Universität zu Köln

Dohr, Robin, Mattissen, Dorothea, Stang, Michael and Ruschewitz, Uwe ORCID: 0000-0002-6511-6894 (2025). Phosphate conversion process on Al–Si-coated steel: characterization and impact on the heat-treatment performance. RSC Applied Interfaces, 2 (5). pp. 1485-1492. Royal Society of Chemistry. ISSN 2755-3701

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Abstract

Energy- and time-efficient hot-stamping processes are essential for the production of lightweight, high-performance automotive components. Aluminum–silicon (Al–Si)-coated steel, widely used in hot-stamping, suffers from low heating efficiency due to its reflective surface, limiting process speed and energy efficiency. In this study, a phosphate conversion process including alkaline cleaning and subsequent phosphate conversion was applied to aluminum–silicon (Al–Si-)-coated steel sheets to improve the heating rate during heat treatment. Surface characterization of the sheets was performed using field-emission scanning electron microscopy (FE-SEM), X-ray fluorescence (XRF), powder X-ray diffraction (PXRD), Fourier-transform infrared (FT-IR) spectroscopy with an integrating sphere, and X-ray photoelectron spectroscopy (XPS). Alkaline cleaning increased the surface concentration of silicon, which in turn decreased reflectance compared to untreated sheets, as confirmed by total reflectance measurements. Subsequent phosphate conversion formed a hopeite (Zn 3 (PO 4 ) 2 ·4H 2 O) layer, further reducing the reflectance. In situ temperature curves, recorded using a thermocouple embedded in the sheets, revealed a significant reduction of the heating time for the cleaned and phosphated sheets. Longer phosphating durations progressively shortened the heating time by up to 50%, which correlated with an increase in surface emissivity of nearly 100%, thereby doubling the efficiency of the heating process. Characterization of the phosphated sheets after heat treatment revealed that the phosphate coating has reacted with aluminum from the Al–Si-layer to form ZnAl 2 O 4 (gahnite) and AlPO 4 (tridymite-type). The dehydration of the hopeite layer was also studied under the heat-treatment conditions, which showed an amorphous, hydrated intermediate phase after 2 s at 920 °C and the formation of crystalline Zn 3 (PO 4 ) 2 after 10 s.

Item Type:	Article
Creators:	Creators Email ORCID ORCID Put Code Dohr, Robin UNSPECIFIED UNSPECIFIED UNSPECIFIED Mattissen, Dorothea UNSPECIFIED UNSPECIFIED UNSPECIFIED Stang, Michael UNSPECIFIED UNSPECIFIED UNSPECIFIED Ruschewitz, Uwe UNSPECIFIED https://orcid.org/0000-0002-6511-6894 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-811667
Identification Number:	10.1039/D5LF00079C
Journal or Publication Title:	RSC Applied Interfaces
Volume:	2
Number:	5
Page Range:	pp. 1485-1492
Number of Pages:	8
Date:	1 September 2025
Publisher:	Royal Society of Chemistry
ISSN:	2755-3701
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Inorganic Chemistry
Subjects:	Chemistry and allied sciences
['eprint_fieldname_oa_funders' not defined]:	Publikationsfonds UzK
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/81166