ISO 21809-3 · field joint coating
Cathodic disbondment, and why it is the field joint's pass or fail
The sleeve went on straight, the adhesive bead showed at both ends, and the coating still released from the steel.
The current that lifts a good sleeve
A buried line is held at a protective potential by its own cathodic protection system. Where the coating is whole, nothing reaches the steel and nothing happens. Where there is a holiday, current arrives at bare metal and the reaction runs at that spot. Water and oxygen are reduced there, and hydroxide builds up in the thin film under the coating. The steel itself stays bright, which is the point of the protection. What gives way is the bond either side of the defect, because alkali attacks the adhesive rather than the pipe.
That is why this one test decides the joint. Leave the steel unabraded and you get a peel failure. Hold the torch in one place and you get a shrink-back failure. Both of those belong to the crew, and both are worked through on why wraparound repairs fail. Cathodic disbondment is different in kind. It goes after a joint that nobody did anything wrong on. The current was switched on deliberately, it runs for the design life of the line, and it is indifferent to how well the sleeve was fitted.
ISO 21809 covers external coatings for buried and submerged pipelines in the petroleum and natural gas industries. Its earlier parts describe the mainline coating, applied to bare pipe in a plant under controlled conditions. Part 3 is the field joint. That is the only length of the line welded outdoors and coated outdoors, with the trench open and the weather in whatever state it is in. It gets its own part of the standard because it is the length of pipe least like the rest of the pipe.
The three-layer wraparound sleeve manufactured by Gala Thermo Shrink Pvt. Ltd. carries a single line for this. Cathodic disbondment, 3 rad, with ISO 21809-3 in the method column. The figure is published on the manufacturer's page for the sleeve. The same standard turns up again on the pre-insulated pipe sealing cap, where it is cited for adhesion and peel strength, at 14 N/cm minimum. One standard, two properties, two different parts. Check which property a competitor is quoting before you put two figures side by side.
Five words the result is written in
- Holiday
- A break in the coating that leaves steel exposed. Accidental in a trench, drilled on purpose in a test.
- Artificial holiday
- The drilled defect the test cell is built around. Everything reported afterwards is measured outwards from its edge.
- Disbondment radius
- How far the coating released from that edge by the end of the run. Smaller is the better result, and the number is a distance, not a score.
- Rad
- The column heading against the 3. Read it as radius. It is not a rating, not a class and not a mark out of ten.
- Cutback
- The bare steel either side of the girth weld, where the mainline coating stops short. This is the surface the sleeve has to bond back onto.
Three conditions the published row leaves out
The value sits on the manufacturer's own product page for the sleeve, and no separate datasheet for it appears among the manufacturer's published sheets. The page gives the result and the method. It does not give the test temperature, the applied potential or the duration, and it prints no length unit against the 3. Those three conditions set the answer. A thirty-day run held hot and a thirty-day run at ambient are both ISO 21809-3, and they are not the same test. Ask for the test report itself, not the row copied out of it.
The results that decide the same joint
| Test | What it answers | Published value | Method |
|---|---|---|---|
| Cathodic disbondment | Does the bond survive the line's own protection current | 3 rad | ISO 21809-3 |
| Peel strength, PE to steel | Does the adhesive still hold once the joint is warm | 100 N/cm at 25 °C, 30 N/cm at 50 °C | ASTM D2671 |
| Peel strength, PE to FBE | The same question against a fusion-bonded epoxy mainline coating | 120 N/cm at 25 °C, 35 N/cm at 50 °C | ASTM D2671 |
| Peel strength, PE to PE | The same question against a polyethylene mainline coating | 110 N/cm at 25 °C, 30 N/cm at 50 °C | ASTM D2671 |
| Impact resistance | Does the coating survive what the backfill drops on it | Greater than 15 J | ASTM G14 |
| Abrasion resistance | Does it survive being dragged and lowered in | 30 mg | ASTM D1044 |
| Chemical immersion, 24 hours | Retained elongation in 10% HCl, 10% NaOH and 10% NaCl | 100%, 97%, 98% | ASTM D2671 |
All values as published for the GTWS three-layer sleeve. Each is the manufacturer's figure, with the method it was measured by.
What to ask for before you compare two disbondment figures
A radius on its own cannot be compared with another radius. These are the conditions that move it, and any supplier holding a real report can give you all seven.
- The report itself, and the laboratory that issued it. A line in a brochure is a claim, not a result.
- The test temperature. Disbondment runs faster hot, and a joint on an operating line is not sitting at room temperature.
- The duration in days, because the radius keeps growing for as long as the cell is powered.
- The applied potential, stated against the reference electrode it was measured to.
- The diameter of the artificial holiday, and how the surface around it was prepared before the coating went on.
- The substrate under the adhesive: bare steel, fusion-bonded epoxy or polyethylene. Peel already differs by substrate, and so does this.
- Whether the specimen carried a closure patch or was a plain coupon. The overlap is the one feature a flat coupon never tests.
Where the rest of this joint is worked through
Each of the adjacent tests is explained once, on the page it belongs to, rather than a second time here.
- The three-layer sleeve itselfConstruction, what the hot-melt adhesive bonds to, and the full published property table.
- Pipeline sleeve size chartFifteen codes from Ø159 to Ø1016 mm, each with the closure patch supplied against it.
- Why wraparound repairs fail in the groundPeel at 50 °C, closure geometry, and what a released sleeve looks like when it is dug up.
- How to install a wraparound sleeveAbrasion, preheat, and the adhesive bead that shows the bond actually wetted out.
- Reading a datasheet without taking it on trustThe method key: which test measures what, mapped against the values published here.
- Pre-insulated pipe sealing capThe other part on this site citing ISO 21809-3, for adhesion rather than disbondment.
Send the pipe OD, the mainline coating and the operating temperature
Those three settle the sleeve code and tell you which peel column applies to your joint. Technical questions are answered here. Prices, lead times and stock belong to the manufacturer, and this site quotes none of them.