC). Stress corrosion cracking has not played a major role in the corrosion of the fuel stored in the basins. Ihe alloys used as cladding materials are pure aluminium — 1100,6061 and 6063 — and these are not susceptible to SCC. 

Titanium, like any other metal, is subject to corrosion in certain environments. The corrosion resistance of titanium is the result of a stable, protective, strongly adherent oxide film. This film forms instantly when a fresh surface is exposed to air or moisture. Additions of alloying elements to titanium affect the corrosion resistance because these elements alter the composition of the oxide film. 

The oxide film of titanium is very stable, though relatively thin, and is attacked by only a few substances, most notable of which is hydrofluoric acid. Because of its strong affinity for oxygen, titanium is capable of healing ruptures in this film almost instantly in any envirorunent where a trace of moisture or oxygen is present. 

Anhydrous conditions, in the absence of a source of oxygen, should be avoided because the protective film may not be regenerated if damaged. 

The protective oxide film of most metals is subject to being swept away above a critical water velocity. After this takes place, accelerated corrosion attack occurs. This is known as erosion-corrosion. For some metals, this can occur at velocities as low as 2-3 ft/s. The critical velocity for titanium in seawater is in excess of 90 ft/s. Numerous corrosion-erosion tests have been conducted and all have shown that titanium has outstanding resistance to this form of corrosion. 

General corrosion is characterized by a uniform attack over the entire exposed surface of the metal. The severity of this kind of attack can be expressed by a corrosion rate. With titanium, this type of corrosion is most frequently encoimtered in hot, reducing acid solutions. In environments where titanium would be subject to this type of corrosion, oxidizing agents and certain multivalent metal ions have the ability to passivate the titanium. Many process streams, particularly sulfuric and hydrochloric acid solutions, contain enough impurities in the form of ferric ions, cupric ions, etc., to passivate titanium and give trouble-free service. Refer to Table 20.6 for compatibility of titanium with selected corrodents. 

Zinc has the ability to form a protective layer comprising basic carbonates, oxides, or hydrated sulfates, depending on the nature of the environment. When the protective layers have formed and completely cover the surface of the metal, corrosion proceeds at a greatly reduced rate. A large number of zinc corrosion products have been identified by Gilbert (1952) and by Biestek (1974). 

Some of the factors affecting film formation and subsequent changes have been analyzed in experiments in recent years in relation to atmospheric exposure and are, therefore, discussed in Chapter 2. In dry air, a film of zinc oxide is initially formed by the influence of the atmospheric oxygen (e.g., at a speed of approximately 40 nm/24 h when a part leaves the galvanizing bath). The subsequent reactions with the atmosphere are complicated and more often than not depend on the local climate or microclimate. Therefore, the formation of an insoluble zinc patina is irregular both in place and in time. But when the zinc surface becomes wet with rain, mist, or dew, the atmospheric carbon 

The first layer formed influences the corrosion of the zinc throughout its life, although the factor is less significant than the nature of the environment in which it is exposed. Zinc corrosion products occupy a larger volume than the zinc from which they originate and, consequently, a small loss of zinc can often give a large volume of corrosion products. 

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. 

Iron can be beneficial in coatings. The iron-zinc alloys formed in hot dipping or sherardizing can be up to 30% more resistant in mildly acidic conditions, but some workers report lower corrosion resistance with some galvannealed coatings. The iron-zinc alloy layers, while continuing to protect 

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This type of corrosive attack occurs when dissimilar metals (i.e., with a different are in direct electrical... 

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. 

Water Treatment. Sodium sulfite is an agent in the reduction of chlorine or oxygen in water. Dissolved oxygen in boiler water tends to enhance pitting and other types of corrosion. In boilers operated at below 4.82 MPa (700 psi), a residual concentration of 30 ppm of sodium sulfite is generally effective. Catalytic amounts of cobalt are often added to accelerate the reaction of oxygen with sulfite (321,322) (see Water, industrial water treatment). 

Service Life. The service life offered by a coolant is dependent on many factors, including the initial condition of the coolant and the cooling system, the type of water used for dilution, the metals of constmction in the system, the type of corrosion inhibitors and SCAs used, the system operating... 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Parting, or Dealloying, Corrosion This type of corrosion occurs when only one component of an alloy is removed by corrosion. The most common type is dezincification of brass. 

Most Common Types of Prohes There are three most common types of corrosion monitoring probes used. Other types of probes are used, but in smaller numbers. 

Electrical re.slstance probe.s. These probes are the next most common type of corrosion probes after coupons. This type of probe measures changes in the electrical resistance as a thin strip of metal gets thinner with ongoing corrosion. As the metal gets thinner, its resistance increases. This technique was developed in the 1950s by Dravinieks and Cataldi and has undergone many improvements since then. 

Multiinformational Prohes Corrosion probes can provide more information than just corrosion rate. The next three types of probes yield information about the type of corrosion, the kinetics of the corrosion reaction, as well as the local corrosion rate. 

A final type of measurement is the detection of localized corrosion, such as pitting or crevice attack. Several corrosion-measuring probes can be used to detec t localized corrosion. Some can detect locahzed corrosion instantaneously and others only its result. These types of corrosion may contribute little to the actual mass loss, but can be devastating to equipment and piping. Detec tion and measurement of localized corrosion is one of the areas with the greatest potential for the use of some of the newest electrochemicaUy Based corrosion monitoring probes. 

This is an example of measuring the wrong thing. In this case, the probes work adequately, the monitoring system is adequate, as is the monitoring interval, but detection of the type of corrosion cannot be made based on the available data. Different types of probes and testing are required to detect the corrosion problem. 

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. 

One must always bear in mind that potential dependence is not the same in different types of corrosion. Thus critical potential ranges for different kinds of corrosion can overlap or run counter to one another. This is particularly important... 

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. 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

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 diagnosis of possible damage should be made before beginning repairs with other construction measures [48,49]. There should be a checklist [48] of the important corrosion parameters and the types of corrosion effects to be expected. Of special importance are investigations of the quality of the concrete (strength, type of cement, water/cement ratio, cement content), the depth of carbonization, concentration profile of chloride ions, moisture distribution, and the situation regarding cracks and displacements. The extent of corrosion attack is determined visually. Later the likelihood of corrosion can be assessed using the above data. 

Magnesium anodes are widely used in conjunction with enamel coatings. This type of corrosion protection is particularly economical and convenient in small-and medium-sized boilers. The anode only has to ensure protection of small de-... 

When an alloy fails by a distinct crack, you might suspect stress-conosion cracking as the cause. Cracking will occur when there is a combination of corrosion and stress (either externally applied or internally applied by residual stress). It m.ay be either intergranular or trans-granular, depending on the alloy and the type of corrosion. 

Grooving is a type of corrosion particular to environmental conditions where metals are exposed to acid-condensed phases. For example, high concentrations of carbonates in the feed to a boiler can produce steam in the condenser to form acidic condensates. This type of corrosion manifests as grooves along the surface following the general flow of the condensate. 

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