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[Coating Preparation] Corrosion-Resistant Coating
Release time:
2019-12-12 16:50
Source:
Thermal spray coating technology can serve as a protective measure for machine components operating in corrosive environments. However, given the wide variety of corrosive environments in which these components may find themselves—ranging from dry atmospheres and marine atmospheres to urban atmospheres, industrial atmospheres, soil environments, freshwater environments, seawater environments, corrosive gases, and corrosive liquids—in different corrosive settings, it is essential to select the most appropriate thermal spray process and coating material to meet specific protection requirements. Among these processes, arc spraying has become the most widely used technique for preparing anti-corrosion coatings made of Al, Zn, and their alloys. In particular, arc-sprayed coatings are highly effective in preventing corrosion of iron and steel in air, freshwater, or saltwater environments. This technology has been successfully applied in various fields, including steel frame structures, bridges, transmission towers, offshore engineering projects, mine shafts, inner walls of storage tanks, ship hulls, and the “four pipes” of boilers in thermal power plants. Currently, arc spraying is one of the fastest-growing and most widely adopted thermal spray technologies in China.
1. Parameter Settings for Arc Spraying of Al and Its Alloys
The basic parameter settings for spraying Al and its alloys using the BP400 arc spraying equipment manufactured by Praxair, Inc. of the United States are shown in the table below.
Parameter Settings for Arc Spraying of BP400 Al and Its Alloys
|
Voltage (V) |
Current (A) |
Spraying efficiency (pounds/hour) |
|
|
100 |
6 |
|
28-32 |
240 |
13 |
|
|
360 |
19 |
2. Parameter Settings for Arc Spraying Zn and Its Alloys
The basic parameter settings for spraying Zn and its alloys using the BP400 arc spraying equipment manufactured by Praxair of the United States are shown in the table.
Parameter Settings for Arc Spraying of Zn and Its Alloys Using BP400
|
Voltage (V) |
Current (A) |
Spraying efficiency (pounds/hour) |
|
|
100 |
24 |
|
22-26 |
240 |
50 |
|
|
360 |
73
|
Zinc coatings applied by spraying have relatively poor acid resistance and are suitable for environments with a pH range of 6 to 12. If the workpiece is exposed to acidic conditions, an aluminum coating would offer better corrosion resistance.
When applying a Zn coating by spraying, it’s important to note that zinc fumes are toxic. Be sure to ensure adequate ventilation and pay special attention to respiratory protection.
3. Pore-sealing treatment for corrosion-resistant coatings
Since thermal-sprayed coatings invariably contain a certain degree of porosity, the morphology, size, and distribution of these pores vary depending on the thermal-spray process and the coating material used. The porosity levels can range from less than 1% to over 15%. These pores may be interconnected, and the porous characteristics extend from the coating surface all the way through to the substrate. However, when the coating thickness meets certain requirements, the pores will not penetrate through the entire coating and thereby reach the substrate. To prevent or inhibit corrosion at the coating-substrate interface and to extend the service life of the coating, it is common practice to perform pore-sealing treatments on corrosion-resistant coatings in order to fill these pores.
As a material for sealing pores in corrosion-resistant coatings, it should possess the following characteristics:
(1) Sufficient permeability;
(2) Resistance to chemicals or solvents;
(3) Certain mechanical properties;
(4) Certain temperature resistance;
(5) Does not affect the performance of the coating or substrate;
(6) Safe to use.
In actual operation, after thermal spraying, the coating should first undergo a pre-treatment using an etching solution (typically phosphoric acid) to seal the pores. This creates a composite phosphate film on the surface, which helps enhance the adhesion of the pore-sealing agent.
The experimental results indicate that applying low-viscosity sealing agents to a steel substrate can enhance the corrosion resistance of aluminum and zinc coatings. These sealing agents include ethylene resin, silicone resin, phenolic resin, modified epoxy-phenolic resin, and polyurethane, among others.
