Role of dew points and Fe pre-coats on the galvanizing and galvannealing of dual phase steel

In this study, the galvanizing and galvannealing ability of dual phase steel sheet (DP 590), with and without Fe pre-coatings, is investigated with varying dew points using a hot dip process simulator (HDPS). Good quality defect free adherent galvanized coating on the substrate surface is obtained b...

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Bibliographic Details
Published in:Surface & coatings technology 2021-09, Vol.422, p.127573, Article 127573
Main Authors: Walunj, Mahesh G., Mandal, Gopi K., Ranjan, Rakesh K., Pais, Rajesh, Mishra, Suman K., Venugopalan, T., Pathak, Lokesh C.
Format: Article
Language:English
Subjects:
Annealing
Coated electrodes
Corrosion currents
Corrosion potential
Corrosion resistance
Corrosion tests
Dew point
Dip coatings
Dual phase steels
Galvanizing
Galvannealing
Gas mixtures
Immersion coating
Iron pre-coat
Manganese
Metal sheets
Oxidation
Protective coatings
Substrates
Thermal cycle
Wetting
XPS
Zinc base alloys
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Summary:In this study, the galvanizing and galvannealing ability of dual phase steel sheet (DP 590), with and without Fe pre-coatings, is investigated with varying dew points using a hot dip process simulator (HDPS). Good quality defect free adherent galvanized coating on the substrate surface is obtained by appropriate selection of the inter-critical annealing atmosphere consists of N2-5%H2 gas mixture with +10 °C dew point without substrate pre-coating. However, the production of defect-free coating on DP 590 substrates is highly challenging when the substrates are without any pre-coating. It is observed that the selective oxidation of manganese on the substrate surface during intercritical annealing is primarily responsible for the appearance of bare spots on the galvanized surface in case of the coated specimen produced without pre-coating. A significant improvement in the reactive wetting behaviour between the liquid zinc alloy and pure Fe results in defect free adherent coatings (both galvanized and galvannealed) with electrodeposited Fe pre-coating on the substrate surface irrespective of dew points. Both the galvannealed specimens prepared with and without iron pre-coatings, exhibit partial passive-active metal characteristics in potentiodynamic polarization corrosion test. Iron pre-coating on the substrate surface is observed to improve the quality of galvannealed coatings by reducing defects and enhancing the formation of more compact and dense delta (δ) phase compared to galvannealed coatings produced without pre-coating under identical conditions. This leads to the improvement in corrosion resistance of galvannealed coating produced with pre-coating by lowering the corrosion potential and corrosion current density compared to that of galvannealed specimens produced without iron pre-coating. [Display omitted] •Selective oxidation of Mn at varying dew points governs galvanizability of DP steel.•Iron pre-coating prevents selective surface oxidation and improves coatability.•Iron pre-coating results in formation of continuous Fe-Al interfacial layer.•Pre-coating improves coating quality and adhesion irrespective of dew points.•Pre-coating improves corrosion resistance of galvanized and galvannealed coatings.
ISSN:0257-8972
1879-3347
DOI:10.1016/j.surfcoat.2021.127573