These values are largely based on an examination of several structures in which it was found there was a low risk of corrosion up to 0.
The Norwegian Code, NS , allows an acid-soluble chloride content of 0. Both these codes are under revision. Other codes have different limits, though the rationale for these limits is not well established.
Corrosion of prestressing steel is generally of greater concern than corrosion of conventional reinforcement because of the possibility that corrosion may cause a local reduction in cross section and failure of the steel. The high stresses in the steel also render it more vulnerable to stresscorrosion cracking and, where the loading is cyclic, to corrosion fatigue.
However, most examples of failure of prestressing steel that have been reported3. Nevertheless, because of the greater vulnerability and the consequences of corrosion of prestressing steel, chloride limits in the mix ingredients are lower than for conventional concrete.
Based on the present state of knowledge, the following chloride limits in concrete used in new construction, expressed as a percentage by weight of portland cement, are recommended to minimize the risk of chloride-induced corrosion.
If the materials meet the requirements given in either of the relevant columns in the previous table they are acceptable.
If they meet neither of the relevant limits given in the table then they may be tested using the Soxhlet test method. Some aggregates, such as those discussed previously,3. The Soxhlet test appears to measure only those chlorides that contribute to the corrosion process, thus permitting the use of some aggregates that would not be allowed if only the ASTM C or ASTM C tests were used. If the materials fail the Soxhlet test, then they are not suitable. For prestressed and reinforced concrete that will be exposed to chlorides in service, it is advisable to maintain the lowest possible chloride levels in the mix to maximize the service life of the concrete before the critical chloride content is reached and a high risk of corrosion develops.
Consequently, chlorides should not be intentionally added to the mix ingredients even if the chloride content in the materials is less than the stated limits. In many exposure conditions, such as highway and parking structures, marine environments, and industrial plants where chlorides are present, additional protection against corrosion of embedded steel is necessary. Since moisture and oxygen are always necessary for electrochemical corrosion, there are some exposure conditions where the chloride levels may exceed the recommended values and corrosion will not occur.
Concrete which is continuously submerged in seawater rarely exhibits corrosioninduced distress because there is insufficient oxygen present. Similarly, where concrete is continuously dry, such as the interior of a building, there is little risk of corrosion from chloride ions present in the hardened concrete.
However, interior locations that are wetted occasionally, such as kitchens or laundry rooms or buildings constructed with lightweight aggregate that is subsequently sealed e. Whereas the designer has little control over the change in use of a building or the service environment, the chloride content of the mix ingredients can be controlled.
Estimates or judgments of outdoor dry environments can be misleading. The bridges were located in an arid area where the annual rainfall was about 5 in. Within 5 years of construction, many were showing signs of corrosion-induced spalling and most were removed from service within 10 years.
For these reasons, the committee believes a conservative approach is necessary. The maximum chloride limits suggested in this report differ from those suggested by ACI Committee When making a comparison between the figures, it should be noted that the. As noted previously, Committee has taken a more conservative approach because of the serious consequences of corrosion, the conflicting data on corrosion threshold values, and the difficulty of defining the service environment throughout the life of a structure.
Various nonferrous metals and alloys will corrode in damp or wet concrete. Surface attack of aluminum occurs in the. Anodizing provides no protection. Much more serious corrosion can occur if the concrete contains chloride ions, particularly if there is electrical metalto-metal contact between the aluminum and steel reinforcement, because a galvanic cell is created. Serious cracking or splitting of concrete over aluminum conduits has been reported.
Other metals such as zinc, nickel, and cadmium, which have been evaluated for use as coatings for reinforcing steel, are discussed elsewhere in this chapter. Additional information is contained in Reference 3.
Where concrete will be exposed to chloride, the concrete should be made with the lowest water-cement ratio consistent with achieving maximum consolidation and density. The effects of water-cement ratio and degree of consolidation on the rate of ingress of chloride ions are shown in Fig. Concrete with a water-cement ratio of 0. A low water-cement ratio is not, however, sufficient to insure low permeability. As shown in Fig. The combined effect of water-cement ratio and depth of cover is shown in Fig.
