ANALYSIS AND MICROHARDNESS PROFILE OF HOT DIPPING COATING ON LOW-ALLOY STEEL

This research is an effort to understand the morphology of the coating produced during hot dipping process in pure zinc bath based on iron–zinc phase diagram. In this investigation, zinc coating on low-alloy steel AISI 4340 samples was applied by hot dipping method followed by an annealing process. Morphological characterizations of the steel surface layer were accomplished by optical microscopy (OM) and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDS). Results revealed that the coating is bonded to the steel surface through a sequence of Fe–Zn layers namely alpha ([Formula: see text], delta ([Formula: see text] and zeta ([Formula: see text] with uniform sloping hardness profile.

Effects of Chemical Compositions on Plating Characteristics of Alkaline Non-Cyanide Electrogalvanized Coatings

The effects of zinc and sodium hydroxide concentrations in an alkaline non-cyanide zinc bath on the electrodeposition characteristics of zinc deposits are systematically investigated. Using microstructural and phase analyses of specimens with specifically designed geometries, the study indicates that the bath formulations critically control the electrogalvanizing characteristics and affect the coating surface morphology, deposition rate, throwing power, coating uniformity, and residual stresses developed during and after electrogalvanizing. The coatings produced from baths with a moderate Zn-to-NaOH ratio of 0.067–0.092 appear to provide uniform and compact deposits, moderately high deposition rate, and relatively low residual stresses.

Development of Bath Chemical Composition for Batch Hot-Dip Galvanizing—A Review

Obtaining zinc coatings by the batch hot-dip galvanizing process currently represents one of the most effective and economical methods of protecting steel products and structures against corrosion. The batch hot-dip galvanizing process has been used for over 150 years, but for several decades, there has been a dynamic development of this technology, the purpose of which is to improve the efficiency of zinc use and reduce its consumption and improve the quality of the coating. The appropriate selection of the chemical composition of the galvanizing bath enables us to control the reactivity of steel, improve the drainage of liquid zinc from the product surface, and reduce the amount of waste, which directly affects the quality of the coating and the technology of the galvanizing process. For this purpose, the effect of many alloying additives to the zinc bath on the structure and thickness of the coating was tested. The article reviews the influence of various elements introduced into the bath individually and in different configurations, discusses the positive and negative effects of their influence on the galvanizing process. The current development in the field of the chemical composition of galvanizing baths is also presented and the best-used solutions for the selection and management of the chemical composition of the bath are indicated.

Microstructure Characterization and Corrosion Resistance of Zinc Coating Obtained in a Zn-AlNiBi Galvanizing Bath

The article examines the impact of the addition of Al, Ni, and Bi to a zinc bath on the microstructure and corrosion resistance of hot dip galvanizing coatings. The microstructure on the surface and the cross-section of the coatings obtained in the Zn-AlNiBi bath were examined. The corrosion resistance of the coatings was assessed by the standard neutral salt spray test (EN ISO 9227), the sulfur dioxide test in a humid atmosphere (EN ISO 6988), and the electrochemical test. The corrosion resistance of Zn-AlNiBi coatings was compared with the corrosion resistance of coatings attained in the bath of “pure” zinc. The corrosion tests showed higher corrosion wear of the coating obtained in the Zn-AlNiBi bath and a higher value of the corrosion current density for this coating. It was found that the cause of the reduction of the corrosion resistance of the coating, in contrast to the coating obtained in the “pure” zinc bath, may be the presence of bismuth precipitates in the coating, which may form additional corrosion cells.

Structural Aspects of Decreasing the Corrosion Resistance of Zinc Coating Obtained in Baths with Al, Ni, and Pb Additives

The article presented the results of tests determining the synergistic effect of Al, Ni, and Pb additions on a zinc bath on the structure and corrosion resistance of coatings obtained on low silicon steel. Analyzed coatings were produced on S235JRG2 steel with Si content of 0.02 mass%. The corrosion resistance of the coatings was compared with the corrosion resistance of the coating obtained in the "pure" zinc bath. Structure at high magnifications (SEM) was determined, as well as coating thickness and chemical composition in microspheres. The corrosion resistance of the coatings was established comparatively in standard corrosion tests in neutral salt spray and a humid atmosphere containing SO2. It was found that the addition of Pb to the zinc bath reduced the corrosion resistance of the coatings. In the coating structure obtained in the Zn-AlNiPb bath, lead precipitation was observed in both the outer layer and the intermediate layer of the coating. Grain boundaries were the preferred site for lead precipitation. The presence of Pb precipitates favored conditions for the creation of additional corrosion cells, which led to a decrease in the corrosion resistance of the coatings.

The Resistance of Welded Joints of Galvanized RHS Trusses with Different Vent Hole Geometries

A worldwide-accepted technique to protect steel lattice girders with welded hollow sections against corrosion is the hot-dip galvanizing process. In this process, vent holes are required in braces to fill the inner part protecting them from corrosion, to allow the immersion of the structure in the zinc bath and to recover the excess fluid after the bath. The cross-section reduction due to the vent hole could lead to a decrease in the effective brace resistance; this is not easily quantified, because there are neither prescriptions nor recommendations in the design codes to assess this effect. Therefore, the hollow structural sections could be underutilized due to doubts regarding the safety of this type of joint. This research was conducted in order to categorize different geometries and positions of vent holes in order to determine the best in terms of joint efficiency. A validated finite element model considering welds on lattice girders joints was extended to take into account different vent hole shapes. This research concludes that the presence of ventilation holes such as the ones considered in this study does not significantly affect the joint resistance, and that all the analyzed hole shapes could be proposed as a valid solution for machining vent holes. The conclusions drawn up from this work could be useful for structural steel designers, providing them with valuable design recommendations.