Anti-corrosion coatings

Corrosion is a natural phenomenon, the return of a material from a high-energy state to a low-energy state, so it is impossible to stop completely. We can only control the corrosion rate through reasonable material selection and effective anti-corrosion measures to keep it within the allowable and controllable range.

1. What is the definition and interpretation of coatings?

Definition: A list of liquid or solid materials that can form a solid coating with chemical protection, decorative or special properties (such as insulation, anti-corrosive properties, marking) on the surface of an object.


Among them, liquid coatings account for the vast majority, and solid coatings are a new type of coatings that have developed rapidly in recent years. They have the characteristics of environmental protection, short construction period and excellent performance. In liquid coatings, organic coatings have an absolute advantage in the field of coatings due to their excellent performance and good performance.


Because most of the earliest paints used vegetable oil and natural paint as the main raw materials, they are also called paints. However, with the advancement of science and technology, more and more coatings are directly synthesized from chemical raw materials, which have nothing to do with "oil", and many coatings are already water-soluble coatings or solid coatings, so the word "paint" has been used. Not all coatings are represented.

2. What is the basic composition of coatings?

Regardless of the type and performance of the coating, it is composed of three or four basic components, and the functions are shown in the following table:


Basic Ingredients

Typical Representative

Main Effect

Film-forming substances (also known as binders)

Vegetable oils (eg: tung oil), natural resins, various synthetic resins (eg: epoxy resin, polyurethane resin)

Making the coating form a complete and continuous solid coating film and firmly bonding with the substrate is the foundation of the coating and determines the basic characteristics of the coating

Dispersion medium

Volatile organic solvents (e.g. aromatic hydrocarbons, esters, ketones), water

The film-forming substance is dispersed to form a viscous liquid, which helps to improve the construction performance of the coating and some characteristics of the coating film. The dispersion medium itself cannot form a film

Pigments and Fillers

(Extender Pigment)

Taibai, chrome yellow, chrome green, iron red, talc, calcium sulfate, zinc powder, aluminum powder, etc.

Coloring and improving film properties, enhancing the protective, decorative and anti-corrosive pigment effect of the application, can also reduce costs.


Drying agent, leveling agent, anti-skinning agent, curing agent, plasticizer

Improve paint production, application, storage, use characteristics


3. What are the requirements for anti-corrosion coatings?

3.1 Have good adhesion

The adhesion of the coating, especially the wet adhesion, can replace the water and oxygen on the interface and improve the protective performance of the coating. Therefore, coatings with good adhesion generally have better corrosion resistance. Stronger adhesion is often required for corrosion resistant coatings used as primers. Therefore, epoxy and alkyd coatings are often used alone as primers.

3.2 Have strong anti-penetration ability

Strong anti-penetration ability can effectively prevent the penetration of corrosive media. In addition to the inherent characteristics of the coating resin itself, the anti-penetration ability is related to the innate characteristics of the coating resin. In order to improve the anti-penetration ability, fillers such as mica and iron oxide are often added to improve its performance.

3.3 Stability to corrosive media

Not only the film-forming substances of the coating are required to have good stability, but also the additives and fillers added should be stable and not chemically react with the medium or undergo physical changes.


4. What is the classification of coatings?

There are thousands of various coating products, and there are many classification methods. The common classifications are:


4.1 According to whether there are pigments, there are two types: varnish and color paint.

4.2 Divided by form: water-based paint, solvent-based paint, solid powder paint, high solid content paint.

4.3 According to the application: architectural paint, wood paint, marine paint, drinking water silo paint, etc.

4.4 According to the construction process: primer, putty, intermediate paint, topcoat and finish paint.

4.5 According to the effect of use: insulating paint, anti-rust paint, anti-fouling paint, anti-corrosion paint, fire-resistant paint.

4.6 According to the main film-forming substances of the paint, it is divided into: natural resin paint, asphalt paint, phenolic paint, epoxy paint, polyurethane paint.


