API 570 Soil-to-Air Interface and Buried Piping Inspection
Short answer: API 570 treats the soil-to-air interface (SAI), where pipe enters or leaves the ground, as a zone needing specific attention. In the 4th edition, the zone generally runs from at least 12 inches below to 6 inches above grade. SAIs are inspected visually on the piping class's external interval. Buried process piping (not DOT-regulated) is inspected from cathodic protection effectiveness, coating condition and direct examination. If there is no effective CP, it is excavated or pigged on a 5, 10 or 15-year cycle set by soil resistivity.
This guide explains what the code expects at the SAI and below grade, how surveys, screening and excavation fit together, which NDE methods actually measure wall loss, where API 570 stops and pipeline regulation starts, and what the records should show. Figures quoted here come from the API 570 4th edition (2016), which was reviewed for this guide. The current edition is the 5th (February 2024), and the buried-piping provisions and numbers should be confirmed against your licensed copy (API 570 announcement, api.org) and your jurisdiction.
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What the soil-to-air interface is and why it corrodes
API 570 defines the SAI as the area on buried or partly buried pipe, near where it comes out of the soil, in which external corrosion can occur or speed up. The extent of the zone depends on soil moisture, oxygen content and operating temperature. The 4th edition says it is generally taken as at least 12 inches below to 6 inches above the soil surface. Pipe lying along the ground and touching the soil is included too.
The SAI is aggressive for several reasons that tend to act together:
- Wet-dry cycling and oxygen. Just below grade the soil stays moist but still has oxygen, which drives corrosion. Above grade, rain, irrigation and splash keep the surface wet.
- Coating stress. Coatings and wraps are often damaged or disbonded at the transition. Ground movement, frost and mowing equipment all work on the coating right where it changes from buried to exposed.
- Debris and ponding. Gravel, weeds and trapped water collect against the pipe at grade.
- Cathodic protection does not reach above grade. CP protects the buried surface, not the atmospheric side of the interface. The short band where the pipe leaves the soil is often the least well protected part.
- Hidden attack. The worst metal loss is often just below the surface, where it cannot be seen without digging.
API 570 lists soil-to-air interfaces and soil corrosion of buried piping among the areas of deterioration owner-users must give specific attention. It also expects CMLs to be established where SAI corrosion is a concern. API 574 carries more detail on SAI inspection practice.
How API 570 sets SAI inspection
In the 4th edition's recommended-maximum-interval table, soil-to-air interfaces have no fixed thickness interval. Their visual external inspection is "by class". In practice the SAI is inspected on the external visual interval of the piping class it belongs to: 5 years maximum for Class 1 and Class 2, and 10 years for Class 3 (Class 4 is optional). The class basis is covered in API 570 piping service classes 1-4.
A good SAI inspection does more than look at the pipe where it enters the ground:
- Visual check of coating and wrap at and just above grade: disbondment, cracking, holidays, missing wrap and rust staining.
- Shallow excavation where the coating is suspect or the pipe is bare. Removing soil below grade exposes the zone where loss usually concentrates. The owner-user's procedure should say how deep, since the code's zone definition gives a minimum depth.
- Cleaning and measuring the exposed surface: pit depth gauging, UT thickness, and UT corrosion mapping where loss is widespread.
- Screening from above grade, using guided-wave or other ultrasonic screening, where digging is impractical (paved areas, congested racks). API 570 notes such technologies are useful at locations such as soil-to-air interfaces.
- Recoating after inspection. Any coating removed must be renewed and inspected.
Where corrosion continues to be found at an SAI, setting a CML there allows wall loss to be trended with a calculated corrosion rate, like any other location in the circuit.
Buried piping: scope and what the inspection plan must be based on
API 570's buried-piping section applies to buried process piping that is not regulated by the US Department of Transportation. Inspection of buried piping is hard because the corrosive environment is soil and the pipe cannot be seen. The code therefore requires owner-users to inspect buried piping to find its external condition, and to base the plan on three things: how effective any cathodic protection is, whether the pipe was coated, and inspection information from one or more sources. The 4th edition lists these sources:
- findings during maintenance on connected piping of similar material;
- examination of representative portions of the actual piping;
- buried piping in similar circumstances;
- permanently installed thickness monitoring devices;
- remote visual inspection where possible;
- CP survey results, or guided-wave examination used to find areas for follow-up with more quantitative thickness methods.
The code names API 574, API 651 and several NACE (now AMPP) documents — SP0102, SP0169, SP0274 and RP0502 — as important non-mandatory references. API 651 covers cathodic protection of aboveground storage tank bottoms, but its CP inspection guidance is referenced for buried piping. NACE SP0169 is the core external-corrosion-control practice for buried metallic piping; see our summary of NACE SP0169 for buried piping CP.
