External corrosion system

External corrosion system that includes corrosion problems such as those occurring in the soil surrounding the buried pipe, the coating, the cathodic protection system,. .. 

NACE Standard RP-01-69, Recommended practices Control of external corrosion on underground or submerged metallic piping systems. 

Recommended Practice Control of External Corrosion on Underground or Submerged Metallic Piping Systems, RP-OI-69, NACE, Houston (1983)... 

A system for inspecting all packages received, and routinely in-store, for leaks, damage, external corrosion, etc. 

In this example, assume the team obtained pipe samples of some of the remaining pipe and finds evidence of external corrosion. The team concluded that the feed line failed due to higher than normal pressure combined witb corrosion of tbe piping system (an AND-gate). These relationships are shown in Figure 9-26. 

Known facts Possible Scenarios], Isopentane Pipe samples show external corrosion especially in heat affected zones Pressure indicator on system went up to 120 psig. Pipe samples found with split running along pipe Maintenance records indicate correct materiai and schedule used for repairs Area has clearly understood gasket chart... 

Cast Iron and Ductile Iron Cast iron and ductile iron provide more metal for less cost than steel in piping systems and are widely used in low-pressure services in which internal and external corrosion... 

Gas prooessmg Gas treating Foaming was caused ly a corrosion inhibitor used in the boiler feed water. Steam condensate used for solvent makeup contained the inhibitor. Beware of external corrosion inhibitors entering a foaming system. 

Large steel aboveground storage tanks (ASTs) are located on large tank farms of oil producers (Fig. 3.15). Maintenance teams take care of external painting and both internal and external corrosion inspections. Corrosion inspection and protection of ASTs is important for the preservation of large capital investments, the reduction of maintenance and inspection costs, and the assurance of system integrity for release... 

A large percentage of mains (57%) and services (46%) are metallic systems (steel/cast iron/copper), and corrosion is a major problem. For distribution pipe, external corrosion is the primary problem and internal corrosion has also been observed in some instances. The methods of corrosion monitoring on cathodically protected piping are similar to the methods used in the case of transmission pipelines... 

External corrosion of water systems may be caused by general corrosion, stray current corrosion MIC, and/or galvanic corrosion. Corrosion mitigation techniques include the application of protective coatings, wrapping pipe in a plastic cover, and the application of CP. The areas of major external corrosion impact are generally those where localized attack may take place, such as in the proximity of other systems like galvanic corrosion or in areas where stray currents may occur. 

External corrosion of the cars is primarily because of atmospheric exposure. While corrosion damage is still a concern, car appearance takes precedence therefore, the car manufacturers/lessees often choose to apply an exterior paint system to address the aesthetic issues. The paint systems are typically direct-to-metal (DTM) epoxy or epoxy with a urethane coat. This epoxy substrate adds protection against ultraviolet radiation. 

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

Table 5.11 summarizes the major accidents reported to the US Department of Transportation by the operators for the 6-year period between 1994 and 1999. The data show that for transmission pipeline systems, inclusive of hazardous liquid and natural gas, approximately 25% of all reported accidents were because of corrosion (see Table 5.10). Of the hazardous liquid pipeline accidents caused by corrosion, 65% were because of external corrosion and 34% were because of internal corrosion.

A problem of particularly large economic significance is corrosion on buried pipelines, and there are special reasons to worry about the external corrosion. Examples from some communities show that a major part of the water in the public fresh water pipeline system gets lost due to leakage, mainly because of corrosion. 

Other corrosion problems in oil and gas production are corrosion in the water injection system, particularly at welds, galvanic elements and irregularities that disturb the flow, and external corrosion on hot process piping, with or without heat insulation [8.27]. Corrosion in seawater is dealt with in Section 8.3 and at various places in other chapters of the book. 

Considering the corrosion system as a whole, the sum of the cathode current must equal the sum of the anode currents for reason of electroneutrality. Thus, the cathode and anode reactions affect one another reciprocally, and the slowest, most inhibited one determines the overall reaction rate. In many cases the cathode reaction is the slowest, so the cathode controls corrosion. Lack of external current can only arise at a given mixed potential, which is the free corrosion potential. 

In addition to the well-known application of cathodic corrosion protection to underground pipelines, there has been an increased use for the internal protection of containers and pipes. Initially, galvanic anodes were used to this effect, like the ones currently used, for example, to protect the interiors of tankers and boilers. However, since these anodes are often subject to heavy inherent corrosion, especially with the highly aggressive media often found in the chemical industry, they have largely been replaced by external current systems with insoluble anode material. 

NACE RP0169 Control of External Corrosion on Underground or Submerged Metallic Piping Systems. 

This paper focuses on how to model the deterioration of static pressurized process equipment to enable efficient inspection and maintenance planning. Such equipment tends to gradually deteriorate over time from erosion, corrosion, fatigue and other mechanisms, and at some point of time inspection, repair or replacement is expedient with respect to safety, production and costs. The deterioration of the equipment is influenced by many factors such as type of equipment, system design, operation and process service, material and environment. For hydrocarbon systems, the most critical deterioration mechanisms are corrosion due to CO2 and H2S, microbially influenced corrosion, sand erosion and external corrosion (DNV 2002). In general, CO2 is the most common factor causing corrosion in oil and gas system of low alloy steel (Singh et al. 2007). 

Filament winding has been used to provide a protective laminate on the outside of steel pressure pipes where external corrosion can take place. An example of this use is in the protection of the splash zone of steel riser pipes used on sea based oil and gas production platforms. Here, care has to be taken in the design of such a composite structure since the coefficient of expansion of the filament wound glass wrap can be lower than that of the steel core. If such a composite structure is produced using a heat cured resin system (say 120°C cure) and then subjected to subzero temperatures... 

---