# Applications of Thermal Spray Corrosion-Resistant Coatings

As a core technique in the field of surface engineering, thermal spraying technology involves rapidly projecting molten or semi-molten metal, ceramic, and other materials onto a substrate surface to form coatings with specific functional properties. Among these, corrosion-resistant coatings have become the "protective armor" in industries such as energy, transportation, and marine engineering, thanks to their ability to effectively isolate corrosive media and significantly extend the service life of equipment. This article will systematically explore the practical value of thermally sprayed corrosion-resistant coatings from four perspectives: technical principles, material selection, typical applications, and emerging trends.

### I. Technical Principle: Building Multi-Layered Protective Barriers

The mechanism behind the corrosion-resistant coating in thermal spraying involves both physical isolation and electrochemical protection pathways.

In terms of physical isolation, the coating effectively blocks direct contact between corrosive media—such as water, chloride ions, and acidic gases—and the substrate, thanks to its dense structure. For instance, plasma-sprayed alumina coatings can create a continuous, pore-free ceramic barrier, with porosity controlled below 1%, significantly reducing the corrosion rate.

In terms of electrochemical protection, certain coating materials—such as zinc and aluminum—have electrode potentials lower than those of the base metal. As a result, they preferentially dissolve in corrosive environments, acting as sacrificial anodes to provide protection. For instance, when arc-sprayed zinc-aluminum alloy coatings are applied, their potential is approximately 0.3V lower than that of the steel substrate, enabling them to oxidize first in seawater environments and thereby safeguarding the underlying metal from corrosion.

Additionally, the bonding strength between the coating and the substrate directly affects the protective performance. The High-Velocity Oxygen-Fuel (HVOF) spraying technique uses a high-temperature, high-speed flame to achieve metallurgical bonding between the coating and the substrate, resulting in a bonding strength of over 60 MPa—far surpassing the mechanical bonding levels achieved by conventional spraying processes.

### II. Material Systems: Diversified Choices to Suit Complex Operating Conditions

The selection of materials for thermally sprayed corrosion-resistant coatings requires a comprehensive consideration of operating conditions, cost-effectiveness, and process compatibility, and is primarily categorized into three major types: metals, ceramics, and composite materials.

**Metal Coatings** primarily consist of zinc, aluminum, and their alloys. Zinc coatings exhibit excellent performance in freshwater and atmospheric environments, with their corrosion products—such as basic zinc carbonate—possessing self-sealing properties. Meanwhile, aluminum coatings demonstrate superior corrosion resistance in marine environments, thanks to the protective alumina film that forms on their surface, effectively preventing chloride ion penetration. Zinc-aluminum alloy coatings (e.g., Zn-15Al) combine zinc’s sacrificial anodic behavior with aluminum’s passive film characteristics, enabling a corrosion-resistant lifespan in salt-spray tests up to 6 times longer than that of pure zinc coatings.

**Ceramic Coatings** are typically represented by oxides (Al₂O₃, Cr₂O₃) and carbides (WC-Co). Plasma-sprayed yttria-stabilized zirconia (YSZ) coatings retain their structural stability even at high temperatures of 1100°C, with an oxygen diffusion coefficient that is reduced by more than 90% compared to the untreated substrate. These coatings are widely used in the hot-end components of gas turbines.

**Composite Coatings** achieve complementary performance through material compounding. For instance, the NiCr-Cr₃C₂ metal-ceramic coating offers both excellent wear resistance and corrosion resistance. After HVOF spraying, its hardness can reach over HRC60, and in an 850°C high-temperature HCl gas environment, its annual wear rate is as low as just 4 μm—making it an ideal solution for corrosion protection of the "four tubes" in power plant boilers.

### III. Typical Applications: Addressing Industry Pain Points

1. **Energy Industry**: Thermal power generation boilers' "four tubes" (water-cooled walls, superheaters, reheaters, and economizers) have long faced challenges from high-temperature corrosion and wear. After a certain ultra-high-pressure boiler was coated with an HVOF-sprayed NiCr-Cr₃C₂ layer, the annual wear rate of the water-cooled wall tubes dropped dramatically—from 1.5–2.0 mm to just 0.037 mm. Moreover, under corrosive gas conditions at 850°C containing 4,000 ppm HCl and 500 ppm SO₂, the annual wear rate was further reduced to only 0.004 mm, extending the service life by more than five times.

2. **Marine Engineering**: Offshore platform pile legs and ship hulls are subjected to long-term seawater corrosion. By applying an arc-sprayed zinc-aluminum-magnesium alloy coating, a MgAl₂O₄ passivation film is formed, significantly enhancing corrosion resistance—up to 3 times better than traditional zinc coatings—in the high-salinity environment of the South China Sea. As a result, the maintenance cycle has been extended from 3 years to 10 years.

3. **Petrochemical Industry**: Oil production rods are prone to sulfide stress corrosion cracking in oil wells containing H₂S and CO₂. After applying a supersonic arc-sprayed nickel-based alloy coating, the corrosion rate was reduced from 0.5 mm/year to 0.02 mm/year, resulting in an annual maintenance cost reduction of 500,000 yuan per well.

4. **Aerospace**: Aircraft engine turbine blades must withstand temperatures as high as 1,200°C