Corrosion flow effects

Flow effects may be pronounced. High-turbulence areas can become preferred attack sites (Fig. 7.17). Erosion-corrosion phenomena are important (Fig. 7.18) (see Chap. 11, Erosion-Corrosion ). 

To ensure maximum continuity of the tin-iron compound layer between tin and steel. This layer is itself corrosion resistant and appears to act as a nearly inert screen limiting the area of steel exposed as tin is removed by corrosion. Its effectiveness is measured by the Alloy-Tin Couple (A.T.C.) test, in which the current flowing is measured between a sample of tinplate from which the unalloyed tin layer has been removed, and a relatively large tin electrode immersed in an anaerobic fruit juice. ... 

In developing the arguments that are presented later in this review, it is necessary to keep in mind the relative scales (dimensions) at which each phase occurs. This is important because the effect of flow on localized corrosion is largely (though not totally) a question of the relative dimensions of the nucleus and the velocity profile in the fluid close to the surface. However, the velocity profile is a sensitive function of the kinematic viscosity, which in turn depends on the density and the dynamic viscosity. Because the kinematic viscosity of water drops by a factor of more than 100 on increasing the temperature from 25 °C to 300 °C, the conclusions drawn from ambient temperature studies of the effect of flow on localized corrosion must be used with great care when describing flow effects at elevated temperatures. 

The approach developed by Newman for the treatment of both mass-transfer and electric-field effects in boundary-layer flows has had considerable success.L2 6 However, many flows of practical interest have separation and recirculation regions, features not amenable to a boundary-layer analysis. Fortunately, there has been significant progress in the heat-transfer and other communities in computational fluid dynamics (CFD), providing numerical methods applicable to problems important to electrochemistry. The pioneers in using CFD for electrochemical applications are Alkire and co-workers, who have been largely interested in flow effects in localized corrosion. The literature is briefly reviewed in the next section. 

Since these experiments were not carried out under ideally defined flow conditions the dependence of corrosion rate on flow rate will be discussed only in a qualitative manner. Under laminar flow conditions and mass transfer control one would have expected the corrosion rate to increase with the square root of the velocity while under turbulent conditions proportionality would prevail. However, in Fig.15 one finds that the corrosion rate varies approximately with the 0.2 to 0.3 power of the flow rate. It appears therefore that the observed dependence on the flow rate does not obey conventional mass transfer theory. A flow effect might be expected in uninhibited hydrochloric acid because hydrogen bubbles, formed on the surface of the metal, are faster and more easily removed at higher flow rates. While this argument could be applied in discussing Fig.15, we find in Fig.16 that the flow effect at similar corrosion rates is much less pronounced under deaerated conditions. We therefore have to conclude that the observed flow effect is not mechanical and cannot be related to pure mass transfer control either. In Fig.17, the flow dependence of the corrosion rate is shown for 2-butyne-l,4-diol in deaerated UN hydrochloric acid. Note that the corrosion rate appears to be noticeably affected only at the higher flow rates. Finally, in Fig.18, we observe that increased flow rate can either increase or decrease the corrosion rate in the presence of an inhibitor. This effect was observed reproducibly only in 6N hydrochloric acid with 2-butyne-l,U-diol under deaerated conditions for 0.2% and 0.1% inhibitor concentration. This behavior indicates that the corrosion rate is controlled by the superposition of two partial reaction rates each of which is mass transfer dependent to a certain extent. In terms of the model delineated in Table 6, it is suggested that the three-dimensional polymeric layer made up by inhibitor molecules is in fact a three-dimensional chelate made up of iron ions and inhibitor molecules. The corrosion rate is then... 

LPR testing is typically done in accordance with ASTM Standard G 96. Yeske [154] measured the corrosion rates of carbon steels in alkaline sulfide environments using the LPR technique and described the use of the silver/silver sulfide reference electrode. These techniques were develop>ed further by Crowe and Yeske [755] and used with real mill liquors for on-line monitoring of corrosion rates. The corrosion rates were found to be related to liquor composition with a strong flow effect. Crowe [156] also used LPR testing for on-line corrosion monitoring in white liquor clarifiers. Leinonen et al. [757] used the LPR method to monitor corrosion in a batch digester. 

Alloying elements of Mn, Al, Si, P, minimize flow induced corrosion of steels (annealed) in 3.0% NaCl, whereas C promotes corrosion. The effect of alloying elements on flow induced corrosion are shown in Fig. 9.14 [26]. 

