What matters most
External corrosion may reflect soil, moisture, coating condition, aeration differences, backfill, dissimilar contacts, or external electrical influences. Internal corrosion may reflect fluid chemistry, dissolved gases, flow, temperature, deposits, stagnation, treatment, and biological activity. Map and preserve both surfaces before selecting tests.
Key takeaways
- Record whether damage appears to initiate internally, externally, or from interacting conditions before cleaning.
- Review coating or lining, soil and backfill, drainage, fluid chemistry, deposits, flow, temperature, treatment, and operation.
- Tubercles, pits, or deposits do not prove one mechanism by appearance alone.
- A thinned wall may ultimately rupture mechanically even though corrosion created the loss of section.
Map the damage before cleaning
Record internal and external surfaces, clock position, flow direction, distribution, morphology, deposits, coating or lining, joints, nearby repairs, and the apparent penetration path. Cross-section and wall-thickness mapping can help identify the side and pattern of loss.
Cleaning can obscure the origin, particularly at a small perforation. Preserve deposits and comparison areas before destructive preparation.
Evaluate the external exposure
Review native soil and trench backfill, moisture, drainage, irrigation, leaks, groundwater, coating condition, burial details, joints, electrical continuity, dissimilar materials, utility crossings, and nearby current sources.
A sample collected several feet away or at the surface may not represent the pipe-zone environment. Sampling locations should follow the observed damage and site variability.
Evaluate the internal exposure
Review fluid source and chemistry, treatment, dissolved gases, temperature, pressure, flow, velocity, stagnation, solids, deposits, recirculation, shutdowns, and process changes. Compare the timing of failures with operating events.
Different positions around the pipe can experience different solids deposition, gas pockets, flow, or wetting. The clock-position pattern is useful evidence.
Consider interacting causes
One mechanism can create wall loss while another condition triggers final rupture. Coating damage may localize external exposure; deposits may create local chemistry; stress may open a thinned region; and the leak itself may alter soil moisture after penetration.
The report should distinguish the corrosion mechanism, contributing conditions, and final failure event where the evidence supports that separation.
Choose tests from the competing hypotheses
Potential work includes soil chemistry and resistivity, deposit and fluid analysis, dimensional or UT mapping, material verification, microscopy, metallography, hardness, coating assessment, and microbiological sampling under suitable protocols.
Do not order every test automatically. Select the work that can distinguish the credible internal and external hypotheses.
Connect findings to inspection and repair decisions
Recommendations may include broader inspection, monitoring, repair extent, material compatibility, coating or lining evaluation, operating review, or a specialized CP or fitness-for-service scope. The decision should reflect whether the observed location is representative of the system.
Calicore can investigate both sides of the pipe and coordinate specialty testing. Pressure integrity or CP design is performed by the appropriate qualified professional.
This article is general educational information, not a project-specific engineering conclusion. Site conditions, service environment, material, geometry, governing documents, and available evidence must be evaluated for the actual project. Calicore Engineering does not currently provide cathodic protection system design.
Frequently asked
Questions about this topic
Can the leak opening show which side initiated corrosion?+
Sometimes, but deformation, secondary corrosion, cleaning, and the final rupture can obscure the origin. Preserve and section the material carefully.
Does tuberculation prove microbiologically influenced corrosion?+
No. Tubercles and deposits can form through multiple processes. MIC requires a combined evaluation of morphology, environment, microbiology where appropriate, chemistry, and alternative mechanisms.
Can internal and external corrosion occur together?+
Yes. Both surfaces may lose section, and their combined effect can control the final wall thickness and failure location.