New rules in DASt-Richtlinie 022 for avoiding liquid metal assisted cracking (LMAC) of prefabricated structural steel components during hot-dip galvanizing

Hot-dip galvanizing is one of the most efficient ways of providing durable corrosion protection for prefabricated steel components. It contributes greatly to the sustainability of steel structures.
Technical rules for the design of steel components for zinc coating and for the execution of the hot-dip galvanizing process have been developed over a period of 100 years. The state of the art has been implemented in the standards EN ISO 1461 [1] and EN ISO 14713 [2] and in recommendations of the galvanizers. It was understood that the technology applied would cover the specific requirements that result from the steel used, the usual way of structural detailing, the fabrication up to the point of galvanizing the steel component and the pretreatment and hot-dip galvanizing process in the galvanizing plant so that any relevant cracking that might impair the structural safety should not normally occur.
Over the years 2000-2006, however, the use of newly developed zinc alloys for the zinc baths [3] - which were introduced at the same time as the use of steel components with larger dimensions and material thicknesses and also with steels of higher strength and with new production techniques, - resulted in significant damages in the shape of the formation of cracks that were filled with zinc and the alloying elements of the zinc melt. The cracks had formed in the zinc bath because zinc melt alloying elements could always be found between the grain boundaries of the steel. Hydrogen embrittlement, which up until then had often been considered as a prevailing hazard, could be excluded; “liquid metal assisted cracking” (LMAC) or “liquid metal embrittlement” (LME), which up to then had been not considered in the design and execution of hot-dip galvanized steel components and was even not properly mentioned in EN ISO 1461 and EN ISO 14713, could be identified as the relevant cause.
A survey of the relevant literature revealed that LMAC in hot-dip galvanizing had been investigated in studies and tests since 1930 [7]-[23], but only in particular non-coordinated projects with different approaches depending on the individual problem. These studies provided the first qualitative information, but due to shortcomings in the documentation were not applicable for concluding a general concept. Therefore, new research projects had to be carried out to analyse the LMAC phenomenon systematically, aiming to establish a method that allows quantification of the influences of structural detailing, steel type and semi-finished products, fabrication of steel components in the workshop and pretreatment and hot-dip galvanizing in the galvanizing plant in such a way that they can be implemented in a “limit state concept” for LMAC. That would enable clear rules to be derived for all parties involved in the supply of hot-dip galvanized structural steel components.
This work is interdisciplinary because metallurgy, materials mechanics, structural engineering approaches with respect to structural safety, galvanizing technology and simulation techniques for time-step analysis of fabrication and galvanizing processes have to be applied in order to arrive at quantifiable data for “actions” on the one side and “resistances” on the other side of the limit state equation. This “limit state” approach in particular requires quantifiable data regarding the steel resistance to LMAC cracking in the hot zinc bath, for which a particular small-scale test had to be developed that is able to quantify the reduction in plastic strain capacity (i.e. the strain withstood up to the time of relevant crack formation) under ambient liquid zinc alloy. This test supplies the relevant data (ultimate equivalent plastic strain capacity) needed for the limit state equation.
This article provides a brief survey of the research results that allow the identification of the causes of the cracks observed. It also outlines conclusions for the design of prefabricated steel components for hot-dip galvanizing and rules for the galvanizing process that have been implemented in the German guideline DASt-Richtlinie 022 [4]. These conclusions and rules could also be used as a basis for supplementing the rules in EN ISO 1461 [1] and EN ISO 14713 [2] as well as EN 1993 - Eurocode 3 [5] and EN 1090 [6].