The effect of laser surface textures on the adhesion strength and corrosion protection of organic coatings - Experimental assessment using the pull-off test and the shaft load blister test

Adhesion is necessary for an effective corrosion protection of metallic surfaces through organic coatings. Even the best anticorrosive coatings fail if it is not well adhered to the substrate. Laser surface texturing (LST) process has been widely used for enhancing adhesion between coatings and steel surfaces in various applications. In addition, recent researches have proven that LST is an effective option for pretreatment of steel surfaces prior to organic coating deposition. This work reports an investigation on the adhesion strength of organic coatings deposited onto AISI-A36 carbon steel laser-textured surfaces. Nanosecond LST via the direct laser writing (DLW) method was used to fabricate V-shaped grooves on the steel surfaces. The distance between consecutive grooves was varied to obtain different surface topographies with controlled surface textured area (TA) ranging from 10 % to 60 %. Conventional bead-blasted surfaces were used as a reference. Surface morphology and topography were characterized through roughness and tortuosity. The frequently used pull-off test (ISO 4624) and a shaft load blister test (SLBT) were used to determine the adhesion of the epoxy coating. In addition, accelerated corrosion tests were carried out to assess the effects of the laser textured surface conditions on the corrosion propagation. After running the pull-off test, all the surfaces' conditions exceeded the minimum pull-off strength value of 5 MPa stated in ISO 12944-9. The results for the SLBT showed that the adhesive strength is highly influenced by the laser-textured area. The geometrical aspects of the groove structure promote phenomena such as mechanical interlock and mechanical hooking of the coating, which affect the adhesion failure mechanism of the organic coating during the test. For lower TA, the crack propagation occurs mainly at the coating/steel interface. However, increasing the TA shows that the crack propagation is stopped near the V-shaped grooves due to the complex shape and must deviate inside the coating for further propagation. Finally, adhesion strength results were correlated with coating delamination rates and an indirect relationship was found.