Pure Zinc Bath Research Articles

Numerical Simulation of Traditional and Technological Zinc‐Based Coatings: Part II

Herein, the bending behavior of hot‐dip galvanizing hot‐rolled hypersandelin plates, together with the corresponding crack pattern, is numerically investigated by using a hybrid model developed in Ansys LS‐DYNA environment, by combining the discrete element method (DEM) with finite element method (FEM). The experimental bending tests here simulated and available in the literature are performed by considering two types of bath, that is, a pure zinc bath and a technological bath, consisting in a zinc bath with 3% Sn addition by weight. The results related to the bending behavior are compared with both experimental data and other FE numerical results, previously obtained by some of the present authors. Moreover, the numerical crack patterns are compared with the experimental observations of the longitudinal section of the plates at the end of testing, by means of a light optical microscope. A quite satisfactory agreement is observed.

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.

Microstructure and Electrochemical Performance Evaluation of Zn, Zn-5 wt.% Al and Zn-20 wt.% Al Alloy Coated Steels

A comparative study has been done on three coatings obtained on steel surface using pure Zn, two Zn-Al alloy baths. The coatings obtained using both Zn alloy baths are thinner than the same obtained using pure zinc bath as Al reacts with steel and forms very thin layer of Fe2Al5 phase which hinders iron-zinc reaction. Coating obtained using pure zinc bath predominantly consists of eta (~ 100 wt.% Zn) and zeta (FeZn13) phases in combination with less extent of delta (FeZn10) and gamma (Fe3Zn10) phases, whereas coatings obtained using Zn-Al alloy baths exhibit zinc-rich phase covered by aluminum rich phase with Al2O3 at the top. Al-rich phase content is higher for Zn-20 wt.% Al alloy coating. The coatings obtained on steel surfaces using both the Zn alloy baths show better ductility compared to pure zinc bath. This can be attributed to the presence of brittle Zn-Fe alloy phase in the coating obtained using pure zinc bath. The coatings obtained using both the Zn alloy baths were found to have a greater passivation characteristics compared to the coating obtained using pure zinc bath without compromising sacrificial characteristic of the coating. Electrochemical and salt spray tests results showed 3 and 6 times improved corrosion resistance properties of Zn-5 wt.% Al and Zn-20 wt.% Al alloy coatings than the pure zinc coating.

Structure and Performances of 7075 Aluminum Alloy Surface Subject to Hot-Dip Zinc

The hot-dip galvanized coating was produced on the 7075 aluminum alloy plate by immersion in a pure zinc bath at different temperature from 440°C to 470°C for different modification time (between 1 min and 10 min). The hot-dip galvanized of the treated and un-treated samples were investigated by using scanning electron microscopy (SEM) with EDX analysis, X-ray diffraction (XRD), hardness measurements and potentiodynamic polarization curves. There were three layers fabricated on the 7075 substrate surface, specifically, zinc-rich layer, two-phase mixed layer Al-Zn and interdiffusion layer. Moreover, the 7075 aluminum alloy with the hot-dip galvanized coating exhibit higher hardness and better anti-corrosion properties in comparison with the 7075 substrate.

Corrosion Resistance of Fe-B Alloys Immersed in Molten Zinc Bath

Fe-B alloys with different boron contents were dipped into a pure molten zinc bath to investigate their anti-corrosion property. Scanning electron microscope (SEM) and energy dispersive spectrum(EDS) were used to detect the corrosion products. It is found that the corrosion mechanism of α phase is corrosion and dissolution ,while the Fe2B phase is crack mechanism because of its brittleness and excellent corrosion resistance.The Fe2B phase in eutectic structure can protect the matrix from molten zinc and restrict the reaction of Fe-Zn. With the increase of boron content, the corrosion resistance performance of the alloys improves gradually.

Influence of Boron Concentration on the Corrosion Resistance of High Boron White Cast Iron to Liquid Zinc Bath

The influence of boron concentration on corrosion resistance of high boron white cast iron dipped into a pure liquid zinc bath at 460°C was investigated. The results reveal that high boron white cast iron containing 3.5 wt.%B exhibits excellent corrosion resistance due to the dense continuous netlike or parallel Fe2B phase which hinders the Fe/Zn interface reaction. The corrosion rate decreases significantly when the boron concentration increases, but the corrosion rate declines slightly when the boron concentration exceeds 3.5wt.%. EDS results indicate the coarse and compact δ phase generated near the matrix and a large amount of massive and blocky  phase occurred close to the liquid zinc. The corrosion process includes Fe/Zn interface reaction and the spalling and fracture of Fe2B. The failure of Fe2B is mainly caused by the microcrack of phase transformation.

