As with other failure modes, evaluating the potential for corrosion follows logical steps, replicating the thought process that a corrosion control specialist would employ. This involves (1) identifying, at all locations, the types of corrosion possible, both on internal, external surfaces; (2) identifying the vulnerability of the pipe material—how probable and how aggressive is the potential corrosion; and (3) evaluating the corrosion prevention measures used,.
Quantifying this understanding is done using the same PoF triad that is used to evaluate each failure mechanism: exposure, mitigation, and resistance, each measured independently. The independent measurements of exposure and mitigation are critical to the understanding of corrosion damage potential. For example, a subsurface environment of Louisiana swampland may presents a very corrosive environment, while a dry Arizona desert environment typically has a very low corrosion rate. The mitigation—the coating system and the cathodic protection system—are more critical to damage prevention in Lousiana. Perhaps, the damage potential in the Louisiana system with very robust corrosion prevention could be made roughly equivalent to the Arizona desert situation where minimal corrosion preventions are needed since the environment is very benign. But it is important to understand when the damage potential is low because of exposure or due to mitigation.
The two factors that must be assessed to define the corrosion exposure are the material type and the environment. The environment includes the conditions that impact the pipe wall, internally as well as externally. Because most pipelines pass through several different environments, the assessment must allow for this by sectioning appropriately.
Corrosion mechanisms are among the most complex of the potential failure mechanisms. There are a wide variety of available mitigation methods and supporting inspection techniques. As such, many more pieces of information are efficiently utilized in assessing this threat. Because corrosion is usually a highly localized phenomenon, and because inspection opportunities often provide only general information, uncertainty is often high.
Types of corrosion failure mechanisms
The corrosion assessment should recognize the two locations where corrosion can occur—the external surface of the component or the internal surface.. Since these two are significantly different both in terms of exposure and mitigation, they are usually best assessed independently.
for evaluation purposes, the two corrosion (types) locations are further broken down as follows:
External Corrosion
- Exposure to Atmosphere
- Burial in Soil
- Submersion in Water
- AC-induced
- Interferences
Internal Corrosion
- Stream-based
- Under-deposit
MIC is a potential exacerbating factor in most of these and is therefore assessed within each, instead of as an independent aspect.
From a chemistry perspective, these corrosion processes are often very similar.
External Corrosion
A pipeline component can be susceptible to external corrosion damage via atmospheric corrosion, subsurface corrosion, or both.
Atmospheric corrosion deals with pipeline components that are in contact with the atmosphere. it is a normally a less aggressive corrosion mechanism but there are dramatic exceptions. Alternately wet and dry areas, such as splash zones near water bodies or an annular space inside buried casings, have caused aggressive corrosion and pipeline failures. Failure potential due to atmospheric corrosion are lower in most segments because 1) most pipelines are predominantly buried and, hence, have few portions exposed to the atmosphere, 2) atmospheric corrosion rates are usually low, and 3) there are increased inspection opportunities for above-ground components.
Subsurface corrosion includes both onshore and offshore installations and is the result of potentially very aggressive mechanisms, including various types of galvanic corrosion cells and interference potential from electrical sources and other buried structures. There are also challenges in gaining knowledge of actual corrosion on subsurface components. Subsurface pipe corrosion is often the most information-rich area of risk assessment, reflecting the numerous data-collection practices and the complicated mechanisms underlying this type of corrosion.
Modern metallic distribution pipeline systems (steel and ductile iron, mostly) are installed with coatings and/or cathodic protection when soil conditions warrant. This is equivalent to practices in modern tranmission pipelines. However, in many older metal systems, especially older urban distribution systems, little or no corrosion barriers were put into design considerations.
As a special form of subsurface external corrosion, AC-induced corrosion is best examined independently. Also warranting special attention in subsurface systems are nearby sources of DC electricity that can interfere with protective systems or generate new corrosion potential.
Erosion is an external corrosion mechanism (in the broad definition of ‘corrosion’). Often due to moving water, it is most often included in geohazards. The potential for undermining (loss of support), impingement forces, and others is normally more likely than material loss due to erosion. However, one can envision scenarios involving susceptible component materials in an aggressive flowing (or even stagnant) fluid environment that warrants assessment as a bona fide external degradation mechanism. UV degradation of plastics and other material-property changing mechanisms can be included here and/or in the resistance estimations. See chap xx discussion of PoF modeling nuances.