Different sealing agents should be selected according to the operating temperature of various environments. Among them, the maximum operating temperature for ethylene resins is 65℃; phenolic resins can operate at temperatures ranging from 150℃ to 260℃. Silicone resin sealing agents containing aluminum as a pigment can withstand temperatures up to 480℃ and are primarily used for sealing applications on components such as exhaust pipes and mufflers. Coal tar sealing agents containing aluminum can operate at temperatures as high as 870℃ to 980℃ and are mainly used for sealing aluminum and nickel-chromium coatings that function under these high-temperature conditions.
When used as a sealing agent, paraffin wax not only exhibits resistance to seawater, freshwater, and most acids and bases, but is also frequently employed as a sealing and lubricating coating on the surfaces of mechanical parts in the food and chemical industries, thereby addressing existing contamination issues. Commonly used sealing agents are listed in the table below.
Common sealing agents
|
Pore-sealing agent code |
Instructions |
Praxair-TAFA brand grade |
|
a |
Dedicated pore-sealing agent |
P7 |
|
b |
Etching and washing coating |
4526 |
|
CT |
Coal tar epoxy resin |
4538 |
|
d |
Transparent or ivory-colored Plasite 7155 |
|
|
f |
1-Transparent Plasite 7155 |
|
|
1- Ivory-colored Plasite 7155 |
|
|
|
g |
1-Transparent Plasite 7155 |
|
|
1-White Plasite 7133 |
|
|
|
h |
1 part VC + 1 part tributyltin oxide |
|
|
p |
Paint |
|
|
s |
Silicone alcohol resin (260℃) |
4545 |
|
yes |
Aluminum-silicon resin (538℃) |
4503 |
|
VC |
Ethylene resin polymer |
4550-4556
|
To ensure good permeability, the sealing agent should be diluted before performing pore-sealing treatment, and a vacuum impregnation method should be employed. This method leverages the vacuum environment to extract air from the coating’s pores and uses capillary action to draw the sealing agent into the evacuated pores, thereby enhancing the coating’s corrosion resistance.
4. Selection of Corrosion-Resistant Coatings
When using arc-sprayed Al and Zn coatings for corrosion protection, a simple and concise coating selection scheme is shown in the table. In the "Coating Selection" column of the table, the numbers preceding Al and Zn indicate the thickness of the respective coating, measured in mils. For example, the number 3 means the coating thickness is 3 mils, or 0.003 inches (75 μm). The letters following Al and Zn denote the pore-sealing treatment method required for that particular coating.
Brief Selection Guide for Anti-Corrosion Coatings
|
|
Expected effect |
Coating Selection |
||
|
Zinc |
Al |
|||
|
Atmospheric corrosion |
Rural atmosphere |
Lowest cost |
3Zn |
3Al |
|
Longest lifespan |
3Zn/vc |
3Al/vc |
||
|
Beautiful appearance, long lifespan |
3Zn/vc |
3Al/vc |
||
|
Industrial atmosphere |
Lowest cost |
3Zn |
3Al |
|
|
Longest lifespan |
3Zn/vc |
3Al/vc |
||
|
Beautiful appearance, long lifespan |
-- |
3Al/vc |
||
|
Coastal and salt-mist environments |
Lowest cost |
3Zn/vc |
3Al/vc |
|
|
Longest lifespan |
6Zn/vc |
6Al/vc |
||
|
Beautiful appearance, long lifespan |
6Zn/vc |
6Al/vc |
||
|
High temperature |
<260℃ |
-- |
7Al/s |
|
|
<538℃ |
-- |
7Al/sa |
||
|
>538℃ |
-- |
7Al |
||
|
Immersion corrosion |
Chemical industry |
The only choice |
-- |
6Al/f |
|
Food |
The only choice |
-- |
6Al/g |
|
|
Seawater |
Polluted |
-- |
6Al/vc |
|
|
Pollution-free |
-- |
6Al/h |
||
|
High temperature |
-538℃ |
-- |
7Al/sa |
|
|
Soft water |
Lowest cost |
10Zn |
3Al/vc |
|
|
3Zn/vc |
|
|||
|
Subject to severe corrosion |
-- |
3Al/vc |
||
|
-- |
3Al/g |
|||
|
Longest lifespan |
3Zn/vc(pH7) |
3Al/vc(pH7) |
||
|
Drinking water |
|
10Zn |
-- |
|
|
Surface control |
Algal growth |
Prevent growth |
50Cu/Al |
|
|
Bivalve attachment |
Prevent growth |
50CuNi/Al
|
||
Based on the characteristics of different environments, these environments can further be subdivided into 10 types, as shown in the table. Depending on the conditions of the working environment and the desired service life required for the first maintenance coating, the appropriate corrosion-resistant coating selection can be determined according to the table below. In the "Coating Selection" column, the numbers preceding Al and Zn indicate the thickness of the coating, measured in mils; for example, a number 3 means the coating thickness is 3 mils, or 0.003 inches (75 μm). The letters following Al and Zn specify the type of sealing treatment required for that particular coating.
