Cross Country Pipelines - Bonding of Coating to Pipeline 1

[shaafishaikh]

Good day all!

Recently we carried out a peel test of the asphalt enamel coating on our cross country water pipeline. The coating was observed to be disbonded from the steel pipeline. Our pipeline is protected using an impressed current CP system along with sacrificial Mg anodes at many locations.

As far as my understanding goes, the purpose of a coating is to isolate the steel pipeline from the soil, and to act as a barrier to moisture and other corrosive elements.

As such, can anyone please explain why bonding of the coating to the pipeline is considered to be so vital?

Thanks,
Shaafi

PS: Any literature on pipeline coatings and their types, merits and demerits would be great.

 

[CoryPad]

Without bonding, the coating can be undercut by water and corrodents which will lead to corrosion of the base metal. This is a good book on corrosion and coatings:

http://www.crcpress.com/product/isbn/9780849372780

 

[shaafishaikh]

Thanks for the response.
Im afraid I dont quite understand the terms "undercut by water and corrodents".

 

[Sloss]

Water gets under the coating and has O2 dissolved in it so all the ingredients for corrosion are present - the third requirement is already there - the steel pipe.

'Bonded' in this sense means 'tightly adhered' - that my be slightly confusing the issue.

 

[shaafishaikh]

So this means that the coating is not impermeable?

 

[Sloss]

A coating for this application is impermeable when applied correctly.

I think you may be confusing 'coating bonding' with 'electrical bonding'.

 

[shaafishaikh]

Thanks!
But if it is impermeable, how does whater get under the coating?

Coating bonding would mean the adhesion of the coating to the steel pipeline.
And electrical Bonding would mean?

 

[Sloss]

I think this is a wind up.......

Anyone else wish to explain?

 

[weldstan]

No coating is truly impermeable. Cathodic disbonding may also have resulted. As disbonding increases, greater current density will be required to maintain cathodic protection.

 

[brimmer]

Man.... Disbonded (but intact asphalt coating - I have attached a photo showing this. As you can see, the coating is perfectly intact, however disbonded (not adhered), look at the corrosion deposits on the pipeline and what has been happending. As far as my understanding goes, the purpose of a coating is to isolate the steel pipeline from the soil, and to act as a barrier to moisture and other corrosive elements - yes, but mositre will get under the coating through holidays (small holes in the coating, see other attached photo showing this), and other areas of coating damage (from original construciton). The coating will never be 100% perfect after construction, and also degrade over time (for example asphalt coating will become brittle and disbond over time in dry higher temp conditions). So if water enters under the coating through a holiday, and it is dosbonded or not adhered, there is nothing to stop the water from traveling to other areas under the coating, becoming trapped, casuing corrosion. Also, if the coating is not intact, and there is a temperature gradient between the pipeline and surrounding soil, you can get condensation build up under the coating on the steel. So without bonding a coating is useless, in fact it generally makes the situation worse as it is now trapping water under it rather than allowing it flow off. Also, some coating shield cathodic protection. If you have a holiday and the pipeline has adequate CP current to protect it, it will at the holiday location. But if there is a pocket of disbondment adjacent to the holiday, and the coating is shielding CP, the CP current will not penetrate into the area under the pocket of disbondment and not provide protection. Kind of get the idea why bonding is so important? NACE produces good litature on this topic of coating failures / disbondment, but really any literature on coatings will explain this.

 

[brimmer]

and the holiday photo as ENG-TIPS won't let me attach 2 files at the same time to 1 post

 

[LittleInch]

Shaafi,

Brimmer has provided a very good explanation. A coating is only as good as the surface preparation when it was applied and is only of any value when it is well attached to the steel. As soon as it disbonds it allows moisture into the gap through small holes in the coating. Asphalt enamel as it gets old ( > 20 to 30 years) also gets more permeable to the extent that if there is disbondment, water will fill the gap behind it and start corroding your pipeline.

If your line has impressed current and Mg anodes at several locations it sounds like the coating is in very poor condition and if you have disbondment as well then it looks like you'll need a new pipeline quite soon or an extensive re-coating exercise...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way