Al-Zn Bath Research Articles

ＺｎおよびＺｎ‐Ａｌめっき組織と食塩水中におけるその耐食性

The corrosion behavior of Zn and Zn-Al plating samples was investigated based on the measurement of anodic and cathodic polarization curves, the change in potential with use of a constant current of 2mA, the observation of structures in a cross section after a corrosion test, and the identification of corrosion products in the samples. Two plating samples were examined. One was fabricated by the dipping of the steel in a Zn bath at 743K for 60s, and another was fabricated by dipping steel in a secondary Zn-Al bath at 693K for 60s after first dipping in a Zn bath. The potentiostatic corrosion was carried out in a 3% by NaCl aqueous solution at 303K with a potential scan rate of 30mv/min and Ag/AgCl was used for the reference electrode. The galvanostatic corrosion was carried out in a 0.5% by NaCl aqueous solution with a constant current of 2mA/dm2. The change in the potential of the sample was measured, After the corrosion test, the samples were examine using EPMA and X-RD, Polarization curves obtained indicated that the reaction of Zn and Zn-Al coatings was controlled by control of the cathodic reaction. The rate of the cathodic reaction was faster for the Zn-Al coating than for the Zn coating. The same tendency was observed with Zn-Al alloy layer (Fe4Al13-Zn) and Zn alloy layer (ζ). During galvanostatic corrosion, corrosion proceeding through the η phase was observed between the lamellar ζ phase in the Zn plating sample. The contrast, the Fe4Al13-Zn phase dispersed in a Zn-Al matrix in the Zn-Al plating sampls, and the corrosion path was more complicated than with in the Zn plating sample. The time to reach galvanosttic corrosion of the base metal is thus believed to be determined by the difference in the corrosion path.

The effect of iron oxide as an inhibition layer on iron-zinc reactions during Hot-Dip galvanizing

A study was conducted on the effect of a uniform oxide layer on the galvanizing reaction in 0.20 wt pct Al-Zn and pure Zn baths at 450 °C. In the 0.20 wt pct Al-Zn bath, poor wettability of the oxide layer was observed. No significant liquid Zn penetration of the oxide occurred and, therefore, attack of the steel substrate to form localized Fe-Zn growth did not occur. It was found that the iron oxide acted as a physical barrier or inhibition layer in the pure Zn bath, similar to the Fe2Al5 inhibition layer that forms at the steel interface in Al-Zn baths. The inhibition effect of the oxide in the pure Zn bath was temporary, since cracks and other macrodefects in the oxide acted as fast diffusion paths for Zn. Localized Fe-Zn growth (outbursts) formed at the steel/coating interface, and the number of outbursts was generally inversely proportional to the oxide layer thickness at constant immersion times. Increased immersion time for a constant oxide layer thickness led to an increase in the number of outbursts. These results simulate the diffusion short circuit mechanisms for Fe2Al5 inhibition layer breakdown in Al-containing Zn baths.

Effect of substrate grain size on iron-zinc reaction kinetics during hot-dip galvanizing

In the galvanizing process, it has been proposed that the grain size of the substrate steel influences the Fe-Zn alloy phase reaction kinetics and growth rate during immersion in the liquid Zn bath. Two grain sizes (nominally 15 and 85 µm) were developed in a decarburized low-carbon (0.005) steel and hot-dipped galvanized in 0.00 wt pct Al-Zn and 0.20 wt pct Al-Zn baths to study the effect of substrate grain size on Fe-Zn phase formation. Uniform attack of the substrate steel occurred in the 0.00 wt pct Al-Zn bath, since an Fe2Al5 inhibition layer did not form. No barrier to nucleation of the Fe-Zn phases exists in this Zn bath, and therefore, the substrate steel grain size had no significant effect on the kinetics of phase growth for the gamma, delta, and zeta phase layers. In the 0.20 wt pct Al-Zn bath, discontinuous Fe-Zn phase growth (outburst formation) occurred due to the initial formation of the Fe-Al inhibition layer. The nucleation of the Fe-Zn phases was significantly retarded in this bath for the large (85 µm) substrate grain size. Whereas outbursts were found in the 15-µm grain size substrate after 10 seconds of immersion time, it required 1200 seconds to nucleate just a few outbursts in the 85-µm substrate. These results support the mechanism that Fe-Al inhibition layer breakdown occurs along fast diffusion paths for Zn in the inhibition layer that correspond to the location of substrate steel grain boundaries where reaction with Fe can occur.

