Research and application of electrostatic spray anti-corrosion treatment

The development history of electrostatic spraying is introduced, the advantages of electrostatic spraying are analyzed, especially the anti-corrosion treatment of transformer housing, and the experimental comparison of electrostatic spraying treatment with other coating treatment methods is carried out, aiming to explore the application value and practical significance of
electrostatic spraying anti-corrosion treatment.

0 Preface

The rapid development of the factory has led to the deterioration of environmental pollution, especially in the chemical, fertilizer, smelting, petroleum and other environments, and the corrosive process contains a lot of chemical gases
such as sulfur dioxide, nitrogen dioxide, coal gas and ammonia.
Steel products are highly corrosive, which obviously significantly shortens their service life, causing economic losses and even serious accidents coatingol.
com
coatings online.
For example, sulfuric acid around phosphorus plants, SO2 gas concentration can reach 5~10mg/m3, but general electrical equipment should not exceed 0.
3mg/m3, electrochemical corrosion is faster, especially in the south with
higher humidity.
Joint research, the distribution transformer used in the southern chemical industry is completely separated from the paint film, and after one or two years of normal use, there will be a comprehensive paint removal, rust, scale comprehensive peeling, resulting in many transformers often for these reasons, not until the end of the service life to be forced to be replaced, and the transformer shell can operate normally in a corrosive environment for a long time, the anti-corrosion effect is good, and frequent maintenance
is not required.
However, the product is currently used
more in household and chemical production.
In addition to some imported products, many factories use general-purpose distribution transformers, and the potential demand is huge
.
Therefore, this study mainly studies and discusses the anti-corrosion treatment of electrostatic spraying, so as to provide a reference
for clarifying its application value.

1 Overview of electrostatic spraying

Electrostatic spraying, commonly known as "electrostatic powder spraying", since the 80s of the 20th century, electrostatic spraying technology has been the most common technology
for decorating metal surfaces.
Compared with conventional painting treatment, it has the advantages
of bright color, advanced technology, resource saving, safety and practicality.
Therefore, it is often used for simple work and upholstery
.
It works by filling high-voltage electrostatic equipment
with plastic powder.
The plastic powder is sprayed on the surface of the workpiece by electric field, and the powder is evenly sucked into the surface of the workpiece to form a plastic coating; After high-temperature baking and polishing, the plastic particles melt into a dense protective layer, which is tightly glued to the surface of the workpiece and has an anti-corrosion effect, which does not require thinner or primer; Its structure is harmless to the human body and the environment, the coating quality is good, corrosion resistance, aging resistance, and high
mechanical strength.
Compared with traditional coating techniques, the application drying time is short, the coating corrosion resistance is strong, there is no leakage, the construction is simple, the technical requirements are low, and the cost is also low
.
Therefore, it is widely
used.

2 Advantages of electrostatic spraying anticorrosion

Electrostatic coatings in electrostatic spray plastic powders are changing traditional decoration methods, such as hot-dip galvanizing and electrogalvanizing
.
Because plastic powder is made of
synthetic resin.
It is not only light and thin, but also has the characteristics of insulation, and the finished product has a beautiful
appearance after spraying.
However, the workpiece cannot be cooled quickly after baking, and must be cooled slowly so that there is not much stress
.
In addition, a corresponding increase in thickness can significantly reduce the pressure
generated during cooling.
As the thickness of the overlay increases, the heat transfer system inevitably decreases
.
Therefore, the heating from the outer surface to the inner surface takes longer, and the isomerization of the outer surface can be severe, so the baking time will be longer
.
Composite coatings mask paint or varnishes that dry after spraying, curing and cooling on the surface of the workpiece, which not only eliminates defects but also improves overlapping boundaries
.
However, the powder coating on the surface cannot be sprayed on the solvent, because the electrostatic spraying machine has electrical conductivity, while the solvent-based coating will affect the quality of the electrostatic mold because it is non-conductive and has strong stability; If the order is reversed, the sprayed topcoat can easily peel off and blister, as the coating leaves a small amount of solvent, even
when drying.