Thus, 40 mm 1. Equivalent protection was afforded by 70 mm 2. On the basis of this work, ACI Even greater cover, or the provision of additional corrosion protection treatments, may be required in some environments. These recommendations can also be applied to other reinforced concrete components exposed to chloride ions and intermittent wetting and drying. Even where the recommended cover is specified, construction practices must insure that the specified cover is achieved.
Conversely, placing tolerances, the method of construction, and the level of inspection should be considered in determining the specified cover. The role of cracks in the corrosion of reinforcing steel is controversial. Two schools of thought exist. The cracks thus accelerate the onset of the corrosion processes and, at the same time, provide space for the deposition of the corrosion products. The other viewpoint is that while cracks may accelerate the onset of corrosion, such.
Since the chloride ions eventually penetrate even uncracked concrete and initiate more widespread corrosion of the steel, the result is that after a few years service there is little difference between the amount of corrosion in cracked and uncracked concrete. The differing viewpoints can be partly explained by the fact that the effect of cracks is a function of their origin, width, intensity, and orientation. Where the crack is perpendicular to the reinforcement, the corroded length of intercepted bars is likely to be no more than three bar diameters.
Studies have shown that cracks less than about 0. Consequently, it has been suggested that the control of surface crack widths in design codes is not the most rational approach. Controlled cracks are those which can be reasonably predicted from a knowledge of section geometry and loading. For cracking perpendicular to the main reinforcement, the necessary conditions for crack control are that there be sufficient steel so that it remains in the elastic state under all loading conditions, and that at the time of cracking, the steel is bonded i.
Examples of uncontrolled cracking are cracks resulting from plastic shrinkage, settlement, or an overload condition.
Uncontrolled cracks are frequently wide and usually cause concern, particularly if they are active. However, they cannot be dealt with by conventional design procedures, and measures have to be taken to avoid their occurrence or, if they are unavoidable, to induce them at places where they are unimportant or can be conveniently dealt with, by sealing for example. Since external sources of chloride ions are waterborne, a barrier to water will also act as a barrier to any dissolved chloride ions.
The requirements for the ideal waterproofing system are straightforward;3. The number of types of products manufactured to satisfy these requirements makes generalization difficult.
Any system of classification is arbitrary, though one of the most useful is the distinction between the preformed sheet systems and the liquid-applied materials. Although it is more difficult to control the quality of the liquid-applied systems, they are easier to apply and tend to be less expensive. Given the different types and quality of available waterproofing products, the differing degrees of workmanship, and the wide variety of applications, it is not surprising that laboratory3.
Sheet systems generally perform better than liquid-applied systems in laboratory screening tests because quality of workmanship is not a factor. Although there has been little uniformity in both methods of test or acceptance criteria, permeability usually determined by electrical resistance measurements has generally been adopted as the most important criterion.
However, some membranes do offer substantial resistance to chloride and moisture intrusion, even when pinholes or bubbles occur in the membrane. Experience has ranged from satisfactory3. The frequency of blisters occurring is controlled by the porosity and moisture content of the concrete3. Water or water vapor is not a necessary requirement for blister formation, but is often a contributing factor. Blisters may also result from an increase in concrete temperature or a decrease in atmospheric pressure during or shortly after application of membranes.
The rapid expansion of vapors during the application of hot-applied products sometimes causes punctures which are termed blowholes in the membrane. Membranes can be placed without blisters if the atmospheric conditions are suitable during the curing period. Once cured, the adhesion of the membrane to the concrete is usually sufficient to resist blister formation. To insure good adhesion, the concrete surface must be carefully prepared and be dry and free from curing membranes, laitance, and contaminants such as oil drippings.
Sealing the concrete prior to applying the membrane is possible, but rarely practical. Venting layers have been used in Europe, but rarely in North America, to prevent blister formation by allowing the vapor pressures to disperse beneath the membrane. The disadvantages of using venting layers are that they require controlled debonding of the membrane, leakage through the membrane is not confined to the immediate area of the puncture, and they increase cost.
Laboratory studies have demonstrated that polymer impregnated concrete PIC is strong, durable, and almost impermeable. The properties of PIC are largely determined by the polymer loading in the concrete.
Maximum polymer loadings are achieved by drying the concrete to remove nearly all the evaporable water, removing air by vacuum techniques, saturation with a monomer under pressure, and polymerizing the monomer in the voids of the concrete while simultaneously preventing evaporation of the monomer. In the initial laboratory studies on PIC, polymerization was accomplished by gamma radiation. Consequently, chemical initiators, which decompose under the action of heat or a chemical promoter, have been used exclusively in field applications.
