Corrosion cells concentration cell

Figure 6.20 Two types of crevice corrosion oxygen concentration cell and metal ion concentration cell774...

Brass Pitting Cavitation De-alloying Stress corrosion cracking Concentration cell... 

Crevice corrosion, and concentration cell corrosion, susceptibility can be measured directly, by exposing test specimens to the environment. ASTM F 746, G 48, and G 78 can be used for evaluating these forms of corrosion. These methods were designed to measure crevice corrosion in severe electrolytes such as salt solutions, but can be modified for freshwaters. 

Biological Corrosion The metabohc activity of microorganisms can either directly or indirectly cause deterioration of a metal by corrosion processes. Such activity can (1) produce a corrosive environment, (2) create electrolytic-concentration cells on the metal surface, (3) alter the resistance of surface films, (4) have an influence on the rate of anodic or cathodic reaction, and (5) alter the environment composition. 

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

Cleaning Specimens after Test Before specimens are cleaned, their appearance should be observed and recorded. Locations of deposits, variations in types of deposits, and variations in corrosion produces are extremely important in evaluating localized corrosion such as pitting and concentration-cell attack. 

Depth of localized corrosion should be reported for the actual test period and not interpolated or extrapolated to an annual rate. The rate of initiation or propagation of pits is seldom uniform. The size, shape, and distribution or pits should oe noted. A distinction should be made between those occurring underneath the supporting devices (concentration cells) and those on the surfaces that were freely exposed to the test solution. An excellent discussion of pitting corrosion has been pubhshed [Corro.sion, 25t (January 1950)]. 

Local variations in temperature and crevices that permit the accumulation of corrosion products are capable of allowing the formation of concentration cells, with the result of accelerated local 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. 

As rust accumulates, oxygen migration is reduced through the corrosion product layer. Regions below the rust layer become oxygen depleted. An oxygen concentration cell then develops. Corrosion naturally becomes concentrated into small regions beneath the rust, and tubercles are born. 

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. 

Almost all cooling water system deposits are waterborne. It would be impossible to list each deposit specifically, but general categorization is possible. Deposits are precipitates, transported particulate, biological materials, and a variety of contaminants such as grease, oil, process chemicals, and silt. Associated corrosion is fundamentally related to whether deposits are innately aggressive or simply serve as an occluding medium beneath which concentration cells develop. An American... 

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. 

Wastage was caused by classic long-term underdeposit corrosion. The combined effects of oxygen concentration cells, low flow, and contamination of system water with high chloride- and sulfate-concentration makeup waters caused corrosion. 

Zinc is susceptible to attack from oiQ gen concentration cells. Shielded areas or areas depleted in oxygen concentration tend to corrode, forming voluminous, white, friable corrosion products. Once the zinc layer is breached, the underlying steel becomes susceptible to attack and is severely wasted locally (Figs. 5.12 and 5.13). 

Two sections of steel condenser tubing experienced considerable metal loss from internal surfaces. An old section contained a perforation the newer section had not failed. A stratified oxide and deposit layer overlaid all internal surfaces (Fig. 5.14). Corrosion was severe along a longitudinal weld seam in the older section (Fig. 5.15). Differential oxygen concentration cells operated beneath the heavy accumulation of corrosion products and deposits. The older tube perforated along a weld seam. 

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). 

Microstructural examinations revealed V-shaped openings along the tube seam, some extending into as much as 50% of the tube wall thickness. The incompletely closed seam provided a crevice in which differential concentration cells developed (see Chap. 2, Crevice Corrosion ). The resulting localized corrosion caused the observed pits. 

Chemical removal of deposits and corrosion products revealed the appearance of the groove (Fig. 14.5). The crevice formed by the incompletely fused weld seam fostered the establishment of differential concentration cells (see Chap. 2). This resulted in localized corrosion and eventual perforation through the greatly thinned tube wall at the bottom of the crevice. The tubercle, which is composed of corrosion products, is a simple result of the corrosion process occurring locally within the crevice. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

Heterogeneous surface areas consist of anodic regions at corrosion cells (see Section 2.2.4.2) and objects to be protected which have damaged coating. Local concentrations of the current density develop in the area of a defect and can be determined by measurements of field strength. These occur at the anode in a corrosion cell in the case of free corrosion or at a holiday in a coated object in the case of impressed current polarization (e.g., cathodic protection). Such methods are of general interest in ascertaining the corrosion behavior of metallic construction units... 

Both reactions indicate that the pH at the cathode is high and at the anode low as a result of the ion migration. In principle, the aeration cell is a concentration cell of H ions, so that the anode remains free of surface films and the cathode is covered with oxide. The J U curves in Fig. 2-6 for anode and cathode are kept apart. Further oxidation of the corrosion product formed according to Eq. (4-4) occurs at a distance from the metal surface and results in a rust pustule that covers the anodic area. Figure 4-2 shows the steps in the aeration cell. The current circuit is completed on the metal side by the electron current, and on the medium side by ion migration. 

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