Towards a better understanding of the cathodic disbondment performance of polyethylene coatings on steel

Polyethylene is one of the most widely used corrosion-protection thermoplastic coating materials that protects steel by providing a barrier to oxygen, water, and corrosive ions and performs very well. In order to further improve medium density polyethylene (MDPE) properties with regard to cathodic disbondment (CD), i.e., the loss of adhesion that takes place in the presence of cathodic protection (CP), various blends with functionalized polyethylenes containing polar functional groups such as maleic anhydride-grafted-polyethylene (MAH-g-PE) and other thermoplastic elastomers such as EPDM have been formulated to increase CD performance. Using compression molding and strew coating, the above modified polyethylenes have been applied on steel plate and pipe, respectively. The CD performance and 90° peel strength (dry and wet) of these formulations are investigated in this study. It is observed that dry bond strength does not correlate with CD performance. The improvement of CD performance for compression-molded materials and strew-coated samples is demonstrated. Also discussed, for comparison, is the performance of MDPE applied by a commercially relevant fluidized bed technique. Wet adhesion strength is shown to be an important parameter in assessing the CD performance of coatings. The disbondment mechanism of failed polymer surfaces has also been investigated using X-ray photoelectron spectroscopy, which allows us to monitor the mass transfer of the sodium cations along the crevice during CD.