Concentration cell corrosion severe

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. 

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... 

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. 

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. 

The precautions generally applicable to the preparation, exposure, cleaning and assessment of metal test specimens in tests in other environments will also apply in the case of field tests in the soil, but there will be additional precautions because of the nature of this environment. Whereas in the case of aqueous, particularly sea-water, and atmospheric environments the physical and chemical characteristics will be reasonably constant over distances covering individual test sites, this will not necessarily be the case in soils, which will almost inevitably be of a less homogeneous nature. The principal factors responsible for the corrosive nature of soils are the presence of bacteria, the chemistry (pH and salt content), the redox potential, electrical resistance, stray currents and the formation of concentration cells. Several of these factors are interrelated. 

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 of fouled end-covers and/or tube sheets. This is a frequent problem, especially when cooling water flows through the shell. It can take several forms, such as massive concentration cell/pitting... 

Production of differential aeration cell. A scatter of individual barnacles on a stainless steel surface creates oxygen concentration cells. The formation of biofilm generates several critical conditions for corrosion initiation. Uncovered areas will have free access to oxygen and act as cathodes, while the covered zones act as anodes. Underdeposit corrosion (crevice corrosion) or pitting can occur. Depending on the oxidizing capacity of the bacteria and the chloride ion concentration, the corrosion rate can be accelerated. However, the presence of a biofilm does not necessarily mean that there will always be a significant effect on corrosion. (Dexter)5... 

The surface finish of the component also has an impact on the mode and severity of the corrosion that can occur. Rough surfaces or tight crevices can facilitate the formation of concentration cells. Surface cleanliness can also be an issue with deposits or films acting as initiation sites. Biological growths can behave as deposits, or can change the underlying surface chemistry to promote corrosion. 

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. 

For many locations, the phosphonate/dispersant combination provides effective scale control without necessity of acidification. How does this type of treatment function as a corrosion inhibitor One possible explanation is that if a surface is clean, free from deposits of any type that can set up local concentration cells, then any corrosion occurring will be uniformly distributed and reduced in severity. This permits normal service life of system equipment. This approach to corrosion control is called the "clean system concept." Copper inhibitors (thiazoles and triazoles) function well in these combinations. Addi-... 

Distribution and intensity of corrosion is therefore usually determined by the access of oxygen to the various parts of the surface. Uneven oxygen access causes concentration cells (Section 7.5). Very often the corrosion is diffusion controlled, i.e. the diffusion-limiting current density is of special interest. Several examples of corrosion rate determined by the oxygen reduction rate are dealt with in Chapter 7. 

If an electrolsrte can enter the crevice formed by the faying surfaces of two almninmn surfaces an oxygen concentration, and subsequently chemical concentration, cell can form and cause accelerated locahzed attack. As such, corrosion protection often is required in joints, even when not needed on the freely exposed almninmn. The severity of crevice corrosion depends on the electrolyte and how readily it is replenished. It also is influenced by the geometric shape of the crevice, and the ratio of active crevice cathode area to the adjacent external cathode area. The best protective measures are to design so that crevices will drain, and to effectively seal crevices to prevent ingress of the electrolyte. 

Microbiologically influenced corrosion (MIC) is used to designate corrosion resulting from the presence and activities of microorganisms within biofilms on a material surface. Such microorganisms can accelerate and control corrosion reactions by several mechemisms formation of differential or concentration cells, formation of metabolites, such as sulfides md organic and inorganic acids metal oxidation and reduction, and deactivation of corrosion inhibitors. 

In the above cell, HCl is in two different concentrations. The activity (molality x activity coefficient) ai is greater than activity 02 fli > <12-Several types of concentration cells are encountered in corrosion. For example, a concentration cell is formed if one end of a pipe is exposed to soil and the other end to air. The end of the pipe in air is exposed to a high concentration of oxygen than the end of the pipe in the soil. The formation of a concentration cell leads to differential aeration corrosion in buried structures in the soil. 

For many waste streams, electrical efficiencies are compromised owing to the corrosivity of the solution toward the precipitated metals and/or the low concentrations of metals that must be removed. The presence of chloride in the solution is particularly troublesome because of the formation of elemental chlorine at the anode. Several commercial cells have become available that attempt to address certain of these problems (19). 

Severe attack frequently occurs at a water-line, which in practice can range from structural steel partly immersed in a natural water to a lacquered tin can used for containing emulsion paint. This can be illustrated by adding increeising amounts of sodium carbonate to a sodium chloride solution in which a steel plate is partly immersed (Fig. 1.48c, d and e). With increase in concentration of the inhibitor, attack decreases and becomes confined to the water-line. The attack at the water-line is intense and is characterised by a triangular pasty mass of corrosion products bounded on the upper surface by a dark-brown membrane that follows the contour of the water-line. The mechanism of water-line attack is not clear, but it is likely that the membrane of corrosion products results in the formation of an occluded cell, in which the anolyte and catholyte are prevented from mixing. These occluded cells are discussed in more detail subsequently. 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

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. 

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