Fusion-bonded epoxy (FBE) is a proven coating technology used to protect steel pipelines from corrosion, both in buried and atmospheric environments. In these types of service, FBE is commonly used with multilayer polyolefin (3LPO) systems that include polyethylene or polypropylene to provide greater corrosion protection and/or thermal insulation.
All of these coating materials have long track records of success. However, during the installation of pipelines with multilayer coatings, it is not uncommon for coating delamination to occur in the bare steel cutback area of the girth weld.
Figure 1. It is not uncommon for FBE to disbond from the steel in girth weld areas when multilayer polyolefin coating systems are used.
Literature from numerous studies conducted over the past three decades can assist pipeline owners and installers, coating applicators and inspectors in understanding this type of coating failure. We will take a look at known failure mechanisms and some preventive measures to address this phenomenon. (Learn about some other failure mechanisms and cures in 5 Coating Defects That Can Be Avoided By Adhering To Coating Specs.)
Cutback Stresses and Adhesion Loss Mechanisms
According to Chang et al., one of the mechanisms that can occur is residual stress from heating and cooling during and after a three-layer coating application. Typically, the coatings and the steel are heated to temperatures in excess of 392°F (200°C). As the pipe cools, the coating wants to contract more than the underlying steel, but it is held in place through adhesion, which creates residual stress. This is understood as a difference in the coefficient of thermal expansion (CTE) between the materials.
Table 1. Thermal Expansion Coefficient of Coating Layers and Steel. Source: Chang et al.
Further analysis shows an intensified level of stress at the interface of the FBE and the steel in cutback areas with multilayer coating systems. If adhesion is reduced, these forces can result in FBE disbondment. However, reducing the chamfer angle and adding an FBE tail can significantly reduce stress and the risk of delamination.
Figure 2. Finite element analysis shows that the coating configuration in the cutback area significantly determines the amount of stress at the FBE/steel interface. Utilizing a chamfer angle of 30 degrees and an FBE tail reduces the stress. Source: Chang et al.
Thermal expansion of polyolefin generates high residual stress in a 3LPO coating system. This occurs because polyolefin has low water absorption, which means that stress levels will remain high even after exposure to the atmosphere. In the cutback area, peel and shear stresses are high for polyolefin, and if a reduction in adhesion occurs between the FBE and the steel, the resulting stress can cause delamination.
Appearance of Disbondment: The Initiation Point
The delaminated area looks like half of a blister or half of a cathodic disbondment. The semicircle begins at the interface between the FBE and the steel in the cutback area. The shape of the disbonded area implies that there is an initiation point where an electrochemical reaction begins. (Related reading: Corrosion Electrochemistry: The 6 Electrochemical Reactions Involved in Corrosion.)
Figure 3. Disbondment grows in a semicircle from an initiation point.
- Anodic crevice corrosion – An anodic reaction occurs at the coating/steel interface where a larger volume of corrosion product forces the coating away from the steel.
- Cathodic blistering and delamination – A cathodic reaction causes the formation of hydroxide, which disrupts the coating bond.
Figure 4. Anodic delamination on the left, and cathodic on the right. Source: Chaung et al.
Knowing whether the disbondment front is cathodic or anodic is not critical. The solution is to prevent the formation of the electrochemical cell from the outset.
The key to avoiding coating disbondment at the pipe end is to minimize environmental exposure. This is particularly true for pipe storage near the ocean. Minimizing the time between coating application, pipe assembly, girth weld coating application and installation can reduce risk. For long storage times, preserve the FBE/steel interface with a suitable technology, such as a wrap, peelable coating or vapor phase inhibitor. (More tips can be found in the article Temporary Corrosion Protection During Storage, Transportation and Handling.)
Delamination of the FBE layer can be solved by removing the parent coating back to the point of sound adhesion, then applying the girth weld coating of choice. If delamination is extensive, specialized equipment may be required, or a pipe might even need to be cut out and rewelded.
Figure 5. To make a proper girth weld coating application, the delaminated coating must be removed back to well-adhered mainline coating.
The most common cause of coating delamination in the cutback area is environmental exposure, particularly in marine environments. The degradation process initiates at a damaged area of the FBE/bare steel interface and a corrosion cell starts. The underlying mechanism is likely an electrochemical reaction, either anodic or cathodic. Other factors such as residual stresses may act as intensifiers once adhesion is reduced by environmental exposure.
To minimize the possibility of coating delamination in the cutback area:
- Use world-class coating materials and application processes.
- Minimize the time that the cutback interface is exposed to the atmosphere without protection.
- In prolonged storage time, implement suitable protective measures such as wraps, peelable coatings or vapor phase inhibitors.
- Include an FBE tail to minimize the influence of stresses.
- Keep the chamfer angle down to 30 degrees.
Understanding the mechanisms and taking preventive actions can reduce the likelihood of coating delamination during pipeline installation.