Thin organic coatings for hot dip galvanised steel and cold rolled products

The research themes in the current thesis stem from collaboration with an industrial project in which thin organic coatings (TOCs) were being developed for application as primer coatings for cold rolled (CR) and hot dip galvanised (HDG) steels. As current legislation requires the replacement of chromate, a known carcinogen, with environmentally friendly alternatives there is a need to develop inhibitors that may provide corrosion protection to an equally high standard. Electrochemical techniques have been employed to assess inhibitors in both immersion situations and in-coating with the presence of a penetrative electrolyte. Chapter 3 A study using the SKP technique where the effect of in-coating phenyl phosphonic acid (H2PP) for the prevention of corrosion-driven underfilm delamination on HDG substrates is studied. Experiments are carried out in the presence of large defects and the possibility of inhibitor leaching is explored. Chapter 4 Results are presented from an extensive study using the Scanning vibrating electrode technique (SVET), backed up by open circuit potential (OCP) measurements to, firstly, determine the corrosion activity on HDG when fully immersed in electrolyte in the full range of pH. These results are then used as a baseline to compare the effectiveness of H2PP and sodium phosphate inhibitors when added to the electrolyte. Chapter 5 A further study on the inhibitor H2PP is carried out on iron substrates. Both cathodic and anodic delamination process are studied. Results obtained using the Scanning Kelvin probe (SKP) are presented where the effect on cathodic delamination of in-coating additions of the H2PP inhibitor made to a polyvinylbutyral (PVB) model primer coating is studied. This is done in two ways, the first being in the presence of a large defect and the second a realistic scribed defect experiment. Subsequent SVET experiments are presented where the influence of H2PP on the corrosion of bare iron fully immersed in electro containing the inhibitor is assessed. Finally, the effect of in-coating H2PP additions on the anodic delamination process filiform corrosion is studied. Chapter 6 Firstly, an alternative study utilises the SKP, which features profoundly in this thesis, to assess the interaction of PEDOT:PSS films with a range of metal surfaces. Secondly, an assessment of the effectiveness of PEDOT on preventing underfilm delamination on both HDG and iron substrates is presented. On iron both cathodic and anodic FFC delamination are studied. Chapter 7 An exploration of the use of time-lapse photography as an alternative, or complementary, technique for assessing the corrosion prevention of thin organic coatings (TOCs) under development. The aim is to provide a technique that achieves accurate results with a fast throughput when compared with industry standard humidity tests. A range of coated products is tested, from laboratory to the pilot-line phase, to fully explore the validity of the technique.