5. How to choose corrosion-resistant coatings?


5.1 Main film-forming substances

The main film-forming substances of anti-corrosion coatings are different, and their anti-corrosion properties, applicable conditions, construction characteristics, service life and price levels are also different.


Therefore, before choosing coatings, you should have a certain understanding of the characteristics of various coatings. The following table shows the characteristics and main applications of common synthetic resin coatings:


Main film-forming substances

Coating properties

Main applications

Alkyd resin

Bright, weather resistance, strong adhesion, but soft texture, poor water resistance and alkali resistance

Construction, vehicles, machinery, ships, light industrial products

Amino resin

Bright, glossy and color retention good, hard, but requires heat curing

Construction, vehicles, machinery, ships, light industrial products

Perchlorinated vinyl resin

Weather resistance, superior corrosion resistance, water resistance, oil resistance, and flame resistance, but poor adhesion, low temperature resistance, low solid content, and thin film thickness

Chemical Equipment

Vinyl resin

Light color, good flexibility, high corrosion resistance, but low solid content and thin film thickness

Cement, environmental or industrial chemicals equipment


Bright, light color, good gloss and color retention, strong weather resistance

Building exterior walls, machinery, light industrial products

Epoxy resin

Tough, wear-resistant, excellent adhesion, anti-corrosion protection, dark single color, poor weather resistance

Chemical Equipment


Good flexibility, wear resistance, troublesome construction



Wear resistance, excellent corrosion resistance, good insulation

Aircraft, ships, chemical equipment, wood

Silicone resin

Weather resistance, high-temperature resistance, good insulation, but poor adhesion, not resistant to gasoline, brittle

Coating electrical materials, heat-resistant equipment

Inorganic polymer

High temperature and flame resistance

Ships, bridges


5.2 Corrosive medium environment

The performance characteristics of each coating are different, and its applicable conditions are also different.


The following table provides some recommendations for the selection of coatings in corrosive environments commonly found in the petrochemical industry.

Corrosive medium environment

Recommended preferred coating varieties

Recommended thickness (μm)

Field application example

general atmosphere

Phenolic resin coatings, asphalt coatings, alkyd resin coatings


Power plant boiler room, oil refining air separation, fertilizer boiler room, other equipment for drying indoor environment

Chemical atmosphere

Asphalt coatings, perchloroethylene coatings, vinyl resin coatings, epoxy resin coatings, polyurethane coatings, polyester coatings, inorganic zinc-rich coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings, high chlorinated polyethylene


Refinery, fertilizer, chemical fiber, various production units in chemical plants

Acid gas, acid mist

Perchloroethylene coatings, vinyl resin coatings, epoxy resin coatings (primer and intermediate coatings only), chlorosulfonated polyethylene coatings, chlorinated rubber coatings, polyurethane, special cyanide gels


Chemical fiber oxidation, melamine in chemical plants, chemical water in power plants

Alkaline environment

Perchloroethylene coatings, vinyl resin coatings, epoxy resin coatings, silicone coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings,


Fertilizer Synthesis, Fertilizer Urea

Sewage environment containing acid, alkali and salt

Perchloroethylene coatings, vinyl resin coatings, epoxy resin coatings, silicone coatings, chlorosulfonated polyethylene coatings, chlorinated rubber coatings,


Refinery sewage

Oil resistant paint

Epoxy resin coatings, polyurethane coatings (static-conducting varieties must be used for the inner wall of refined oil storage tanks)


Refinery oil storage tank inner wall

Water and moisture resistant coatings

Phenolic resin coating, bitumen coating, vinyl perchloride coating, vinyl resin coating, epoxy resin coating, polyurethane coating (including special cyanide), chlorosulfonated polyethylene coating, chlorinated rubber coating, SBS waterproof membrane (for roofing ), chlorinated polyethylene waterproofing membrane (for roofing)

Chlorosulfonated polyethylene, epoxy, polyurethane and phenolic coatings ≥ 200; asphalt paint ≥ 300; SBS waterproofing membranes should be models with a polyester base surface layer with a thickness of more than 4mm covered with sand.

