well as improved casting technique. methods for mitigating corrosion the potential for corrosion, leading to pipe failure, is significantly impacted by the corrosivity of soil. unprotected pipes in highly corrosive soil tend to have shorter lifespans. the lifespan of ductile iron pipe installed in an aggressive environment without appropriate protection may be between 21 and 40 years. the introduction of corrosion mitigation methods for ductile pipe, including the use of polyethylene sleeving, can reduce corrosion by controlling the effect of corrosive soil on piping. in the united states, the american national standards institute and american water works association have standardized the use of polyethylene sleeving to protect ductile iron pipe from the effects of corrosion. a 2003 report by researchers from the national research council of canada noted that "both good and poor performances" of polyethylene sleeving had been reported. however, a study in the ductile iron pipe research association's florida test site found that, compared with uncoated pipes exposed to a corrosive environment, pipes encased in loose polyethylene sleeving were "in excellent condition". based on a 2005 meta analysis of 1,379 pipe specimens, loose polyethylene sleeving was found to be highly effective at mitigating corrosion. the only environment for which the analysis found the polyethylene sleeving did not provide effective corrosion control was for "uniquely severe" environments, a classification of a rare but extremely corrosive environment. the analysis found that a lifespan of 37 years could be expected in these "uniquely severe" environments. pipes manufactured under international organization for standardization (iso) standards are typically coated with zinc, to provide protection against corrosion. in instances of more aggressive soils, polyethylene sleeving is installed over the zinc coated pipe to provide added protection. cathodic protection may also be used to prevent corrosion and tends to be advocated by corrosion engineers for pipes in corrosive soils as an addition to external dielectric coatings. engineers and water authorities in the united states are divided on the use of different coatings or cathodic protection. mixed results have been found for all methods of protection. however, this may be due to the impact of variations in local soil corrosiveness and temperature or by damage occurring during installation, which can impact effectiveness of protective coatings. internal linings ductile iron pipe is somewhat resistant to internal corrosion in potable water and less aggressive forms of sewage. however, even where pipe material loss and consequently pipe wall reduction is slow, the deposition of corrosion products on the internal pipe wall can reduce the effective internal diameter. a variety of linings are available to reduce or eliminate corrosion, including cement mortar, polyurethane and polyethylene. of these, cement mortar lining is by far the most common. polyurethane (pur) polyurethane is an option offered as an internal lining for ductile iron pipes in lieu of cement mortar. however, as pur provides only passive protection it becomes vitally important that the coating is not damaged during handling and installation. manufacturers will specify strict handling, transport and installation procedures to ensure pur coatings are protected. if pipes are deformed polyurethane's elasticity, does in some situations allow the coating to remain intact. corrosion experts polyurethane coatings were first used in 1972. in comparison with other coatings, the internal polyurethane lining exhibits a high resistance to various different media such as drinking water, wastewater, de-mineralised water, industrial water and gas, as well as to aggressive solutions such as sulphuric acid. polyurethane is a thermosetting plastic with no solvents, with a three-dimensionally linked molecular structure giving it mechanical stability. the polyurethane lining used for internal coating has the following standard properties is standardised by en 15655:2009 (ductile iron pipes, fittings and accessories - internal polyurethane lining for pipes and fittings - requirements and test methods). cement mortar main article: cement-mortar lined ductile iron pipe the predominant form of lining for water applications is cement mortar centrifugally applied during manufacturing. the cement mortar comprises a mixture of cement and sand to a ratio of between 1:2 and 1:3.5. for potable water, portland cement is used; for sewage it is common to use sulfate resisting or high alumina cement. cement mortar linings have been found to dramatically reduce internal corrosion. a dipra survey has demonstrated that the hazen-williams factor of cement lining remains between 130 and 151 with only slight reduction with age. external coatings unprotected ductile iron, similarly to cast iron, is intrinsically resistant to corrosion in most, although not all, soils. nonetheless, because of frequent lack of information on soil aggressiveness and to extend the installed life of buried pipe, ductile iron pipe is commonly protected by one or more external coatings. in the us and australia, loose polyethylene sleeving is preferred. in europe, standards recommend a more sophisticated system of directly bonded zinc coatings overlaid by a finishing layer be used in conjunction with polyethylene sleeving. loose polyethylene sleeving (lps) loose polyethylene sleeving was first developed by cipra (since 1979, dipra) in the us in 1951 for use in highly corrosive soil. it was employed more widely in the us in the late 1950s and first employed in the uk in 1965 and australia in the mid-1960s. loose polyethylene sleeving (lps) remains as one of the most cost effective corrosion protection methods available today with a proven track record for reliability and effectiveness. lps comprises a loose sleeve of polyethylene that completely wraps the pipe, including the bells of any joints. the sleeving inhibits corrosion by a number of mechanisms. it physically separates the pipe from soil particles, preventing direct galvanic corrosion. by providing an impermeable barrier to ground water, the sleeve also inhibits the diffusion of oxygen to the ductile iron surface and limits the availability of electrolytes that would accelerate corrosion. it provides a homogeneous environment along the pipe surface so that corrosion occurs evenly over the pipe. the sleeve also restricts the availability of nutrients which could support sulfate-reducing bacteria, inhibiting microbially induced corrosion. lps is not designed to be completely water-tight but rather to