Commercial Industrial Finishing Equipment by Slocum Equipment, Inc.


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Powder Coating Basics

Powder coating is an advanced method of applying a decorative and protective color finish to a wide range of materials and products that are used by both industries and consumers. The powder used for the process is a mixture of finely ground particles of pigment and resin, which is sprayed onto a surface to be coated. The charged powder particles adhere to the electrically grounded surfaces until heated and fused into a smooth coating in a curing oven. The result is a uniform, durable, high-quality, and attractive finish.

Powder Coating Pre-treatment
Pre-treatment means surface preparation for the powder coating to be applied to metals. Surface preparation includes:

Mechanical cleaning includes methods like scratch brushing and sand blasting. This abrasive action not only removes the surface impurities but also eliminates scratches and surface irregularities. Cleaning is very good, however, coating must be done immediately because the cleaned surface is in a highly reactive state and corrosion occurs very soon.

Chemical cleaning includes removal of dirt, oil and grease, and the oxidation products present on the surface by means of chemicals. The chemicals may be applied by wiping, spraying or dipping. The nature of chemicals used depends upon the base metal and will be discussed later in detail.

Application of conversion coating. These are applied for three purposes:
1. Provide temporary in-process corrosion protection before application of powder coating.
2. Promote good adhesion of the powder coating to the substrate.
3. Impart under-paint corrosion inhibitor to improve the life of the powder coating.

Note: The type of the conversion coating used depends upon the base metal and will be discussed later

What type of metal are you working with?
Mild Steel
Aluminum
Galvanized Material
Die-Cast Materials
Importance of Pre-treatment

Pre-treatment for Mild Steel

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Mild steel quickly begins to rust when exposed to humidity in the atmosphere. Mild steel material is known for its tendency of rapidly oxidize when exposed to humidity and oxygen in the atmosphere. The result of oxidation is the formation of an oxide layer on the surface which we commonly call ‘RUST’. To avoid this oxidation or rusting, the mild steel material is not allowed to be directly exposed to the atmosphere. For this, we generally find rust preventive oil on the surface of mild steel material. The oil does not allow the material to come in direct contact with air and thus delays oxidation. However, this does not entirely eliminate the problem of rusting and it only delays the process of rust formation. Some mild steel materials are hardened for strengthening and similarly some are welded during fabrication. Both of these processes leave black scale on the surface. In short, mild steel parts generally have rust and/or grease-oil and/or black scale on the surface. All of these cause severe problems for powder coat bonding if not removed before application of the powder. Mild steel should be cleaned if there is a presence of oil, grease, rust or black scale on the surface.

DE-GREASING

Solvent Based Degreasing - Petroleum based solvents have excellent degreasing abilities but are not commonly used because they are highly inflammable.

Alkaline degreasing powders - This is generally a blend of alkalies and surfactants. This is advised to be used under hot conditions. However, alkaline powders are also used under cold conditions as knock out degreasing stage.

Acidic Degreasing - These are solvent based liquids which are acidic in nature. They remove rust as well as grease under cold conditions. The bath of these chemicals is however, much expensive as their concentration is much higher than alkaline degreasing.

DE-RUSTING
When removal of rust and light scale from the surface cannot be achieved by sanding, acidic chemical cleaners are commonly used. These chemicals are generally a blend of mineral acids like phosphoric acid, sulfuric acid and hydrochloric acid with added inhibitors. Higher acidity will result in faster removal of rust. However, this will affect the life of the coating applied later. Phosphoric acid based de-rusting chemicals are most suggested for pretreatment of mild steel before applying coatings.

DE-SCALING
Removal of heavy scale and heavy rust from the surface. These are strong mineral acid based chemicals with added inhibitors.

USE OF CONVERSION COATING
The bare metal being extremely susceptible to atmospheric corrosion should be applied with conversion coating. Before powder coating, once the rust, oil and scale are removed, the bare metal is again exposed to environmental oxidation factors. To avoid this, the material may be coated with a conversion coating or primer to prevent further oxidation before applying paint and also gives adhesion for paint.