Circulating water field, cooling tower, pool, factory building

Temperature resistant coating

When ≤100℃: phenolic resin coating, alkyd resin coating, epoxy resin coating, polyurethane coating, inorganic zinc-rich coating


All kinds of equipment and pipes

100℃~200℃: epoxy resin coating, inorganic zinc-rich coating


Heating furnaces, equipment, flue gas ducts, chimneys

Above 200℃: Silicone coating


Gasifier, high temperature reactor, regenerator

Special paint for water cooler

Special paint for TH847, DH22, JST-2L heat exchangers


Carbon steel water cooler tube bundles

Buried pipeline

Asphalt glass cloth, coal tar asphalt glass cloth, epoxy coal tar glass cloth, polyethylene tape, epoxy powder coating


Various buried pipelines


5.3 The use of environmental and construction conditions


For example, in the buried pipeline anti-corrosion should give priority to asphalt coatings with excellent water resistance; in the heavy corrosive environment, high viscosity, one-time film thickness of anti-corrosion coatings should be used; perchloroethylene, nitrocellulose paint and other volatile fast-drying coatings are not suitable for spraying construction; chlorosulfonated polyethylene is not suitable for back and forth rolling method construction.


5.4 The compatibility of the primer and substrate


The selection of anti-corrosive coatings should also take into account the compatibility of the coatings, there should be good compatibility between the primer and the substrate, and the primer and topcoat should not be mutually soluble and bite the bottom phenomenon.


For example, chlorosulfonated polyethylene can be used as primer and topcoat on concrete metal surfaces, but it is not suitable for a primer on the steel surface to make ferrous metal coating; epoxy paint can be used as primer in most cases, but it is not suitable for topcoat in the open air environment, because epoxy paint is poor in UV resistance; coatings with good corrosion resistance but poor adhesion (e.g. perchloroethylene) can only be used as a topcoat, and primer must be chosen for adhesion. The primer must choose the paint with excellent adhesion (epoxy, alkyd).


5.5 The economic factors


Sometimes the price of paint is low but short life is not economic, because the construction cost of re-corrosion in some occasions may be much higher than the difference of paint cost. In general, for bridges, towers, large structures and other design anti-corrosion layer to ensure more than 10 to 20 years, general chemical equipment more than 5 years. Heavy corrosion environment in general anti-corrosion coating use cycle is difficult to do long-term, mainly because of the lack of technical reliability, and economically uneconomical.


6. Coating construction control points

After the determination of coating varieties, the quality of the corrosion protection coatings is a key factor in determining the anti-corrosion effect and life of the coating. Among the various factors affecting the quality of the coating, the treatment of the surface of the substrate and the control of the conditions during the construction of the coating (including ratio, thickness, pinhole, temperature, humidity) account for 80% of the impact on the quality of the coating.


Therefore, the control of protective coating quality and life must focus on the following key aspects.

6.1 Surface treatment before painting construction

The purpose of surface treatment is to ensure the necessary cleanliness, porosity and roughness of the substrate surface.


To achieve the necessary cleanliness is to remove rust, oxide, oil, dust, water, etc. from the surface of the substrate to ensure the basic adhesion between the paint and the substrate and to avoid impurities from mixing in to form pores and pinholes. A certain degree of porosity and roughness can improve the adhesion of the coating. However, excessive roughness will cause insufficient local thickness, easy to pinhole, and the paint consumption also increases.

6.2 Choose a reasonable coating method

Commonly used anti-corrosion protective coatings methods include: brush coating, roller coating, dip coating, curtain coating, air spraying, airless spraying, etc. Each method has its own characteristics and scope of application, which should be determined in combination with the site environment (including temperature, moderation, and wind speed) and the variety of coatings. Generally speaking, brush coating and roller coating are suitable for anti-corrosion construction under most conditions. Brushing is relatively flexible, and suitable for all kinds of complex parts, and the roller-coating construction speed is faster. Dip coating and curtain coating are mainly suitable for anti-corrosion of workpieces and small equipment in workshops. Spraying is suitable for large-area construction, but the loss is large, and the thickness of the facade is uneven and easy to flow.

