Study of the Temperature Effect on the Structure and Thickness of Hot-Dip Zinc Coatings on Fixing Products

Abstract

Hot-dip galvanizing of steels is usually performed by immersing the metal in a bath with molten zinc in the temperature range from 450 to 460 °C. In some cases it is necessary to obtain a minimal coating thickness. For example, high-strength bolts and other fixing products require a minimal thickness of the coating because a too thick zinc coating requires additional work on re-threading, which leads to spalling of coatings, a loss of corrosion resistance and, consequently, failure of the entire product. The main aim of this work was to study the influence of elevated temperatures of hot-dip galvanizing on the thickness and microstructure of zinc coatings on bolts and nut preform. The microstructure and elemental composition of the coating were studied by scanning electron microscopy and energy dispersion X-ray microanalysis. It was found that the coating thickness obtained in the range between 475 and 535°C decreases with temperature and reaches a minimum at 535°C. The structure of the coating after high-temperature hot-dip galvanizing was fundamentally different from the structure of the coating made at standard temperatures 450-460°C. This coating formed at 535°C was dense, homogeneous, non-porous and composed of a mixture of the δ and ζ-phases without distinct phase boundaries. That’s why it was recommended to maintain the bath temperature in the range between 533°C and 537°C. It allows us to obtain a hot-dip galvanized coating of a minimal thickness and a good quality on fixing products.

The Impact of Heat Treatment on the Behavior of a Hot-Dip Zinc Coating Applied to Steel During Dry Friction

“…In fact, the microstructure of the hot-dip zinc coating can differ significantly from that in the model. Such factors as the method of preparation of the galvanized surface [ 11 , 12 , 13 , 14 , 15 ], the chemical composition of the base surface [ 16 , 17 , 18 ], the chemical composition of the zinc bath [ 19 , 20 , 21 , 22 , 23 , 24 ], the parameters of galvanization [ 25 , 26 , 27 , 28 , 29 ], etc., may cause microstructural differentiation even within the same coating ( Figure 1 b). Most of the mentioned parameters influence the reactivity of steel, and only the optimal selection of galvanizing conditions enables the formation of a proper zinc coating.…”

Comparison of the Tribological Properties of the Thermal Diffusion Zinc Coating to the Classic and Heat Treated Hot-Dip Zinc Coatings

“…However, in some cases, the high-temperature (ab. 535 °C) hot-dip zinc galvanizing [ 17 ] that allows for removal of the excess zinc from the surface of bolts is applied, but such a treatment temperature can result in issues with the material of the bolts—steel tempering and losing the mechanical properties. As an alternative to electro-galvanizing, lamellar or thermal diffusion processes can be applied.…”

The Impact of Zinc Coating Specifications on Corrosion Resistance аnd Durability of Steels

“…Coating applied in this manner has to correspond to a number of requirements: sufficient corrosion resistance, wear resistance, plasticity, rigid adherence with substrate, durability, etc. Therefore, a number of problems associated with coating quality after hot dip galvanization [1][2][3][4] based on chemical composition, structure and other characteristics of both steel used and the coating itself are currently studied. The objective of this study was to investigate the influence of the coating characteristics (type and grade of coating, its thickness and chemical compound) on the corrosion resistance and service life of steels after hot dip galvanization.…”