Multifunctional monomers are often used to increase the rate of polymerization. The physical properties of PIC are determined by the extent to which the ideal processing conditions are compromised.
Since prolonged heating and vacuum saturation are difficult to achieve, and increase processing costs substantially, most field applications have been aimed toward producing only a surface polymer impregnation, usually to a depth of about 25 mm 1 in.
Such partially impregnated slabs have been found to have good resistance to chloride penetration in laboratory studies, but field applications have not always been satisfactory. Some of the disadvantages of PIC are that the monomers are expensive and that the processing is lengthy and costly. The principal deficiency identified to date has been the tendency of the concrete to crack during heat treatment.
Most monomers have a low tolerance to moisture and low temperatures; hence the substrate must be dry and in excess of 4 C 40 F. Improper mixing of the two or more components of the polymer has been a common source of problems in the field. Aggregates must be dry so as not to inhibit the polymerization reaction. Workers should wear protective clothing when working with epoxies and some other polymers because of the potential for skin sensitization and dermatitis.
A bond coat of neat polymer is usually applied ahead of the polymer concrete. Blistering, which is a common phenomenon in membranes, has also caused problems in the application of polymer concrete overlays.
A number of applications were reported in the s. More recently, experimental polymer overlays based on a polyester-styrene monomer have been placed, using heavy-duty finishing equipment to compact and finish the concrete. The overlay may be placed before the first-stage concrete has set or several days later, in which case a bonding layer is used between the two lifts of concrete. The advantage of the first alternative is that the overall time of construction is shortened and costs minimized.
In the second alternative, cover to the reinforcing steel can be assured, and small construction tolerances achieved because dead load deflections from the overlay are very small. No matter which sequence of construction is employed, materials can be incorporated in the overlay to provide superior properties, such as resistance to salt penetration and wear and skid resistance, than possible using single stage construction.
Where the second stage concrete is placed after the first stage has hardened, sand or water blasting is required to remove laitance and to produce a clean, sound surface. Resin curing compounds should not be used on the firststage construction because they are difficult to remove. Etching with acid was once a common means of surface preparation.
Several different types of concrete have been used as concrete overlays including conventional concrete,3. The water-cement ratio is reduced to the minimum practical usually about 0. The concrete is air-entrained and a water-reducing admixture or mild retarder is normally used. The use of such a high cement factor and low workability mixture dictates the method of mixing, placing, and curing the concrete. Following preparation of the first-stage concrete, a bonding agent of either mortar or cement paste is brushed into the base concrete just before the application of the overlay.
The base concrete is not normally prewetted. The overlay concrete is mixed on site, using either a stationary paddle mixer. The concrete must be compacted and screeded to the required surface profile using equipment specially designed to handle stiff mixtures.
Such machines are much heavier and less flexible than conventional finishing machines and have considerable vibratory capacity. The permeability of the concrete to chloride ions is controlled by its degree of consolidation, which is often checked with a nuclear density meter as concrete placement proceeds. Wet burlap is placed on the concrete as soon as practicable without damaging the overlay usually within 20 min of placing , and the wet curing is continued for at least 72 hr.
Curing compounds are not used, since not only is externally available water required for more complete hydration of the cement, but the thin overlay is susceptible to shrinkage cracking and the wet burlap provides a cooling effect by evaporation of the water. Low-slump concrete was originally proposed as a repair material for concrete pavements3.
More recently, concrete overlays have been used as a protection against reinforcing steel corrosion in new bridges. In general, the performance of low-slump overlays has been good.
The water of suspension in the emulsion hydrates the cement and the polymer provides supplementary binding properties to produce a concrete having a low water-cement ratio, good durability, good bonding characteristics, and a high degree of resistance to penetration by chloride ions, all of which are desirable properties in a concrete overlay. The latex is a colloidal dispersion of synthetic rubber particles in water. The particles are stabilized to prevent coagulation, and antifoaming agents are added to prevent excessive air entrapment during mixing.
