Interface and volume transport on technical cataphoretic painting: A comparison of steel, hot-dip galvanised steel and aluminium alloy

Water diffusion through the polymer and the ion transport along the coating-metal interface is examined on steel, hot-dip galvanised steel (HDG) and AlMg(Mn) with clear and pigment cataphoretic paintings (E-coats). A general velocity decrease is identified from AlMg(Mn) to HDG steel to steel via extensive impedance studies combined with force modulation analyses. The polymer matrix reveals a slight Young's modulus drop and an increased disorder in the pigment-polymer network on rough substrates after 50 days of salt-water uptake. Further signs of plasticising are not observed but these circumstances already require a Non-Fickian approach. During the film formation, the substrate dissolves due to the extreme cathodic polarisation under high alkaline conditions, which is why the E-coat is contaminated by the solvated metal on zinc and aluminium and - contrary to expectations - with its highest contamination on steel. The Kelvin Probe data illustrates the prominent differences between the clear and the pigment E-coat interfaces as well as the concentration- und substrate-effects under humid oxygen-deficient conditions. Additionally, the interfacial bonding properties of the E-coat with aluminium oxide are used to assess the pH-activities during the sub-surface flux to complete this comprehensive study about the substrate dependencies.