Concentration cell corrosion causes

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. 

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. 

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. 

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). 

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. 

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. 

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... 

A) Oxygen concentration cell corrosion beneath a deposit (B) Oxygen concentration cell corrosion in a beaker containing an aerated piece of steel and an unaerated piece of steel. A potential develops between the aerated and unaerated steel pieces. Steel exposed to the lower dissolved oxygen concentration corrodes. Beneath a deposit (A) the oxygen-poor environment causes wastage. 

Sulfates are causative agents (along with oxygen, hydrogen, etc.) of the various types of concentration cell corrosion that result in pits, crevice attack, or tuberculation. 

Abrasion removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents, forming sites for impact hydraulic splitting. Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack. 

Oxygen is essential for corrosion to occur in the pH range commonly found in soils. Soils such as clays restrict oxygen movement therefore, these soils are prone to anaerobic conditions. Often, however, soils such as clays tend to promote galvanic corrosion cells that are caused by differences in oxygen concentration (differential concentration cell corrosion). Bacterial corrosion often occurs in clay soils because anaerobic conditions and organic matter promote bacterial growth. A common type of bacteria that causes... 

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. 

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. 

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. 

The carbon dioxide produced can contribute to the corrosion of metal. The deposits of ferric hydroxide that precipitate on the metal surface may produce oxygen concentration cells, causing corrosion under the deposits. Gallionalla and Crenothrix are two examples of iron-oxidizing bacteria. 

In extreme cases irritant chemicals can have a corrosive action. Corrosive substances can attack and weaken materials of construction, as mentioned in Chapter 3. They can also attack living tissue (e.g. to cause skin ulceration and in severe cases chemical burns), kill cells and possibly predispose to secondary bacterial invasion. Thus while acute irritation is a local and reversible response, corrosion is irreversible destruction at the site of the contact. The outcome is influenced by the nature of the compound, the concentration, duration of exposure, the pH (see Figure 4.1) etc. Thus dilute mineral acids may be irritant whereas at higher concentrations they may cause corrosion. 

The problem with relying solely on anodic area corrosion inhibition is the risk of local film damage, which concentrates the corrosion current flow and permits a highly active anodic cell to be developed and causing accelerated corrosion to take place. This in turn leads to severe metal wastage, often in the form of deep pitting. 

Corrosion. Fink 16) of Battelle Memorial Institute has presented the results of a literature study combined with views of experts on the corrosion of metals by sea water. The study revealed a paucity of data on corrosion at elevated temperatures. The Cl ion is the chief culprit in causing corrosion, but an important factor is dissolved oxygen and it is probable that oxygen-free sea water would have very little corrosive action, at least at ordinary temperatures. Natural sea water may have very different corrosion effects from synthetic sea water because of the organic content. Fouling of the surface by organic deposits can lead to severe pitting due to concentration-cell effects. Consequently corrosion by actual water is not readily simulated in the laboratory by synthetic sea water. 

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