During the heat curing process of powder coating, the solvent remains under the coating, the resistance evaporates outside the boundary, and the bubbles in the coating are decreasing
.
As for anti-corrosion treatment, although the paint layer has strong anti-corrosion properties, metal parts often need to be strictly pre-treated on the surface, and rust or oil stains
that affect the adhesion of the coating must not appear.
The transformer oil tank is in a vacuum drying state and is running, which will inevitably produce oil pollution
.
If the pretreatment and cleaning are not clean, the adhesion of the coating will weaken and the ink will peel off
.
Hot-dip galvanizing is the immersion of prefabricated ferrous metal parts in a molten galvan pot for a short time
.
Although it has anti-corrosion properties, its biggest difference is the uniformity of thickness, and the crystal density is not as good as hot-dip galvanized film
.
Widely used in non-ferrous metal galvanized crystal anti-corrosion, good compactness, surface paint on the cell membrane anti-corrosion effect
.
The disadvantage is that the coating is less thick and rusts faster when exposed to rain than in other locations
.

3 Characteristics of common surface treatment processes for exposed steel parts of transformer housing

Spraying anti-corrosion paint on the tank shell is the most basic measure, generally the transformer shell is only coated with a primer and a top coat, and the anti-medium corrosion type transformer should be made two bottom two sides, the anti-corrosion type can be made two bottom four sides, or even two bottom six sides
.
In order to ensure the adhesion of the paint, the shell is rusted and destained, and the surface is phosphated to form a layer of porous iron phosphate on the surface, which can make the primer enter the pores and improve the anti-corrosion ability
.

There are many types of anti-corrosion coatings, commonly used are: anti-corrosion polyurethane coatings, which have excellent performance
in water resistance, oil resistance, acid and alkali resistance, wear resistance and other aspects.
Among them, silver-gray polyurethane coatings have better sunlight resistance and aging resistance than other coatings, and are usually used in outdoor photocorrosive areas
.

Fluorosulfonated polyethylene is light and corrosion resistant, has excellent corrosion resistance to acid, alkali and salt, as well as excellent physical and mechanical properties, good paint film resistance, wear resistance, oil resistance, heat resistance, cold resistance, ultraviolet resistance, aging resistance, dense paint film, strong adhesion, slightly better water permeability, is the first choice for anti-corrosion coatings, and has achieved good results
in the application of transformer external transformer anti-dielectric corrosion.

R type electrostatic powder coating, epoxy resin can be automatic assembly line operation, high production efficiency, good quality, its coating has excellent heat resistance, moisture resistance, oil resistance, corrosion resistance, paint film adhesion is strong, but the resistance to sunlight absorption is lower than epoxy resin, so it is mostly used in domestic products, due to the limited volume of electrostatic spraying equipment, only small transformers
can be used in batches.

Special anti-corrosion coating G-615 not only has excellent physical and mechanical properties, such as flexibility, disassembly impact resistance, durability, strong adhesion, but also resistant to sulfuric acid, nitric acid, such as 30% sulfuric acid, nitric acid, hydrochloric acid, or 40% sodium hydroxide solution, or immersed in saturated salt 560 water does not change, the coating still maintains the original color, but also has waterproof, oil-resistant, high temperature, cold resistance and other effects, can be used in the range of 110-30 °C, is the best coating
in anti-corrosion transformers.

3.
1 Introduction of plastic powder

Plastic powder is composed of organic polymer materials, which are the main components
of synthetic resins.
Coatings offer many
advantages.
In addition to resins, plastic powder coatings also contain additives such as preservatives, curing agents, and pigments to improve the effect of
powder coatings.
Powder coatings can be divided into two categories
: heat-resistant and heat-resistant.
Synthetic thermoplastic resins are the main materials for high melting point films and are usually suitable for thicker protective and anti-corrosion layers; The latter uses different heat-resistant synthetic resins as film materials [6].

Under the action of the curing agent, the crosslinking reaction occurs at temperature, and the curing agent is treated into a solid structure so that the final product does not melt or melt.

