Introduction: Thermal Sprayed Metal Coatings—The Ultimate Corrosion Protection Solution to Make Steel Structures Last Over Half a Century
Thermal spraying (TSZ zinc spraying/TSA aluminum spraying) is the process of melting metal wire (zinc/aluminum/zinc-aluminum alloy) via arc high temperature (>4000°C) → atomizing with compressed air → high-speed spraying onto steel substrate to form a dense metal coating. TSZ/TSA is not only a mechanical barrier layer—but also a sacrificial anode and passivation layer. The corrosion products of the zinc/aluminum coating (ZnO/Zn(OH)₂/Al₂O₃) densely fill the pores—forming a self-healing barrier. In CX marine environments, the composite system of TSZ/TSA + organic sealing coating has a service record of >50 years (e.g., Forth Road Bridge in the UK / zinc sprayed in 1964 / first local maintenance in 2008).
I. Technical Comparison between TSZ and TSA
| Parameter | TSZ (Zinc Spraying / Zn99.99) | TSA (Aluminum Spraying / Al99.5) |
|---|---|---|
| Coating Thickness (μm) | 80-150 | 100-200 |
| Melting Point (°C) | 420 | 660 |
| Sacrificial Anode Potential (V vs Cu/CuSO₄) | -1.05 (Active) | -0.85 (Weaker) |
| Density of Corrosion Products | Medium (Porous ZnO) | High (Dense Al₂O₃ / Self-sealing) |
| Suitable Environment | C3-C5 (Atmospheric / Immersed) | CX (Extreme Marine / Recommended) |
| Reference Cost (RMB/m²) | 150-300 | 200-400 |
II. Key Parameters of Thermal Spray Process
| Parameter | Range | Effect on Coating Quality |
|---|---|---|
| Arc Voltage (V) | 18-32 (zinc) / 24-36 (aluminum) | Too high → excessive metal oxidation / Too low → insufficient melting |
| Atomization Pressure (MPa) | 0.4-0.7 | Too low → coarse atomization / more porosity / Too high → rapid cooling / reduced adhesion |
| Spray Distance (mm) | 100-200 | Too close → substrate overheating / deformation / Too far → excessive cooling / increased porosity |
| Substrate Pretreatment | Sa3 (white metal blast cleaning / Rz 50-100μm) | Substrate roughness is the basis for mechanical anchoring of the coating |
Technical deepening: systematic optimization methods for process parameters (DOE experimental design)
The optimization of coating production processes should not rely on the “trial-and-error method” but should adopt the scientific method of DOE experimental design. Taking the dispersion process as an example—factors affecting quality (linear velocity/time/filling rate/temperature), 4 factors each at 3 levels—full factorial requires 81 experiments—DOE uses orthogonal experiment L9 (9 times) or response surface methodology (27 times) to greatly reduce the number of experiments—while obtaining the main effects and interactions of each factor. For example, it is found that “the interaction of linear velocity × time is significant”—high linear velocity + short time and low linear velocity + long time can achieve the same dispersion effect—but the former saves over 20% energy.
In DOE analysis, interpretation of the P-value — P95% confidence). The final output of DOE is a set of prediction models (polynomial regression equations) — input line speed/time/temperature → predict fineness/viscosity/gloss — providing formulation engineers with a ”digital formulation optimization” tool.
Industry practice: from “master craftsman’s feel” to “parameter standardization”
The common challenge in the coatings industry — when experienced veteran workers retire, their “feel” (mixing resistance / fineness gauge scraping / visual inspection of wet film gloss) is taken away — new employees cannot replicate it. Transform the “feel” into quantifiable standard parameters (1) mixing resistance → viscometer reading; (2) fineness gauge scraping → fineness gauge reading (μm); (3) wet film gloss → gloss meter (GU value). The “standard parameter card” for each process is posted next to the equipment — new employees operate according to the “card” rather than “by feel”. “Parameter standardization” is a key step for coating factories to move from “workshop” to “factory”.