Surveys from above ground: what they find and their limits
Before anyone digs, API 570 describes several above-grade tools. Each answers a different question, and most only infer corrosion.
| Technique | What it tells you | Limit | Frequency in the 4th edition |
|---|---|---|---|
| Above-grade visual surveillance of the piping route | Leak signs: ground contour changes, soil discoloration, softened asphalt, pools, bubbling, odour | Finds leaks, not wall loss | About every six months (owner-user should) |
| Close-interval potential survey (pipe-to-soil potentials) | Where CP may be ineffective and active corrosion cells may exist | Infers corrosion from potential; does not measure wall loss | For poorly coated CP pipe with inconsistent potentials, about every 3-5 years |
| Coating holiday survey (e.g., DCVG) | Locations of coating defects; coating deterioration trend | Coating condition only | When other corrosion controls appear ineffective |
| Soil resistivity (Wenner four-pin per ASTM G57; single-pin or soil box where needed) | Relative soil corrosivity; lower resistivity = more corrosive | Relative ranking, not wall loss | For non-CP pipe longer than 100 ft, at intervals based on likelihood of change |
| CP monitoring | Whether protection levels are adequate | Needs trained CP personnel; records must be kept | Regularly; critical components such as rectifiers more often |
CP surveys and soil testing are specialist corrosion-engineering activities, usually done by CP technicians qualified under AMPP programmes. They do not replace direct examination. They tell you where to look.
Inspection intervals for buried piping without effective CP
For buried piping that is not cathodically protected, the 4th edition requires the external condition to be found either by pigging (in-line tools that measure wall thickness) or by excavation. The frequency comes from a table based on soil resistivity:
| Soil resistivity (ohm-cm) | Inspection interval, buried piping without effective CP (4th ed.) | Leak-test alternative interval (non-CP piping) |
|---|---|---|
| Below 2,000 | 5 years | Half the inspection interval |
| 2,000 to 10,000 | 10 years | Half the inspection interval |
| Above 10,000 | 15 years | Half the inspection interval |
Confirm these values in the current edition. Important qualifiers in the 4th edition:
- CP does not end the matter. Significant external corrosion found by pigging or other means may require excavation and evaluation even on cathodically protected piping.
- Internal corrosion counts too. If the above-grade part of the line shows internal corrosion, the intervals and methods for the buried part should be adjusted. Dead legs can see accelerated internal attack.
- Excavation extent. Periodic excavations expose 6 to 8 ft lengths at one or more locations judged most susceptible. The pipe is inspected around its full circumference for the type and extent of corrosion and for coating condition.
- Follow the damage. If coating damage or corrosion is found, more piping is excavated until the extent is known. If the average wall is at or below required thickness, the pipe is repaired or replaced.
- Cased crossings. Check whether water or soil has entered the casing, that the casing ends extend beyond the soil and are sealed where not self-draining, that the carrier pipe is coated, and that there is no metallic or electrolytic contact between casing and carrier.
Choosing the dig site is the critical judgement. Low points, wet areas, road and rail crossings, foreign-line crossings, coating holidays from surveys, low CP potentials and SAIs are typical candidates.
Leak testing as an alternative or supplement
The 4th edition allows leak testing as an alternative or supplement to inspection. The liquid test is at least 10% above maximum operating pressure. For piping without CP, it is run at half the soil-resistivity table intervals; for CP-protected piping, at the same intervals. The test is held for eight hours. Pressure is noted four hours after the initial pressurisation, and the line is repressurised to test pressure if needed and isolated. A drop of more than 5% in the remaining period calls for inspection to find the leak and assess the corrosion. Other leak-integrity methods mentioned include temperature-corrected volumetric or pressure methods, acoustic emission, and tracer gases such as helium or sulfur hexafluoride (checking suitability for the process).
A leak test shows the line holds pressure today. It does not measure remaining wall or corrosion rate. Many owner-users pair leak testing with targeted direct examination so the corrosion state is actually known.
NDE methods for buried piping and SAIs
- Guided-wave testing (GWT). API 570 describes guided-wave examination (formerly long-range UT or GWUT) as a screening method. From one installation it may cover 15 ft or more, depending on corrosion, coatings, soil, product and fittings in the path. It suits road crossings, sleeves, wall penetrations and SAIs, where only a small excavation or an above-grade collar location is available. It flags areas of interest; it does not size the minimum wall. Follow-up with quantitative methods is expected. See guided wave testing.
- UT thickness and corrosion mapping at excavations and SAIs, to quantify remaining wall and pit depth after cleaning. See piping circuit and CML inspection.
- Visual examination and pit gauging of the exposed surface and coating, recorded with photographs and dimensions.
- In-line inspection (pigging) where the line is piggable. This measures wall thickness along the whole buried length and is recognised by the code as an alternative to excavation. Most plant process piping is not piggable without modification. Atlantis does not provide in-line inspection tools.
- Radiography of exposed sections where access suits it, by crews licensed by the radiation authority where the work takes place.
- Remote visual (crawler cameras) for internal condition where entry points allow.
The common pattern is: screen widely, then measure precisely where screening, surveys or history point.
Where API 570 stops: DOT pipelines and other regimes
API 570's buried-piping section expressly excludes DOT-regulated piping. Gas transmission and distribution lines are regulated by PHMSA under 49 CFR Part 192, and hazardous liquid pipelines under 49 CFR Part 195. Each has its own corrosion-control, integrity-management and recordkeeping rules. At many facilities the regulatory boundary sits at an isolation valve or the property line, so a single physical line can change regime. Mapping that boundary is part of any buried-piping programme; see API 570 vs ASME B31.4/B31.8 pipeline boundary.
