Electrodeposition coating is an important method for applying rust-preventive primers to metal parts.
The workpiece is immersed in a coating bath and a coating film is formed electrically, making it easier to coat internal surfaces and gaps that are difficult to reach by spray coating.

This capability is a major benefit of electrodeposition coating.

However, a point of caution is necessary.

Electrodeposition coating does not automatically provide adequate film thickness everywhere.
Even when external flat surfaces have sufficient film thickness, the film may be too thin on internal surfaces, in recesses and pocket-shaped sections, at joints and welds, and near jig contact points.

The areas that truly matter for rust protection are not limited to easily visible external surfaces.
Rust is more likely to develop in areas that coating material has difficulty reaching, that are difficult to dry, or that tend to retain moisture and dirt.

Therefore, in electrodeposition coating, it is necessary to examine not only appearance but also internal film thickness and throwing power.

1. The value of electrodeposition coating lies in its ability to form a film on internal surfaces

In spray coating, coating material is sprayed from the outside.
It readily reaches surfaces visible from the gun but may have difficulty reaching interiors, deep areas, and gaps.

In electrodeposition coating, by contrast, the entire workpiece is immersed in a coating bath.
The film is then formed electrically, making it easier to coat complex-shaped parts.

Electrodeposition coating is therefore effective for the following types of parts.

• Box-shaped metal parts
• Welded structural parts
• Frame components
• Parts with holes or gaps
• Automotive and motorcycle parts
• Machinery parts
• Metal products for which rust prevention is important

However, being able to form a film more readily on internal surfaces is not the same as guaranteeing adequate internal film thickness.

Actual film thickness varies with workpiece shape, the electrodeposition bath, electrical application conditions, jigs, surface preparation, and liquid flow.

2. What is throwing power?

Throwing power describes the ability of electrodeposition coating to form a film on internal surfaces and in deep areas.
It is also referred to in Japanese as coverage into surrounding and recessed areas.

Simply put, the greater the throwing power, the more readily a film forms not only on the outside but also on the inside and deep within the workpiece.

In electrodeposition coating, electrical current flows more readily to external surfaces, where a film also tends to form more easily.
However, current may have difficulty reaching internal and deep areas, resulting in insufficient film thickness.

Examining throwing power helps determine whether sufficient film has formed in the following areas.

• Internal surfaces of pipes
• Interiors of box-shaped sections
• The depths of pocket-shaped structures
• Inside holes
• Areas around welds
• Joints
• Backs of flanges
• Areas easily shielded by jigs

To assess the rust protection of electrodeposition coating, it is essential to ask whether the film reaches deep inside.

3. External film thickness alone cannot establish rust protection quality

Inspection after electrodeposition coating may involve measuring external flat surfaces to check that film thickness is within specification.
This is important.

However, judging rust protection solely from external film thickness is risky.

External flat surfaces are relatively easy to coat.
Even when their film thickness is adequate, internal surfaces and gaps may have insufficient film thickness.

Rust protection problems tend to originate in the following areas.

• Areas where water tends to collect
• Areas that are difficult to dry
• Areas with a thin coating film
• Areas difficult to reach during surface preparation
• Weld heat-affected zones
• Edges and ends
• Internal corners
• Areas shielded by jigs

In other words, when measuring film thickness in electrodeposition coating, it is important to measure areas prone to rust rather than simply the most visible areas.

4. Insufficient internal film thickness can cause rust later

If internal film thickness is insufficient, rust may develop during product use or storage.

Particular care is needed under the following conditions.

• High-humidity environments
• Outdoor storage
• Coastal areas
• Structures that tend to retain moisture
• Environments where condensation occurs
• Exposure to humidity during transport
• Products that are difficult to dry after washing

In Vietnam, the hot and humid climate, rainy season, and transport and storage environments may cause insufficient internal film thickness to manifest later as rust.

Even if a product passes visual inspection, inadequate internal coating may mean that its rust protection is insufficient.

Therefore, electrodeposition coating requires checking internal film thickness in addition to passing visual inspection.

5. Workpiece shape has a major influence on throwing power

Throwing power is not determined solely by the electrodeposition coating material or bath control.
It also varies greatly with workpiece shape.

For example, the following shapes require care.

• Deep box-shaped sections
• Long, narrow pipes
• Pocket-shaped structures with small openings
• Complex welded structures
• Parts with overlapping sections
• Recesses that are difficult for liquid to enter
• Shapes that prevent air from escaping easily
• Shapes partly shielded by jigs

In electrodeposition coating, immersion in the coating bath allows coating material to enter the interior.
However, liquid entry and sufficient current flow to build film thickness are separate issues.

