Research and application of electrostatic powder spraying anti-corrosion treatment

This paper introduces the development history of electrostatic spraying, analyzes the advantages of electrostatic spraying, especially briefly introduces the anti-corrosion treatment of transformer housing, and experiments compare electrostatic spraying treatment with other coating treatments, in order to explore the application value and practical significance
of electrostatic spraying anti-corrosion treatment.

Author | Zhu Tongkun

Shandong Xinya Industry Co.
, Ltd

1 Preface

The rapid development of the plant 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, gas and ammonia.
Steel products are highly corrosive, which obviously shortens their service life, causing economic losses and even serious accident coatingol.
com
s.

For example, sulfuric acid around the phosphorus plant, 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 high humidity.

Joint research, the distribution transformer used in the southern chemical industry and the paint film are completely separated, after one or two years of normal use, there will be comprehensive paint removal, rust, scale comprehensive peeling, resulting in many transformers often because of these reasons, less than the end of the service life to be forced to replace, 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 universal 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 reference
for clarifying its application value.

2 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 colors, 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 through the 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, tightly glued to the surface of the workpiece, with anti-corrosion effect, this process 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 the traditional coating technology, the construction 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
.

3 Advantages of electrostatic spray anti-corrosion

Electrostatic coating in electrostatic spray plastic powder is changing traditional decoration methods, such as hot-dip galvanizing, electrogalvanizing
.

Because plastic powder is made
of synthetic resin.
It is not only light and thin, but also has insulation characteristics, and the finished product looks beautiful
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, 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 varnish that has been sprayed, cured and cooled to 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 conductivity, while the solvent-based coating will affect the quality of the electrostatic mold due to its non-conductivity and strong stability; If the order is reversed, the sprayed topcoat can easily peel off and blister because the coating leaves behind a small amount of solvent, even when dry
.

The powder coating heats the curing process, the solvent remains under the coating, the resistance evaporates outside the boundary, and the bubbles in the coating are falling
.
As for the 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 no rust or oil stains
that affect the adhesion of the coating must appear.

The transformer tank is in a vacuum-dry state and is running, which inevitably produces oil contamination
.
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 zinc pot for a short period of 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 anticorrosion, good compactness, surface paint on cell membrane anti-corrosion effect
.

The disadvantage is that the thickness of the coating is smaller, and it will rust
faster than other locations when it encounters rain.

4 Characteristics of common surface treatment processes for exposed steel parts outside the transformer shell

Spraying anti-corrosion paint on the tank shell is the most basic measure, the general transformer shell is only coated with a primer and a topcoat, and the anti-medium corrosion transformer should be two bottom two sides, the anti-corrosion type can be done two bottom four sides, or even two bottom six sides
.

In order to ensure the adhesion of the paint, the shell is derusted and decontaminated, 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 are better than other coatings with sunlight and aging resistance, and are usually used in outdoor photocorrosive areas
.

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

R type electrostatic powder coating, epoxy resin can be automatically assembly line operation, high production efficiency, good quality, its coating has excellent heat resistance, moisture resistance, oil resistance, corrosion resistance function, paint film adhesion is strong, but the resistance is small The absorption of sunlight 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, durable, 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 resistance, high temperature resistance, cold resistance and other effects, can be used in the range of 110-30 °C, belongs to the best coating
in anti-corrosion transformers.

4.
1 Introduction of plastic powder

Plastic powder is composed of organic polymer materials and is the main component of
synthetic resin.
Coatings offer many
advantages.
In addition to resin, 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 non-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
.
Under the action of the curing agent, the crosslinking reaction occurs at temperature, and the curing agent is processed into a solid structure so that the final product does not melt or melt.

4.
2 Powder coating method and film forming treatment

Powder coating is different from solvent coating, there are two problems: one is 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 dust layer adhering to the surface of the workpiece from peeling off, electrostatic adsorption or melt adhesive
must be used.

There are many coating methods, such as curing bed method, electrostatic spraying method, hot melt spraying powder electrophoresis method, etc
.

Electrostatic spraying is a commonly used method
.
It is characterized by cold coating, generally with a layer
of 60μm ~ 100μm thick at the same time.
The coating is uniform, without overhang, with sharp edges and rough surfaces, forming a continuous smooth coating
.

If the metal parts are made of thermosetting powder, after spraying, after a certain hardening temperature and resistance time, fine dust particles will chemically react with the resin and hardener to form a layer
through sintering, melting and leveling.