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Internal Corrosion
Internal corrosion deals with the potential for corrosion originating within the pipeline. Some significant pipeline failures have been attributed to internal corrosion. Internal corrosion results in wall loss and is caused by a reaction between the inside pipe wall and the interior environment, ie, the product being transported and its flow regime. Internal corrosion may not be the result of the product intended to be transported, but rather a result of impurities in the product stream. Erosion is an internal corrosion mechanism (in the broad definition of ‘corrosion’).
MIC
MIC
The term microbiologically influenced corrosion (MIC) is used to designate the localized corrosion affected by the presence and actions of microorganisms. MIC was described in a previous section.
External corrosion manifestations of MIC are typically characterized by pitting and crevice corrosion, according to some experts (Koch). (Beavers) Soils with sulfates or soluble salts are favorable environments for anaerobic sulfate-reducing bacteria [69]. Also see PRMM page xxx.
Erosion
Erosion is also considered here as a potential time dependent mechanism. For instance, an exposed concrete pipe in a flowing stream can be subject to erosion as well as mechanical forces. Erosion on an interior component wall is caused by high velocity flow streams containing abrasive particles and can be particularly damaging at impingement points such as elbows.
Corrosion rate
The time to failure is related to the resistance of the material and the aggressiveness of the corrosion mechanism—the mitigated corrosion rate. The material resistance is a function of material strength and dimensions, most notably wall thickness and the stress level. This chapter examines the process of estimating first the unmitigated- and then the mitigated corrosion rate. A separate estimate is produced for internal and external corrosion potential.
The unmitigated corrosion rate is the ‘exposure’ in the exposure-mitigation-resistance modeling triad. Exposure estimates should consider the formation of a protective layer or film of corrosion by-products that often occurs and precludes or reduces continuation of the damage. Similarly, temperature effects, rare weather conditions, releases of chemicals, or any other factors causing changes in the corrosion rate should be considered.
Corrosion is a volumetric loss of material but convention measures most corrosion as depth penetration (pitting). Mils per year (mpy, one mil = 1/1,000 inch) and mm/year are common units of pitting corrosion rates in metals.
while plastics are often viewed as corrosion proof, sunlight and airborne contaminants (perhaps from nearby industry) are two degradation initiators that can affect certain plastic materials and can be efficietly modeled as corrosion in a risk assessment.
Unmitigated Corrosion Rates
As the phrase implies, an unmitigated corrosion rate is a measure of the corrosion progression that may occur in the absence of any corrosion control actions. Normally, a pitting rate is used as the most conservative measure, since pitting rates are usually the most aggressive. When a general (non-pitting) corrosion rate is also active, the resistance measurement (see Chapter XX) takes into account loss of component integrity by loss of metal, in addition to loss of integrity by a pitting-induced leak.
There is much research available showing corrosion rates under various laboratory scenarios. Even though laboratory results are often not directly transferable to field conditions, they nonetheless provide valuable insight into plausible corrosion rates, especially when extreme conditions, unlikely to be seen in actual field characteristics, are simulated in the laboratory and suggest maximum rates.
Note that corrosion rates are very situation specific. Any type of corrosion might lead to a failure under the right circumstances, even when history suggests it to be relatively rare failure mechanism.
Recall the previous discussion of measurements and inferences. Proper risk assessment uses all available information. In the case of corrosion rates, information often appears in both general forms—measurements and inferences. The final estimate emerges from an examination of both, after adjustments for information age and accuracy have been made. The assessment chooses the best estimate based on the strength of evidence–newer and more accurate information is chosen over older, less accurate information. Note the nuances that have to be considered. For example, highly accurate measurements, but taken some distance from the point of interest where conditions may not be consistent (ie, internal corrosion coupons); or measurements taken at a point in time no longer refletivw of recent conditions.
Also see PRMM page xxx.
Corrosion Mitigation
Corrosion mitigation is specific to the type of corrosion and, often, to the location. details are discussed in subsequent sections. Here, the philosophy of modeling corrosion mitigation is discussed.
Similar to other mitigation where an OR gate can combine mitigation measures acting independently, a multi-layer defense against corrosion uses the same modeling approach. The common mitigation against external corrosion for a buried metal pipeline is a two-part defense of coating and cathodic protection (CP). These two are usually employed in parallel and provide redundant protection. Some practitioners rate these measures as equally effective, in theory at least. Since each can independently prevent or reduce corrosion, an OR gate can be used in assessing the combined effect. The notion of independence here refers to a modeling protocol, not to an idea that the two are not related in considerations of real world design, economics, maintenance, etc.
