Forms of Corrosion

The differing forms of corrosion can be divided into the following eight categories based on the appearance of the corrosion damage or the mechanism of attack  

Uniform or general corrosion Galvanic corrosion Pitting corrosion 

Crevice corrosion, including corrosion under tubercles or deposits, filiform corrosion, and poultice corrosion 

Erosion-corrosion, including cavitation erosion and fretting corrosion Intergranular corrosion, including sensitization and exfoliation Dealloying 

It is obvious from the data that galvanic corrosion of zinc in rural atmosphere can be five times the rate in corresponding atmospheric corrosion and three times that in marine atmospheres. Mild steel appears to be the most efficient cathode among the metals studied. More detailed discussions are given in the literature.92 

As noted earlier pitting corrosion of zinc is usually not encountered, with the exception of corrosion in nonuniform soils, distilled water containing vertical samples of zinc panels, and galvanized steel in hot water tanks. In hot water tanks containing soft water, 

Intergranular corrosion has been observed in Zn-Al, Zn-Pb and Zn-Mg alloys and some data in the case of Zn-Al alloys are given in Table 4.83. 

The intergranular corrosion of Zn-Al alloys may be attributed to the preferential attack at the Al-rich phase present in the grain boundaries. 

Typical applications of zinc-coated steel and products are given in Table 4.84. 

The variables in the corrosion environment, which inclnde flnid velocity, temperatnre, and composition, can have a decided inflnence on the corrosion properties of the materials that are in contact with it. In most instances, increasing flnid velocity enhances the rate of corrosion dne to erosive effects, as discnssed later in the chapter. The rates of most chemical reactions rise with increasing temperatnre this also holds for most corrosion sitnations. Increasing the concentration of the corrosive species (e.g., H ions in acids) in many sitnations produces a more rapid rate of corrosion. However, for materials capable of passivation, raising the corrosive content may resnlt in an active-to-passive transition, with a considerable reduction in corrosion. 

Cold working or plastically deforming dnctile metals is nsed to increase their strength however, a cold-worked metal is more snsceptible to corrosion than the same material in an annealed state. For example, deformation processes are nsed to shape the head and point of a nail conseqnently, these positions are anodic with respect to the shank region. Thus, differential cold working on a strnctnre shonld be a consideration when a corrosive environment may be encoimtered dnring service. 

Uniform attack is a form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease. 

The galvanic series in Table 17.2 indicates the relative reactivities in seawater of a number of metals and alloys. When two alloys are conpled in seawater, the one lower in the series experiences corrosion. Some of the alloys in the table are grouped in brackets. Generally the base metal is the same for these bracketed alloys, and there is little danger of corrosion if alloys within a single bracket are conpled. It is also worth noting from this series that some alloys are hsted twice (e.g., nickel and the stainless steels), in both active and passive states. 

The rate of galvanic attack depends on the relative anode-to-cathode snrface areas that are exposed to the electrolyte, and the rate is related directly to the cathode-anode area ratio—that is, for a given cathode area, a smaller anode corrodes more rapidly than a larger one because corrosion rate depends on cnrrent density (Equation 17.24)—the current per unit area of corroding siud ace—and not simply the ciurent. Thns, a high cnrrent density results for the anode when its area is small relative to that of the cathode. 

---

There are two serious problems associated with continuous tar distillation. Coal tar contains two types of components highly corrosive to ferrous metals. The ammonium salts, mainly ammonium chloride, associated with the entrained Hquor remain in the tar after dehydration, tend to dissociate with the production of hydrochloric acid and cause rapid deterioration of any part of the plant in which these vapors and steam are present above 240°C. Condensers on the dehydration column and fractionation columns are also attacked. This form of corrosion is controlled by the addition of alkaU (10% sodium carbonate solution or 40% caustic soda) to the cmde tar in an amount equivalent to the fixed ammonia content. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Atmospheric exposure, fresh and salt waters, and many types of soil can cause uniform corrosion of copper aHoys. The relative ranking of aHoys for resistance to general corrosion depends strongly on environment and is relatively independent of temper. Atmospheric corrosion, the least damaging of the various forms of corrosion, is generaHy predictable from weight loss data obtained from exposure to various environments (31) (see Corrosion and CORROSION CONTKOL). 