4.
3 Electrostatic spray pretreatment

Before spraying the powder, it is very important
to process the surface of the workpiece.
Pretreatment plays an important role
in increasing coverage intensity, achieving the same coverage performance, and extending coverage life.

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 and holes.
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 work 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.

5 Electrostatic spray anti-corrosion coating experiment

Take 6 300mm×200mm×1mm steel samples, first phosphate treatment, 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 and compare it with some common coated surface treatment coatings: the anti-corrosion ability of electrostatic spraying is the best
in some commonly used coated (coated) coatings.
The pretreatment adopts phosphating or electrogalvanized coating, and the adhesion of the spray coating can reach up to 0 grade
.

6 Application of electrostatic spray anti-corrosion coatings

● Previously, the housing of the electromagnetic unit tank of the transformer housing was tested for superheated zinc, 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, there are closed grooves on the bottom and lid of the capacitor separator, which can cause zinc deposits that are difficult to remove
.
Components of the condenser separator cannot be hot-dip galvanized
.

(2) The hot-dip galvanizing 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, many substances
are suspended on the surface of the zinc liquid in the tank.
Once the groove is outside the groove, the internal and external surface area of zinc is serious, especially the concave parts such as rounded holes, which are not easy to remove
.

●Based on the above factors, the exposed and ever-changing steel parts adopt the following surface treatment process
.

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

(2) After the tank is pre-phosphated, electrostatic spray is used for heat curing, 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 gone through more than 40 years of development, however, transformer housings have gone through many stages
in terms of method, output performance, accuracy and construction.
At the same time, the anti-corrosion treatment of steel parts on the surface of the transformer has also experienced several development stages
such as painting, galvanizing, and hot-dip galvanizing.

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

7 Workflow of electrostatic spray anti-corrosion coating

Electrostatic spray technology includes prefabricated, 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.

7.
1 Preprocessing

It is important
to prepare the pretreatment of the workpiece surface before spraying.
Pretreatment plays an important role
in increasing coverage intensity, achieving the same coverage performance, and extending coverage life.
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 sandblasting, shot blasting and other mechanical treatment
.
on the surface of the workpiece; Chemical treatments such as degreasing, acid refining, phosphorylation of the surface of the workpiece, etc.
, usually have lubricants, oxide films, etc
.
On the surface of the artifact, and on the surface of the plastic powder to produce coatings, peeling, 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 that prevents corrosion and increases the clutch.

The phosphorylation film formed during phosphorylation is 2~3m thick, which can increase the viscosity
of one or another layer.

For uniform and long-lasting use, the workpiece surface needs to be pre-treated
.
Commonly used pre-treatment includes 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
.
Common pretreatments include immersion and spraying
.
The immersion type requires multiple soaking tanks; Spraying requires the installation of a spray line on the spray line
.

7.
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 uniformly covered with plastic powder
supplemented by electrostatic adsorption.
Electrostatic spraying equipment is spray gun, spraying cup and electrostatic spray power supply
.
The falling powder can be recycled, reused after screening, and the coating particles are adsorbed on the surface of
the workpiece by spray technology.

7.
3 High temperature curing

The powder coating is heated to a certain temperature and maintained until it melts, becomes flat and smooth, and achieves the desired effect
for the work.
The working method is to put the partially completed electrostatic spray workpiece into a dry oven, heat it to the preheating temperature, generally 185°, and keep it for 15min accordingly; Turn on the oven to cool the finished product
.
Heating and control systems include electricity, natural gas, coal, etc
.

7.
4 Cooling

Cooling is the process of restoring cooling to normal temperature as a result of the spraying process
.

7.
5 Decoration

Decorative treatment is the direct treatment of the workpiece after electrostatic spraying to achieve various special appearance effects
such as wood beads, patterns, and gloss.

8 Concluding remarks

Electrostatic spraying process technology has important advantages over traditional painting technology: the use of electrostatic spraying, which compresses clean air into diffusion interfaces 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%, which is much better than solvents: the appearance quality of spraying mechanical equipment has excellent performance, the strength of plastic coating and mechanical adhesion plastic coating increases, becomes more stable, acid resistance alkaline and mechanical properties are excellent, the adhesion performance of metal surface is better, and has "three prevention" (anti-mildew, anti-humid heat, anti-salt spray) and good electrical insulation performance;

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, simplified application, 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 technology, the cost is much lower and the quality is much
higher.
Accepted by more and more users, many enterprises often use this technology in various metal industries, thereby obtaining great economic and social benefits, and has been recognized and valued
by customers.

Source | Chemical Design Newsletter - Research and Development, Vol.
48, No.
6, Jingcheng
, Anhui