3.
2 Powder spraying method and film forming treatment

Powder coatings are different from solvent coatings in that there are two problems: first, how the powder is evenly dispersed and adhered to the surface of the workpiece; The other is how to form a film
.
To prevent the peeling of the dust layer adhering to the surface of the workpiece, electrostatic adsorption or melt must
be used.
There are many coating methods, such as cured bed method, electrostatic spraying method, hot melt spray powder electrophoresis method, etc
.
Electrostatic spraying is a commonly used method
.
It is characterized by cold coating, generally with a layer
of 60μm~100μm thickness at the same time.
The coating is uniform, without overhang, with sharp edges and rough surface, which forms a continuous smooth coating
.
If the metal part is made of thermosetting powder, after spraying, after a certain hardening temperature and withstand time, the fine dust particles will chemically react with the resin and hardener, forming a layer
by sintering, melting and leveling.

3.
3 Electrostatic spraying pretreatment

Before spraying powder, it is very important
to process the surface of the workpiece.
Pretreatment plays an important role
in improving coverage intensity, achieving the same coverage performance and extending coverage usage time.
The surface of the workpiece is often contaminated with oil, oxide film and other substances that affect the gold-plated clutch, resulting in defects
such as coating covering, holes, etc.
The commonly used pretreatment process is mechanical treatment, for example, shot peening, sandblasting, etc.
on the surface of the workpiece; Chemical treatments such as degreasing, leaching and phosphorylation
of working surfaces.
The phosphorylated film formed by phosphorylation is usually a chemical conversion film with a thickness of 2~3m, which is an intermediate transition layer and a coupling agent
for reinforcing sprays or other coatings.

4 Electrostatic spray anti-corrosion coating experiment

Take 6 300mm×200mm×1mm steel samples, first phosphating, followed by electrostatic nozzles of various thicknesses, and then salt spray test
.
The results show that in the neutral salt spray test, the electrogalvanized coating basically retains the "large surface black corrosion" in the sixth cycle, and the electrostatic coating remains unchanged
.
Check the coupling and corrosion protection in electrostatic spraying, compare with some common plating (coating) layer surface treatment coatings: the corrosion resistance of electrostatic spraying is the best
among some commonly used coated (coated) coatings.
The pretreatment adopts phosphating or electrogalvanized coating, and the spray coating adhesion can reach up to class
0.

5 Application of electrostatic spray anti-corrosion coating

1) Previously, the shell of the electromagnetic unit tank of the transformer housing underwent an overheated zinc test, but it was closed
for two reasons.

(1) The sealing position of the fuel tank directly affects the closing effect
.
Zinc deposits are difficult to recover or remove, and similarly, the bottom and lid of the capacitor separator have closed grooves, which can cause zinc deposits that are difficult to remove
.
The components of the condenser separator cannot be hot-dip galvanized
.

(2) The hot-dip galvanized tank is usually rectangular, 8m long and 1m
wide.
The sink with a diameter of 0.
60~0.
74m is not easy to rotate
during the hot-dip galvanizing process of the water tank.
In addition, there are many substances
suspended on the surface of the zinc liquid in the tank.
Once the groove is outside the groove, the inner and outer surface areas are seriously zinc, especially concave parts such as rounded corners, which are not easy to remove
.

2) Based on the above factors, the exposed and constantly changing variable steel parts adopt the following surface treatment process
.

(1) When hot-dip galvanizing is not possible, the bottom cover and condenser are directly electrostatically sprayed after galvanizing (coating thickness ≥ 15μm), but pay attention to protecting the installation hole and the lower galvanized layer to ensure the collateral effect
of the installation hole.

(2) After the tank body is pre-phosphated, it is thermocured by electrostatic spraying, and the thickness is 70m~100m after ten years of use, and no corrosion or rust
of the coating is observed.
Capacitive voltage transformers have evolved over 40 years, however, transformer housings have gone through many stages
of development in terms of method, output performance, accuracy and structure.
At the same time, the anti-corrosion treatment of steel parts on the surface of transformers has also undergone several stages of development such
as painting, galvanizing, and hot-dip galvanizing.

(3) The capacitor voltage divider adopts a special model of primary terminal block and upper cover plate, and the terminal block adopts hot-dip galvanized
.
At these stages, plastic corrosion with transformer housings is more convenient and reliable
.

6 Workflow of electrostatic spray anti-corrosion coating

Electrostatic spraying technologies include prefabrication, electrostatic plates, and high-temperature cooling
.
One of the important forms that does not affect the coating is corrosion, and electrostatic spraying technology can prevent and reduce damage to the machine phosphorylation is an important form of
corrosion protection.