The most common form of corrosion is uniform corrosion, in which the entire metal surface degrades at a near uniform rate (1 3). Often the surface is covered by the corrosion products. The msting of iron (qv) in a humid atmosphere or the tarnishing of copper (qv) or silver alloys in sulfur-containing environments are examples (see also SiLVERAND SILVER ALLOYS). High temperature, or dry, oxidation, is also usually uniform in character. Uniform corrosion, the most visible form of corrosion, is the least insidious because the weight lost by metal dissolution can be monitored and predicted. 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

This form of corrosion can result because of a deficiency of oxygen in the crevice, acidity changes in the crevice, buildup of ions in the crevice, or depletion of an inhibitor. 

This form of corrosion can be prevented in some instances by ehm-inating high stresses. Stresses developed during fabrication, pai4icu-larly during welding, are frequently the main source of troiible. Of course, temperature and concentration are also important factors in this type of attack. 

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

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 three major forms of concentration cell corrosion are crevice corrosion, tuberculation, and underdeposit attack. Each form of corrosion is common in cooling systems. Many corrosion-related problems in the cooling water environment are caused by these three forms of wastage. The next three chapters—Chap. 2, Crevice Corrosion, Chap. 3, Tuberculation, and Chap. 4, Underdeposit Corrosion — will discuss cooling water system corrosion problems. 

Deposits cause corrosion both directly and indirectly. If deposits contain corrosive substances, attack is direct interaction with the aggressive deposit causes wastage. Shielding of surfaces below deposits produces indirect attack corrosion occurs as a consequence of surface shielding provided by the deposit. Both direct and indirect attack may involve concentration cell corrosion, but indirect attack almost always involves this form of corrosion. 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

The importance of biologically influenced corrosion (particularly micro-biologically influenced corrosion) has been underestimated for many years. Recently, more attention has been paid to biological forms of corrosion— yet more attention does not always mean an improved situation. 

Many forms of corrosion resemble each other. However, it is just as true that each form of corrosion produces a unique fingerprint by which it can be differentiated from all other forms of attack. Biologically influenced corrosion is no exception. 

This case history illustrates the paradox so often encountered in microbiologically influenced corrosion. Clearly, two corrosion mechanisms were operating in the system, namely, acid attack and microbiologically influenced corrosion. To what degree each mechanism contributed to wastage is difficult to quantify after the fact. This was especially the case here, since other areas of the rolling oil system were attacked by a predominantly acidic form of corrosion. 

Dealloying can be reduced (as can any other form of corrosion) by good system operation and the judicious use of appropriate materials and chemical treatment. Specific categories needing attention follow. 

Chemical corrosion inhibition can reduce all forms of corrosion including dealloying. In particular, filmers such as tolyltriazole are effective in reducing corrosion of yellow metals. 

Graphitic corrosion has two distinct features that are useful in distinguishing it from other forms of corrosion. First, it affects an unusually limited number of metals the only metals commonly affected are gray cast iron and nodular cast iron. Second, metal that has experienced graphitic corrosion may retain its original appearance and dimensions. Consequently, graphitic corrosion frequently escapes detection. 

Dillon, C. P. (ed.). Forms of Corrosion, Recognition and Prevention, NACE Handbook I, National Association of Corrosion Engineers, Houston, Texas, 1982. 

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. 

Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

Gaseous corrosion is a general form of corrosion whereby a metal is exposed to a gas (usually at elevated temperatures). Direct oxidation of a metal in air is the most common cause. Cast iron growth is a specific form of gaseous corrosion in which corrosion products accumulate onto the metal surface (and particularly at grain boundaries) to the extent that they cause visible thickening of the metal. The entire metal thickness may succumb to this before loss of strength causes failure. 

During design the life expeetaney, due to ereep or other forms of corrosion, should be estimated and examination or replacement planned. Cheap fittings, such as studs, bolts, and nuts, should be replaced in good time. Not to do so is penny-pinching and expensive in the end. 

It is easy to buy safety equipment. All we need is money, and if we make enough fuss we get the equipment in the end. It is much more difficult to make sure the equipment is kept in full working order when the initial enthusiasm has faded. All procedures, including testing and maintenance procedures, are subject to a form of corrosion more rapid than that which affects the steelwork and can vanish without trace once managers lose interest. A continuous auditing effort is needed to make sure that procedures are maintained. 

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

Corrosion may take various forms and may combine other forms of damage (erosion, wear, fatigue, etc.) to cause equipment failure. The forms of corrosion most encountered in drilling equipment are uniform corrosion and galvanic corrosion. 

---