CONVERSION COATING FOR MILD STEEL
The most common types of conversion coating include Zinc phosphate coating and Iron phosphate coating. Zinc phosphate coating is crystalline heavy coating gray in color. While iron phosphate coating is amorphous conversion coating that ranges in color from iridescent blue to gray. Iron phosphate coating gives minimum sludge formation and hence smoother finish than zinc phosphate. Apart from this the bonding characteristics are also good. Iron phosphate processes are much easier to operate than zinc phosphate processes and require fewer process stages (zinc phosphate coating requires a pre-dip in activation chemical prior to phosphating to have micro crystalline grain structure), but iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates. Iron phosphate systems are therefore used for a range of products requiring a durable finish that are not exposed to severely corrosive environments. Zinc phosphating processes have been developed to provide exceptional coated part durability in corrosive applications. Typical industries using zinc phosphate processes include automotive, appliance and truck and bus.

POST TREATMENT
After a metal surface receives a conversion coating, the surface should be rinsed with clean water to remove unreacted conversion coating chemicals and a post-treatment is applied. The post-treatment can provide a two- to ten-fold increase in corrosion resistance and humidity resistance when compared to conversion coatings without final rinses. Post-treatments are generally based on chromic acid.

MILD STEEL PRE-TREATMENT PROCESS
Seven (or eight) stage treatment for zinc/iron phosphate include following stages:
1. De-greasing
2. Water rinse
3. De-rusting
4. Water rinse
5. Activation - This gives more compact and uniform coating of zinc phosphate. This step can be omitted for iron phosphate process.
6. Phosphating - This forms coating of zinc phosphate (4 to 6 microns) OR iron phosphate (1 to 1.5 microns) on the clean surface of mild steel.
7. Water rinse
8. Passivation - This will seal the pores of phosphating
9. Drying

CLEANER-COATER SYSTEM 3-IN-1 CHEMICAL
Cleaner-coater system for mild steel is a more simplified treatment than the 8 tank process. This is done in a single tank with a 3-in-1 chemical. In this system, all three processes of degreasing, de-rusting and iron phosphating are done with single chemical, thus making the process time effective. However, its corrosion resistance and bonding properties are not as good as that of 8 the tank process.

Pre-treatment for Aluminum

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Aluminum is known for it’s tendency of undergoing natural oxidation forming a thin layer of aluminum oxide on the surface. Aluminum should be cleaned if it has a presence of oil and/or layer of aluminum oxide on it’s surface. When aluminum anodizes, it forms a layer of aluminum oxide under the influence of electric current. This is not a good base to apply powder or any other coating. Hence, it must be removed before powder coating. Apart from this, there may be oil on the surface which may have come during fabrication process. This must be removed to have proper bonding of the powder coating.

ALUMINUM PRE-CLEANING METHODS
Pre-cleaning methods for aluminum include Alkaline Cleaning and Acidic Cleaning.

Alkaline Cleaning: The aluminum material is very sensitive to alkaline attack. Therefore, alkaline cleaners are milder for aluminum than for mild steel. However, if there is an anodizing layer on aluminum, caustic soda based strong alkaline cleaner should be used for removal. This leaves black smut on the surface. To remove this aluminum is dipped in weak bath of nitric acid.

Acidic Cleaning: Acidic cleaners are based on phosphoric acid. They are capable of removing anodizing layer without formation of black smut and reduces the additional step of rinsing in nitric acid.

CONVERSION COATING FOR ALUMINUM
Conversion coatings may be used to prevent further oxidation of cleaned aluminum surfaces. Conversion coatings for aluminum include Chromium Phosphate coating (Green in color) and Chromate coating (Yellow in color).

ALUMINUM PRE-TREATMENT PROCESS
The six-stage alkaline cleaning treatment for aluminum include the following stages:
1. Alkaline Degreasing
2. Water Rinse
3. Nitric acid
4. Water Rinse
5. Chromating
6. Water Rinse

Alternatively, four stage acidic cleaning treatment for aluminum is as follows:
1. Acidic Degreasing
2. Water rinse
3. Chromating
4. Water Rinse

CLEANER-COATER SYSTEM-3 IN 1 CHEMICAL
The cleaner-coater system for aluminum is a much more simplified treatment than a 6 tank process. This is done in a single tank with a 3-in-1 chemical. In this system, all three processes of degreasing, oxide layer removal and aluminum phosphating are done with single chemical, thus making the process time effective. However, its corrosion resistance and bonding properties are much more inferior, compared to a conventional chromatizing process.

