For the last few years, an oil company has carried out laboratory testing of tri-layers PE and PP pipe coatings in hot wet environment.
The objective was to simulate the disbondment from the steel substrate noted in some cases in the field (with no corrosion associated), and understand the physico-chemical mechanism at the origin of the loss of adhesion experienced.
Durability of the bond between the steel and the FBE primer/Influence of water diffusion
Influence of the FBE type and ageing temperature on the disbondment
Influence of internal stresses on the disbondment of three layers coatings.
This paper will sum up the results obtained so far from this RD effort, with a special highlight on the following points:
In 2003-2004, TOTAL carried out In-Line Inspection on an oil export pipeline onshore Gabon. External corrosion was identified and excavations were done first in a few locations (with the most severe identified corrosion) to repair the coating and better understand the origin of the corrosion.
Corrosion cases were associated with disbonded heat shrinkable sleeves at field joint locations.
On both sides of the disbonded heat shrinkable sleeves, the 3LPE coating had lost adhesion and was easily removed from the pipe. The adhesion failure was between the steel and the epoxy (FBE), and no corrosion was noted underneath the disbonded coating.
What was found during the repair was as follow:For further readings on the description of this case study (and other reported since then), the reader can refer to the following references1-2-3 :
At that time, it was decided within the company to try to better understand the disbondment phenomena, and find some ways to improve the durability of the bond between the steel and the epoxy primer (FBE), as it seemed to be the weakest link of the system.
The first study was launched in 2004-2005 via the ISITV (Institut des Sciences de l'Ingénieur de Toulon et du Var) and the IFP (Institut Français du Pétrole).
The main outcomes of this research were presented in the following references 4-5-6-7-10.
The water sensitivity and water uptake of the epoxy primers were strongly dependant of their formulation
We were able to simulate the loss of adhesion of a three layer PE coating in the laboratory with ageing tests in hot water. Similar modes of failures (between steel and epoxy) were recorded at 60°C.
The glass transition temperature of the epoxy primer was measured after ageing in water, and was found to decrease from about 100°C down to 80°C (inflection point). It was then believed that this “wet Tg” was one parameter at the origin of the observed loss of adhesion.
The loss of adhesion was observed even without any artificial defect created in the coating before ageing immersed in water. That meant the diffusion of water and other species through the coating was another parameter of prime importance in the process.
The main findings at that time were: