The types of corrosion include wet corrosion occurring in the presence of moisture and oxygen at room temperature and dry corrosion occurring by the reaction with oxygen due to heating during refining and hot rolling. The majority of the corrosion that usually occurs is wet corrosion.

Principles of Corrosion

1. Wet Corrmosion

Wet corrosion proceeds by an electrochemical reaction. Anode (low potential part) and cathode (high potential part) are formed in the presence of moisture and oxygen, and a local battery is generated between the two electrodes, which accelerates the dissolution oxidation of the metal. In general, unoxidized metals appear to be uniform in texture, but the structure is highly uneven due to thermal and mechanical processing and the addition of various elements. If moisture and oxygen are present on such uneven surfaces, corrosion proceeds by the flow of corrosion current from the anode (low potential part) to the cathode (high potential part).
Anode : Fe → Fe2++2e-
Iron is ionized and dissolved, emitting electrons Cathode: O2 + 2H2O + 4e- → 4OH-
Cathod : O2 + 2H2O + 4e- → 4OH-
Fe2+, which is an anode product generating a hydroxyl ion by receiving electrons in the presence of oxygen and moisture, is converted to rust by reaction with OH-, which is a product of cathode, or reaction with moisture.
Fe2++ 2OH- → Fe(OH)2
Fe2++ 2H2O → Fe(OH)2 +2H+
2Fe(OH)2 +¹₂O2 + H2O → 2Fe(OH)3
2Fe(OH)3 → Fe2 O3 + 3H2O
Fe(OH)2, Fe(OH)3, and Fe2O3 that are generated are the major components of red rust.
Like this, wet corrosion is generated as a local battery is generated between the anode (low potential part) and cathode (high potential part), and a corrosion current flows in the presence of moisture and oxygen. The local battery is caused by the contact of dissimilar metals, difference in the oxygen concentration, difference in the liquid temperature, etc.

① Contact of dissimilar metals
Even the same kind of metal is uneven in terms of the structure, a local battery is produced due to a potential difference. If the metal in contact with iron has a higher electric potential than the iron (if the ionization tendency of the contacted metal is low), the iron is ionized and dissolved, emitting electrons.
If the metal in contact with iron has a lower electric potential than the iron (if the ionization tendency of the contacted metal is high), the contacted metal is ionized and dissolved, emitting electrons. For example, when iron and zinc are in contact, zinc is ionized by becoming the anode and corrodes first.
(Anti-corrosion principle of inorganic zinc rich primer) ② Difference in oxygen concentration
A local battery is generated by the oxygen concentration difference on the iron surface. The area with a low oxygen concentration becomes the anode, while the area with a high oxygen concentration becomes the cathode. The area with a low oxygen concentration is ionized and emits electrons. ③ Difference in liquid temperature
A temperature difference occurs in the solution in contact with the iron surface, and a local battery is generated. The area in contact with a high-temperature solution becomes the anode, while the area in contact with a low-temperature solution becomes the cathode. The area in contact with a high-temperature solution is ionized and emits electrons.
In this way, corrosion occurs as corrosion current flows in the presence of moisture and oxygen. The factors that promote this further are salt (sea salt factor), sulfur compounds, and nitrogen compounds. Therefore, depending on the environment where the metal is placed, the rate of corrosion is very different. In general, the corrosion rate of the steel caused by the environment is as follows.

The corrosion rate of the urban environment can be accelerated by air pollution becoming worse each day, and the major causes are the main components of exhaust gas, NOx and SOx. NOx and SOx are combined with atmospheric moisture to form sulfuric acid and nitric acid, which are acidulous. As they are major causes of acid rain, the rate of corrosion increases according to the degree of air pollution. This is even more so in industrial zones.

2. Dry Corrosion

The corrosion caused by the reaction between iron and oxygen caused by high-temperature heating during hot rolling at the time of casting steel is called dry corrosion. The rust produced at this time is mill scale that forms a thick iron oxide film.
Mill scale itself has a fine and stable structure in many cases, but cracks and peeling occur easily by impact at the time of molding and repeated cold and warm temperatures. If the steel surface is exposed by cracks and peeling, the mill scale becomes the cathode (high potential part) and the steel becomes the anode (low potential part). Consequently, a local battery is formed easily so that corrosion current flows, and corrosion of the steel proceeds rapidly. Therefore, since even the mill scale that has a stable structure easily peels off and rust proceeds fast, it is necessary to apply anti-corrosive paint after removing it through proper surface treatment.
Especially, if zinc rich paint is used without the sufficient removal of mill scale, it is difficult to expect the anti-corrosive effect by the self-sacrifice method because the zinc rich paint does not directly come into contact with the steel surface.

It is desirable to consider the anti-corrosion method from the design stage of the structure, sufficiently taking into consideration the type and shape of the metal, purpose of use, environmental conditions, anti-corrosion period, and economic feasibility. Since wet corrosion generally occurs more frequently, if water and oxygen are not present, there will be no generation of local battery, and corrosion will not occur. Therefore, preventing the metal surface from corrosion by blocking water and oxygen is called anti-corrosion. The primary goal of the anti-corrosion method by coating is to block corrosion factors (water, oxygen, acid, salt) by forming a film. Most paints are capable of alleviating corrosion through interception of the corrosion factors to some extent by forming a coating film. However, it is difficult to expect a long-term anti-corrosive effect.
Therefore, paints made by using various anti-corrosive pigments and resins to which penetration of corrosion factors is difficult in order to prevent corrosion of the metal are called anti-corrosive paints (rust preventive paints). Therefore, anti-corrosion principles can be classified according to which anti-corrosion pigment is used for the paint. The classification is as follows.

Principles of Anti-corrosion

1. Cathodic Protection Method (Self-sacrifice Method)

Corrosion of iron is carried out by the movement of electrons. The cathodic protection method is to delay the corrosion of iron as much as possible by containing a large amount of pigment with a higher ionization tendency than iron in the paint and making it corroded first.

① Typical pigment with the cathodic protection method
ZINC POWDER ② Typical Product
Inorganic Ethyl Silicate Primer DHDC-1800
Inorganic Ethyl Silicate Shop Primer DHDC-1650
Epoxy Zinc Rich Primer DHDC-1610
Epoxy Zinc Rich Primer High Build DHDC-1610HB

2. Reaction Inhibition Effect

A method of delaying or preventing corrosion of iron by reacting with harmful elements (water, oxygen) in advance by adding a chemically active pigment, namely passivation of iron.

① Typical pigment with a reaction inhibition effect
RED LEAD, ZINC PHOSPHATE, ZINC CHROMATE ② Typical product (examples of epoxy anti-corrosive primer)
Zinc phosphate epoxy anti-corrosive primer DHDC-0690ZP
Zinc chromate epoxy anti-corrosive primer DHDC-0690ZC
Red lead epoxy anti-corrosive primer DHDC-0690RL

3. Blocking Effect

A method of preventing the corrosion of iron by blocking the penetration of harmful elements by adding a pigment with a structure to which the penetration of harmful elements is difficult.
.
① Typical pigment with a blocking effect
MIO (Micaceous Iron Oxide), GLASS FLAKE, Aluminum ② Typical product
Epoxy MIO (silver gray) DHDC-6000MIO
Chlorinated Rubber MIO (silver gray) DHDC-4000MIO
Phenol MIO (silver gray) DHDC-2000MIO

4. Iron Oxide Anti-rust Effect

A method of preventing the corrosion of steel by forming a stable iron oxide film on the steel by adding chemically stable oxide to paint

① Typical pigment
IRON OXIDE RED ② Typical product (examples of epoxy anti-corrosive primer)
Iron Oxide Epoxy Anti-corrosive Primer DHDC-0690
Iron Oxide Epoxy Anti-corrosive Primer DNY-130
Quick-drying Epoxy Anti-corrosive Primer Speed Poxy 100

The most fundamental roles of surface treatment are

1) To remove all foreign matter from the substrate that may cause early failure in coating specifications, and 2) To clean the substrate so that the paint can be adhered well to the substrate. The most important factor that determines the success or failure of the coating is the surface treatment. Since the adhesion between the paint and the substrate is reduced by foreign matter attached to the surface of the substrate, coating often becomes unsuccessful. The most well-known foreign matter include oil, rust, mill scale and chemicals such as chlorides and sulfates. The surface treatment of the metal can be described as “completely removing any residual material that deteriorates adhesion to the metal surface or that is not compatible.”
For the surface treatment, there are mechanical and chemical methods. The mechanical method is generally used for the surface treatment in heavy duty coating. The influence of the surface treatment on the coating effect is shown in the table below.

The effect of each factor on the coating life upon coating

※ Others (coating environment, proficiency, etc.)

1. Steel

On the metal surface, oil, rust, mill scale and dust are always generated or adhered. According to the degree of surface treatment, the adhesion and durability of the coating film are greatly influenced, thereby determining the life of metal. Therefore, surface treatment is a very important coating pretreatment step. The better the surface treatment for the metal, the better the durability of the coating film even when the same paint is used. So, the prescribed surface treatment is essential for a good coating finish.

(1) Abrasives

Steel grit, shot and sand are suitable as abrasives used for blast. The abrasives that can form adequate surface roughness and that are clean and dry should be used.
The surface roughness according to the type and size of abrasives is presented in the following table.

(2) Surface roughness

The surface roughness is closely related to the surface treatment and should be specified in the coating specification separately from the surface treatment grade.

(3) Surface cleaning

In order to prevent any foreign matter from remaining on the blasted metal surface, vacuum cleaning and high-pressure dry air should be used to remove steel, grit, sand, dust, etc.

(4) Surface treatment specifications

For surface treatment specifications, the following SSPC, SIS, BS and NACE specifications should be followed.
• Steel Structures Painting Council
• Swedish Standards Institution
• British Standards Institution
• National Association of Corrosion Engineers

(5) Anti-corrosion grade of steel disc

(6) ASTM/SSPC Rust Judgment Standard (Film Appearance)

• 9(0.03%)
• 8(0.01%)
• 7(0.03%)
• 6(1%)
• 5(3%)
• 4(10%)
• 3(17%)
• 2(33%)
• 1(50%)

2. Nonferrous Metal

1) Aluminum
Solvents, steam and chemical treatment methods are used. In this way, an etching primer such as a wash primer should be applied after surface treatment. 2) Galvanized steel
① A suitable solvent should be used to remove oil and other foreign matter from the surface. If white zinc salt is formed on the galvanized surface, it should be removed by rinsing because it inhibits adhesion.
② Before coating general paint, an epoxy primer (electro, hot-dip galvanized surface) containing a wash primer (electro-galvanized surface) or zinc phosphate pigment should be applied.
③ After a long period of time of coating with a wash primer or zinc phosphate epoxy primer, paint peeling may occur, as zinc salt will form again on the galvanized surface. Therefore, it is necessary to apply a proper top coat before a long period of time passes. 3) Copper and lead
The best way to adjust the copper and lead substrate is to treat the surface carefully using an abrasive with low pressure after solvent cleaning or treat the surface using a nonferrous metal abrasive. 4) Other nonferrous metals
Before applying other nonferrous metals, an etching primer such as a wash primer should first be applied after solvent cleaning.

3. Concrete

Surface Treatment Suitable for Cement and Concrete

1) Curing and drying: The substrate should be dried for about 30 days at 21℃. 2) Dust and grease accumulated on the substrate surface should be removed by mechanical surface treatment or surface etching with blast cleaning and a hydrochloric acid solution (10~15%). 3) Water content limit: below 6% 4) pH level: pH7~pH9 5) Cracks or crevices should be cut into a V-shape and then filled with the proper resin mortar or putty. 6) For concrete surfaces plastered with a trowel, etc., the soft cement layer (LAITANCE) formed on the surface should be removed by mechanical surface treatment or acid etching. 7) If a release agent (FORM RELEASE COMPOUND) is used that is not compatible with the paint specification, the release agent should be completely removed. 8) For the surface treated before coating, the drying condition and neutralization condition in the acid treated areas should be checked, and a test patch can be performed on the substrate in advance to check the adhesion condition.

4. Minimum Surface Treatment by Paint Type

Theoretical Application Amount and Actual Application Amount

1. Theoretical Application Amount

In general, the recommended dry film thickness on the physical properties data sheet is presented based on a simulated ideal flat surface. The amount of application on the data sheet is also the theoretical application amount calculated based on it. The theoretical application amount is an amount that does not consider the surface condition to be coated, coating method, coating environment, and the amount of paint loss during coating. The theoretical application amount can be calculated simply by the following formula.

S (㎡/ℓ) = 10V/Tm
Here, S: Recommended Application Amount, meaning how many ㎡ can be coated with 1ℓ.
V: Solid Volume Ratio (%), meaning volume solid.
Tm: Recommended Dry Film Thickness (μ).
Example) What is the recommended application amount for a paint with a solid volume ratio of 55% to form a dry film thickness of 50?
<Explanation>
10×55/50 = 11.0(㎡/ℓ) The recommended application amount is 11.0㎡/ℓ.

2. Actual Application Amount

Despite the manufacturer’s detailed data on the paint, it is very difficult to accurately calculate the amount of paint used in the actual site. Due to various factors in the actual site, more paint than the theoretical paint consumption is used. The difference between the theoretical paint consumption and the actual paint consumption is called loss, and when expressed as a percentage (%) is called loss rate.

1) Surface Roughness Coefficient

The amount of paint loss depends on the unevenness of the surface after the surface treatment, and the coefficient according to the unevenness of the surface is shown in the following table.

2) Coating Condition Coefficient

There are many differences in paint loss according to the coating equipment and other environmental conditions. The coefficient according to this is shown in the following tab

3) Actual Application Amount

Actual Application Amount
Actual Application Amount (㎡/ℓ) = Recommended Application Amount (㎡/ℓ) × Surface Roughness Coefficient × Coating Condition Coefficient

4) Example

The undercoat with a solid volume ratio of 55% is intended to be coated with a dry film thickness of 50㎛. The surface treatment is blast (henvily pitted steel), the coating equipment is a spray, and the outdoor coating is done in an area without wind.
Here, What is the actual application amount?

① Recommended Application Amount = 10×55/50 = 11.0 (㎡/ℓ) ② Actual Application Amount = 11.0 × 0.8 × 0.7 = 6.2 (㎡/ℓ) ③ Loss rate = 11.0-6.2/50 × 100 = 43.6%

1. Coatability Table

O : Coatable, △ : Coatable depending on conditions (apply after making an inquiry), × : Not Coatable
※ The above items are general conditions. Therefore, it is necessary to confirm the coatability with our technical department for the special coating system.

2. Coating Interval

The data show general coating intervals. If the maximum recoatable time in the catalog for the product has elapsed, make sure to contact the relevant technical department before carrying out coating.