How Pressure-Treated Lumber Can Cause Metal Corrosion (and What to Do About It)
Pressure-treated lumber is a popular choice for outdoor construction — decks, fences, pergolas — because it resists rot, moisture, and insects. But not everyone knows that the chemicals used to preserve the wood can actually react with certain metals, causing fasteners, connectors, and brackets to corrode far faster than normal.
In this post, we’ll walk through why this happens, what wood treatments are involved, what metals are most at risk, and how you can protect your project.
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Why Pressure-Treated Lumber Affects Metal
Pressure-treated lumber is infused with chemical preservatives that extend its life, but those same chemicals, especially copper-based compounds, are highly corrosive to many metals. When moisture is present (rain, humidity, ground contact), it accelerates a chemical reaction known as galvanic corrosion between the treated wood and the metal.
This corrosion can weaken fasteners and structural connections, leading to premature failure — a serious issue when safety and durability are on the line.
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Different Wood Treatments and Their Effects on Metal
Not all pressure treatments are the same. Here’s a quick rundown of the most common types:
1. ACQ (Alkaline Copper Quaternary)
• What it is: A widely used wood preservative containing a high concentration of copper combined with quaternary ammonium compounds.
• Effect on Metal: Highly corrosive to unprotected steel, aluminum, and even cheaper galvanized coatings. Copper in ACQ aggressively reacts with metals, especially in damp environments.
2. CA-C (Copper Azole - Type C)
• What it is: Another copper-based treatment, but with different co-biocides (like tebuconazole) instead of quaternary ammonium compounds.
• Effect on Metal: Still quite corrosive, although slightly less so than ACQ. However, it still demands corrosion-resistant fasteners and connectors.
3. MCA (Micronized Copper Azole)
• What it is: A newer treatment that uses tiny, ground-up copper particles (instead of dissolved copper) with azole fungicides.
• Effect on Metal: Somewhat less corrosive than ACQ and CA-C, but still enough that you should use rated hardware. The micronized process slows down the leaching of copper ions, but corrosion is still a risk.
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Metals Most at Risk
Depending on what type of pressure treatment was used, the wrong metal fasteners can fail surprisingly quickly. Here’s how different metals perform:
• Plain carbon steel: Extremely vulnerable. Will corrode rapidly.
• Electro-galvanized steel (thin zinc coating): Not suitable — corrodes quickly in treated lumber.
• Hot-dipped galvanized steel (heavy zinc coating): Better, but only if it meets specific standards (ASTM A153 or better).
• Aluminum: Highly reactive with copper preservatives. Should generally be avoided with treated lumber.
• Stainless steel: The best option. Grades like 304 and 316 stainless steel resist copper-based corrosion extremely well.
• Copper and brass: More compatible but rarely used for fasteners due to cost and strength concerns.
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How to Protect Your Project
If you’re working with pressure-treated lumber, especially ACQ- or CA-C-treated wood, take these steps:
1. Use the Right Hardware:
• For ACQ and CA-C lumber, only use hot-dipped galvanized fasteners (meeting ASTM A153) or, better yet, stainless steel (304 or 316 grade).
2. Create a Barrier:
• Install plastic or rubber barriers between the metal and the wood when possible.
• Some builders also use protective coatings or sealants on fasteners for an extra layer of defense.
3. Design for Drainage:
• Keep water from pooling on your structures. Good airflow helps keep wood and metal dry and slows down corrosion.
4. Regular Inspections:
• Check annually for rust, fastener deterioration, or wood damage. Early detection can prevent major failures.
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Final Thoughts
Pressure-treated lumber is incredibly durable against natural decay, but its chemical preservatives can be surprisingly harsh on the metals that hold your structure together. Understanding the type of treatment your wood has (especially if it’s ACQ, CA-C, or MCA) and selecting corrosion-resistant fasteners is essential for building something that truly lasts.
A little attention upfront to the right materials can save you a lot of costly repairs — and safety risks — down the road.
Notes:
• Hot-dipped galvanized fasteners should have a thick, consistent coating and meet ASTM A153 standards.
• Stainless steel is the safest choice, especially in wet or coastal environments.
• Always verify the type of treatment if you’re using lumber bought at — most labels on the end of boards will tell you if it’s ACQ, CA-C, or MCA.