Function of corrosion coupon
Figure b shows the condition after an anodic inhibitor has been applied. The rate of the anodic reaction has been decreased. This causes a decrease in I corr accompanied by a shift in E corr to a more positive anodic potential. Figure c shows the effect of a cathodic inhibitor. Here, the rate of the cathodic reaction has been decreased, accompanied again by a decrease in I corr , but this time the shift in E corr is in the negative cathodic direction. Passivation Inhibitors. Examples of passivators anodic inhibitors include chromate, nitrite, molybdate, and orthophosphate.
All are oxidizers and promote passivation by increasing the electrical potential of the iron. Chromate and nitrite do not require oxygen and thus can be the most effective. Chromate is an excellent aqueous corrosion inhibitor, particularly from a cost perspective.
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However, due to health and environmental con-cerns, use of chromate has decreased significantly and will probably be outlawed in the near future. Nitrite is also an effective inhibitor, but in open systems it tends to be oxidized to nitrate. Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Molybdate can be a very effective inhibitor, especially when combined with other chemicals. Its main drawback is its high cost. Orthophosphate is not really an oxidizer per se, but becomes one in the presence of oxygen.
If iron is put into a phosphate solution without oxygen present, the corrosion potential remains active and the corrosion rate is not reduced. However, if oxygen is present, the corrosion potential increases in the noble direction and the corrosion rate decreases significantly. A negative attribute of orthophosphate is its tendency to precipitate with calcium hardness found in natural waters. In recent years, deposit control agents that prevent this deposition have been developed.
Due to its relatively low cost, orthophosphate is widely used as an industrial corrosion inhibitor. Precipitating Inhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes which are insoluble at this high pH pH units above bulk water , but whose deposition can be controlled at the bulk water pH typically pH. A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate.
Calcium carbonate and calcium orthophosphate are also precipitating inhibitors.
Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole BZT and tolyltriazole TTA. These compounds bond directly with cuprous oxide Cu 2 O at the metal surface, forming a "chemisorbed" film.
The plane of the triazole lies parallel to the metal surface; thus, each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A very recent study supports the idea of an electronic stabilization mechanism.
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The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. In addition to bonding with the metal surface, triazoles bond with copper ions in solution. Thus, dissolved copper represents a "demand" for triazole, which must be satisfied before surface filming can occur.
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Although the surface demand for triazole filming is generally negligible, copper corrosion products can consume a considerable amount of treatment chemical. Excessive chlorination will deactivate the triazoles and significantly increase copper corrosion rates.
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Due to all of these factors, treatment with triazoles is a complex process. Adsorption Inhibitors. Adsorption inhibitors must have polar properties in order to be adsorbed and block the surface against further adsorption. Typically, they are organic compounds containing nitrogen groups, such as amines, and organic compounds containing sulfur or hydroxyl groups. The size, orientation, shape, and electrical charge distribution of the molecules are all important factors. Often, these molecules are surfactants and have dual functionality. They contain a hydrophilic group, which adsorbs onto the metal surface, and an opposing hydrophobic group, which prevents further wetting of the metal.
Glycine derivatives and aliphatic sulfonates are examples of compounds which can function in this way. The use of these inhibitors in cooling systems is usually limited by their biodegradability and their toxicity toward fish. In addition, they can form thick, oily surface films, which may severely retard heat transfer.
For many years, silicates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of silicate chemistry, their mechanism of inhibition has not yet been firmly established. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense.
Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that silica and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. Silicates are slow-acting inhibitors; in some cases, 2 or 3 weeks may be required to establish protection fully. It is believed that the polysilicate ions or colloidal silica are the active species and these are formed very slowly from monosilicic acid, which is the predominant species in water at the pH levels maintained in cooling systems.
Figures through show the effects of several operating parameters on the corrosion tendency in aqueous systems. As shown in Figure , corrosion rate increases with conductivity. Figure shows the effect of pH on the corrosion of iron. In cooling water, the potential for calcium carbonate precipitation increases with higher pH and alkalinity; thus the corrosion rate decreases slightly as pH is increased from 4 to Above pH 10, iron becomes increasingly passive. Figure shows the effect of oxygen concentration on corrosion at different temperatures. As discussed previously, oxygen is the main driving force for corrosion of steel in cooling water.
The increase in corrosion with temperature at a given oxygen concentration is due to more rapid oxygen diffusion occurring at higher temperatures. In the determination of treatment levels, solubility data is important.
The Langelier Saturation Index, which defines the solubility of calcium carbonate, is commonly used. Solubility data for calcium orthophosphate and zinc orthophosphate may be needed if the treatment contains phosphate and zinc. Every cooling water system should include a method of monitoring corrosion in the system.
Data obtained from these devices can be used to optimize an inhibitor treatment program to maintain the plant equipment in the best possible condition. When heat transfer data cannot be obtained on operating exchangers, monitoring devices can be useful for evaluating the success of a treatment program without a plant shutdown. Corrosion Coupons. Preweighed metal coupons are still widely used as a reliable method for monitoring corrosion in cooling systems. Coupon weight loss provides a quantitative measure of the corrosion rate, and the visual appearance of the coupon provides an assessment of the type of corrosion and the amount of deposition in the system.
In addition, measurement of pit depths on the coupon can indicate the severity of the pitting. Coupons should be installed properly in a corrosion coupon bypass rack with continuous, controlled water flow past the coupons. The metallurgy should match that of the system. One disadvantage of coupons is their lack of heat transfer, resulting in a lower temperature than that of the actual heat exchanger tubes. In addition, only a time-weighted average corrosion rate is obtained. Corrosion Rate Meters. Additional corrosion monitoring tools have been developed by various instrument manufacturers and water treatment companies.
Instantaneous corrosion rate meters can measure the corrosion rate at any given point in time. Instrument methods fall into two general categories: electrical resistance and linear polarization. With either technique, corrosion measurements are made quickly without removal of the sensing device. The electrical resistance method is based on measuring the increase in the electrical resistance of a test electrode as it becomes thinner due to corrosion.
This method is desirable because the probes can be installed in both aqueous and nonaqueous streams.