Styrene-butadiene latexes have been used most widely. The rate of addition of the latex is approximately 15 percent latex solids by weight of the cement. The construction procedures for latex-modified concrete are similar to those for low-slump concrete with minor modifications. The principal differences are: 1. The base concrete must be prewetted for at least 1 hr prior to placing the overlay, because the water aids penetration of the base and delays film formation of the latex.
A separate bonding agent is not always used, because sometimes a portion of the concrete itself is brushed over the surface of the base. The mixing equipment must have a means of storing and dispensing the latex. The latex-modified concrete has a high slump so that conventional finishing equipment can be used.
Air entrainment of the concrete is believed not required for resistance to freezing and thawing. A combination of moist curing to hydrate the portland cement and air drying to develop the film forming qualities of the latex are required. Typical curing times are 24 hr wet curing, followed by 72 hr of dry curing.
The film-formation property of the latex is temperature sensitive and film strengths develop slowly at temperatures below 13 C 55 F. Curing periods at lower temperatures may need to be extended and application at temperatures less than 7 C 45 F is not recommended.
Hot weather causes rapid drying of the latex-modified concrete, which makes finishing difficult and promotes shrinkage cracking. Some contractors have placed overlays at night to avoid these problems. The entrapment of excessive amounts of air during mixing has also been a problem in the field. Most specifications limit the total air content to 6. Higher air contents reduce the flexural, compressive, and bond strengths of the overlay.
Furthermore, the permeability to chloride ions increases significantly at air contents greater than about 9 percent. Where a texture is applied to the concrete as, for example, grooves to impart good skid resistance, the time of application of the texture is crucial. If applied too soon, the edges of the grooves collapse because the concrete flows. If the texturing operation is delayed until after the latex film forms, the surface of the overlay tears and, since the film does not reform, cracking often results.
High material costs and the superior performance of latexmodified concrete in chloride penetration tests have led to latex-modified concrete overlays being thinner than most low-slump concrete overlays. Typical thicknesses are 40 and 50 mm 1.
Although latex-modified overlays were first used in ,3. Performance has been generally satisfactory, though extensive cracking and some debonding have been reported,3.
The most serious deficiency reported has been the widespread occurrence of shrinkage cracking in the overlays. Many of these cracks have been found not to extend through the overlay and it is uncertain whether this will impair long-term performance. Stainless steel reinforcement has been used in special applications, especially as hardware for. Stainless-steel-clad bars have been evaluated in the FHWA time-to-corrosion studies, and found to reduce the frequency of corrosion-induced cracking compared to black steel in the test slabs, but did not prevent it.
However, it was not determined whether the cracking was the result of corrosion of the stainless steel or corrosion of the basis steel at flaws in the cladding. In general, metals with a more negative corrosion potential less noble than steel, such as zinc and cadmium, give sacrificial protection to the steel. If the coating is damaged, a galvanic couple is formed in which the coating is the anode.
Noble coatings such as copper and nickel protect the steel only as long as the coating is unbroken, since any exposed steel is anodic to the coating. Even where steel is not exposed, macrocell corrosion of the coating may occur in concrete through a mechanism similar to the corrosion of uncoated steel. Results of the performance of galvanized bars have been conflicting, in some cases extending the time-to-cracking of laboratory specimens,3.
Marine studies3. In general, it appears that only a slight increase in life will be obtained in severe chloride environments. The epoxy coating isolates the steel from contact with oxygen, moisture, and chloride. The process of coating the reinforcing steel with the epoxy consists of electrostatically applying finely divided epoxy powder to thoroughly cleaned and heated bars. Many plants operate a continuous production line and many have been constructed specifically for coating reinforcing steel.
Integrity of the coating is monitored by a holiday detector installed directly on the production line. The use of epoxy-. The chief difficulty in using epoxy-coated bars has been preventing damage to the coating in transportation and handling. Cracking of the coating has also been observed during fabrication where there has been inadequate cleaning of the bar prior to coating or the thickness of the coating has been outside specified tolerances. Padded bundling bands, frequent supports, and nonmetallic slings are required to prevent damage during transportation.
Coated tie wires and bar supports are needed to prevent damage during placing. Accelerated time-to-corrosion studies have shown that nicks and cuts in the coating do not cause rapid corrosion of the exposed steel and subsequent distress in the concrete.
