ATMOSPHERIC CORROSION BEHAVIOR OF HOT-DIP GALVANIZED AND CONTINUOUS GALVANIZED STEEL

Abstract

Five years of outdoor atmospheric corrosion tests of hot dip galvanized steel samples were performed, for products of continuous galvanizing and after fabrication batch galvanizing processes. For the purpose of comparison between corrosion performances of these two different galvanizing process products, an industrial coastal area (Gemlik-Bursa/TURKEY) was chosen for outdoor testing, which fits into C4 type corrosive atmosphere definition, according to ISO 9223. Samples were studied in laboratory with accelerated salt spray exposure test and electrochemical methods. Corrosion products formed on exposed samples and cross section of coatings are analyzed by SEM. Lead is observed to change the corrosion characteristics of the coatings with change in constituents of environments. In saline electrolytes, alloying of lead is found to accelerate corrosion rate. This metal deposits as cluster on top layer of the galvanized coatings and acts as strong cathodes with respect to the zinc and accelerates the corrosion rate. It was determined that differences in dip and continuous galvanization processes cause dramatic differences in the elemental composition, morphology and regional hardness values of coatings. In the comparison of corrosion resistance, lower performance of the dip galvanized coating, although it is much thicker, has been shown due to the differences mentioned above.