Investigation of Zn-alloy coatings for corrosion protection of installation systems

Abstract

It is known that Zn and Zn-alloy coatings protect steel from corrosion. A standard is to apply such coatings via hot dip galvanizing (HDG). The process works either by dipping finished parts (batch HDG) or continuously immersing steel strip into a liquid zinc bath.The continuously hot dip galvanization is suitable for products like strut channels. In this case the steel strip for the strut channel gets coated before the roll forming step. Smaller parts like connectors are produced mainly by batch hot dip galvanizing. To increase the lifetime of different parts of the system there are some options: for the strut channels made from continuously coated steel an increase of the coating thickness and adding alloying elements like Al or Mg to the coating bath is the simplest way. However, for the connectors the thickness obtained by batch HDG is limited by the process itself. In this work two coating processes are evaluated to increase the corrosion protection of the connector parts: the double dip HDG process and the zinc diffusion process. The double dip approach adds a second dip step, into a suitable Zn-alloy bath after the first dip step to the batch HDG process. In the zinc diffusion process, fine powder containing zinc and other additives is put together with the parts in an airtight container, heat is introduced and through thermal diffusion zinc combines with the metal substrate and forms an intermetallic layer. The coated samples are investigated with metallographic analytics including SEM and EDX, standard electrochemical methods and accelerated lab corrosion tests. Based on the results parameters like dipping time and alloy composition for the double dip approach should be optimized. Existing zinc diffusion samples are compared to the double dip zinc approach. The zinc diffusion coatings appear to be more suitable for protecting the channels for corrosion than the double dip samples. The double dip approach is promising as alloying is proposed to have an impact regarding corrosion protection.