FAQ
Q1: Why is TSA recommended over TSZ in CX environments despite being more expensive? Aluminum’s Al₂O₃ oxide layer is extremely dense (nanoscale/self-sealing) — in the extreme CX marine environment, the self-healing and sealing effect of aluminum coatings far outperforms zinc coatings — although TSA’s initial cost is 30-50% higher, its maintenance-free benefit of >50 years far exceeds TSZ (which may require local touch-up after 20-30 years) — in the life cycle cost analysis (LCCA) for CX environments, TSA wins.
Q2: Why must thermal spray substrates be Sa3 (white metal blast cleaning) rather than Sa2.5?Thermal spraying is pure mechanical anchoring (molten metal particles impact the substrate → flatten → mechanically lock into the rough peaks and valleys) — no chemical bonding — adhesion depends entirely on substrate roughness. Sa3 (>99% of surface free of any contamination + ultra-high roughness Rz 50-100μm) is the mandatory minimum standard for thermal spraying — the insufficient adhesion of Sa2.5 may cause the coating to delaminate in whole sheets.
Q3: What is the role of organic sealer coating in the TSZ/TSA system?Thermally sprayed metal coatings themselves have a certain porosity (3%-8%)—seawater can penetrate through the pores. The organic sealer coating (epoxy/polyurethane/low-viscosity penetrative type) penetrates into the pores of the metal coating → (1) fills the pores to block the seawater pathway; (2) provides additional barrier protection. The composite system of TSZ/TSA + sealer coating has a much longer anti-corrosion service life than TSZ/TSA alone.
Q4: Competition and complementarity between thermal spraying and hot-dip galvanizing (HDG)?Thermal spraying——immersing steel structures into molten zinc (450°C)/thickness 50-100μm——suitable for factory-prefabricated small parts. Thermal spraying——can be applied on-site (large structures/bridges/towers)+controllable thickness+repairable. HDG’s factory conditions are superior to TSZ’s on-site conditions——but TSZ’s thickness (>100μm) and surface roughness (mechanical anchoring) are incomparable to HDG.
Q5: Safety and health protection for thermal spray workers?Arc temperature >4000°C generates intense UV radiation (arc eye/skin burns) + metal fumes (zinc fume/aluminum dust/respiratory hazards) + high-decibel noise (>110dB)Workers must wear full protective radiation suits + welding masks + supplied-air respirators + earplugs — thermal spraying is one of the highest safety-risk trades in the coatings industry.
Q6: Can thermal spray coatings be inspected non-destructively on site?(1)Coating thickness—magnetic induction/eddy current thickness gauge + steel substrate reference—measure 5 points and take the average;(2)Adhesion—ISO 4624 pull-off method (≥5MPa/thermal spray coating);(3)Porosity—potassium ferricyanide (K₃[Fe(CN)₆]) spot test—coating shows blue spots with potassium ferricyanide solution—blue spot density <5 points/cm² is acceptable (NOB).
Q7: Can thermal spray coatings be repaired?Yes—mechanical damage (scratches/impacts during lifting/transport)—use an angle grinder to polish the damaged area → clean → manual arc spraying (handheld spray gun) for local repair—the overlap between the repaired area and the surrounding intact coating should be >50mm. The quality of the repaired coating is slightly lower than the original coating (site conditions are not as good as the workshop)—but >80% of the original coating performance.
Q8: What is the environmental impact of TSZ and TSA coatings?The zinc/aluminum metal dust generated during the thermal spraying process is a source of PM10/PM2.5—barriers and dust collection devices must be installed in the spraying area—the collected metal dust is treated as general industrial solid waste (non-hazardous waste / recyclable for smelting). The corrosion products (Zn/Al oxides) of TSZ/TSA coatings in service are slowly released into the environment under rainwater washing—there is a lack of research reports on the long-term impact on aquatic organisms.