An effective modeling approach quantifies external corrosion potential by coupling exposure (corrosion aggressiveness) with the probability of one or more active corrosion points on the pipeline segment. This probability is based on an estimate of the frequency of active corrosion locations, derived from estimates of coating holiday rates plus the efficiency with which CP prevents those holidays from experiencing corrosion.
Underpinning this procedure is the belief that the simultaneous occurrence of multiple defects is appropriately modeled as the product of the independent defect rates.
Corrosion Failure Resistance
The resistance to failure by corrosion is efficiently measured as an effective wall thickness. The wall thickness is a critical part of all stress-carrying capacity calculations that underpin resistance estimates in thin-shelled, pressure containing structures. This wall thickness, taken with the mitigated corrosion rate, yields a time to failure, or remaining life estimate. For example, a 0.250” effective wall thickness, experiencing 10 mpy pitting corrosion, would be expected to leak in 25 years. A rupture could occur sooner, depending on the lateral corrosion damage and the stress level. This is modeled in a parallel analysis, with the leak-driven or the rupture-driven TTF estimate providing the final TTF value for the PoF estimate.
The ‘effective’ adjective allows inclusion of any weaknesses (previous damages, manufacturing or construction defects, stress concentrators, etc) or vulnerabilities (selective seam corrosion, heat affected zones of welds, etc) which, when modeled as equivalent reductions in pipe wall thickness, show reduced remaining life estimates and corresponding increases in PoF. This is fully discussed in XX.
sequence of eval
regardless of the type of corrosion or its location on the pipeline system, the risk assessment protocol is the same. That protocol is as follows:
Estimate exposure levels
the corrosion rate for the PXX level of conservatism desired is first estimated.
for all types of corrosion, the risk assessment evaluation protocol is the same. The first step involves evaluating the pipe’s environment. For each corrosion type, external and internal, an assessment is made of the corrosivity at the material’s interface with its immediate environment. Once a database is built, this process can be at least partially automated. The following discussion illustrates a typical approach to characterizing each component’s environmental exposures (the threats to the pipe from its immediate environment).
To differentiate two general types of external corrosion, typically with quite different pitting rates, the computerized risk model first searches for indications of contact with the atmosphere, including locations with depth of cover = 0, casings, tunnels, spans, valve vaults, manifolds, and meters. Under an assumption of a mostly-buried pipeline, these occurrences are noted in the database and identify locations for atmospheric corrosion.
Next, location-specific characteristics that typically harbor more aggressive atmospheric corrosion rates are identified. These include supports, hangars, splash zones, tree sap depositions, and many others. These are treated as external corrosion’hot spots’.
If the pipe is not exposed to the atmosphere, then the model assumes it is immersed in soil or water and is treated as being in a subsurface corrosive environment. soil corrosion rates are measured or estimated at all points along the pipeline. Provisions can alsos be added to capture scenarios where a component is exposed to both atmospheric and soil corrosivities, such as a pipeline laid atop the ground.
The model assumes that all portions of the system are exposed to the product being transported and, hence, to any internal corrosion potential promulgated by that product. Therefore, all portions have general exposure to internal corrosion. Especially where corrosion rates can change over both time and space—eg, where contaminant and velocity excursions impact internal corrosion —a probability-weighted corrosion rate can be used.
Next, location-specific characteristics that exacerbate internal corrosion such as areas of accumulations of solids and liquids, are identified, perhaps by elevation profiles, velocity profiles, and product stream analyses. These are ‘hot spot’ locations for increased internal corrosion rates, analagous to the external corrosion hot spots.
Estimate mitigation effectiveness
For barrier-type mitigation, such as coatings, and perhaps even inhibitors, the probability of a gap in protection per unit of surface area to be protected is estimated. For subsurface corrosion, both soil burial and submersion in water, the probability of unprotected surface area is similarly estimated. These mitigation effectiveness estimates are very challenging. Much information is often available, but inferential and/or location-specific in nature, requiring interpretation and extrapolation to assessed areas with less information.
exposure and mitigation estiamtes are then combined to yield probabilistic damage rates, after mitigation, typically in units of mpy or mm per year. All combinations of unmitigated exposure and mitigation effectiveness are considered along the assessed pipeline. Hot spots with weak mitigation will show highest damage rates.
These mitigated damage rate estimates are now combined with estimates of effective pipe wall thickness to estimate TTF. This is value, again usually changing along the pipeline, is often of more interest than the final PoF. TTF can more effectively drive risk management decision-making, including integrity re-assessment intervals.
Finally, choosing and applying a representative relationship between TTF and PoF yields the estimate for corrosion PoF for the future year of interest.