External corrosion

CORROSION, EXTERNAL - A chemical deterioration of the metal on the fireside of boiler heating surfaces. 

Painting the exposed external surfaces of an AST provides corrosion protection, improved appearance, and reduced evaporation loss. Selection of coating type depends on the tank operating temperature and the presence of insulation that contains minerals and salts that may cause corrosion. External coatings must be able to withstand the effects of weather, ultraviolet light, and industrial or marine atmospheres. 

Metal pipe Small leak Multiple leaks Large leak/rupture Internal corrosion External corrosion Clap or sleeve aronnd pipe or replace small pipe section Replace pipe section Replace pipe section Apply cement lining. Insert PVC tubing in pipe Evalnate structural integrity. If fit for service, then apply coating to protect metal and/or apply cathodic protection to rednce corrosion rate... 

Glass reinforced plastic (GRP) piping is the material of choice for handling corrosive fluids and is especially suitable when corrosive external conditions exist. Corrosive external conditions are typically caused by corrosive soils or by chemical fumes. In some cases, corrosive external conditions govern the selection of materials. 

Carbon dioxide (COj) corrosion Hydrogen sulfide (HjS) corrosion Preferential weld corrosion Erosion and erosion-corrosion Crevice corrosion Flange face corrosion Cavitation Dead-leg corrosion Under-deposit corrosion Microbial corrosion Oxygen corrosion Galvanic corrosion External corrosion Corrosion under insulation (CUI)... 

This includes a complete examination and involves disassembly and inspection of each part and replacement where necessary. Conduct maintenance at least annually or more often if conditions warrant. Accomplish hydrostatic testing of portable fire extinguishers to protect against unexpected in-service failure. Failure can occur due to internal corrosion, external corrosion, and damage from abuse. Perform hydrostatic testing using trained personnel with proper equipment and facilities. OSHA Standard 29 CFR 1910.157 (Table 1) provides test intervals for extinguishers. 

Figure 4.8 shows the structure of a bow-tie diagram, which consists of an event tree that creates a top event, the safeguards and controls for which are analyzed with a fault tree. At the left of the diagram is the hazard, say a vessel containing hydrocarbons stored under pressure. Threats are events such as corrosion, external impact, and operating error, which could create an undesired event, in this case a release of hydrocarbons from the vessel. Between the threats and the undesired outcome are barriers such as operator training, relief valves and instrumentation. 

The simplest defect is an external flaw, generally occurring as a result of corrosion external to the pipe. In repairing these types of flaws, the repair assumes that the cause of the corrosion has been arrested, and there may be remaining steel substrate which the FRP can be bonded to. The repair typically consists of restoring the lost circular... 

The techmque was first employed, in real-world conditions, for monitoring external corrosion in the large diameter steel tubing used for oil well casings. In the late fifties, T.R. Schmidt at Shell Developments, pioneered the technique in those demanding applications, although the technique itself was invented, by W.R. MacLean, (Ref. 1) somewhat earlier. T.R Schmidt has written a history (Ref. 2) of much of the early work in the technology, which contains many references, others which may be of interest are held on the NTIAC database (Ref 3). 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Refrigerated storage tanks are iasulated usiag great care to minimize heat loss and access of air and moisture to the iasulation or metal surface. In double-wak tanks, the annular space is usuaky fiked with perlite and the external surface of the outer tank is painted for corrosion protection. 

Diffusion alurninide and sihcide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 10 /yr in North America and about 50 x 10 worldwide. 

Acid Coolers. Cast Hon trombone coolers, once the industry standard (101), are considered obsolete. In 1970, anodically passivated stainless steel sheU and tube acid coolers became commercially available. Because these proved to have significant maintenance savings and other advantages, this type of cooler became widely used. Anodic passivation uses an impressed voltage from an external electrical power source to reduce metal corrosion. 

Aluminum geodesic dome roof tanks are becoming popular. These are often the economic choice. They offer superior corrosion resistance for a wide range of conditions, and are clear span stmctures not requiring internal supports. They can also be built to any required diameter. However, domes caimot handle more than a few inches of water column internal or external pressure. 

The exposed surface area of a tank is relatively large, thus heated tanks are almost always insulated. Another reason for insulating is that the external corrosion rate of the steel owing to atmospheric conditions increases with increasing temperature. Insulation, if propedy installed, reduces external... 

In a battery, the anode and cathode reactions occur ia different compartments, kept apart by a separator that allows only ionic, not electronic conduction. The only way for the cell reactions to occur is to mn the electrons through an external circuit so that electrons travel from the anode to the cathode. But ia the corrosion reaction the anode and cathode reactions, equations 8 and 12 respectively, occur at different locations within the anode. Because the anode is a single, electrically conductive mass, the electrons produced ia the anode reaction travel easily to the site of the cathode reaction and the 2iac acts like a battery where the positive and negative terminals are shorted together. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Pitting corrosion may occur generaHy over an entire aHoy surface or be localized in a specific area. The latter is the more serious circumstance. Such attack occurs usuaHy at surfaces on which incomplete protective films exist or at external surface contaminants such as dirt. PotentiaHy serious types of corrosion that have clearly defined causes include stress—corrosion cracking, deaHoying, and corrosion fatigue (27—34). 

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