Key Takeaways & Executive Findings
- •• • At 380 °C for 6 h, the layer reaches ~36.1 μm with 84.1% δ phase and no observable cracks; this specific window delivers the best combination of corrosion resistance (icorr 1.16×10⁻⁶–9.78×10⁻⁷ A/cm²) and process efficiency, avoiding the through-thickness cracking that occurs beyond 6 h and the interfacial cracking caused by excessive Γ phase at 340 °C. • • Temperature dominates growth kinetics: increasing from 340 °C to 400 °C at 6 h raises layer thickness from 10.80 μm to 43.90 μm (a ~3× increase), while extending holding time from 2 h to 10 h at 380 °C grows the layer from 12.10 μm to 81.60 μm; the measured diffusion coefficient at 380 °C is 2.341×10⁻¹³ m²/s, providing a quantitative basis for furnace time–temperature scheduling. • • Phase constitution is temperature-sensitive: all layers contain δ and Γ phases, but higher temperatures increase the Zn/Fe ratio and δ-phase fraction, which correlates with denser layers and improved corrosion resistance; conversely, low-temperature processing (340 °C) produces excessive interfacial Γ phase that initiates cracking at the substrate–layer interface, a critical failure mode for load-bearing structural components. • • The three-stage growth mechanism—active Zn atom generation from ZnCl2 decomposition, bidirectional Zn/Fe interdiffusion along substrate defects, and continuous inward advancement of the intermetallic front—identifies NH4Cl activator chemistry and substrate defect density as levers for industrial scale-up; holding times beyond 6 h are counterproductive because through-thickness cracks degrade uniformity and corrosion protection.
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Abstract
Powder sherardizing on Q235 structural steel was conducted in a 70wt.% Zn–0.8wt.% NH4Cl–29.2wt.% α-Al2O3 activated pack at 340–400 °C for 2–10 h to establish the temperature-dependent growth kinetics, phase evolution, and corrosion performance of Zn–Fe intermetallic layers. Cross-sectional SEM/EDS and XRD show that all layers consist of δ and Γ phases, with Γ concentrated near the substrate; excessive Γ at 340 °C initiates interfacial cracking. Layer thickness increases monotonically with temperature and time, rising from 10.80 μm at 340 °C to 43.90 μm at 400 °C after 6 h, and from 12.10 μm at 2 h to 81.60 μm at 10 h at 380 °C. The Zn/Fe ratio and δ-phase fraction increase with temperature, yielding denser layers and improved corrosion resistance; at 380–400 °C, corrosion current densities fall to 1.16×10⁻⁶–9.78×10⁻⁷ A/cm². The diffusion coefficient at 380 °C is 2.341×10⁻¹³ m²/s. Prolonged holding beyond 6 h produces through-thickness cracks. DSC and microstructural evidence support a three-stage growth mechanism: formation of active Zn atoms via ZnCl2 decomposition, bidirectional Zn/Fe interdiffusion along substrate defects, and continuous inward advancement of the Zn–Fe intermetallic front. The optimal processing window is 380 °C for 6 h, yielding a ~36.1 μm crack-free layer with 84.1% δ phase and superior corrosion resistance.
1. Introduction
Conventional zinc coatings on structural steel—electroplating, hot-dip galvanizing, and zinc-rich paints—face persistent limitations in thick-section or complex-geometry components: hydrogen embrittlement risk, dimensional distortion at molten-zinc temperatures, and inadequate throwing power into recesses. Powder sherardizing, a solid-state thermal diffusion process, circumvents these constraints by forming a Zn–Fe intermetallic layer at temperatures below the steel's tempering threshold, yet industrial adoption has been hindered by insufficient quantitative understanding of how furnace temperature governs phase selection, layer growth, and corrosion performance in activated NH4Cl packs.
This study addresses that gap by systematically mapping the 340–400 °C processing window for Q235 steel in a 70wt.% Zn–0.8wt.% NH4Cl–29.2wt.% α-Al2O3 pack. Through SEM/EDS, XRD, DSC, potentiodynamic polarization, and EIS, the authors establish the thickness–temperature–time relationships, identify the δ/Γ phase evolution, quantify the diffusion coefficient at 380 °C (2.341×10⁻¹³ m²/s), and pinpoint the cracking mechanisms that arise from excessive Γ phase at low temperature and from prolonged holding beyond 6 h. The resulting three-stage growth model and the recommended 380 °C/6 h window provide actionable parameters for optimizing sherardizing cycles without sacrificing corrosion resistance.
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YU Tianjian, LI Haiyang, ZHANG Shizhao, LIU Shujing, WANG Shuaixing, LIU Xiaohui, DU Nan (2026). Influence of Temperature on Diffusion Behavior and Infiltration Layer Structure of Sherardizing on Structural Steel. Surface Technology (表面技术). https://doi.org/10.16490/j.cnki.issn.1001-3660.2026.10.005
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Frequently Asked Questions
What is the dominant failure mechanism when sherardizing Q235 steel at the lower bound of the investigated temperature range (340 °C)?
At 340 °C, the layer contains an excessive fraction of Γ phase concentrated near the substrate, which initiates cracking at the substrate–layer interface. This interfacial cracking compromises adhesion and load transfer, making 340 °C unsuitable for structural applications despite producing a continuous layer.
Why does extending holding time beyond 6 h at 380 °C degrade layer quality, and what is the quantitative threshold?
Holding beyond 6 h at 380 °C produces through-thickness cracks and progressively reduces layer uniformity. Although thickness increases from 12.10 μm at 2 h to 81.60 μm at 10 h, the crack network undermines corrosion protection and mechanical integrity, establishing 6 h as the practical upper limit for crack-free layers.
How does the corrosion current density of the optimized 380–400 °C layers compare with conventional zinc coatings, and what does this imply for industrial service life?
The 380–400 °C layers exhibit corrosion current densities of 1.16×10⁻⁶ to 9.78×10⁻⁷ A/cm², which are low and indicative of good barrier protection. The increase in corrosion resistance diminishes above 380 °C, so 380 °C for 6 h offers the best trade-off between corrosion performance and energy consumption.
What is the measured diffusion coefficient at 380 °C, and how does it inform scale-up from laboratory to production furnaces?
The diffusion coefficient at 380 °C is 2.341×10⁻¹³ m²/s. This value allows calculation of layer thickness for arbitrary holding times via the parabolic growth law, enabling furnace time–temperature scheduling that avoids both under-thickness and crack-inducing over-processing.
What are the critical stages of the sherardizing growth mechanism, and which stage is most sensitive to activator chemistry?
The growth mechanism comprises three stages: (1) formation of active Zn atoms primarily from ZnCl2 decomposition, (2) bidirectional Zn/Fe interdiffusion along substrate defects, and (3) continuous inward advancement of the Zn–Fe intermetallic front. Stage 1 is most sensitive to NH4Cl content because the activator governs ZnCl2 generation and thus the supply of active Zn atoms.
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