6.3 Environmental requirements for the construction of anti-corrosion coatings

Most paint construction requires above 5 ℃. If the temperature is too low, it is difficult to dry and cure, and the adhesion of the paint film is also poor. Construction cannot be carried out under conditions of wind, rain, snow and humidity greater than 70%.


In addition, the surface of the qualified substrate must be painted with primer in time, otherwise, the anti-corrosion effect will be greatly affected after being polluted or damp.

6.4 Requirements for primer adhesion

In key anti-corrosion projects, it is often required to test the adhesion of the primer. Generally, there are laboratory circle methods and field cross-cut methods. The on-site cross-cut method can better represent the real anti-corrosion quality. The method is to use a set of special scratcher blades to scratch twice in the vertical direction to ensure that the paint film is completely cut through, and then use a soft brush to gently brush off the scratched paint film fragments on the small squares drawn. The more small squares that remain, the better the adhesion.

6.5 Number of coating layers

Pinholes and defects will inevitably occur in coatings during the construction process. Therefore, when the overall thickness is the same, the coating obtained by applying multiple layers tends to be denser. Anti-corrosion coatings used in important environments are generally applied with no less than 5 layers.


During construction management, it is often encountered that the construction unit has painted twice before notifying Party A's personnel to check. At this time, it is difficult to confirm whether all parts have been brushed twice enough. A relatively simple control method is to require the construction unit to have a different color for each pass, so that it is easy to find out where there are not enough passes. Another method is to require the construction unit to brush only once at a time, and make some small marks in advance in hidden places and parts that are easy to be brushed. If the marks can still be seen, the result is that the construction unit should be punished.

6.6 Thickness of coating

Generally speaking, the anti-corrosion coating must meet a certain thickness requirement to have the anti-corrosion effect. The basic requirement is that the total thickness of the paint film should be more than 2 times the roughness. In addition, thicker is required in strongly corrosive environments. In general, the total thickness of the paint film should be greater than 150 μm under atmospheric corrosion conditions, and generally reach 250 to 300 μm in a liquid environment.


However, the thickness of the coating is also related to the variety of coatings. The anti-penetration ability of epoxy coatings with a thickness of 110 μm is better than that of asphalt coatings with a thickness of 1 mm.


After the anti-corrosion construction is completed, the thickness test must be carried out by the instrument. Human senses cannot confirm whether the coating thickness is up to standard. Construction units often use a wet film thickness gauge to accurately control the final thickness.


There are certain quantity requirements for final thickness testing. The test ratio is as follows: evenly and randomly select the measuring points, one measuring point every 20 meters of the pipeline, and each pipeline test shall not be less than 3; . The qualified standard for the thickness of the anti-corrosion coating is: more than 90% of the measuring points reach the specified thickness (the specified thickness is determined by the design drawings or implemented with reference to the requirements of the appendix of these detailed rules, and the thickness of the remaining old coating should be removed), and the remaining measuring points that do not reach the specified thickness. The minimum thickness shall not be less than 80% of the specified value. If it fails to meet the qualified standard, the random test shall be doubled, and the deficiencies shall be repainted. When more than 20% of the measuring points do not reach the specified thickness, all of them must be brushed and re-inspected.

6.7 Detection of pinholes

Pinhole testing is also required for some critical anti-corrosion coatings to ensure reliable anti-corrosion coatings. For example, the anti-corrosion of heat exchanger coatings must be tested for pinholes. There are two methods: salt water conduction method and high voltage electric spark test method. The detection voltage must be determined according to the coating thickness.

6.8 Brushing interval

There must be a certain interval between each layer during the construction of the anti-corrosion coating. Generally, the next layer of primer can be applied after the surface of the primer is dry. For chlorosulfonated polyethylene, special cyanide and other coatings, it is required to be completely dried before brushing the next layer.


