Environmentally Friendly Layered Double Hydroxide Conversion Layers: Formation Kinetics on Zn–Al–Mg-Coated Steel

Abstract

Phosphate- or chromate-based industrially produced conversion layers, while effectively increasing adhesion for organic coatings and corrosion resistance, come at the cost of environmentally problematic and harmful treatment solutions and waste. In this respect, layered double hydroxide (LDH)-based conversion layers offer an environmentally benign alternative without toxicologically concerning compounds in the treatment solution. Here, we study an LDH conversion layer on Zn–Al–Mg-coated steel (ZM-coated steel), which was produced by immersion into a carbonate- and magnesium-containing alkaline solution. The mechanism and kinetics of the conversion layer formation were investigated with in situ open circuit potential measurements, cyclic voltammetry (CV), and scanning electron microscopy (SEM). Acceleration of the LDH layer formation through high convection in the treatment solution was found. This was attributed to a higher oxygen availability at the metal/solution interface because no diffusion-limited state during the layer formation is reached due to high convection. The importance of oxygen within the kinetics indicates a corrosion-like mechanism, with cathodic and anodic sites on the steel sample. The LDH formation happens by co-precipitation of ions present in the treatment solution and dissolved ions from the ZM-coated steel. With CV, SEM, and X-ray diffraction, the growth of the LDH conversion layer was investigated with respect to the immersion time. It was found that after 30 s, the sample surface was almost fully covered with an LDH layer, and with the increasing immersion time, the layer grows in thickness. Increased understanding on the kinetics and mechanism of the LDH conversion layer formation on ZM-coated steel gives rise to a targeted optimization of the treatment solution and process parameters.

Corrosion protection of Zn–Al–Mg‐coated steel by a layered double hydroxide conversion layer

“…A more detailed characterization of the prepared LDH conversion layer including the mentioned XPS measurements is given in our previous study. [ 40 ]…”

“…[38,39] However, the corrosion protection of ZM-coated steel by an in situ grown LDH conversion layer is not reported in detail in the literature yet. In previous work, we studied the mechanism and kinetics of the LDH conversion layer formation on ZM-coated steel, [40] where a minimum treatment time for obtaining an almost fully covering LDH layer on the substrate surface of 30 s was found. Longer formation times lead to an increase in layer thickness.…”

Contributions to a More Realistic Characterization of Corrosion Processes on Cut Edges of Coated Metals Using Scanning Microelectrochemical Techniques, Illustrated by the Case of ZnAlMg-Galvanized Steel with Different Coating Densities

“…Frequently associated with the formation of stable layered double hydroxide (LDH) [23,24], the improved corrosion protection of ZnMgAl coatings, compared to pure zinc or binary zinc coatings, still presents unclear features. While salt spray and conventional electrochemistry of these alloys are frequently performed for their characterization [11,25,26], a comprehensive understanding of chemical interaction mechanisms requires the synthesis of micrometer-and nanometer-scale data from the activated sites [27], which is remarkable considering the microstructural heterogeneity of ternary-alloyed coatings [28].…”

Superhydrophobic Corrosion Inhibitor Intercalated Hydrotalcite Coating for Enhanced Corrosion Resistance of the Aluminum

“…Layered double hydroxides­(LDH) are as known as hydrotalcite, is a kind of green environmental protection material, its general formula is [M 1– x 2+ M x 3+ (OH) 2 ] x + [(A n – ) x / n ·mH 2 O] x − . LDHs is an anionic layered compound, as shown in Figure , the structure of LDHs is very similar to brucite, and the unit layer is formed by a large number of common edges of MgO 6 octahedron.…”