The Influence of Cutting Metal Elements on The Nature and Corrosion Resistance of The Zinc Coating

Abstract

Oxy-acetylene blowpipe cutting (OAB) belongs to one of the most popular and economic methods of elements formation. In particular cases, e.g. in hot-dip galvanizing, it may cause difficulties with achieving the required thickness and adhesiveness of zinc coatings. It determines their corrosion resistance and the period of their exploitation. In practice the problem of zinc coatings quality is significant, especially taking into consideration the corrosion protection of network equipment elements used in electrical overhead power lines. The aim of the dissertation is to determine the method of cutting and preparing fittings for overhead power lines guaranteed to obtain zinc coating of a uniform thickness and high corrosion resistance. The research was focused on a double eye link type SLINK 626502006 made of S355JR steel. The research will be conducted in a few directions: •indication of the reasons for the diversification of the Zn coating thickness achieved on the surface shaped by thermal cutting, •determining the growth kinetics of the Zn coating on the surfaces cut by various methods, •elimination of the coating thickness diversification by application of other (apart from OAB) cutting method (water-abrasive, laser), more thorough cleaning of the steel surface area before galvanizing (electropolishing, grinding). The study was divided into two parts: preliminary and fundamental. Preliminary research was based on the current production process of links fittings for overhead power lines (double-eye links). The aim was to evaluate the current situation - the formulation of the problem and the causes of a difference in thickness of the zinc coating on the surface shaped by OAB cutting. Based on the results of preliminary studies, the directions of the fundamental research were determined. Fundamental research was conducted in two stages. In the first stage the influence of cutting on the nature and corrosion resistance of the zinc coating was evaluated. For comparison with the OAB cutting methods: laser cutting and abrasive water-jet cutting were selected. In the second stage the aim was to determine the effect of additional treatment performed after the OAB cutting on the structure and corrosion resistance of the zinc coating. For comparison, normalizing annealing, grinding and electropolishing were applied. After cutting and additional technological treatment prepared materials were subjected to an abrasive blasting – steel and chemical treatment. The hot – dip Zn galvanizing process was conducted in industrial conditions. Parameters were fixed (temperature 460oC,dipping time 2,5min). It has been proved that the existence of HAZ in the OAB cutting material and, as a consequence, bigger hardness have an impact on the diffusion pace on the edge of steel/coating, and on the processes of substrate melting which assist zinc coating growth. The reduced iron diffusion rate to Zn coating on the cut side surface is probably the reason of both thickness of the entire coating and its individual sublayers diversification due to the reduction of the solubility of the steel basis resulting from the creation of HAZ zone. The results of the conducted research are the grounds for the selection of proper solutions in order to remove zinc coating failures, which occur on steel surfaces after thermal cutting. They may be used by design offices and galvanizing plants, as well as by companies producing fittings for overhead power line.