4,588,022). A heat exchanger as claimed in claim 1 wherein 'said cathode means are bare. 1 and the current flow entering the cathode at either end is only half of that of FIG. A method as claimed in claim 14 in which on startup, power is supplied initially to only one of said end shell spaces. 10 shows a baffle bushing adapted to provide power to such a longitudinal cathode and shows a power connection away from baffles. One typical arrangement is the provision of two cathodes in the hot end of the exchanger and a single cathode in the cold end. steel and stainless steel in 98% sulfuric acid. were ambitious save the children focus on children associated with armed forces and groups children in, Protection - . Cathodic protection is commonly used to protect numerous structures against corrosion, such as ships, offshore floaters, subsea equipment, harbours, pipelines, tanks; basically all submerged or buried metal structures. As long as more alkalinity is saved than consumed, an increased surface pH and hence corrosion protection is ensured even under severe time-variant anodic stray current interference. ), the easier it will be to passivate the metal by oxidation and it will be difficult to reactivate the passivated metals by reduction. Anodic protection. A heat exchanger as claimed in claim 1 wherein said cathode means are disposed in whole or in part in the core of a disc-and-donut heat exchanger. The region of applied potential in which the passive film exists is known as the `passive` zone and varies in width with acid strength and temperature and narrows as the acid temperature rises or the acid concentration drops. Anodic protection mechanism: the anodic corrosion inhibitor acts as an oxidizing ion. A heat exchanger as claimed in claim 1 wherein said connection means include radial connections through the shell to cathode means located in the shell spaces of said exchanger. anodic aluminum oxide(aao). Acid enters shell space 16 of exchanger 10 through shell nozzle 18 and leaves though nozzle 20. S. Nei, Key issues related to modeling of internal corrosion of oil and gas pipelines A review. At. It is another object to provide a cathode system with improved cathode voltage control along the exchanger so corrosion resistance can be maximized. A typical exchanger may have as many as twenty or thirty baffles. For two cathodes, the preferred locations are central to nozzle 114 and opposed 116, so that the cathodes impede acid flow around the bundle the least possible. 11 shows diagrammatically a hot end cathode constituted as cathode 134 and part of cathode 136 which extends through the full length of exchanger 10 and thus constitutes the cold end cathode as well. During this process, corrosion occurs on the anode, whereas the cathode is protected. The voltage levels at which transpassive corrosion can occur are dependent on the same factors which affect the passive film. by using filters or scrapers, Preventing the formation of deposits or corrosion, e.g. On the other hand, intermediate power input is possible as is external inspection of the cathode. Anodic protection of 304SS exposed to an aerated H2SO4 at 300C at 0.500 vs. SCE. Interference Over Time on the Corrosion of Cathodically Protected Pipelines,, On the Mechanism of AC Assisted Corrosion of Buried Pipelines and Its CP Mitigation,, AC-Corrosion on Cathodically Protected Pipelines: A Discussion of the Involved Processes and Their Consequences on Mitigation Measures,, Determining the A.C. FIG. 4,588,022 to Sanz, issued May 13, 1986. In a heat exchanger for a corrosive fluid, said heat exchanger having an elongated shell, said shell having dome spaces, a first end shell inner surface defining a first end shell space and a second end shell inner surface defining a second end shell space, a plurality of elongated tubes extending longitudinally within said shell, said corrosive fluid being located between said shell and the exterior surfaces of said tubes and a heat exchange fluid flowing within said tubes to exchange heat with said corrosive fluid, baffle means within said shell for directing the flow of said corrosive fluid in a tortuous path within said shell; an anodic protection system for protecting the exterior surfaces of said tubes, said anodic protection system comprising: power supply means for supplying a positive potential, means for connecting said positive potential to said shell, elongated cathode means extending longitudinally in said shell and insulated from said shell and tubes, said cathode means being of a material having substantial electrical resistance, connection means to said cathode means. A representative central main cathode 22 is shown as a dotted line, entering exchanger 10 through water box 24 at an end of exchanger 10 and stopping just short of tube sheet 26 at the opposite end of exchanger 10 in shell space 16. C. de Waard, D.E. . These deposits can plug the holes in the sheath and insulate it from the acid. 10 is a schematic view showing power supply to cathodes in the shell space at a baffle and between baffles according to the invention; FIG. Considering the relevant associated costs of mitigation, the use of objective, technically correct, and justifiable assessment criteria is of highest importance. Corrosion will occupy a bigger space which exerts stresses on concrete. 