What corrosion does to a fastener
Steel rusts when it is exposed to oxygen and moisture. Rust eats away at the metal, weakens the fastener, and can stain the surrounding wood or siding with dark streaks. In structural joints, a corroded screw or nail may fail long before the wood around it shows any trouble, and the failure is often hidden inside the joint. Choosing hardware that suits the environment from the start is far cheaper than repairing a joint later.
Indoor, dry locations are forgiving, so ordinary bright or plain steel fasteners are normally fine. Moisture, salt, chemicals and some kinds of wood change that quickly.
Common coatings and materials
Fastener finishes differ in how thick and durable they are. The following are general descriptions, and actual performance depends on the product and the manufacturer’s rating.
- Zinc plating (electroplated). A thin silver or yellow zinc layer. It offers modest protection and is suitable for indoor and dry locations. It is generally not recommended for long-term outdoor exposure.
- Hot-dip galvanizing. The fastener is dipped in molten zinc, creating a much thicker, rougher coating. It lasts much longer outdoors, and is a typical choice for treated lumber, fences and decks. The coating can be damaged by driving or by tools, so avoid scraping it more than necessary.
- Proprietary exterior coatings. Many screws sold for decks and exterior work have multilayer ceramic or polymer coatings. Look for a label saying they are suitable for outdoor use or for the type of treated wood you have.
- Phosphate coating. Dark gray or black, common on drywall screws. It provides little rust protection and is meant for dry interiors.
- Stainless steel. Stainless contains chromium, which forms a protective surface film. It resists corrosion well in humid or coastal conditions and is often chosen for boats, outdoor furniture and areas exposed to salt. Different grades vary in how well they resist salt, so for harsh marine exposure check the manufacturer’s information. Stainless fasteners can still discolor, and cheaper types may show surface rust spots.
- Brass, bronze and copper. Common for decorative and marine use, and resistant to rust, though softer than steel.
Treated lumber and compatible hardware
Pressure-treated lumber is infused with preservatives to resist rot and insects. Some of those preservatives contain copper compounds. In damp conditions, copper and bare steel or aluminum can speed up corrosion of the metal. For this reason, treated wood generally calls for hot-dip galvanized, stainless steel or specifically rated fasteners instead of plain steel or thin electroplated ones.
Lumber treatment products have changed over the years, and the recommendations differ by product and by manufacturer. The tag stapled to the lumber and the instructions from the fastener maker are the right sources to check. Joist hangers and connector plates deserve the same attention as the screws and nails that hold them. Hardware used with connectors should be rated for use together, because mixing coatings can cause problems. If in doubt, ask the supplier or a qualified professional. Other considerations when building outdoors, such as choosing outdoor lumber, belong in a wider planning discussion.
Some woods, such as cedar and oak, contain natural acids and tannins that can react with steel and leave black stains, so stainless or coated fasteners are often preferred for them.
Mixed metals and galvanic corrosion
When two different metals touch in the presence of moisture, they can form a weak electrical cell. One metal, usually the less noble one, corrodes faster than it would on its own. The classic example is aluminum in contact with steel or copper in a damp or salty location. The aluminum can pit and degrade around the fastener.
Ways to reduce the problem:
- Match metals where possible: stainless fasteners with stainless parts, galvanized with galvanized.
- Keep dissimilar metals apart with a plastic or rubber washer or sleeve, or a suitable coating.
- Keep the area dry. Corrosion needs moisture. Sealing joints and allowing drainage helps considerably.
- Check manufacturer guidance. Gutter, roofing and siding products often specify the fasteners to use.
Care and storage
- Store fasteners dry. Loose screws in a damp garage or shed can rust before they are used. Keep them in sealed containers, with a desiccant pack if the space is humid.
- Keep them labeled. Mixed bins of old fasteners make it hard to know what is galvanized and what is plain.
- Do not reuse rusty hardware in a structural joint. Replace it.
- Protect cut and scratched areas. When you cut a galvanized bolt, the bare cut end is unprotected. A cold galvanizing compound or paint can help.
- Tools matter too. The same moisture that rusts hardware also affects your tools, and the basics of rust prevention for tools apply here as well.
When it’s already rusty
A lightly rusted bolt can sometimes be cleaned with a wire brush and treated, but the metal underneath may already be weakened. Seized, rusty bolts are common in automotive and plumbing work, and penetrating oil often helps with loosening them. If a stripped or broken screw has been damaged by corrosion, the approaches in the article on removing stripped screws may apply.
For safety-related joints, such as deck ledgers, stair railings, ladders and vehicle parts, replace corroded fasteners with new ones of the right rating, and have the joint inspected by a qualified person if you are unsure.
Safety
Wear eye protection and gloves when brushing off rust or cutting fasteners, and a dust mask when sanding treated wood. Follow label directions for any coatings or compounds you apply. This article gives general information and is not professional advice.