Effect of phosphorous surface segregation on iron-zinc reaction kinetics during hot-dip galvanizing

Phosphorous was ion implanted on one surface of a large grain (10 to 20 mm) low-carbon steel sheet in order to study the effect of surface segregation on the formation of Fe-Zn phases during galvanizing. Both an Al-free and a 0.20 wt pct Al-Zn bath at 450 °C were used in this investigation. It was found that P surface segregation did not affect the kinetics of Fe-Zn phase growth for the total alloy layer or the individual Fe-Zn gamma, delta, and zeta phase alloy layers in the 0.00 wt pct Al-Zn baths. In the 0.20 wt pct Al-Zn bath, the Fe2Al5 inhibition layer formed with kinetics, showing linear growth on both the P-ion implanted and non-P-ion implanted surfaces. Fe-Zn phase growth only occurred after extended reaction times on both surfaces and was found to directly correspond to the location of substrate grain boundary sites. These results indicate that P surface segregation does not affect the growth of Fe-Zn phases or the Fe2Al5 inhibition layer. It was shown that in the 0.20 wt pct Al-Zn bath, substrate grain boundaries are the dominant steel substrate structural feature that controls the kinetics of Fe-Zn alloy phase growth.

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.

溶融55mass%Al-Zn合金めっき浴中ドロスの抽出分離定量

溶融55mass%Al-Zn合金めっき浴中ドロスの抽出分離定量

The reaction between solid iron and liquid Al-Zn baths

The reaction which occurred between iron panels and Al-Zn baths during hot dipping was investigated. Three baths were studied: 45Al-55Zn, 55Al-45Zn, and 75Al-25Zn (in wt pct) in the temperature range of 570 to 655 °C. The reaction between the iron panel and the Al-Zn bath was very severe and in all cases the iron panel was totally consumed by the bath in less than two minutes. The rapid attack of the iron panels by the Al-Zn baths was attributed to two separate causes depending on growth conditions. First, in some panels the intermediate layer which formed between the iron panel and the molten bath was nonadherent. This resulted in the direct contact of the molten bath with the iron panel at a nonequilibrium interface, which presented a large driving force and little inhibition for the reaction. Second, in panels containing an adherent alloy layer, the layer had channels of liquid Zn which extended from the molten bath to the iron panel. These channels allowed rapid transport of Zn and Al to the iron panel which resulted in a very high reaction rate. The controlling step in the reaction between the iron panel and molten Al-Zn bath was the diffusion rate of Al in the molten bath to the surface of the iron panel. The diffusion coefficient of Al in the molten bath was found to be in the range of 1 × 10-5 to 5 × 10-5 cm2/s. Microstructural, electron microprobe, and X-ray diffraction data are presented to support the above-mentioned mechanisms and conclusions.

Reaction mechanisms for the coatings formed during the hot dipping of iron in 0 to 10 Pct Al-Zn baths at 450° to 700°C

The reaction mechanisms and the structures of the phases formed during the hot dipping of iron in 0 to 10 pct Al-Zn alloy baths at temperatures of 450° to 700°C were studied by X-ray diffraction and electron microprobe analysis techniques. A new mechanism for the inhibition reaction between iron and zinc is proposed. At bath temperatures below 600°C, a thin layer of an Fe-Al-Zn ternary compound forms on the iron surface and inhibits the growth of Fe-Zn phases. Breakdown of inhibition occurs during the dipping process when the ternary compound becomes rich in aluminum and transforms to a more stable structure which is isomorphous with Fe2Al5. While this breakdown is occurring, the zinc atoms react with iron and form the conventional Fe-Zn phases.