Subsequent tests3. Consequently, for long life in severe chloride environments, consideration should be given to coating all the reinforcing steel.
If only some of the steel is coated, precautions should be taken to assure that the coated bars are not electrically coupled to large quantities of uncoated steel. A damaged coating can be repaired using a two-component liquid epoxy, but it is more satisfactory to adopt practices that prevent damage to the coating and limit touch-up only to bars where the damage exceeds approximately 2 percent of the area of the bar.
Studies have demonstrated that epoxy-coated, deformed reinforcing bars embedded in concrete can have bond strengths and creep behavior equivalent to those of uncoated bars. The mechanism of inhibition is complex and there is no general theory applicable to all situations. The effectiveness of numerous chemicals as corrosion inhibitors for steel in concrete3.
The compound groups investigated have been primarily chromates, phosphates, hypophosphites, alkalies, nitrites, and fluorides. Some of these chemicals have been suggested as being effective; others have produced conflicting results in laboratory screening tests. Many inhibitors that appear to be chemically effective produce adverse effects on the physical properties of the concrete, such as a significant reduction in compressive strength.
More recently, calcium nitrite has been reported to be an effective corrosion inhibitor3. Admixtures used to prevent corrosion of the steel by waterproofing the concrete, notably silicones, have been found to be ineffective. Consequently, the principles and performance of cathodic protection systems are described in Chapter 5. It should be noted, however, that the reinforcement in many offshore structures is connected to the cathodic protection system used on the exposed steel.
This results in protection of the reinforcement and current densities of 0. Beaton, J. Ost, Borje, and Monfore, G. Clear, K. Also, Research and Development Bulletin No. Cornet, I. Atimay, E. Lewis, D. Gouda, V. Chamberlin, William P. Hime, William D. Roberts, M. Magazine of Concrete Research London , V. Haynes, Harvey H. Locke, C. Diamond, S. C Peterson, Carl A. Department of Transportation, Washington, D. McGeary, Frank L.
Erlin, B. Manning, D. Beeby, A. Martin, H. Raphael, M. Manning, David G. Boulware, R. Van Til, C. Frascoia, R. Cardone, S. Bishara, A. Tripler, Arch B. Bird, C. Cook, A. Hill, George A.
Franklin, Pergamon Press, New York, , pp. Unz, M. Sopler, B. Arnold, C. Castleberry, J. Clifton, J. Backstrom, T. Pike, R. Virmani, Y. Mathey, Robert G. Corkill, J. MacDonald, M. Legvold, T. Meader, A. Steinberg, M. Bureau of Reclamation, Denver, Smoak, W.
McConnell, W. Santucci, L. Jenkins, J. Felt, Earl J. Westall, William G. McKeel, W. Tyson, S. Jenkins, G. Hilton, N. OConnor, E. Bukovatz, J. Bergen, J. Tracy, R. Paul, Dec. Clifton, James R. Johnston, D. Grifn, D. Fidjestol, P. Generally, a visual inspection of the structure and the environment in which it serves is the first step in any examination. Visual inspections may range from a simple cursory examination to those that are very detailed, wherein all cracks and other visual evidences of physical deterioration are plotted on scaled diagrams of the structure and specific information is gathered on environmental exposure.
This type of inspection may also include taking a limited number of cores to be examined visually for evidence of deterioration due to corrosion. The detailed type of visual inspection is time-consuming and costly, and generally only useful for research studies of structure performance. It does not develop the type of information that is required for scheduling of maintenance. There are several techniques and tools that can be used to more specifically delineate areas of deteriorated concrete and areas of potential or active corrosion of steel.
For purposes of planning a maintenance or rehabilitation program, techniques such as suggested by Stratfull, Jurkovich, and Spellman,4,5 or Manning,4. These techniques have been derived from experience and include judicious use of visual examinations together with collection of specific information on the extent of physical deterioration, active corrosion, chloride ion contamination, and depth of cover over reinforcing steel.
The references noted previously should be studied for more detailed information on these techniques. It is battery-operated and contains a transistorized oscillator that establishes an elecromagnetic field in a search coil.
In the presence of a steel reinforcing bar, the magnetic field is distorted. By calibration, the distance from the bar may be read from the meter dial.