17. 13c shows separate cathode rods in the two ends of the exchanger with current feeds to the outer and inner ends of the two cathodes. FIG. protection. FIG. The shells may be completely filled with tubes, segmental baffles may be used with dome spaces on either side of the tube bundle or annular tube bundles with disc and donut baffles may be used with an empty core and outer annulus free of tubes. A main reference electrode 42 on the shell of exchanger 10 is connected through line 44 to controller 32. Feasibility of anodic protection is firstly demonstrated and tested by Edeleanu in 1954. It forms holes and causes thickness loss. It will be readily appreciated that the relative numbers of cathodes and values of cathode diameters and cathode lengths may be varied depending on the foreseen current requirements in the two ends of the shell and the desired current densities entering the acid from the cathodes. Anodic protection uses a layer of protective coating on the surface of the metal, which is commonly known as the substrate. of NSTI Nanotechnology Conference and Expo, An Objective Discussion of Cathodic Protection Criteria Based on Literature Data,, Beeinflussung von unterirdischen metallischen Anlagen durch Streustrme von Gleichstromanlagen; textgleich mit der AfK-Empfehlung Nr. The article discusses the specific requirements of equipment required for anodic protection. Also, The steel cross-section will be reduced due to corrosion. Can be applied for active-passive metals/alloys only. While there is some contribution of an activation polarization at the very moment the current is applied, the relevant polarization that provides corrosion protection is a result of the concentration change at the steel surface. FIGS. Cathode 80 is partly embraced by a cylinder of glass-filled polytetrafluoroethylene bushing 96. A preferred location is in the outer baffle opening next to the exchanger shell where the acid flow is parallel to the tube and the cathode is accessible through the shell, either for inspection, or to accept power from the power supply. Corrosion Mechanisms of Buried Pipes. For these metals the potential can be shifted electropositively into the passive range through anodic protection (AP). It is also clear that much lower voltage losses can be obtained without significant increase in cathode material. It is also clear that there is an advantage in terms of voltage loss in a relative increase in cathode use in the hot end of the exchanger relative to the cold end. 18. Anodic protection mechanism: the anodic corrosion inhibitor acts as an oxidizing ion. Titanium and chromium can be passivated very easily and their passivation process will occur more often than not, spontaneously, even in the absence of oxidizing agent. A modest gap between the ends of cathodes 80 and 86 is of the order of one baffle spacing, or less, typically 25 to 50 cm. 1. hot plate & magnetic spinner. This change in concentration with respect to oxygen and hydroxide requires time to build up. CECEBE TECHNOLOGIES INC., CANADA, Free format text: Anodic ProtectionIts Operation and Applications JAMES IAN MUNRO AND WINSTON W. SHIM, Corrosion Service Co. Ltd. Anodic protection decreases corrosion rate of the stainless steel, initially from 5mm/year down to 0.025mm/year and therefore less contaminated sulfuric acid can be obtained. First, the corrosion inhibitor will delay the depassivation of rebar and strengthen the passive layer. Where the current demand in the unit is high and it is desired to use power feed through the shell, cathodes may be placed in outer annular space 111. Advertisement In modern sulphuric acid plants, shell and tube heat exchangers are commonly used which are fabricated of stainless steel and anodically protected on the acid side to minimize corrosive attack on the stainless steel. Often the protective voltage and current may be adequate at one end of the exchanger but not at the other. 6. In this embodiment, hot sulphuric acid enters exchanger 10 through a nozzle 18 in the zone where cathodes 86 and 88 are located where the current demand is highest. 7. discuss the goals and principles of protection in a modern computer system, Protection - . These compound will act as a barrier restricting oxygen diffusion to the cathodic area. 8. A systematic study of film growth on aluminium, over a wide range of anodizing conditions, encompassing barrier and porous-type films, has been undertaken to develop a more integrated theory of anodizing than previously possible. Bushing 96 is relatively dimensionally stable and has a concave cone 98 and a projecting convex cone 100. Due to the time constants involved in concentration polarization, the corrosion protection is not lost when CP is interrupted or when anodic current discharge occurs. The cooling water is the dirtier of the two fluids and in most duties, cleaning is needed only on the water side. Bockris, D. Drazic, A.R. i.e. 11. The tube bundle being protected is shown as 45. It is a further object to provide a cathode system in which normal cathode voltage losses due to current flow along the cathode are an order of magnitude less than the anodically passive zone width. 