Hot-dip galvanizing of high and ultra-high strength thin-walled CHS steel tubes: Mechanical performance and coating characteristics

Highlights

• Mechanical and microscopy studies were conducted on galvanized HSS and UHSS tubes.

• Effect of hot-dip galvanizing on performance of thin-walled tubes was quantified.

• A practical process for galvanizing HSS and UHSS tubes was developed.

• Zinc bath duration affects the brittleness of coating when it exceeds 1.5 min.

• Sand-blasted surfaces experience a thicker and more brittle zinc coating.

Abstract

Hot-dip galvanizing is one of the most common corrosion protection techniques for steel. This widely available method increases the lifetime of steel structures by up to 50 years. Although there is a well-established method for hot-dip galvanizing of structural mild-steel, the effects of this method on mechanical properties of high and ultra-high strength steels (HSS and UHSS), with nominal yield strength greater than 700MPa, has not been thoroughly studied. The study of galvanized thin-walled HSS and UHSS tubes is important since (a) galvanizing heat can easily penetrate the small thickness of thin-walled tubes and change their mechanical behaviour, (b) these materials are made through special heat treatment process which might render them sensitive to heat exposure during hot-dip galvanizing, and (c) these materials are high in alloy especially Si equivalent (Si+P) content which categorizes them as reactive steel with rapid reaction with molten zinc. Therefore, to deeply study the performance of galvanized high-grade steels, HSS and UHSS thin-walled tubes were dipped in zinc bath for various durations. For each bath time, a tube with intact and one with sand-blasted surface were used so that the roughness effect of the steel surface on coating performance could be explored. Dog-bone, tube shape, and small beam-shaped specimens were cut out of the larger tubes for tensile, compressive, impact, microscopy, coating thickness, and hardness studies. A mild steel tube was also galvanized and studied as a control specimen for comparison purposes. These studies showed that after galvanizing, HSS and UHSS materials lost their ultimate stress and recovered some of their ductility. On the other hand, galvanized mild steel specimens lost a small portion of their ductility and gained higher yield and ultimate stresses. In addition, it was found that galvanizing did not affect the hardness of HSS materials but decreased the hardness of UHSS materials up to 10%. Coating studies were conducted through thickness measurements, impact tests, and microscopic studies. The measurements showed that the coating thickness increased with respect to bath time and that a thicker coating formed on sand-blasted surfaces. Moreover, impact tests and microscopic studies on plastically bent HSS and UHSS specimens demonstrated that the favourable coating in terms of cracking under severe plastic deformation was formed on non-sand-blasted steel surfaces which were experienced a zinc bath time of 1.5 min. The outcomes of this study have the potential to be incorporated into the codes of practice and design guidelines for suitable hot-dip galvanizing and design of structural components made of galvanized HSS and UHSS.

Introduction

Due to low prices, wide availability, and exceptional mechanical properties, steel is broadly used all around the world in engineering applications. However, any material in high demand has its own shortcomings. The main shortcomings of steel structures are corrosion and behaviour at elevated temperatures. There are many effective isolating techniques to decrease the exposure of steel structures to elevated temperature such as shotcrete, intumescent paint coating, etc [1]. In case of corrosion, when iron is extracted from ore, the energy equation is reversed and the nature tries to balance it. Therefore, steel is prone to corrosion in damp harsh environment such as soil, sea water, humid areas, etc. Steel corrosion imposes extreme costs to different societies. In Australia, the cost of steel corrosion is around 30 billion dollars a year. There are different suitable practices for corrosion protection of steel such as regular paints, zinc rich paints (cold galvanizing), hot-dip galvanizing, electroplating, ceramic coatings, advanced multilayer polymer coatings [2], sacrificial anodes, etc. Zinc rich paints and regular paints have no metallurgical bond to steel, are not abrasion resistant, and lose cathodic protection in atmospheric exposure by time [3]. Electroplating forms a very thin coating layer (around $10\mathrm{\mu }\mathrm{m}$) and is mainly suitable for indoor environments due to low wear and tear resistance [4]. Ceramic and multilayer polymer coatings are not widely available, high in price, and not practical for large structures [2], [5]. On the other hand, hot-dip galvanizing which can increase the lifespan of a steel part or structure up to 50 years, is highly scratch and abrasion resistant due to metallurgical bonds, economical, easy to maintain, and widely available for any structure member dimensions [6].

Zinc was first recognized as a metal in 16th century in Europe. In 1837, the first patent on galvanizing procedure was filed by Stanislaus Tranquille Modeste Sorel, a French engineer [7]. Since then, the use of galvanizing as a suitable corrosion protection method has increased rapidly, as an example, up to 600 kilo-tons of steel parts are galvanized in Australia annually. The zinc layer forming on steel acts as a barrier as well as a cathodic protection metal. Therefore, if a small part of zinc layer is delaminated, the zinc around it prevents the bare steel from corrosion by acting like a sacrificial anode. Based on the wide availability, numerous advantages, and ease of use, there is a good chance that steel materials, profiles, and structural components available on the market may end up in hot-dip galvanizing baths; therefore, it is of paramount importance to possess good knowledge about the behaviour of any type of steel material after hot-dip galvanizing.

Hot-dip galvanizing is done in 8 main steps (see Fig. 1). In the first two steps, the organic pollutants available on the steel parts are cleansed and rinsed. Through the third and fourth steps, oxides and milling waste are removed using a strong acid solution and subsequent rinsing. Then, the elements are dipped in a flux solution to remove fine oxides and to form a protective layer on the steel surface which prevents rusting before zinc bath. Thus, the prepared steel element will be dipped in a molten zinc bath with a temperature ranging from 450 to 500 °C. Note that the typical dipping time for mild-steel elements ranges from 8 to 10 min to ensure that a zinc coating with sufficient thickness is formed [6]. After dipping, the parts are water quenched to maintain a shiny pure zinc layer (no Fe present) on the surface of steel parts. If they are not quenched, there is a chance that the zinc-Fe (Zeta) layer grows to the surface, changing the surface to a matt grey colour.