Pre-treatment for Galvanized Material

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Like aluminum, galvanized material has a tendency to undergo natural corrosion, forming a thin layer of zinc oxide or zinc carbonate on the surface, which we commonly call “white rust”. Galvanized material should be cleaned when there is a presence of oil and/or layer of white rust on it’s surface. These conditions present a poor base to apply powder or any other coating. Hence, it must be removed before powder coating. Apart from this, there may be oil on the surface which may have been introduced during the fabrication process. This must be removed to ensure a proper bonding of the powder coating. A conversion coating should be considered in order to prevent further oxidization of a cleaned galvanized surface.

GALVANIZED MATERIALS PRE-CLEANING METHODS
Pre-cleaning methods for galvanized materials include Alkaline Cleaning and Acidic Cleaning:

Alkaline Cleaning: Alkaline cleaners are capable of removing only oil. They can not remove white rust.

Acidic Cleaning: These cleaners are based on phosphoric acid. They are capable of removing white rust in addition to grease and oil.

CONVERSION COATING FOR GALVANIZED MATERIAL
Conversion coating may be accomplished with a Zinc Phosphate coating or Chromate coating.

GALVANIZED MATERIALS PRE-TREATMENT PROCESS
The eight-stage treatment for galvanized sheets includes the following stages:
1. Alkaline Degreasing
2. Water rinse
3. Acidic Degreasing
4. Water rinse
5. Alkaline Activation
6. Zinc Phosphating
7. Water Rinse
8. Passivation

Pre-treatment for Die-Cast Materials

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Like galvanized sheets, die-cast material has a tendency to undergo natural corrosion, forming a thin layer of zinc oxide or zinc carbonate on the surface, which we commonly call “white rust”. This material is porous, hence must be properly dried after pretreatment. Die-cast materials should be cleaned if there is a presence of oil and/or layer of white rust on it’s surface. Consider conversion coating die-cast materials to prevent further oxidization of cleaned surfaces.

DIE-CAST MATERIAL PRE-CLEANING METHODS

Mild alkaline cleaners: Like aluminum, this material is very sensitive to alkaline attack. There for alkaline cleaners are milder for this material. Strong alkaline cleaners are not used because they over etch the material leaving black smut which can not be removed even in nitric acid completely. This also leaves powdery layer on the material which is not good for the bonding of powder coating.

Acidic Cleaning: These are based on phosphoric acid. They are capable of removing white rust along with oil without formation of black smut if proper timing is given.

CONVERSION COATING FOR DIE CASTING MATERIAL Chromate coating – Golden Yellow in color DIE-CAST

MATERIAL PRE-TREATMENT PROCESS
Pre-treatment of die-cast materials is commonly done, using a four-stage treatment, as follows:
1. Acidic Degreasing / Alkaline Degreasing
2. Water rinse
3. Chromating
4. Water Rinse

Problems Caused by Improper or Lack of Pre-treatment

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Pre-treatment is the base for powder coating. It is said that you cannot make a good coating work with poor pre-treatment. Utmost care should be taken to ensure proper pre-treatment is applied before powder coating. Proper pre-treatment means application of the right process and proper maintenance of the chemicals.

PROBLEMS BECAUSE OF IMPROPER CLEANING
Appearance of oily patches
Appearance of pin holes
Appearance of rusty surface beneath powder coating film

PROBLEMS BECAUSE OF IMPROPER CONVERSION COATING
Reduced resistance to weather
Reduced mechanical resistance
Poor finish of the powder coating film

PROBLEMS BECAUSE OF IMPROPER DRYING
Appearance of pin holes
Poor bonding of the powder coating film
These problems can be avoided by the choice of right process – use of correct chemicals for each material and proper maintenance of the chemicals.