6.
1 Preprocessing

It is important to
prepare the surface of the workpiece before spraying.
Pretreatment plays an important role
in improving coverage intensity, achieving the same coverage performance and extending coverage usage time.
The surface of the workpiece is often contaminated with oil, oxide film and other substances that affect the gold-plated clutch, causing defects
such as plating, peeling, and holes.
Commonly used pretreatment methods include mechanical treatment
such as sand blasting and shot blasting.
on the surface of the workpiece; Chemical treatments such as degreasing, acid refining, phosphorylation of the workpiece surface, etc.
, usually lubricants, oxide films, etc
.
On the surface of the artifact, and on the surface of the plastic powder is produced coatings, peels, debuggers, branching and rust removers such as porous pores
.
The atomization and phosphorylation of the workpiece is an important link
.
The goal is to remove grease, dust and rust from the surface of the workpiece, forming a layer of phosphate, preventing corrosion and increasing the clutch.

The phosphorylation film formed during phosphorylation is 2~3m thick, which can increase the viscosity
of one layer or another.
For uniform and long-lasting use, the workpiece surface needs to be pretreated
.
Commonly used pre-treatments include mechanical treatment of the workpiece surface, such as molding, sandblasting and chemical treatment such as degreasing, acidification, phosphorylation, etc
.
The phosphorylated film formed by phosphorylation is usually a chemical conversion film with a thickness of 2~3m, which is an intermediate transition layer and a coupling agent
for reinforcing sprays or other coatings.
Main conversion steps: degreasing, rust removal, phosphorylation, passivation
.
Typical pre-treatments include immersion and spray types
.
The immersion type requires multiple soaking tanks; Spraying requires a spray line to be installed on the spraying line
.

6.
2 Electrostatic spraying

The powder coating is evenly sprayed on the surface of the workpiece, and the high-voltage electrostatic field is used to make the negative current paint particles move in the opposite direction along the electric field, and the surface of the workpiece is evenly covered with plastic powder
with electrostatic adsorption.
Electrostatic spraying equipment is spray gun, spray cup and electrostatic spraying power supply
.
The fallen powder can be recycled, reused after screening, and the coating particles are adsorbed on the surface of
the workpiece by spraying technology.

6.
3 High temperature curing

The powder coating is heated to a certain temperature and held at that temperature until it melts, becomes flat and smooth, and achieves the desired effect
for the job.
The working method is to put the partially completed electrostatic spraying workpiece into the oven in a dry state, heat to the preheating temperature, generally 185°, and maintain it for 15min accordingly; Turn on the oven to cool the finished product [11].

Heating and control systems include electricity, natural gas, coal, etc
.

6.
4 Cooling

Cooling is the process of cooling the results of the spraying process
back to room temperature.

6.
5 Decoration

Decorative treatment is a direct treatment of the workpiece after electrostatic spraying to achieve a variety of wood beads, patterns, gloss and other special appearance effects
.

7 Conclusion

Electrostatic spraying process technology has important advantages over traditional painting technology: the use of electrostatic spraying, which compresses clean air into a diffusion interface without dilution, is good for
the environment and people 。 Electrostatic spraying can also be reused, the proportion of used plastic powder is very high, so the total amount of electrostatic plastic powder used is 98%, much better than solvents: appearance quality spraying mechanical equipment performance is excellent, plastic coating and mechanical adhesion plastic coating strength increases, becomes more stable, acid alkali resistance and mechanical properties are excellent, metal surface adhesion performance is better, and has "three prevention" (anti-mildew, anti-humid heat, anti-salt spray) and good electrical insulation properties; The hardening time of plastic layers and coatings is significantly shortened, and the corrosion of wear-resistant plastic floors is significantly improved
.
The workpiece does not need a liner, and there is no leakage during spraying; Easy to operate, the process is simplified to apply, the use of plastic powder can reduce the amount of paint, the required coating requirements can be achieved by spray; Low
technical requirements for the operator.
Compared to traditional paint techniques, the cost is much lower and the quality is much
higher.
It has been accepted by more and more users, and many enterprises often use this technology in various metal industries, thus obtaining great economic and social benefits, and has been recognized and valued
by customers.