Shapes that prevent air from escaping may also develop internal air pockets that keep coating material out.
In that case, no film will form in the affected area, however well the electrodeposition conditions are adjusted.

Workpiece shape is an important condition determining electrodeposition coating quality.

6. Jigs and electrical contact also affect internal film thickness

In electrodeposition coating, workpieces are attached to jigs or hangers through which current is supplied.
If jig contact is poor, current will not flow steadily.

Poor electrical contact causes insufficient film thickness or unevenness.
It may affect film thickness on internal as well as external surfaces.

Check the following items for jigs.

• Do the contacts provide reliable metal-to-metal contact?
• Has coating film or rust accumulated on the contacts?
• Are the hangers deformed?
• Is the workpiece securely fixed in place?
• Does the jig shield important areas of the product?
• Is the electrical contact position appropriate?
• Is current distributed unevenly among multiple workpieces?
• Has a jig cleaning and stripping interval been established?

Jigs are more than conveying components.
In electrodeposition coating, they are important equipment that determines current flow and film thickness.

7. The condition of the electrodeposition bath also affects throwing power

Throwing power is affected not only by workpiece shape and jigs but also by the condition of the electrodeposition bath.

Check the following bath control items.

• Solids content
• pH
• Conductivity
• Bath temperature
• Solvent content
• Coating material replenishment
• Circulation
• Filtration
• Bath stability

If the electrodeposition bath is outside its appropriate range, film formation becomes unstable and sufficient film may not form on internal surfaces.

Changes in conductivity, solids content, and bath temperature, in particular, affect current flow and the rate of film formation.
Even if external film thickness appears normal, internal coverage may have deteriorated.

When investigating internal film thickness problems, bath control values must therefore also be examined.

8. Electrical application conditions change film thickness distribution

In electrodeposition coating, film thickness varies with voltage, current, and application time.

However, increasing these conditions does not produce uniform film thickness everywhere.

Current tends to concentrate where it flows most easily, so a film forms first on external surfaces.
As the film forms, electrical resistance increases in those areas and the film gradually begins to form elsewhere.

This balance determines film thickness distribution.

Check the following electrical application conditions.

• Voltage
• Current
• Electrical application time
• Voltage ramp-up method
• Workpiece loading conditions
• Jig contact
• Spacing between workpieces
• Current distribution according to shape

Unsuitable electrical application conditions can produce a thick film outside but a thin film inside, or excessive film thickness in certain areas.

9. Air pockets are a major obstacle to internal coating

In electrodeposition coating, the workpiece is immersed in the coating bath.
However, air may remain inside, depending on the product shape.

These trapped volumes of air are called air pockets.

An air pocket prevents coating material from entering the affected area.
If coating material cannot enter, an electrodeposited film cannot form.

Air pockets tend to occur in the following shapes.

• Pocket-shaped parts
• Parts with small openings
• Shapes that make it difficult for air to escape upward
• Pipe interiors
• Gaps in welded structures
• Parts with complex bends
• Parts with insufficient drain or vent holes

Countermeasures require considering the following points.

• Review the direction in which the workpiece enters the bath.
• Change the hanging angle.
• Design air vent holes.
• Design drain holes.
• Change the jig orientation.
• Check immersion time.
• Consider oscillation and liquid flow.

Product and jig design also affect rust protection quality in electrodeposition coating.

10. Select film thickness measurement locations from a quality assurance perspective

Measuring only easily accessible locations is insufficient in electrodeposition coating.

Measurement locations should be selected using the following criteria.

• Areas prone to rust
• Areas difficult to coat
• Areas where water tends to collect
• Areas important to customer requirements
• Areas that become hidden after processing or assembly
• Areas with a history of complaints
• Areas prone to film thickness differences

For example, measuring only external flat surfaces may overlook insufficient internal film thickness.
Failing to check film thickness at edges or welds may also overlook the risk of rust during actual use.

Film thickness is measured to ensure rust protection, not merely to perform an inspection.

11. Visual inspection and film thickness measurement serve different purposes

Visual inspection is also important in electrodeposition coating.
Dirt inclusions, unevenness, pinholes, sagging, and discoloration must be checked.

However, visual inspection alone may not reveal insufficient internal film thickness.

Visual inspection primarily assesses the condition of visible surfaces.
Film thickness measurement, on the other hand, assesses the amount and distribution of coating film.

For rust protection in particular, insufficient film thickness is a problem even if the appearance is good.

Electrodeposition coating therefore requires separate control of these two aspects.

