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The objective of this project was to evaluate possible metal coating replacements for chromate passivation or conversion layers on hot-dip galvanized (HDG) and electro-zinc (EZ) steel strip products.

The experimental coatings, obtained by electron beam PVD, included titanium, aluminium, copper, manganese and tungsten, in various ranges of thickness. In addition to the HDG and EZ some mild steel samples, immediately after being Zn-coated by PVD, were utilized as substrates.

Characterisation results indicated Ti and Al as the most promising Cr replacement coatings. In particular, thin (100-500 nm) coatings can be utilized as conversion layers. In fact, they reduced average corrosion rates to a level comparable to that found for conventional chromed samples. Furthermore, their performance in the painted condition was in most cases equivalent to that of chromed reference materials. On the contrary, the performance in the painted condition of thicker (1-2 µm) coatings was not generally satisfactory. However, in the unpainted condition, they behaved similarly to or better than the chromed reference materials. These results suggest that such coatings would be more appropriate as passivation layers for unpainted applications.

Among the thicker coatings, the best overall properties, and in particular coating adhesion, compactness and compositional uniformity, were obtained with freshly PVD Zn-coated substrates. This result implies the possibility of realizing an industrial continuous PVD line without the need of critical pre-treatment (to remove surface oxidation). In fact, the passivation coatings could be deposited immediately after the deposition of the Zn-layer, i.e. prior to the build-up of any significant oxidation.tion.

Additional information

Authors: AREZZO F, CSM, Roma (IT);LEWUS M O, British Steel, Rotherham (GB)
Bibliographic Reference: EUR 20040 EN (2001), pp.115. Euro: 17.50
Availability: Available from EUR-OP sales agents. URL:
ISBN: ISBN:92-894-2069-3
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