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Problems concerning hot dip galvanizing of steel

At sleeves for structural engineering have been discovered damages in the course of a quality control. Fractographic and metallographic investigations of a cracked bushing has been performed. Energy Dispersive X-Ray (EDX) analysis indicated that the decoration of the crack is related to the presence of Zinc on the surface. Numerous secondary cracks completely filled with Zinc and Tin could be evidenced on the metallographic polished section (REM). Also Lead enrichment could be detected in the Zinc layer, but only on the hot dip galvanized and cracked surfaces. The fracture present in the bushing can therefore clearly be attributed to liquid metal induced embrittlement.
The experiments with steel samples (S355 J2 N) of different geometries dipped in Zinc bath of different composition have been performed. Analysis of the results after galvanizing the cold bent U-shaped parts indicated that the degree of samples cold deformation as well as the bath composition have a significant influence on the crack formation during galvanizing.
The Yoke test samples have proven to be simple in handling and an easy to produce sample shape that can be processed rapidly in practical operation. Especially the galvanizing companies can this way partly replace expensive bath analysis with a visual examination of yoke specimen after hot dip galvanizing.