Two styles of equipment are available. The first is a handheld device4. Automatic data recording equipment is added to faciliate the speed with which a survey can be conducted. The knowledge of cover depth is essential if it is desired to obtain samples of the concrete at the level of the reinforcing steel for chloride ion analysis.
It is also useful in determining the potential for corrosion and subsequent concrete deterioration since it has been well established that structures in corrosive environments with inadequate concrete cover are subject to early deterioration. These devices range from simple chain drags or lightweight hammers to more sophisticated devices such as the Delamtect. The automated Delamtect is useful for surveying large numbers of bridge decks or other horizontal surfaces such as parking garage floors if a record of the area of delamination is desired.
However, the simpler chain drag is adequate for locating delaminated areas during repair operations. The accuracy of the method is good when proper concrete prewetting is used. The significance of the measurements can be summarized as follows for structures exposed to air: Potentials more negative than Potentials more positive than Potentials in the range of It is in the uncertain range that potential differences across a structure, and other detection methods, must often be relied on to deduce the probable condition.
In Federal Highway Administration studies, potential differences rarely exceed mV when corrosion was not active, or was active only at extremely low rates. In reinforced concrete undergoing. It is the potential difference between the anode and cathode that most closely relates to corrosion rate rather than simply the magnitude of the anode potential. A common example in which highly negative potentials are not indicative of high corrosion rates is a totally water-saturated reinforced concrete structure.
In such a structure, oxygen availability to the noncorroding steel is severely restricted and cathodic polarization results. This drives both the anode and the cathode potentials to very negative values, and yet corrosion rate is most often quite low. By careful measurement of potentials on a closely spaced grid pattern, high versus low corrosion rate situations can be identified by studying potential differences.
Large potential differences generally indicate high corrosion rates. Two wet chemical analysis techniques are used to isolate chloride from the concrete, one to determine acid-soluble chloride and the other to determine water-soluble chloride. As discussed in Chapter 3, the chloride ion content measured by the water-soluble test is very sensitive to the test procedures.
The preferred method of sample procurement for chloride measurement is to obtain concrete in powdered form without the aid of liquid coolants that could leach out water-soluble chloride. This can be done by using impact drilling equipment and collecting the pulverized material.
Alternatively, a 3-in. Measurements for acid-soluble and water-soluble chloride can be made on this type of sample using the standard test procedures. Additional guidance is given in References 4.
In many existing concrete structures, the exact cement content is not known. Thus, chloride levels can be expressed in terms of percent by weight of concrete, or, sometimes, pounds of chloride per cubic yard of concrete.
The latter requires an assumed or measured unit weight of the concrete. A table in Reference 4. For greater precision, nonevaporable water contents water chemically combined through cement hydration can be measured on each powdered sample from a given concrete and used as correction factors for aggregate induced distortions in measured chloride levels.
In any interpretation of chloride data, sound engineering judgment must be used to assess the actual potential for corrosion. As stated earlier, free moisture and oxygen as well as chloride must be available to induce corrosion. If it can be. Such conditions may prevail, for example, in concrete that is continuously submerged or in internal members in buildings where air conditioning units maintain constantly low humidities.
However, the difficulty of assessing the possibility of corrosion in the service environment is discussed more fully in Chapter 3. One type involves the use of two or three electrically isolated short sections of steel wire or reinforcing steel and the use of linear polarization techniques to estimate instantaneous corrosion rates. Experiences to date with use of these probes have been conflicting. However, based on recent Federal Highway Administration studies,4.
The Federal Highway Administration studies indicate that current flow within physically separated, macroscopic corrosion cells, such as the case of large quantities of steel in chloride-free moist concrete in close proximity and electrically coupled to steel in chloride-bearing concrete, are primarily responsible for the very high rates of corrosion on bridge decks.
In contrast, microscopic self-corrosion of a small section of steel in chloride-contaminated concrete most often results in only relatively low corrosion rates. Since the electrically isolated linear polarization devices only simulate this latter process, valid predictions of the overall effect of corrosion on the structure are not possible.
This phenomenon is called galvanic corrosion. Galvanic corrosion occurs when 2 different metals are electrically connected and are immersed in an electrolyte.
In order for galvanic corrosion to occur, an electrically and ionically conductive path is necessary. This effects a galvanic couple where the more active metal corrodes at an accelerated rate and the more noble metal corrodes at a retarded rate.
