Bolt and connection corrosion: how to protect bolted joints

A bolted connection is protected by addressing three points at once: fasteners whose finish matches the structure (galvanized with a galvanized structure, painted with edge stripe coats on a painted structure), faying surfaces whose coating is qualified for the slip class required by the RCSC specification when the joint is slip-critical, and sealed crevices so water cannot get in. Galvanic couples between dissimilar metals are also avoided, or isolated.

Bolts, nuts, splice plates, gussets and connection angles are often the first places rust appears on a structure, a walkway, a bridge or an equipment support. The coating is thinner on edges, water collects in corners, and fasteners are sometimes installed without protection after the structure is painted. This guide helps maintenance managers and project managers in Quebec and Ontario protect these connections, alongside our steel structure painting service.

Why do bolted connections rust first?

They rust first because they combine everything that weakens a coating: sharp edges, threads, corners that hold water, crevices between plates and, often, fasteners whose protection does not match the structure's. Corrosion starts there earlier and then spreads under the adjacent coating.

The most frequent causes:

• Thin film on edges and threads. Paint pulls away from edges as it dries; without a stripe coat, thickness there is insufficient.
• Unprotected fasteners. Black bolts installed on a painted structure, with no touch-up after tightening.
• Crevices between plates. Water enters by capillary action, rust swells between the plates and can distort them: this is pack rust.
• Damage during tightening. The wrench and washer damage the coating around the head and nut.
• Dissimilar metals in contact. Stainless bolts on carbon steel, or the reverse, without isolation.

Galvanized or painted bolts: which to choose?

Choose a fastener finish consistent with the structure and the environment. On a galvanized structure, hot-dip galvanized fasteners are the reference; on a painted structure, suitable fasteners are installed and then covered with the paint system after tightening.

High-strength bolt grades are defined by ASTM F3125; that standard and the RCSC specification state which may be hot-dip galvanized, because of the hydrogen embrittlement risk. Connection design remains the responsibility of the structure's designer, per CSA S16. Our comparison of cold galvanizing vs hot-dip galvanizing explains the differences between zinc protections.

What rules apply to faying surfaces in slip-critical joints?

In a slip-critical joint, the faying surfaces between plates transfer load by friction: their surface condition and coating are part of the design. The RCSC specification for structural joints using high-strength bolts requires any coating on those surfaces to be qualified by testing for the slip class specified in the contract documents.

What follows in the shop or on site:

• Masking or a qualified coating. Faying surfaces are either masked during painting or coated with a product whose qualification covers the required slip class.
• Capped thickness. The qualified coating must not exceed the thickness at which it was tested; an extra coat can lose the class.
• Cure before assembly. The coating must reach the cure set by the qualification before tightening.
• Galvanized surfaces. They need specific preparation, set by the specification, to reach the required class.
• No improvised touch-up. A faying surface is never painted with an unqualified product to "protect" the steel.

Zinc-rich primer is often used as a faying surface coating, provided the specific product has been qualified. In joints that are not slip-critical, coated faying surfaces are generally allowed; the specification states it.

How are crevices and corners sealed?

Crevices are sealed to keep water from getting between plates, where paint cannot reach. On a new structure, the best seal is a connection designed to drain; on an existing structure, plate edges are sealed after preparation.

Usual approach on a structure in service:

1. Edge cleaning. Removal of rust and non-adherent paint at plate edges and around bolts, to the required grade.
2. Penetrating sealer. A low-viscosity product that penetrates the crevice and displaces moisture.
3. Caulk. A bead compatible with the paint system on edges that remain open.
4. Stripe coat. Brush application on edges, bolt heads, nuts and threads before each full coat.
5. Full system. Coats per the selected system, with thickness control.

Established pack rust is not always fixed by sealing: if the plates are distorting, the connection is assessed before the work. Our article on maintenance painting of steel structures explains how to prioritize these areas, and bridge painting applies the same principles to bridge connections.

How are galvanic couples avoided in connections?

Galvanic couples are avoided by choosing fasteners whose metal is close to that of the connected parts, or by electrically isolating dissimilar metals. In the presence of moisture, the less noble metal corrodes faster in contact with the more noble one.

The area rule decides severity:

• Small anodic part in a large cathodic area. For example a carbon steel bolt in a stainless plate: the bolt corrodes quickly. Avoid.
• Large anodic part with a small cathodic part. For example a carbon steel plate with a stainless bolt: attack spreads over a large area; more tolerable, but isolate in wet or salty environments.
• Isolation. Isolating washers and sleeves, sealant between plates and a continuous coating break the electrical contact or the electrolyte path.

Our galvanic series table shows the relative position of metals. Corrosion protection of mixed-metal connections combines these measures with a suitable protective coatings system.

How are fasteners protected during repainting?

Protect them by treating them as a separate area, with their own preparation, stripe coat and inspection. Poorly prepared fasteners are often the first area to rust after a full repaint.

• Preparation. Power tools or blasting to the required grade; on galvanized bolts, removal of zinc salts and light preparation compatible with the primer. Cleanliness grades are explained in our NACE 2 / SSPC-SP 10 surface preparation guide.
• Primer. A primer compatible with the fastener metal; galvanized fasteners need a primer suited to zinc.
• Stripe coat. By brush, on all heads, nuts, threads and edges.
• Replacement. Badly corroded bolts are replaced one at a time, in a sequence set by the designer so the connection is not weakened.
• Zinc on site. On a structure where blasting is possible, cold galvanizing or metallizing can protect connections like the rest of the steel.

How to get a quote?

Our crews and qualified subcontractors work from LaSalle, including Quebec City, on structures, walkways and industrial equipment supports. Send photos of the connections, the type of fasteners, the structure's finish and the exposure through the request a quote form: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can galvanized bolts be painted?

Yes, with preparation and a primer suited to zinc. Zinc salts and contaminants are removed, then a primer compatible with galvanized surfaces is applied before the topcoat.

Can faying surfaces in a slip-critical joint be painted?

Only with a coating qualified by testing for the required slip class, per the RCSC specification, and at the qualified thickness. Otherwise, the surfaces are masked during painting.

Why does rust swell between two bolted plates?

Because water enters the crevice by capillary action and rust takes up more volume than the steel it replaces. This pack rust can pry apart and distort the plates; sealing the edges prevents it on a sound structure.

Do stainless bolts protect a steel structure better?

Not necessarily. They do not rust themselves, but they create a galvanic couple with carbon steel in wet conditions. They are isolated with washers and sleeves, or galvanized fasteners consistent with the structure are preferred.