CLADDING TECHNOLOGY SHANXI CO., LTDSelf-Shielded Cladding Process for Iron-Based Amorphous Alloy Coatings
Literature Overview
The study by Yan Tao, Fan Zishuan, and Zhang Zhengdong from the Center for Corrosion and Protection at University of Science and Technology Beijing (2013) investigates the preparation of iron-based amorphous alloy coatings using a self-shielded cladding process. This work was published in the journal "Surface Technology" and addresses an important challenge in advanced coating technology: how to produce amorphous (metallic glass) coatings without the need for external shielding gas, thereby simplifying the process and enabling field application.
Core Technical Content
Amorphous alloys, also known as metallic glasses, possess unique properties including excellent corrosion resistance, high hardness, and superior magnetic properties. However, their production typically requires extremely high cooling rates (exceeding 10^5 K/s) to suppress crystallization during solidification. Conventional cladding processes such as arc welding or thermal spray cannot achieve these cooling rates, making the production of amorphous coatings through these methods extremely challenging.
The self-shielded cladding process proposed in this study eliminates the need for external shielding gas by utilizing a specially designed flux or self-protecting flux composition that generates a protective atmosphere around the molten pool. This innovation significantly reduces equipment complexity and enables application in remote or outdoor environments where shielding gas supply is impractical.
Process Design and Parameters
| Parameter | Value / Range | Purpose |
|---|---|---|
| Welding method | Self-shielded flux-cored arc welding | Eliminate external shielding |
| Base material | Q235 carbon steel | Typical industrial substrate |
| Flux composition | CaF2 + Al2O3 + SiO2 + TiO2 | Self-shielding and deoxidation |
| Wire composition | Fe-Si-B-C alloy wire | Amorphous-forming elements |
| Arc voltage | 22–28 V | Control of heat input |
| Travel speed | 200–400 mm/min | High cooling rate achievement |
| Wire feed rate | 4–8 m/min | Deposition rate control |
| Layer thickness | 0.3–1.0 mm | Thin layer for rapid solidification |
The key to achieving an amorphous structure lies in the combination of high travel speed and thin layer deposition. By maintaining the travel speed above 200 mm/min and limiting each layer to less than 1 mm thickness, the cooling rate at the solidification front can exceed 10^4 K/s, which is sufficient to suppress crystallization in Fe-based alloys containing appropriate amounts of boron and silicon.
Microstructural Characterization
X-ray diffraction and transmission electron microscopy analyses confirmed the formation of a predominantly amorphous structure in the cladding layer. The amorphous phase was identified by the characteristic broad diffraction halo in XRD patterns, with no sharp peaks corresponding to crystalline phases. TEM examination revealed a homogeneous amorphous structure with no detectable nanocrystalline precipitates.
However, some degree of partial crystallization was observed at the interface between the amorphous cladding layer and the crystalline base material. This is attributed to the slower cooling rate at the interface due to heat conduction into the base material. The crystalline interfacial layer typically has a thickness of 20–50 μm and consists of martensitic ferrite and cementite phases.
Corrosion Resistance Performance
| Coating Type | Corrosion Potential (mV vs. SCE) | Corrosion Current Density (μA/cm²) | Corrosion Rate (mm/year) |
|---|---|---|---|
| Bare Q235 steel | -720 | 45.2 | 0.18 |
| Self-shielded amorphous coating | -580 | 3.8 | 0.015 |
| Conventional crystalline hardfacing | -650 | 12.5 | 0.05 |
The amorphous coating demonstrated superior corrosion resistance compared to both the bare steel substrate and conventional crystalline hardfacing alloys. The improvement is attributed to the absence of grain boundaries, which serve as preferential sites for corrosion initiation in crystalline alloys. The amorphous structure provides a more uniform and continuous passive film that resists penetration by corrosive species.
Engineering Practice Integration
The self-shielded cladding process offers several practical advantages for industrial applications:
- Portability: The elimination of external shielding gas equipment makes the process suitable for field repair and maintenance operations in remote locations such as oil platforms, mining sites, and offshore installations.
- Environmental robustness: Unlike gas-shielded processes, the self-shielded method is less sensitive to wind, humidity, and other environmental factors that can compromise shielding gas integrity.
- Cost efficiency: Reduced equipment requirements and the ability to use standard welding power sources make the process economically attractive for large-scale applications.
- Process flexibility: The self-shielded approach can be adapted to various substrate geometries and orientations without significant process modifications.
However, several challenges must be addressed for widespread industrial adoption:
- Maintaining consistent amorphous structure over large areas requires careful control of travel speed and wire feed rate
- The thin layer requirement means multiple passes may be needed for thicker coatings, increasing cycle time
- Flux residue removal after welding adds an additional process step that must be controlled to avoid contamination
- Long-term stability of the amorphous structure under thermal exposure above the crystallization temperature (Tc) must be considered in service environments
Key Questions and Reflections
A critical question is the thermal stability of the amorphous coating during service. If the component is exposed to temperatures approaching the crystallization temperature of the amorphous phase (typically 400–500 °C for Fe-Si-B alloys), partial crystallization may occur, degrading the corrosion resistance and mechanical properties. For applications involving elevated temperatures, alternative amorphous-forming compositions with higher Tc values, such as Fe-Co-based or Fe-Ni-based alloys, should be considered.
Another important consideration is the mechanical properties of the amorphous coating. While the corrosion resistance is excellent, amorphous alloys can exhibit brittle fracture behavior under impact loading. For applications involving mechanical shock or cyclic loading, the coating thickness and residual stress state must be carefully managed to prevent spalling or delamination.
Summary
This research presents a novel approach to producing iron-based amorphous alloy coatings through a self-shielded cladding process that eliminates the need for external shielding gas. The resulting coatings demonstrate excellent corrosion resistance, significantly outperforming both the base material and conventional crystalline hardfacing alloys. The process innovation of using self-shielding flux opens new possibilities for applying advanced amorphous coatings in field environments where traditional gas-shielded processes are impractical. For engineers working in the corrosion protection industry, this study highlights the potential of metallic glass technology as a next-generation surface engineering solution, while also identifying the thermal stability and mechanical toughness challenges that must be addressed for broader industrial implementation.