7. Common defects of anti-corrosion coating and treatment measures

Common defects


Treatment measures

Poor adhesion and failure to meet requirements

① Improper surface treatment method.

②The quality of surface treatment does not meet the requirements.

③ Improper rust removal method, producing surface polishing phenomenon.

④painting on damp surface a confused already polluted surface;

⑤painting time interval is too short.

Adopt reliable rust removal methods such as sandblasting, change manual rust removal to mechanical tools, use coarse sand cloth and hard steel wire brush;

② Clarify the rust removal grade to the construction personnel and clarify the acceptance criteria.

Select different rust removal methods according to different substrates, and use degreasing cleaning agent if necessary.

④Extend the interval of painting appropriately.

Slow curing speed or no curing

①Brush the next layer after the primer is completely cured.

②De-watering and dehumidifying to ensure dryness.

③Choose suitable thinner.

①Change the special curing agent or check the effect of curing agent.

②Check whether the amount of curing agent is compatible with the site temperature and moderate.

③Bring the ambient temperature to meet the conditions before construction.

Bottom biting and blistering of the coating layer

①The bottom layer of paint is not completely cured or the upper and lower layers of paint do not match.

②The paint barrel contains water or the surface of the base layer is wet.

③Thinner is too strong.

① Wait for the primer to be completely cured before brushing the next layer.

②De-watering and dehumidifying to ensure dryness.

③Choose a suitable thinner.

The coating is not flat, with small bumps and inconsistent gloss The paint is not fully stirred when configured. Stir well enough

The coating is not flat, with small bumps and inconsistent gloss

The paint is not fully stirred well when configured.

Fully stirred evenly

Uneven coating thickness and obvious painting traces

① Improper selection of painting method

② Too much force when painting.

③The thickness of single layer is too large during construction.

① To ensure uniform coating, choose the coating method according to the order of spraying, brushing, rolling and dipping.

②Reduce the brushing force or dilute the paint appropriately.

③Check whether the construction sequence is reasonable, should ensure that the top first, then the bottom, first to the corners and recesses of the separate painting. The painting direction between two layers of paint should be vertical.

Cracking and wrinkling of the coating surface

① Bad matching between the ground, middle and top layers of paint when painting.

②The painting interval is too short.

③One paint is too thick or exposed to the sun after painting.

①Change the paint species and choose a suitable matching scheme.

②Appropriately extend the painting interval.

③Increase thinner appropriately.

④Prevent exposure to the sun.

Bottom exposure or sagging

① Oil on the surface.

②Mismatch between the previous coating and the next one.

③ A coating is too thick, paint is too thin or spray gun is used improperly.

①Remove oil with solvent or degreaser.

②Check the matching of the coating.

③Dilute properly with thinner or check the spray gun.

④Stir fully to prevent excessive dilution.

The paint is muddy

① Improper selection of thinner.

② Moisture absorption, water content.

③Incomplete reaction in the manufacturing process.

①Replacement of solvent.


③Return to pot reaction.

Rust return some time after painting

① Incomplete rust removal.

②not brushing primer in time after rust removal.

③Insufficient thickness.

① Thorough rust removal to the specified standard.

②Paint primer in time after rust removal and acceptance.

③Ensure the thickness and choose the paint variety with good corrosion resistance.


8. Coating service life

Generally speaking, coatings have a certain service life for corrosion protection. The influencing factors include design service life, paint variety, rust removal method, thickness, use environment, etc. The factors that generally have a greater impact on the service life include coating type and quality, construction process control (mainly good rust removal, correct ratio without oil, water, dust and qualified thickness) and later use in temperature, humidity, environmental corrosion, ultraviolet light, etc.


Usually, the general outdoor steel structure corrosion prevention cycle should meet at least three years, most again in more than years. For power pylons and other structures that are difficult to reconstruct after they are put into use, the general design anti-corrosion cycle should be at least 10 years to 15 years. Tank anti-corrosion layer is generally required to meet more than 6 years. Buried pipeline anti-corrosion layer life generally requires more than 30 years.

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