Corrosion Behaviour of a Fe-Based Alloy in Zn and Zn-55Al Baths

Corrosion behavior in Zn and Zn-55Al baths were investigated on a Fe-based alloy, which was design to apply to hot-dip production lines as the anti-corrosion alloy. The results reveal that the Fe-based alloy shows excellent corrosion resistance both in pure zinc bath and Zn-55Al bath. And the reaction rate and intermetallic phase formation vary in different bath. In pure zinc bath, the intermetallic phases form on the Fe-base alloy is chiefly Fe-Zn phases contains Γ, δ and ζ phases. In Zn-55Al bath, the Fe-based alloy is comprised of Fe2Al5 and FeAl3 phases, and the outer layer is from bath itself. The more thickness the layer of Fe2Al5 phase, the more corrosion resistance of the alloy.

Effects of boron concentration on the corrosion resistance of Fe–B alloys immersed in 460 °C molten zinc bath

In order to investigate the effects of boron concentration on the corrosion resistance of Fe–B alloys in molten zinc, Fe–B alloys, with the boron concentrations of 1.5 wt.%, 3.5 wt.% and 6.0 wt.% respectively, were dipped into a pure molten zinc bath at 460 °C and kept in different time intervals. The results show that, in comparison with 1Cr18Ni9Ti stainless steel, Fe–B alloy with 3.5 wt.%B exhibits excellent corrosion resistance, due to the dense continuous network or parallel Fe 2B phase which hinders the Fe/Zn interface reaction in Fe–B alloys. The energy dispersive spectrum (EDS) results indicate that the coarse and compact δ phase with the length about 40 μm generates near the matrix of Fe–B alloy and massive ζ phase occurs close to the liquid zinc. The corrosion process includes Fe/Zn reaction and the isolation and fracture of Fe 2B. The failure of boride is mainly caused by the microcrack.

Role of silicon in steels on galvanized coatings

In this article, five kinds of silicon-containing steel sheets have been electrodeposited, and then immersed in a pure molten zinc bath at 450 °C for various periods of time. The results by scanning electron microscopy (SEM) show that the coating of the sample (0.09 wt pct Si) with iron-electrodeposited pretreatment eliminates the reactive zones which are found in the coating without iron-electrodeposited pretreatment. The galvanized sample (0.28 wt pct Si) with iron-electrodeposited pretreatment exhibits a compact and coherent coating. The coating of the sample (0.37 wt pct Si) with the iron-electrodeposited pretreatment experiences a transition from a compact and coherent coating to a reactive one. The energy dispersive spectrum (EDS) results reveal that for the galvanized samples with iron-electrodeposited pretreatment, excessive silicon accumulates on the surface of the substrate due to the low solubility of silicon in the Γ, after the iron layer is depleted by the increasing growth of the Fe-Zn intermetallics. With the movement of the substrate/Γ interface toward the substrate, silicon-enriched α-Fe peels offi from the substrate and breaks into the particles. The particles move toward the δ layer through the Γ layer because silicon-enriched α-Fe cannot be absorbed in the Γ layer. When the particles reach the δ/Γ interface, they are dissolved in the δ layer, making the Γ layer thin or even vanish.

Microstructural study of galvanized coatings formed in pure as well as commercial grade zinc baths

This investigation is an effort to have a better understanding of the growth kinetics and morphology of the coating formed during the galvanizing process in pure as well as commercial grade zinc baths. The protective coating that is formed during hot dip galvanizing, normally between 450 and 480°C, consists of a series of Fe-Zn intermetallic layers, which have been identified as gamma (Γ), delta (δ), zeta (ξ) and an outer eta (η) layer, highly rich in zinc. There is apparently no delay in the formation of ξ or δ phases in both pure as well as the commercial grade zinc baths. The gamma (Γ) phase is formed after an incubation time of about 30 s at a bath temperature of 470°C in the pure zinc bath. Its formation is further delayed in the commercial grade zinc bath. The last morphological feature is the formation of a second ξ layer at the ξ/δ interface in the pure zinc bath. In the commercial grade zinc bath two different morphologies of ξ phase are seen starting from the lowest dipping time, and also the overall coating is considerably thicker due to formation of several iron-zinc intermetallics which degrade its ductility and outward appearance. Commercial grade zinc also enhances the dross formation in the bath and deteriorates the quality of the coating. Presence of transverse cracks as well as entrapment of dross particles in the coating is attributed to the less compact coating that is formed in the commercial grade zinc bath.

