Silicon effect color transformation on galvanized steel lattice towers

Abstract

Steel is one of the most recycled materials in the world and modern, as-rolled structural sections are produced with varying combinations of alloy metals to fulfill many purposes. It is very important to specify the right requirements according to the material properties, ductility, toughness, through-thickness properties, weldability, and appearance of galvanized and dulled finished. Surface preparation is the most critical step in the application of any coating. It is essential for hot-dip galvanizing process, namely the iron-zinc metallurgical reaction, that the steel surface to be perfectly cleaned, i.e. free from grease, rust and scale. Any inadequacies in surface preparation will immediately be apparent when the steel is withdrawn from the molten zinc. It is very common that lattice steel towers are made of galvanized steel and sometimes dulled by a passivation process after hot dip galvanizing steel members. Passivation is a process that creates a protective layer on the surface, which may last about six weeks before being consumed. At that time, galvanized steel will begin to weather naturally. Most steels can generally be satisfactorily coated through a hot dip galvanizing process, however, reactive elements in the steel, silicon (Si) and phosphorus (P) in particular, can affect the appearance and thickness of the coating. Steel with low silicon content can provide a more consistent quality of coating in terms of coating thickness and smoothness of the galvanized coating. Where a long term dull appearances are important, steels with silicon content in the range of 0.04 to 0.15 and above 0.22 percent by weight should be avoided. This paper addresses the challenges in hot dip galvanizing process for reactive steel and possible discoloration phenomenon due to Silicon Effect Color Transformation (SECT) on galvanized steel members for lattice transmission tower structures. It also gives the preliminary guidance to utilize some specialized techniques to achieve the desired galvanizing and dulling finish.