Galvanic corrosion is often utilized in sacrificial anodes. For example zinc is often used as a sacrificial anode for steel structure like pipelines. Factors such as relative size of anode smaller is preferred , type of metal and operating conditions temp, humidity , affect galvanic corrosion. This selective dissolution may lead to the dislodgement of grains. Some significant examples include Intergranular corrosion in sensitized stainless steels and exfoliation in aluminium alloys etc:- At the temperature range of oC carbon diffuses to the grain boundary of stainless steel and reacts with chromium to precipitate chromium carbide.
But for stainless steel, the corrosion resistance depends on the Cr content which due to depletion increases the susceptibility to corrosion. This is because of the fact that the highly oxygenated area above the waterline acts as the cathodic part, while the portion just below the waterline act as the anodic part undergoing corrosion.
During stress- corrosion cracking, the metal or alloy is virtually unattacked over most of its surface, while fine cracks progress through it. It generally has serious consequences. The stresses can be internal or applied. An example is that of brass condenser tubes.
The reason for this is the moist atmosphere containing ammonia. The attack is along the grain boundaries which become more anodic with respect to grain interior.
Specific Objective Under this general objective the specific objectives; describing corrosion, identifying the mechanism and electro chemical nature of aqueous corrosion, understand the causes of corrosion and its effect for human health and investigate the ways of corrosion protection are included. A metallic surface is consumed it react with the environment through oxidation reduction redox reaction [6].
Corrosion of potential reduction reaction has higher potential than the corrosion potential of oxidation reaction. The oxidation of a metal at an anode and the reduction of a substance at the cathode, in order for the reaction to occurs the following conditions must exist [7].
A chemical potential difference must exist between adjacent sites on a metal surface between alloys of a different compositions 2. An electrolyte must be present to provide solution conductivity and as a source of material to be reduced at the cathode 3. An electrical path through the metal or between metals must be Available to permit electron flow.
Electro chemical corrosion of iron in contact with water is an example of case that can be used to describe the electro chemical reactions. The electrode at which oxidation takes place is called anode. A chemical substance is reduced when it acquires electrons. The electrode at which reduction takes place is called cathode. Hence oxidation reactions results in the formulation of positive charge ferrous iron at the anode.
The reduction reaction at the cathode must be take place concurrently in order to continue the corrosion process. Several reactions are possible and the one that occurs is determined by the environment. Without the presence of air or oxygen hydrogen ions can be reduced by the www.
Two possible reaction ocurs [7]. The following are the common causes of corrosion [8]. Generally Corrosion can be caused by different types of factors those are reactivity of metal, presence of impurities, presence of electrolyte, presence of air moisture gas like O2 and CO2 [8]. Eye irritation and exacerbation of skin disorder have been associated with pH values greater than eleven. In www. Exposure to low pH values can also result in similar effect.
Below 2. All material compounds are toxic and they affect many organ systems both during pre-metal and post metal development and in adulthood. Mercury compounds are neurotoxic. Same are immunologically active. The main toxicity stems from the binding of mercury to salt dry groups of enzymes and other proteins, there by disruption their structure and function.
This interferes with basic cellular process and damages or kills cell. The different forms of mercury differ in their ability to penetrate membranes and gains the neuron toxicity, that is of greatest importance although same forms of mercury damage the kidneys and same compounds are highly corrosive to skin and mucous membrane [9]. EMF series and galvanic series kenetic could be used for predication of this type of corrosion.
Galvanic corrosion occurs in multiple phase alloy. Lead- Acid battery the basic operation of lead-acid Pb-H2SO4 battery is based on group of positive and negative plate immersed in an electrolyte that consists of diluted sulfuric H2SO4 and water. A uniform layer of rust on the surface is formed when exposed to corrosive environmental atmosphere corrosion is typical example on this type [11].
Depending on the rate of this movement abrasion takes place. This method uses to protect the water, pipers hulks, staves, pipelines [12]. Pieces of Zinc or magnesium alloy are attached to pump bodies and pipes. The protected metal becomes the cathode and does not corrode. The anode corrodes there by proving the desired sacrifice protection. Iron which is oxidizes will immediately be reduced back to iron [12].
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