As part of an industrial doctorate, the near-surface investigation of zinc-magnesium alloyed surface finishes on steel substrates (in short: ZM) is being carried out. The aim is a comprehensive characterization of the material in order to better interpret and predict the bonding mechanisms of conversion systems, so-called thin inorganic/organic adhesion-promoting layers.

The characterization of the ZM layer includes a differentiation between zinc phases, binary and ternary eutectic phases of aluminum, magnesium and zinc as well as ZnMg2 phases. The formation of different phases in the ZM coating is due to the specific element composition in the zinc pot of the system and the defined temperature program in the galvanizing process.

After galvanizing, skin-pass coating is carried out on the plant side to achieve the desired topography and to improve the stretching and deep-drawing properties of the steel strip. Due to the varying degrees of skin-passing in the various production plants, plateau, valley and transition structures in the µm range are created on the ZM coating.

Depending on the atmospheric storage conditions, a thin passive layer forms on the metallic ZM layer in a very short time. Mainly air and CO2 lead to oxide and carbonate compounds. The influence of water promotes the formation of spotty white rust and increases the corrosion potential.

The phase distribution, the characteristics of the topography and the formation of the passive layer are interdependent factors. Confocal Raman spectroscopy is used to characterize the individual features and their interrelationships. After surface scans, so-called mappings, with a defined laser wavelength, false color images can be generated using a filter over the respective specific wavelength, which are used to evaluate the surface distribution.

A striking feature is the differentiated occurrence of lattice vibrations, known as phonons, in the low wavenumber range of the Raman spectrum for each scanned point in the mapping. These element-related excitations occur in crystal lattices and can be assigned to the ZM phases and compounds formed in the passive layer. In addition, bands of the common molecular vibrations of compounds such as carbonates and hydroxides in the higher wavenumber range can be found in the spectra of each scan.

If the ZM material is coated with an anti-corrosion oil during a transport process and subsequently treated with an alkaline cleaner, the lattice and molecular vibrations of the Raman measurements described above enable an evaluation of the type and local distribution of the phases of the ZM layer as well as a remaining passive layer, residual oiling and residues after cleaning. Thus an assessment of the efficiency of the processes is possible. All interfering factors or residual coatings ensure an inadequate connection of the final conversion chemistry and thus the subsequent coating layers / coating systems.