greatly restrict the movement of water to and from the pipe surface. water present beneath the sleeve and in contact with the pipe surface is rapidly deoxygenated and depleted of nutrients and forms a stable environment in which limited further corrosion occurs. an improperly installed sleeve that continues to allow the free flow of ground water is not effective in inhibiting corrosion. polyethylene sleeves are available in a number of materials. the most common contemporary compositions are linear low-density polyethylene film which requires an 8 mil or 200 μm thickness and high-density cross-laminated polyethylene film which requires only a 4 mil or 100 μm thickness. the latter may or may not be reinforced with a scrim layer. polyethylene sleeving does have limitations. in european practice, its use in the absence of additional zinc and epoxy protective coatings is discouraged where natural soil resistivity is below 750 ohm/cm. where resistivity is below 1500 ohm/cm and where the pipe is installed at or below the water table, where there are additional artificial soil contaminants and particularly stray currents again it is recommended for use in addition to zinc and epoxy coating. because of the vulnerability of polyethylene to uv degradation, sleeving, or sleeved pipe should not be stored in sunlight, although carbon pigments included in the sleeving can provide some limited protection. polyethylene sleeving is standardised according to iso 8180 internationally, awwa c105 in the us, bs 6076 in the uk, and as 3680 and as 3681 in australia. zinc in europe and australia, ductile iron pipe is typically manufactured with a zinc coating overlaid by either a bituminous, polymeric, or epoxy finishing layer. en 545/598 mandates a minimum zinc content of 200 g/m2 (at 99.99% purity) and a minimum average finishing layer thickness of 70 μm (with local minimum of 50 μm). as/nzs 2280 mandates a minimum zinc content of 200 g/m2 (with a local minimum of 180 g/m2 at 99.99% purity) and a minimum average finishing layer thickness of 80 μm. no current awwa standards are available for bonded coatings (zinc, coal tar epoxy, tape-wrap systems as seen on steel pipe) for ductile iron pipe, dipra does not endorse bonded coatings, and awwa m41 generally views them unfavourably, recommending they be used only in conjunction with cathodic protection. bituminous coatings zinc coatings are generally not employed in the us. in order to protect ductile iron pipe prior to installation, pipe is instead supplied with a temporary 1 mil or 25 μm thick bituminous coating. this coating is not intended to provide protection once the pipe is installed. water based pipe coatings water based pipe coatings, are an environmentally friendly coating that is applied to the inner & outer diameter of ductile iron pipe. they protect against corrosion from the outside and inside, and also protect the product from contamination. the coating is an emulsion manufactured using asphaltene and water primarily, with other raw materials according to the manufacturer's specifications. they came in use in the early 1990s, replacing coatings based on dangerous and environmental harmful solvents, such as benzenes, toluenes, hexanes and other volatile organic compounds. industry associations and market in the united states, the ductile iron pipe research association represents manufacturers of ductile iron pipe. the association provides research on and promotes the use of ductile iron piping in utility projects (water and sewer), focusing on its strength, recyclability and life cycle cost compared with alternative products such as pvc. the us industry is also represented by the national association of pipe fabricators. outside of the us, the ductile iron pipe industry is supported by associations including the european association for ductile iron pipe systems. following the financial crisis of 2008, the pipe industry as a whole, experienced a decrease in sales in the us due to municipalities deferring replacement of water mains and reduction in new home construction. according to a report published by the freedonia group in 2011, economic recovery from the 2008 crisis is likely to expand ductile iron's market share in the large diameter pipe market. environmental impact ductile iron pipe in the developed world is normally manufactured exclusively from recycled material including scrap steel and recycled iron. the pipe can be recycled after use. in terms of environmental impact, several studies have compared ductile iron pipe's impact on the environment with that of other pipe materials. a study by jeschar et al. in 1995 compared the energy use and carbon dioxide (co2) emissions produced in manufacturing pipes of various materials including concrete, ductile iron, cast iron and pvc, based on pipes with nominal diameter of 100 mm to 500 mm. the energy consumed in manufacturing ductile iron pipe was 19.55 mj per kg and volume of emissions released during manufacture was 1.430 kg co2 per kg, compared to 68.30 mj per kg of energy and 4.860 kg co2 per kg emissions for pvc pipes, and 1.24 mj per kg and 0.148 kg co2 per kg for concrete pipes of the same diameter. another study the following year, by the forschungsinstitut für chemie und umwelt, had similar findings. however, it also took the lifetime of pipes into account. this study found improved environmental performance for ductile iron pipe in terms of energy consumed and emissions produced during manufacture due to its longer life span. a more recent study, published august 2012, by du et al., carried out a life cycle analysis on six types materials used for water and waste water pipes, including ductile iron, pvc, high-density polyethylene (hdpe) and concrete. they found that at diameters of ≤ 24 in, ductile iron pipe had the highest "global warming potential" based on emissions from manufacturing, transportation and installation. at larger diameters, ≥ 30 in, ductile iron pipe had a lower "global warming potential", while pvc had the highest. according to a 2008 study by koo et al., ductile iron pipe had the lowest impact on natural resource depletion, compared to hdpe pipe and pvc pipe. in november 2012, ductile iron pipe manufactured in the united states received certification as a sustainable product from the institute for market transformation to sustainability. notes moser, a. p. and folkman, steven l. (2008) buried pipe design (3rd edition) mcgraw-hill, new york, p. 336-337, isbn 978-0-07-147689-8 romanoff, melvin (1968). "performance of ductile-iron pipe in soils". journal (american water works association). 60 (6): 645–655. doi:10.1002/j.1551-8833.1968.tb03591.x. jstor 41265349.