4 shows a typical decay of current from the time of initial passivation. protection. Reduction in the thickness of the cathode rod, which is gradually corroding, is usually from the hot end and can then limit severely the current entering the exchanger. Current is also introduced through the exchange shell to the three cathodes 80, 86, and 88, respectively, through the shell at points approximately two thirds of the distance from the outer ends to the centre of the exchanger, 138, 140, and 142. In practice, it has been found necessary to keep the cathode to metal gap at least 25 mm in size to avoid current short-circuiting. This option commonly results in a single cold end cathode and between two and four hot end cathodes and is a preferred arrangement. In all cases, the cathodes must be insulated from the metal surface being protected and this is done by use of fluoropolymer sleeves or sheaths in the case of longitudinal cathodes and pin cathodes or glass in the case of the reference electrodes. Because all relevant influencing parameters are considered, an objective assessment of corrosion protection is possible. There is a maximum current needed which is referred to as the `critical current`. Current is fed from the power supply in parallel to these cathodes. It is yet a further object to provide a cathode system in which the current density entering the acid is significantly reduced and in which the cathodes are located in dome spaces in the exchanger. For this case, smaller cathode rods could be used with a cost saving. A spacing of 90 is preferred for four cathodes in the outer annulus. For corrosion to occur, four elements must be present: a host site from which current flows, a destination site where no current flows, a medium capable of conducting current (such as water, concrete . An alternative feature embodies the use of longer cathodes from one end than from the other end, such as for example, a longer cold cathode than the hot cathode. Cathode 80 is disposed in dome space 84 below the tubes. Anodic protection has been applied to protect storage tanks, reactors, heat exchangers and transportation vessels for corrosive solutions. Dome spaces 104 contain cold cathode 80, and hot cathode 86 while dome space 102 contains hot cathode 88. Despic, The electrode kinetics of the deposition and dissolution of iron. It is an object of the present invention to provide a cathode system which is able to handle high current flows while maintaining cathode voltage losses significantly less than the width of the anodically passive zone in the exchanger. Also, it can be used for an existing structure by adding the corrosion inhibitor to repairing mortar or repairing concrete. In this technique . TEFLON polytetrafluoroethylene. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated. This layer will halt the corrosion anodic process. The belief that the mechanism of corrosion is electrochemical will be expressed by de la Rive only in 1830. Based on these concepts it will, however, be possible to operate the CP under optimized conditions while minimizing the corrosion risks caused by conflicting requirements. Where the current requirements in the hot end are very high relative to the cold end, a design incorporating an even larger number of cathodes in the hot end with smaller diameter, the number being picked to give the same metal cross-section for longitudinal current flow while significantly greater surface for current flow into the acid in the hot end is used. Where the tubing is in an annular arrangement, cathodes can be located either in the core or outer annulus of the exchanger. Clearly, if the current available is small, the ability to modify a large surface will be small and the possibility exists that the current may actually add to corrosion and not protect the surface. Interface potential of the structure is increased into passive corrosion domain. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Voltage losses in the cathode are therefore a quarter of those obtained in prior art FIG. Surfaces of, for example, stainless steel in hot concentrated acid in turbulent conditions are not automatically passive and power has to be applied to create the passive film. Many variations are possible for securing such an insulating bushing 96 to baffle 94 and any suitable insulating material can be used, including ceramics as well as plastics. As current is applied, corrosion initially increases to form the anodic film. The main purpose of its use is to ensure . 1, the length of the current flow path in FIG. In aqueous solutions, dissolved carbon dioxide hydrates to form carbonic acid according to (1-5): CO2 +H 2O H 2CO3 (1) (1) C O 2 + H 2 O H 2 C O 3. where K Hyd is the equilibrium constant for . 12. Platinum clad brass can be used for anodic protection cathodes because this cathode has low overpotential and its degradation rate is very low, however it is very expensive. Consequently, in aggressive ions containing-environment anodic protection is applied only for metals which have relatively high protection potential and high pitting potential.