Based on the table below, the appropriate anti-corrosion coating system for the relevant parts under various environmental conditions can be determined. For example, steel signal buoys operating in coastal waters often require frequent cleaning and repainting when no long-term anti-corrosion treatment is applied, resulting in substantial maintenance workload. Therefore, there is a strong desire to find a long-term anti-corrosion solution that minimizes maintenance frequency and maximizes service life. Under these conditions, the selection process for the coating system is as follows: First, since the steel buoy operates in an environment involving prolonged immersion in seawater, its corresponding environmental classification is Environment No. 9; second, among the coating options for Environment No. 9, the one with a first maintenance life exceeding 20 years is Coating Scheme No. 6Alvc; third, the coating scheme can be finalized as follows: apply an aluminum (Al) coating that is 6 mils thick (i.e., 0.006 inches or 150 μm), followed by sealing the pores with an ethylene resin polymer after spraying. This approach will achieve the desired outcome.
|
|
Category |
Specific description |
||||
|
1 |
Outdoor exposed surface |
Non-corrosive inland |
Rain-soaked surface |
Surface subjected to mild corrosion by acids, bases, salts, and SO2. |
||
|
2 |
Corroding inland |
Surface corroded by SO2 or other gases |
||||
|
3 |
Corrosion-free seaside |
Surface is the same as 1, with salt but no salt spray. |
||||
|
4 |
Corrosion by the seashore |
Surface is the same as No. 2—contains salt but no salt spray. |
||||
|
5 |
Outdoor shaded surface |
Same as above—exposed to rain, poor ventilation, and condensation occurring. |
||||
|
6 |
Indoor |
Often dry |
Inside the building |
Partial condensation |
||
|
7 |
Frequently moist |
Most condensation |
||||
|
8 |
Contact with freshwater |
Salt-free water |
Drinking water or non-drinking water |
|||
|
9 |
Seawater |
Soaking area |
Seawater or other saltwater |
Long-term soaking |
||
|
10 |
Splash zone |
Floating or tide-blocking structures exposed to sea breeze and seawater are often subject to salt spray corrosion.
|
||||
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Core spraying equipment: SX-80 plasma spray main unit, with a 15,000℃ flame suitable for multiple materials including zirconia and silicon carbide; powder feeding accuracy of ±1%, and coating thickness tolerance ≤ ±5 μm. Environmental protection support system: Customized soundproof booth + pulse dust collector—effectively addressing spray noise pollution while achieving highly efficient dust recovery, compliant with Thai environmental standards. Automated equipment: 6-axis industrial robot + intelligent spray cart, enabling precise spraying of complex workpieces and automated loading/unloading, boosting production efficiency by 40%.
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Guangzhou Sanxin Thermal Spraying Turntable Shipment
Guangzhou Sanxin Company manufactures single-station and multi-station thermal spray turntables, which can be paired with plasma equipment, supersonic equipment, and arc equipment. We offer rapid, customized production tailored to customer requirements and product dimensions. A thermal spray turntable is a high-precision rotary worktable specifically designed for thermal spraying processes—including flame spraying, plasma spraying, and arc spraying. By providing controllable rotational speed and exceptional stability, it ensures uniform coating thickness on the workpiece surface, thereby enhancing spraying efficiency and quality.
SX-80 plasma equipment is being used at a customer site in Russia.
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