Failure of vinyl coatings under alternating hydraulic pressure

To investigate the effects of substrate surface roughness and alternating pressure on the bonding strength between the vinyl anti-corrosion coating and the substrate, several coated samples were prepared using 304 stainless steel substrate to carry out alternating hydraulic pressure tests. The surface morphology of the coating and substrate, as well as the surface roughness of the substrate, were examined using a laser confocal microscope and a three-dimensional surface profiler. The alternating hydraulic pressure test was conducted inside an autoclave, and the bonding strength was measured using the double-test column method. The results show that the bonding strength of the coating increases with substrate roughness, with a more pronounced effect when the roughness is less than 1 μm. When the roughness increased from 0.189 µm to 0.85 µm, the bond strength of the coating without vacuum treatment increased from 4.97 MPa to 7.07 MPa; further increasing the roughness to 1.251 μm resulted in a bond strength of 7.92 MPa. During the preparation phase, vacuum treatment effectively eliminated internal bubbles and micro-defects within the coating, significantly enhancing the bond strength by 4 to 5 MPa. The alternating pressure causes cracks on the surface of the coating and further accelerates the failure of the protective effect of the coating. After 180 cycles of alternating hydraulic pressure tests, the bonding strength of the coating without vacuum treatment decreased by 43.3% from 6.99 MPa to 3.96 MPa. In contrast, the bonding strength of the vacuum-treated coating decreased by only 18.6% from 11.77 MPa to 9.58 MPa. The increase in coating thickness is related to the accumulation of water penetration and corrosion products at the substrate-coating interface.