Within the plant fence, API 570 is widely treated as recognised and generally accepted good engineering practice for OSHA PSM-covered piping (29 CFR 1910.119(j)). Buried piping in covered processes therefore needs a documented inspection basis like any other process piping. At oil storage facilities covered by EPA's SPCC rule, 40 CFR 112.8(d) separately requires protective wrapping and coating on buried piping installed or replaced on or after August 16, 2002. It also requires integrity and leak testing of buried piping at installation, modification, construction, relocation or replacement. Read the full paragraph, which also covers corrosion protection and exposed sections, for how it applies to your facility.
In Canada, provincial regulators set the framework. In Alberta, plant pressure piping is regulated by ABSA, while pipelines generally fall under the Alberta Energy Regulator and CSA Z662. Other provinces divide responsibility differently. Confirm which regulator and standard govern each buried line before setting a programme.
Records for buried piping and SAIs
The 4th edition asks for buried-piping records to be kept as part of the general piping record system. In addition:
- keep a record of the location and installation date of any temporary clamps, which must all be treated as temporary and permanently repaired at the next opportunity unless a piping engineer approves an extension;
- show buried piping on a drawing (a plot plan or isometric) with its size and external corrosion mitigation;
- keep CP monitoring and maintenance records.
Good practice adds excavation reports (location, length exposed, coating condition, wall and pit data, photographs), GWT screening results with follow-up findings, soil resistivity values, and SAI CML histories. Together these let the next inspector see what was examined, what was found and why the interval was set.
Common mistakes
- Treating a CP reading as proof of no corrosion. Potential surveys infer conditions; they do not measure wall.
- Inspecting the SAI only above grade. The worst loss is often just below the surface.
- Treating guided-wave results as thickness. GWT is screening; size with UT or other quantitative methods.
- Digging in convenient places rather than the most susceptible ones.
- Not recoating properly after an excavation, which creates a new holiday where you just inspected.
- Ignoring the regulatory boundary between API 570 plant piping and DOT pipeline segments.
How Atlantis supports this
Atlantis NDT performs the NDE parts of a buried-piping and SAI programme. That includes guided-wave screening at crossings, sleeves and SAIs; UT thickness, pit gauging and corrosion mapping at excavations; visual examination of coating and substrate; and radiography by licensed crews where it suits. The work is done by ASNT-certified technicians under ASNT Level III oversight. Excavation, CP surveys and in-line inspection are carried out by others. We report findings with location, method, calibration and technician certification to your API 570 authorized piping inspector, who sets intervals and decides acceptance with the piping engineer. See guided wave testing or request a quote within 24 hours.
FAQ
What is soil-to-air interface corrosion on piping?
External corrosion that starts or speeds up where buried or partly buried pipe leaves the soil. Moisture, oxygen, coating damage and debris at grade all contribute, and the worst loss is often just below the surface.
How far below grade does the API 570 soil-to-air interface extend?
The 4th edition says the zone is generally at least 12 inches below to 6 inches above the soil surface, varying with moisture, oxygen and temperature. Confirm in the current edition.
How often should the soil-to-air interface be inspected?
Visually, on the external inspection interval of the piping class: up to 5 years for Class 1 and 2 and 10 years for Class 3 in the 4th edition table. Problem SAIs warrant CMLs and their own trend.
What is the API 570 buried piping inspection frequency?
For piping without effective cathodic protection, the 4th edition sets excavation or pigging every 5, 10 or 15 years for soil resistivity below 2,000, from 2,000 to 10,000, and above 10,000 ohm-cm.
How is buried process piping inspected without cathodic protection?
By pigging or by excavating 6 to 8 ft lengths at the most susceptible locations on the resistivity-based interval. Pipe-to-soil potential surveys can help choose dig sites, and leak testing can supplement the programme.
Can guided wave testing be used on buried piping?
Yes, as screening. API 570 notes it may cover 15 ft or more from one location. Areas it flags need quantitative thickness follow-up.
Does API 570 apply to buried pipelines?
Not to DOT-regulated pipelines, which follow 49 CFR 192 or 195. It covers buried process piping within its scope at plants and terminals.
What leak test pressure does API 570 use for buried piping?
The 4th edition uses a liquid test at least 10% above maximum operating pressure, held for eight hours, with investigation if pressure drops more than 5% after the four-hour check.
What records are required for buried piping?
Standard piping records, plus the location and date of temporary clamps, and a drawing showing the buried piping with its size and corrosion mitigation. Keep CP monitoring records too.
Who performs CP surveys versus NDE on buried piping?
CP surveys are done by qualified cathodic-protection personnel. Direct NDE at excavations and SAIs is done by certified NDE technicians. The owner-user's inspector combines the results.
Related: API 574 piping inspection practices. Ready to plan screening and excavation NDE? Contact Atlantis NDT for a quote.
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