• Appearance quality: Appearance, dirt inclusions, unevenness, and pinholes
• Rust protection quality: Film thickness, internal film thickness, edges, gaps, and baking condition

Passing appearance inspection does not mean passing rust protection requirements.

12. Common problems caused by insufficient internal film thickness

Insufficient internal film thickness can lead to the following problems.

Insufficient internal film thickness may be difficult to identify at shipment.
However, it may manifest as rust after time has passed in the use or storage environment.

This is why checking internal film thickness is important in electrodeposition coating.

13. A practical checking sequence for the shop floor

If insufficient internal film thickness or rust is suspected in electrodeposition coating, checking in the following order helps organize the investigation.

1. Identify where rust has occurred.

Is it on the outside?
On the inside?
At an edge?
At a weld?
Around a hole?
In a place where water tends to collect?

First, identify the affected location.

2. Check film thickness measurement locations.

Check whether the current measurement locations cover the areas where rust occurs.

3. Measure internal film thickness.

Where possible, measure film thickness on internal surfaces, in recesses, and in gaps.

4. Examine the possibility of air pockets.

Check whether the shape can trap air when the product is placed in the coating bath.

5. Examine jigs and electrical contact.

Check contacts, hangers, jig contamination, and electrical contact positions.

6. Examine bath control values.

Check whether solids content, pH, conductivity, bath temperature, and other values are stable.

7. Examine electrical application conditions.

Check whether voltage, current, application time, and voltage ramp-up conditions are appropriate.

8. Examine baking and the storage environment.

Even with sufficient film thickness, inadequate baking or a poor storage environment can lead to rust.

Internal film thickness problems therefore require examining shape, jigs, the bath, electrical application, baking, and storage together.

14. Directions for improvement

The following approaches can improve internal film thickness and throwing power.

Review film thickness measurement locations.

Include internal surfaces, recesses, edges, and welds in the measurement locations, rather than only external flat surfaces.

Review jig orientation.

Consider hanging arrangements that reduce air pockets and allow liquid to enter and current to flow more readily.

Design air vents and drains.

Depending on product shape, air vent holes or drain holes may need to be designed.

Review electrical application conditions.

Adjust voltage, current, application time, and voltage ramp-up conditions to suit the product shape.

Stabilize bath control.

Control solids content, pH, conductivity, and bath temperature to limit variations that affect throwing power.

Maintain electrical conductivity through the jigs.

Prevent coating film buildup, rust, and poor contact on jigs.

Share information on areas important for rust protection at the design stage.

Quality assurance, production engineering, design, and the coating shop should agree on which areas must be protected.

15. How CoatureVietnam can help

CoatureVietnam can support internal film thickness and throwing power in electrodeposition coating from the following perspectives.

Planning film thickness measurement locations

Identify areas important for rust protection and review measurement locations.

Identifying the causes of rust at specific locations

Use the rust locations to examine internal film thickness, surface preparation, electrical application, baking, and storage conditions.

Reviewing jigs and hanging arrangements

Check jigs for liquid entry, air venting, electrical conductivity, and film thickness distribution.

Checking links with bath control values

Clarify the relationship between solids content, pH, conductivity, bath temperature, and film thickness distribution.

Training operators and managers

Help shop-floor personnel understand throwing power as the ability to form a rust-preventive film deep inside, rather than as a difficult technical term.

Proposing process improvements

Consider improvements to stabilize internal rust protection, encompassing surface preparation, water rinsing, the electrodeposition bath, jigs, electrical application, and baking.

16. Summary: In electrodeposition coating, examine surfaces prone to rust rather than only visible surfaces

Electrodeposition coating is an important method supporting rust protection quality in metal parts.
Its major benefit is that it readily forms a film on internal surfaces and in gaps as well as on the outside.

However, electrodeposition coating does not automatically provide adequate film thickness everywhere.

Internal film thickness must be checked.
Throwing power must be examined.
Jigs and electrical contact must be examined.
The presence of air pockets must be checked.
Film thickness measurement locations must be selected from areas important for rust protection.

What matters in electrodeposition coating is not simply making visible surfaces look good.
It is protecting surfaces prone to rust.

Coating the Future with Science and Care

Science means checking internal film thickness, electrical application conditions, bath control values, and jig condition using measurements and facts.
Care means creating a process that carefully protects even hidden areas so that products can remain in use for a long time.

CoatureVietnam does not judge electrodeposition coating solely by visual inspection. It assesses rust protection comprehensively, including internal film thickness and throwing power.

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• Bath control determines electrodeposition coating quality: Understanding pH, conductivity, and solids content
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