High and ultra-high strength steels (HSS and UHSS) have been widely used in engineering applications such as heavy-duty vehicle frames, crane towers and jibs, etc. for a long time. This wide usage is because HSS and UHSS materials own unique features such as high strength to weight ratio, good formability, weldability, etc. These materials hold yield and ultimate tensile strengths ranging from 700 to 1700 MPa and 800 to 2000 MPa, respectively [8]. Lately, civil and structural engineering researchers are focusing on the usage of such steel materials in construction [9], [10], [11]. The main aim of these studies has been developing state of the art light-weight structures with extraordinary capacities to minimize the weight of the structural components and maximize utilization of the HSS and UHSS materials. However, similar to mild-steel structures, the structural elements made of HSS and UHSS might be subjected to harsh environment for which hot-dip galvanizing is inevitable. Nonetheless, the hot-dip galvanizing of HSS and UHSS structural components might be challenging for two main reasons: (i) due to different manufacturing process, the chemical composition of HSS and UHSS is different to that of mild-steel which may affect the performance of formed zinc coating; and (ii) the mechanical properties of HSS and UHSS may change due to heating and cooling resulting from the hot-dip galvanizing​ process.

High strength steel materials might be manufactured through two different heat treatment methods namely, direct quenching (DQ) and quench and tempering (QT) [12]. In DQ, the material is heated to temperatures around 1000 °C and quenched only once to room temperature. On the other hand, in QT, the material is exposed to high-elevated temperatures and quenched to room temperature several times. In these methods a portion of the steel microstructure changes to martensite which increases its yield strength and decreases its ductility. Such materials are sensitive to heat which may reverse the formed martensite to austenite microstructure [13]. Therefore, hot-dip galvanizing may affect the mechanical performance of high-strength steel materials as they are exposed to high temperatures (around 450 °C $-$500 °C) in this process.

Many studies have been conducted on the behaviour of HSS and UHSS members when they are exposed to heat (from 450 °C to extreme temperatures up to 1200 °C) in different scenarios such as fire, welding, etc. [12], [14], [15]. Outinen and Mäkeläinen [16], studied the post fire performance of S690 steel and found a yield strength reduction of up to 7% in specimens exposed to a temperature of 500 °C. Gunalan and Mahendran [17] conducted an experimental investigation to quantify the behaviour of G550 high strength steel (680 yield strength) and reported a 17% reduction in yield strength for a specimen exposed to a temperature of 500 °C. Azhari et al. [12], studied the post-fire behaviour of UHSS (1200 MPa yield) materials when exposed to fire temperatures from 300 to 800 °C. They reported a considerable reduction of 14% in yield strength of UHSS after cooling down from 450 °C.

The other challenge is that HSS and UHSS materials are categorized as high alloy materials that contain high amounts of Si equivalent content and other alloys based on AS/NZS 4680 standard [18]. Different weight percentages of Si equivalent content (Si$+$P) in a steel material causes steel reactivity with molten zinc and changes the zinc coating thickness severely and makes it non-uniform (Fig. 2) [19], [20]. The typical alloy composition for the UHSS, HSS and Mild steel (MS) tubes are presented in Table 1, and as can be seen the Si equivalent content of HSS and UHSS materials are extremely higher than that for a normal grade steel [12]. As can be seen in Fig. 2, for a typical strength steel with Si equivalents higher than 0.25%, the zinc layer thickness increases greatly. These thick zinc coatings are poorly adherent to the steel and brittle which decreases their service life and strength in rigours of transport, handling, and installation. Many available studies and methods investigate controlling the zinc reactivity of high alloy steels. Tang [19] investigated the reactivity control method which was the addition of Al into the zinc bath (Polygalva process) when the Si content is less than 0.2%. However, this method has its own challenges such as bath chemistry management and coating weight and uniformity control. Fratesi et al. [21] investigated the effects of Bi addition into the zinc bath and they found that this can control the steel reactivity when the silicon content is no higher than 0.2%. Habraken [22] proposed a solution for reactive steel galvanization which was using a zinc bath with a temperature higher than 500 °C that decreases the formation of a thick brittle zinc layer on the steel substrate. Chen et al. [23] studied the addition of Ni in zinc bath (Technigalva) which controls steel–zinc reactivity for steels with a Si content lower than 0.25%. All of the existing remedies can cause two main issues in terms of practicality which categorizes them as not industry friendly. First, controlling or changing zinc bath chemistry may be impossible since a huge amount of zinc (zinc baths are typically 3$\times$3$\times$12 metres in dimensions) needs to be modified each time for different steel part chemical compositions. Second, controlling the bath temperature is a huge effort since changing the temperature for each steel part individually requires lots of time and energy which is avoided by industry. Therefore, a study is required to develop a practical solution to overcome the challenges of hot-dip galvanizing of large steel members made of HSS and UHSS materials.

This study aims to investigate the mechanical behaviour of galvanized HSS and UHSS thin-walled CHS tubes as well as the characteristics of zinc coating and its adhesion to HSS and UHSS substrates. For zinc–steel adhesion study, the steel surface roughness was changed using sand-blasting to study its effects on the zinc coating characteristics [25]. In addition, this study pursuits the development of an industry friendly method to galvanize reactive high alloy steel hollow thin-walled sections, such as HSS and UHSS materials, with Si equivalent content more than 0.25%.