アルミニウム押出材は海洋環境に適していますか？

Marine projects fail faster than expected when corrosion is underestimated. Salt, moisture, and heat attack metal every day. Many buyers choose aluminum without checking marine limits. This leads to surface damage, strength loss, and early replacement.

Aluminum extrusions can be suitable for marine environments only when corrosion resistance, alloy choice, and surface protection are correctly matched to seawater exposure conditions.

Marine use is not one single condition. Coastal air, splash zones, and full seawater immersion all create different risks. This article explains what aluminum extrusion really needs to survive in marine environments and how buyers can reduce long term failure.

What corrosion resistance is needed for marine use?

Marine corrosion is aggressive and constant. Many metals look fine at first. After months, damage appears. Aluminum works well, but only with the right resistance level.

Marine aluminum extrusions must resist chloride attack, pitting corrosion, and galvanic corrosion to remain stable in seawater and coastal environments.

Salt in seawater contains chlorides. These ions break down protective oxide layers. Once pitting starts, it spreads fast and weakens the profile.

Types of marine corrosion

Marine environments cause more than one corrosion type. Each must be considered during design.

孔食

This is the most common issue. Small pits form on the surface and grow deeper. They are hard to detect early and reduce load capacity.

隙間腐食

This happens in joints, bolts, and tight gaps. Salt water gets trapped and oxygen levels drop. Corrosion accelerates in these zones.

ガルバニック腐食

This occurs when aluminum contacts dissimilar metals like steel or copper in seawater. Aluminum becomes the sacrificial metal and corrodes faster.

Resistance levels required

Not all marine projects need the same resistance. Exposure level defines requirements.

Exposure Level	Typical Environment	必要な抵抗
Coastal air	Near sea, no splash	ミディアム
Splash zone	Decks, docks	高い
Full immersion	Hulls, frames	非常に高い

Buyers should define exposure clearly. Vague terms like “marine grade” are not enough.

Practical control measures

In real production, corrosion resistance depends on more than alloy name.

良い実践例には以下が含まれます：

• Avoiding sharp corners where water stays
• Designing drainage paths
• Isolating aluminum from steel fasteners

These simple steps reduce corrosion risk without large cost increases.

How do salt spray tests assess suitability?

Many buyers rely on lab tests to judge marine performance. Salt spray testing is the most common method. However, it is often misunderstood.

Salt spray tests evaluate surface corrosion resistance by exposing aluminum extrusions to a controlled salt fog for a defined time period.

What salt spray tests actually show

Salt spray tests simulate an aggressive chloride environment. Samples are placed in a chamber with continuous salt mist.

The test mainly checks:

• Coating quality
• 表面処理
• Uniformity of protection

It does not fully simulate real marine life cycles.

Typical test standards

Different projects use different test durations. Longer time does not always mean better performance in real life.

Test Duration	Typical Meaning
240 hours	Basic coating check
500 hours	Moderate marine use
1000 hours	Severe exposure reference

These numbers help compare suppliers. They should not be the only decision factor.

Limits of salt spray testing

Salt spray tests are static. Real marine environments are dynamic. Drying, sunlight, and temperature changes also affect corrosion.

Because of this, a product that passes long salt spray tests may still fail outdoors if:

• Design traps water
• Coating is damaged in service
• Dissimilar metals are not isolated

How buyers should use test results

Salt spray results work best as a screening tool. They help reject poor coatings early.

Good buyers:

• Combine test results with field data
• Inspect coating thickness and adhesion
• Require consistent batch testing

This approach gives a more reliable view of marine suitability.

Can coated extrusions withstand seawater exposure long-term?

Coatings protect aluminum from corrosion. Still, not all coatings perform the same in seawater. Wrong choices lead to peeling and under film corrosion.

Coated aluminum extrusions can withstand long term seawater exposure only when coating type, thickness, and surface preparation are properly controlled.

Common coatings for marine aluminum

Different coatings serve different needs.

陽極酸化処理

Anodizing thickens the natural oxide layer. Marine anodizing uses greater thickness than decorative finishes. It improves corrosion resistance but remains porous without sealing.

パウダーコーティング

Powder coating creates a barrier layer. It protects well when intact. Damage allows seawater to reach aluminum underneath.

Marine paint systems

Multi layer paint systems provide the highest protection. They are common in offshore structures.

Coating performance comparison

コーティング・タイプ	Marine Durability	メンテナンスの必要性
クリアアルマイト処理	ミディアム	低い
硬質アルマイト	高い	低い
パウダーコート	中～高	ミディアム
Paint system	非常に高い	高い

This table shows trade offs between durability and upkeep.

Failure modes seen in practice

In real projects, coating failure often starts at edges and fastener holes. Mechanical damage during installation is another common cause.

Once coating is breached:

• Corrosion spreads under the film
• Repairs are difficult
• Structural strength reduces over time

Design and handling advice

Long term success depends on more than coating choice.

Best practices include:

• Rounding edges before coating
• Using touch up after drilling
• Regular inspection schedules

These steps extend service life and reduce total cost.

Which alloys offer best marine durability?

Alloy choice is the foundation of marine performance. Some alloys resist seawater naturally. Others fail quickly even with coating.

Aluminum extrusions with best marine durability usually come from 5xxx and selected 6xxx alloy series due to their strong resistance to chloride corrosion.

Why alloy chemistry matters

Alloying elements change corrosion behavior. Magnesium improves seawater resistance. Copper reduces it.

This is why some strong alloys perform poorly in marine use.

Common marine aluminum alloys

合金シリーズ	Marine Performance	典型的な使用例
5xxx	素晴らしい	海洋構造物
6063	グッド	Coastal frames
6061	ミディアム	Mixed environments
2xxx	貧しい	推奨されません

This table gives a quick selection guide.

Strength versus durability balance

Marine projects often require both strength and corrosion resistance. Designers must balance these needs.

5xxx alloys offer great corrosion resistance but lower extrusion complexity. 6xxx alloys allow complex shapes with acceptable marine durability when coated.

Lessons from field experience

Many failures come from copying non marine designs. Indoor alloys are used outdoors without review.

Successful projects:

• Define marine exposure early
• Select alloy first, coating second
• Avoid copper rich alloys

This approach reduces risk and improves long term reliability.

結論

Aluminum extrusions can work well in marine environments when corrosion risks are fully addressed. Alloy choice, coating system, and testing must match real exposure conditions. Clear definitions and good design prevent early failure.