Liquid Metal Corrosion of 316L Stainless Steel, 410 Stainless Steel, and 1015 Carbon Steel in a Molten Zinc Bath

Corrosion tests of 1015 low-carbon steel and two stainless steels (410 and 316L) were conducted in a pure zinc bath (99.98 wt pct Zn) in order to better understand the reaction mechanisms that occur during the degradation of submerged hardware at industrial general (batch) galvanizing operations. Through this testing, it was found that, in general, 316L stainless steel showed the best dissolution resistance among these three alloys, while 1015 carbon steel provided a lower solubility than 410 stainless steel. Investigating the failure mechanisms, both metallurgical composition and lattice structure played important roles in the molten metal corrosion behaviors of these alloys. High contents of nickel combined with the influence of chromium improved the resistance to molten zinc corrosion. Moreover, a face-centered-cubic (fcc) structure was more corrosion resistant than body-centered-cubic (bcc) possibly due to the compactness of the atomic structure. Analogously, the body-centered-tetragonal (bct) martensite lattice structure possessed enhanced susceptibility to zinc corrosion as a result of the greater atomic spacing and high strain energy. Finally, an increased bath temperature played an important role in molten metal corrosion by accelerating the dissolution process and changing the nature of intermetallic layers.

On the wear of a cobalt-based superalloy in zinc baths

Journal bearings made of a cobalt-based superalloy, trade marked STELLITE 6, were evaluated in zinc baths with and without the presence of aluminum and iron. The sleeve and the bushing wore evenly when tested in a pure zinc bath. The surfaces were generally smooth and covered by some rather fine grooves after the test. The wear of the bearings was much more severe when tested in aluminum-containing zinc baths. The worn surfaces of the bearings were fully covered by wide and deep grooves. The coefficient of friction of the superalloy generally increased with increases in the aluminum content of the molten zinc. The iron addition to the molten zinc appeared to affect the friction and wear characteristics of the superalloy. Detailed metallographic and microchemical analyses were performed to elucidate the wear mechanisms. The superalloy was found to react readily with the molten zinc alloys to form intermetallic compounds. In a pure zinc bath, zinc-based intermetallic compounds formed on the bearing surfaces; in baths containing aluminum, a compact intermetallic layer, based on the cobalt-aluminide phase, was detected on the bearing surfaces. Evidence collected in this study indicated that wear debris reacted with aluminum in the molten zinc and transformed itself into hard and abrasive cobalt-aluminide particles. During the test, these particles reattached to the bearing surfaces and built up. Acting as blunt microcutters, these particles plowed the bearing surfaces and created deep grooves. The dominant wear mechanism in the aluminum-containing zinc baths was identified as abrasion; fatigue and corrosive wear, however, served as precursors of the dominant wear mechanism.

Cracking mechanisms in high temperature hot-dip galvanized coatings

The cracking behavior of hot-dip galvanized coatings produced by immersion in a pure zinc bath at 560°C is investigated after 3-point bending tests. Microcracks formed in the as-produced δ 1 phase as a result of tensile residual stresses developed during processing act as nucleation sites for cracks formed during bending. These cracks follow a Mode I path, normal to the substrate/coating interface, are arrested at the Γ/δ 1 interface and continue to propagate parallel to the coating/substrate interface resulting in the delamination and failure of the coatings. Measurements of crack population followed by statistical analysis show that the density of cracks formed during bending in the coating is independent of coating thickness and depends on the applied strain, indicating that crack density is not controlled by crack initiation, but by crack propagation. It is also shown that the density of cracks that reach the surface of the coating depends only on the thickness of the outer Zn-rich layers of the coating that act as barriers to crack propagation.

Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450°C: Part II 0.20 wt% Al-Zn baths

The effect of solute additions of titanium, titanium and niobium and phosphorus on interstitial-free steels on Fe-Zn phase formation after immersion in a 0.20 wt% Al-Zn bath was studied to determine the morphology and kinetics of the individual Fe-Zn phases formed. These results were contrasted to the previous study using a pure zinc (0.00 wt% Al) bath in Part I. It was found that in the 0.20 wt% Al-Zn bath, an iron-aluminide inhibition layer prevented uniform attack of the steel substrate. Instead, localized Fe-Zn phase growth occurred, termed outbursts, containing a two-phase layer morphology. Delta-phase formed first, followed by gamma-phase. Zeta-phase did not form in the 0.20 wt% Al-Zn bath, in contrast with zeta-phase formation in the pure zinc bath. As in the pure zinc bath, the growth kinetics of the total layer was controlled by the Fe-Zn phase in contact with the liquid zinc during galvanizing. For the 0.20 wt% Al-Zn bath, the Fe-Zn phase in contrast with the liquid zinc was the delta-phase, whereas the zeta-phase was in contact with liquid zinc in the pure zinc bath. The delta-phase followed t1/2 parabolic growth, while the gamma-phase showed essentially no growth after its initial formation. Titanium and titanium + niobium solute additions, which enhance grain-boundary reactivity, resulted in more rapid growth kinetics of the gamma- and delta-phases. Phosphorus additions, which decrease grain-boundary reactivity, generally increased the incubation time and retarded the growth rate of the gamma-phase. These results further confirm the concept that solute grain-boundary reactivity is primarily responsible for Fe-Zn phase growth during galvanizing in a liquid Zn-Al bath in which an iron aluminide inhibition layer forms prior to Fe-Zn phase formation.

Validation of the Modelling of a Solid-Liquid Reaction by A Solid-Vapor Reaction

When silicon killed steels are galvanized in a pure zinc bath at 450°C, the first Fe-Zn compound to precipitate is ξ-compound. If the silicon content is higher than 0.07 wt%, the reactivity of steel in the liquid leads to thick coating and is clearly due to a silicon super-saturated liquid layer near the substrate. In this case, anarchical growth of ξ-crystals is observed. In order to validate the model proposed for this mechanism, zinc vapor deposition on different substrates has been used. In this case it was shown that the first intermetallic compound which precipitates is no more ξ-compound but the iron richest one: Γ. This result shows that the reactivity of the different steels facing the zinc vapor is the same. In fact, silicon present in the substrate can be totally dissolved in the Γ-phase according to the Fe-Zn-Si phase diagram at 450°C and Si does not play any more its role in preventing the formation of the ξ-compound.

Factors affecting the quality of hot-dip-galvanized steel sheets

The coating quality and corrosion resistance of steel sheets hot-dip galvanized by the conventional method in a semiautomatic plant were studied. The effect of various treatment processes such as surface pretreatment, dipping techniques and post-treatment processes on the coating quality, type and extent of defects was investigated. It was found that the coating quality was markedly influenced by the mode and the amount of zinc bath additives, in particular aluminium or antimony. The required amounts of such additives should be added at intervals in the form of binary zinc master alloys with a maximum aluminium or antimony content of 10%. The correlation of the bath temperature and the withdrawal speed with the sheet thickness is an important factor for quality improvement. The corrosion rate and electrochemical behaviour of sheets galvanized in a pure zinc bath and in baths containing various amounts of aluminium or antimony (0.02–0.1 wt.%) were examined in 1 N NaCl solution. Addition of aluminium or antimony slightly increases the corrosion resistance of the galvanized steel sheets. The corrosion resistance increases in the following order: pure zinc < ZnAl < ZnSb.

Reaction between Fe&amp;ndash;Si Alloys and Liquid Zinc

Seven Fe–Si binary alloys with different silicon concentrations ranging from 0.10 to 2.84 mass% were immersed in a pure zinc bath in the temperature range between 713 K and 873 K for 600 s. The thickness of the alloy layer and the quantities of iron having reacted with zinc were measured, and the structure of the alloy layer was observed. These observations were compared with those of pure iron to offer information about the effect of silicon in galvanizing. Results obtained are as follows:(1) The alloy layer consists of a gamma layer existing next to steel, followed by a delta 1 layer under all the conditions studied. The alloy layer formed next to the delta 1 varies with the silicon concentration of the Fe–Si alloy and the immersion temperature, and four areas are seen in the area map depending on the silicon concentration and the immersion temperature.(2) The reactivity of the Fe–Zn reaction is higher in the region of 0.10 mass%Si and 713 K–733 K, and also in the region of 1–1.15 mass%Si and 773 K–793 K. The former high reactivity is due to the formation of a thicker zeta layer, and the latter is due to the formation of a thicker (delta 1+eta) layer. On the other hand, this reactivity is low in the region of 0.2–0.3 mass%Si, 713 K–793 K, and in the region of 813 K.(3) The Fe–Zn reaction above 833 K does not depend on the silicon concentration of the Fe–Si alloy.