Temperature dependence of flow effects. It has been stated that increasing the flow rate of the solution generally produces a detrimental effect on the corrosion of tjrpe-347 stainless steel. This effect is temperature-dependent, as shown in Fig. 5-13. Although the results are based on a 0.17 m UO2SO4 solution, an increase in temperature has a similar effect at all concentrations. With a given solution composition (i.e., constant... 

Erosion is the deterioration of a surface by the abrasive action of solid particles in a liquid or gas, gas bubbles in a liquid, liquid droplets in a gas or due to (local) high-flow velocities. This type of attack is often accompanied by corrosion (erosion-corrosion). The most significant effect of a joint action of erosion and corrosion is the constant removal of protective films from a metal s surface. This can also be caused by liquid movement at high velocities, and will be particularly prone to occur if the solution contains solid particles that have an abrasive action. 

Surface Fluorination of Polymers. Fluorocarbon-coated objects have many practical appHcations because the chemically adherent surface provides increased thermal stabiHty, resistance to oxidation and corrosive chemicals and solvents, decreased coefficient of friction and thus decreased wear, and decreased permeabiHty to gas flow. Unusual surface effects can be obtained by fluorinating the polymer surfaces only partially (74). 

Safety Showers. Safety showers and eyewash fountains or hoses should be installed where corrosive or toxic materials are handled. A large-volume, low velocity discharge from directly overhead should effect continuous drenching, ie, a minimum flow of 20 L/min (50 gal /min). Water to outside showers may be heated to a maximum temperature of 27°C by an electric heating cable. The valves for all safety showers should be at the same height and relative position to the shower head, and they should operate in the same way and direction. The shower station should be identified by paint of a bright, contrasting color. In areas where chemicals harmful to the eyes may be encountered, an eyewash fountain or spray should be available in case of splash accidents. 

Three commercial processes that use these various hot carbonate flow arrangements are the promoted Benfield process, the Catacarb process, and the Giammarco-Vetrocoke process (26—29). Each uses an additive described as a promoter, activator, or catalyst, which increases the rates of absorption and desorption, improves removal efficiency, and reduces the energy requirement. The processes also use corrosion inhibitors, which aHow use of carbon—steel equipment. The Benfield and Catacarb processes do not specify additives. Vetrocoke uses boric acid, glycine, or arsenic trioxide, which is the most effective. 

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. 

Impediments to water flow resulting from inadequate equipment design or lodgement of foreign objects in the tubes can exercise a dramatic effect on the erosion-corrosion process. Much of this influence is linked to the creation of turbulence and the simple increase in fluid velocity past obstructions. The importance of these factors is quickly recognized when the phenomenon of threshold velocity is considered. 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

The attraction of rubbed amber and some other effects of electricity were known in ancient times. We know from finding nails in an old wreck that the Romans knew about contact corrosion combined with electric current flow. A skin of lead as a protection against boring worms covered the wooden planks of the ship and was nailed down with copper nails. Galvanic coupIe.s formed between the lead and the copper nails and the less noble lead sheets around the nails corroded in the seawater and fell off. The shipbuilders discovered a simple solution and covered the heads of the copper nails with lead as well. Galvanic current flow between the two metals was eliminated and corrosion was prevented (26). 

VFO works well in gas turbines. In a nine-month test program, the combustion properties of VFO were studied in a combustion test module. A gas turbine was also operated on VFO. The tests were conducted to study the combustion characteristics of VFO, the erosive and corrosive effects of VFO, and the operation of a gas turbine on VFO. The combustion tests were conducted on a combustion test module built from a GE Frame 5 combustion can and liner. The gas turbine tests were conducted on a Ford model 707 industrial gas turbine. Both the combustion module and gas turbine were used in the erosion and corrosion evaluation. The combustion tests showed the VFO to match natural gas in flame patterns, temperature profile, and flame color. The operation of the gas turbine revealed that the gas turbine not only operated well on VFO, but its performance was improved. The turbine inlet temperature was lower at a given output with VFO than with either natural gas or diesel fuel. This phenomenon is due to the increase in exhaust mass flow provided by the addition of steam in the diesel for the vaporization process. Following the tests, a thorough inspection was made of materials in the combustion module and on the gas turbine, which came into contact with the vaporized fuel or with the combustion gas. The inspection revealed no harmful effects on any of the components due to the use of VFO. 

Side entering mixers are used for blending pui-poses. The side entering propeller type mixer is economical and establishes an effective flow pattern in almost any size tank. Because the shaft seal is below the liquid level, its use in fluids without corrosive and erosive properties is usually ideal. 

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