Q9: Growth in the application of thermal spraying in the offshore wind power sector?Offshore wind turbine towers and foundations—designed for a service life of 25-30 years and unable to be maintained at sea—the thermal spray aluminum (TSA) (>150μm) + epoxy sealer coating one-time >30-year maintenance-free solution is the “gold standard” for offshore wind corrosion protection. Global offshore wind installations (>50GW/2025) growth drives sustained increase in TSA demand.
Q10: Future Trend — Cold Spray Technology?Cold spray — metal powder is propelled by supersonic gas flow (>1000 m/s) at 4000°C) to impact the substrate → particles undergo plastic deformation and consolidation — coating porosity is extremely low (70 MPa) — it is an upgraded technology of thermal spray. Cold spray currently has high equipment cost + slow spraying speed — it cannot yet replace TSA in large-scale anti-corrosion applications for steel structures — but it has already been applied in aviation/military high-value components.
FAQ: In-Depth Technical Q&A Supplement
Q11: How do the differences in domestic and international standards for this technology affect product export?Domestic standards (GB) differ from ISO/ASTM standards in test methods and acceptance criteria. For example, salt spray testing—GB/T 1771 (equivalent to ISO 7253) has test conditions basically consistent with ASTM B117—but the rating systems (ISO 4628 vs ASTM D610/D714) differ—when providing test reports for exported products, you must simultaneously indicate the corresponding international standards, otherwise overseas customers cannot make a comparative assessment. It is recommended to list both GB and ISO/ASTM dual-standard indicators in the TDS (Technical Data Sheet) of exported products—to enhance the trust of international customers.
Q12: How to verify the long-term service performance of this technology in actual engineering?Laboratory accelerated testing (salt spray/QUV/cyclic corrosion) provides comparative data—but cannot fully replace actual outdoor exposure testing. Recommendations—(1) Set up outdoor exposure racks at both the factory location and typical customer locations (e.g., coastal C5-M/industrial C4)—conduct annual inspections of coating appearance/adhesion/film thickness changes—establish a company-owned outdoor service database; (2) Collaborate with universities/research institutes—combine enterprise data with academic research—enhance data credibility.
Q13: What should SMEs pay attention to when purchasing related raw materials/equipment?(1) The batch stability of suppliers is more important than unit price—it is recommended to require suppliers to provide COA data for >10 batches—and evaluate batch variation (CpK); (2) For equipment procurement, visit peers who have used the equipment for >2 years to understand the long-term reliability and after-sales service quality of the equipment—rather than relying only on the demonstration data from the equipment supplier; (3) For key raw materials (resin/curing agent)—maintain at least 2 qualified suppliers to guard against single-supply risk.
Q14: What is the current state and trend of digital transformation in this field?The digital transformation of the coatings industry is evolving from “point-based applications” (automation of individual equipment/processes) to ”system integration” (full-chain ERP+MES+PMS). Currently, the digitalization of small and medium-sized coatings factories has the ”highest ROI investment” in automatic batching systems + digitalization of quality control data—with a payback period of 1-3 years—which is the prioritized recommended direction. Future trend—AI + sensors enabling real-time optimization of process parameters—further reducing quality fluctuations between batches.
Q15: How can a newly entered coating engineer quickly master this technology?(1)Combine theory and practiceDo not only read literature without touching actual production—nor rely solely on experience without studying theory;(2)Establish a“failure case archive”Every customer complaint/production anomaly/coating failure—record the root cause and resolution process—this is the most effective learning material;(3)Learn from suppliersTechnical personnel from resin/additive/pigment suppliers are carriers of ”tacit knowledge” in this field—communicate more with them about solutions to specific problems.
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Summary
Thermal-sprayed TSZ (80–150 μm) and TSA (100–200 μm) are the ultimate corrosion protection solution for CX marine environments with >50 years of maintenance-free service. Sa3-grade surface preparation plus an organic sealer coat is the essential configuration to realize the full corrosion protection potential of TSA. Offshore wind power is the fastest-growing application field for TSA. Kexin New Materials provides customers with matching organic sealer coatings for thermal-sprayed TSZ/TSA and technical support for corrosion protection scheme design.