Surface analysis of white spot formation on industrial electrogalvanised automotive steel
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MSSA (Mittal Steel South Africa), which produces electrogalvanised steel for the local automobile industry, experiences a problem with white spot formation when their steel is phosphated. The addition of nickel inhibits white spot formation but produces an unacceptable discolouration of the surface layer. Furthermore the locally produced steel exhibits blister formation when heated to 300ºC. The substrates, electrogalvanised coatings, phosphated samples and annealed samples are studied with Glow Discharge Optical Emission Spectroscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, as well as X-ray diffraction. Combining the results allows for an interpretation of the morphology, topography, composition, crystalline structure, quantitative depth profile as well as the spatial distribution of the elements The white spot formation on the electrogalvanised surfaces is closely related to the presence of contaminants on the electrogalvanised surface and at the interface between the substrate and the electrogalvanised coatings. The accelerated phosphate reaction results in complete dissolution of the electrogalvanised surface, thereby exposing the iron substrate. The white spot consists of an anomalous protruding perimeter with elongated crystals that grow towards the centre of the spot, present inside the spot. Partial dissolution is required in order for the phosphate process to occur. Complex phosphates deposit on the surface comprised of various cations, such as zinc, manganese and nickel. The zinc dissolution is the preferred reaction and therefore there is a slight enrichment of nickel in the sublayers of the phosphate. The manganese deposited on the surface must not be confused with the manganese present in the substrate. The addition of other cations to the electrogalvanised layer results in a change in the structure of the phosphated layer. The presence of cobalt and copper in the electrolyte results in an increase in the deposition of manganese phosphates on the surfaces. The manganese phosphates grow upward, away from the surface as opposed to the zinc phosphates that grow along the sample surface. The growth of the zinc phosphates only continues until the surface is covered. The structure of the electrogalvanised deposits changes with changes in the composition of the electrolyte. The morphology changes from a well-defined rigid structure (as for the zinc electrolyte) to a complex structure consisting of both grains and a fine intricate network of small deposits as various cations such as nickel, copper and cobalt are added to the electrolyte. The surface of the steel substrate clearly shows the rolling direction, as well as numerous dislocations. This compromises the epitaxial growth of the electrogalvanised layer. The alloy elements added to the steel are also present on the surface. These react differently compared to the steel and will therefore impact on the nature of the deposition at these sites. The annealing of the electrogalvanised samples causes both structural and compositional changes in the samples. The movement of the zinc and possible dezincification are most likely responsible for the blister formation. This is further affected by the presence of hydrogen in the sample and the subsequent hydrogen blistering. It is of paramount importance for all the surfaces and parameters to be controlled and monitored carefully to ensure the best coating quality. The presence of any contamination on the surfaces or in the solutions will cause adverse reactions and compromise the final product.