Cell Corrosion

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. 

Attack associated with nonuniformity of the aqueous environments at a surface is called concentration cell corrosion. Corrosion occurs when the environment near the metal surface differs from region to region. These differences create anodes and cathodes (regions differing in electrochemical potential). Local-action corrosion cells are established, and anodic areas lose metal by corrosion. Shielded areas are particularly susceptible to attack, as they often act as anodes (Fig. 2.1). Differences in concentration of dissolved ions such as hydrogen, oxygen, chloride, sulfate, etc. eventually develop between shielded and nearby regions. 

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

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. 

Concentration cell corrosion is caused by chemical concentration differences between areas contacting metal surfaces. An electrochemical cell is established by virtue of the difference between chemical compositions in the water on, and adjacent to, corroding areas. 

Figure 4.1 A) Oxygen concentration cell corrosion beneath a deposit (B) Oxy-...

Severe concentration cell corrosion involves segregation of aggressive anions beneath deposits. Concentrations of sulfate and chloride, in particular, are deleterious. Acid conditions may be established beneath deposits as aggressive anions segregate to these shielded regions. Mineral acids, such as hydrochloric and sulfuric, form by hydrolysis. The mechanism of acid formation is discussed in Chap. 2. 

Substituting one alloy for another may be the only viable solution to a specific corrosion problem. However, caution should be exercised this is especially true in a cooling water environment containing deposits. Concentration cell corrosion is insidious. Corrosion-resistant materials in oxidizing environments such as stainless steels can be severely pitted when surfaces are shielded by deposits. Each deposit is unique, and nature can be perverse. Thus, replacement materials ideally should be tested in the specific service environment before substitution is accepted. 

Underdeposit corrosion is not so much a single corrosion mechanism as it is a generic description of wastage beneath deposits. Attack may appear much the same beneath silt, precipitates, metal oxides, and debris. Differential oxygen concentration cell corrosion may appear much the same beneath all kinds of deposits. However, when deposits tend to directly interact with metal surfaces, attack is easier to recognize. 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Shells, clams, wood fragments, and other biological materials can also produce concentration cell corrosion. Additionally, fragments can lodge in heat exchanger inlets, locally increasing turbulence and erosion-corrosion. If deposits are massive, turbulence, air separation, and associated erosion-corrosion can occur downstream (see Case History 11.5). 

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). 

Scale deposits create conditions for concentration-cell corrosion as they do not form uniformly over the metal surface. Sulfate-reducing bacteria thrive under these deposits, producing hydrogen sulfide and, consequently, increasing the rate of corrosion. Due to the following factors, the drilling fluid environment is ideal for scale deposition [189]. These factors are as follows ... 

The classification given in Table 1.2 is based on the various forms that corrosion may take, but the terminology used in describing corrosion phenomena frequently places emphasis on the environment or cause of attack rather than the form of attack. Thus the broad classification of corrosion reactions into wet or dry is now generally accepted, and the nature of the process is frequently made more specific by the use of an adjective that indicates type or environment, e.g. concentration—cell corrosion, crevice corrosion, bimetallic corrosion and atmospheric corrosion. 

As the corrosion rate, inclusive of local-cell corrosion, of a metal is related to electrode potential, usually by means of the Tafel equation and, of course, Faraday s second law of electrolysis, a necessary precursor to corrosion rate calculation is the assessment of electrode potential distribution on each metal in a system. In the absence of significant concentration variations in the electrolyte, a condition certainly satisfied in most practical sea-water systems, the exact prediction of electrode potential distribution at a given time involves the solution of the Laplace equation for the electrostatic potential (P) in the electrolyte at the position given by the three spatial coordinates (x, y, z). 

There are various different forms and mechanisms of localized concentration cell corrosion. Unfortunately, because one or more types may be manifest at any one time, the results can be quite devastating. 

Although each form of concentration cell may be considered a discrete form of corrosion, in practice, more than one type may occur simultaneously. These forms of corrosion are all characterized by localized differences in concentration of hydrogen, oxygen, chloride, sulfate, and other minerals, but especially oxygen (producing the so-called differential oxygen concentration cell, or differential-aeration cell). The basic mechanisms surrounding each of these specific forms of concentration cell corrosion are discussed next. 

Tuberculation Tuberculation concentration-cell corrosion may be found in all types of boilers, especially HW heating boilers and LP/HP steam-heating boilers. 

Pitting Corrosion Pitting corrosion is a general term for a visible sign of concentration cell corrosion, where further induced-corrosion processes develop, as when chloride attack occurs (although pits also may occur with simple acid corrosion processes). 

All the various forms of concentration cell corrosion described are types of indirect attack induced by the effects of surface shielding, although not all concentration cell corrosion mechanisms involve the presence of oxygen. 

Additional forms of concentration cell corrosion include those involving sodium hydroxide (caustic), chlorides, and sulfates thus, some control over corrosion may be obtained by limiting salinity in the boiler. 

At least as important, however, is the need to ensure clean metal surfaces because, as stated earlier, concentration cell corrosion mechanisms commonly occur under boiler section sludges and deposits. 

Sulfate ions have reactions similar to those of chloride. They are corrosion-causative agents (similar to oxygen and hydrogen) of the various types of concentration cell corrosion. In addition, they also are depassivation agents and may greatly accelerate the risk of stress corrosion mechanisms. Saline corrosion pits resulting from high concentrations of chloride and sulfate salts also may be associated with low pH corrosion because hydrochloric acid and sulfuric acid can form within the pit, under deposits. 

Localized, concentration-cell corrosion (differential aeration corrosion), occurring as Tuberculation corrosion Crevice corrosion Under-deposit corrosion Pitting corrosion All forms of localized, concentration-cell corrosion are indirect attack type corrosion mechanisms. They result in severe metal wastage and can also induce other corrosion mechanisms, e.g. Stress corrosion Corrosion fatigue... 

Other forms of concentration-cell corrosion include Caustic gouging Saline corrosion Combination of free caustic and concentrating effect causes severe metal wastage. High chlorides and sulfates, result in corrosion from depolarization and depassivation effects... 

Antifoulants or metal surface cleaners to reduce the risk of pitting corrosion and other forms of concentration cell corrosion initiated at the metal surface by shielding effects from inorganic deposits. These could also be called deposit control agents. 

Various other forms of concentration-cell corrosion. 

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