Better offshore lighter batch HDG technology using ZnAlMg alloys (BOLT_ZNAIMG) : final report

Nine spinnable alloys were defined for initial trials. After the first galvanizing trial on rods two alloys were identified for further testing on bolts with spinning, namely Zn20Al4Mg and Zn20Al. Although the large number of tests did not yield adequate coating quality for bolts, correlations between different galvanizing parameters and microstructure were established. The ZnAlMg alloy performed significantly better than Zn in short-term, long-term and outdoor corrosion testing. The corrosion performance of the ZnAlMg alloy is comparable or better than other commercially available ZnAlMg alloys. A finer and more homogeneous distribution of phases, obtained by alloying with Mg and use of a quenching treatment, yielded a significantly better corrosion performance. The influence of the galvanizing process on mechanical properties of the steel was examined. A negative effect on sensitivity to LME was established when using ZnAlMg as compared to Zn and ZnAl. The LME sensitivity however only affects the window for processing parameters during the galvanization process and does not jeopardize the use of ZnAlMg coated fasteners under normal working conditions. It was proven in the project that bolts can be galvanized with ZnAlMg without cracking. A pilot line was built as proof of concept of the spinning galvanizing technology. The trial and error approach did not yield adequate visual coating quality on ZnAlMg bolts. However, reasons for the poor visual quality of the coatings were identified and it was proven that the transfer to an industrial scale will improve quality of the galvanized coatings during sole dip galvanization process.