Previous studies of our group showed that Zn-Al-Mg hot-dip galvanized steel sheets have a superior corrosion resistance compared to conventional hot-dip galvanized steel sheets.

This work especially focuses on the stability of this material at different pH values and the formation of corrosion products. Therefore experiments based on the normal salt spray test according to DIN EN ISO 9227 were conducted. During those tests the solution was adjusted to different pH values from acidic to alkaline. All other parameters were left untouched. The samples were compared to conventional hot-dip galvanized steel sheets of the same coating thickness.

The results showed that the corrosion resistance is severely influenced by the pH value of the testing medium. Also the amount and the type of corrosion products formed in the course of the tests varies greatly. While in severely acidic conditions mainly Simonkolleite is formed. In mildly acidic to neutral conditions Simonkolleite and Hydrozincite is formed, while in strongly basic conditions Simonkolleite and ZnO is formed.

Corrosion products were analyzed by SEM, EDX, XRD, IR and RAMAN spectroscopy. A cross section analysis in SEM showed that the severity of the attack of the coating is very much dependent on the pH value of the testing solution. The corrosion products could be identified as Simonkolleite and Hydrozincite. In alkaline conditions the formation of Zincoxide and Zn-Al-Hydrotalcite was also observed.

Subject
Acids, X-ray diffraction, Zinc rich coatings, Materials, Magnesium alloys, Corrosion testing, Acidity, Corrosion products, Zinc oxide, Rust, Corrosion protection, Aluminum alloys, Samples
Keywords:
hot-dip galvanizing, alloy coating, Zn-Al-Mg coating, salt spray test, pH value, corrosion products
© 2012 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2012
Association for Materials Protection and Performance (AMPP)
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