PPGI Edge Corrosion: Cut-Edge Protection That Works

Cut edges of prepainted roofing rust first because the paint film — the barrier that protects the flat surface — is severed by shearing, slitting or drilling, exposing bare steel and the metallic coating at the most electrochemically active point on the sheet. The exposed zinc (or zinc-aluminium) coating then has to protect the steel sacrificially from a very small area with a very unfavourable ratio of coating area to exposed steel. The practical answers are: specify adequate zinc mass, keep the cut edge as clean and dry as possible, close joints and laps with compatible sealants and flashings, and control site handling so that swarf, filings and trapped moisture do not sit on the edge. This article explains the mechanism, the specification levers, and how to write an RFQ that makes cut-edge protection verifiable rather than assumed.

Why the Cut Edge Is the Weak Point

On a prepainted galvanized coil, protection works in two stages. The organic coating blocks water, oxygen and chloride from reaching the metal. Beneath it, the metallic coating — normally zinc, sometimes a zinc-aluminium alloy — protects the steel both as a barrier and by sacrificial action: zinc corrodes preferentially and, in doing so, holds the steel in the cathodic state.

Cutting removes the organic coating along a narrow line. At that line, three things happen at once:

• Bare metal is exposed — steel at the sheared face, plus a thin band of coating and zinc disturbed by the blade.
• The cathodic area is tiny and the anodic area is large. A small exposed steel area coupled to a large painted/zinc area concentrates corrosion current at the cut, which is why edge rust often appears as a distinct line rather than general surface rust.
• Burrs, swarf and shear damage create crevices and iron particles. Iron contamination on a zinc surface is particularly damaging because it forms local galvanic cells.

In roofing, the cut edge rarely exists in isolation. It sits inside a lap, under a flashing, at a ridge or verge, in a gutter return — places where water lingers, where debris collects, and where drying is slow. That combination of a small anode, a large cathode, and a persistently wet micro-environment is the core of the problem.

Zinc Mass: The First Specification Lever

Zinc coating mass, usually expressed in g/m² total both sides (or as coating designation such as Z275), determines how much sacrificial metal is available at the cut. General industry guidance is straightforward: for a given environment, a heavier metallic coating extends the time before cut-edge corrosion becomes visible and before it undermines the paint film from the edge inward.

Two qualifications matter more than the headline number:

• Coating mass is not uniform across a coil. Edge and centre values differ, and the minimum local value — not the nominal average — is what protects the worst point. If cut-edge performance is critical, ask for the inspection basis: how coating mass is measured, at what positions across the width, and what minimum is accepted.
• Alloy coatings behave differently from pure zinc. Zinc-aluminium and zinc-aluminium-magnesium coatings can show different edge behaviour and different cut-edge protection characteristics from conventional hot-dip zinc. Do not assume equivalence; ask the supplier to state the coating type explicitly and to describe its documented edge performance rather than substituting a generic “galvanized” description.

Where the environment is aggressive — coastal, industrial, or high-rainfall with long wetting periods — the metallic coating is the primary defence at the cut, because the paint cannot cover it. This is also the reason the substrate choice in galvanized steel coil matters even when the finished product is painted: the base coating grade sets the ceiling on edge durability.

Paint System, Edge Coverage and Forming Damage

What the coating can and cannot do

A thicker or more durable paint system improves resistance to chalking, colour change and film breakdown on the face, and it slows the rate at which the edge zone degrades. It does not, by itself, seal a cut edge. Some paint systems include additives intended to improve edge protection, and some primers are formulated for better adhesion and cut-edge tolerance, but the mechanism is still largely sacrificial protection from the metallic coating.

Forming damage is cumulative

Every operation between coil and roof changes the edge condition. Slitting produces burrs. Roll forming bends the sheet and can micro-crack the film at tight radii. Punching and drilling throw swarf. Cropping leaves a sheared face. The relevant question is not only “what coating did we buy?” but “what condition is the edge in when it reaches the roof?”

Useful controls that are generally recognised in industry practice:

• Specify a maximum burr height or a deburring requirement where the edge will be visible or exposed.
• Keep tooling sharp and correctly set; worn blades tear rather than cut and produce worse edges.
• Remove swarf and filings from sheets before stacking, and never let them sit on a wet surface.
• Avoid dragging sheets across one another, which abrades both the film and the metallic coating.

Sealants, Flashings and Joint Design

Because the cut edge cannot be painted after installation in most roofing systems, the practical route is to keep water away from it and to close the gap where it sits.

Lap sealants and butyl tapes

Lap sealant or butyl tape placed in the overlap reduces water ingress into the lap and limits the wetting of the cut edge. Compatibility matters: the sealant must be compatible with the paint system, must not contain aggressive components that attack the coating, and must tolerate the expected service temperature range without hardening or flowing out of the joint. Ask the supplier for the recommended sealant type for the specific paint system rather than assuming a generic product is suitable.

Flashings and detailing

Flashings, cappings, closures and gutter details exist to direct water away from cut edges. Poor detailing — a flashing that traps water against a cut edge, a closure that blocks drainage, a verge that allows capillary action into a lap — will defeat a good coating specification. Where cut edges must remain exposed, orient them so they drain and dry, not so they collect water and debris.

Fasteners and dissimilar metals

Fasteners, washers and any other metal in contact with the sheet should be compatible. Dissimilar metal contact in a wet environment creates galvanic couples that accelerate corrosion near the fixing, and the cut edge around a drilled hole is often the first place this shows. Confirm the recommended fastener material and coating for the environment.

Site Practice: The Part That Is Usually Underestimated

Cut-edge corrosion is frequently a site problem as much as a material problem. Common contributors:

• Wet stacking and storage. Coils or sheets stored where condensation forms between layers can show edge staining before installation. Store under cover, off the ground, with ventilation and drainage.
• Long site exposure before fixing. Sheets left in bundles for extended periods, especially in coastal or industrial air, accumulate contamination on edges that is difficult to remove later.
• Cutting on site with abrasive tools. Abrasive discs leave hot sparks and iron particles embedded in the coating and edge; these become corrosion initiation sites.
• Walking on sheets and trapping debris. Swarf, soil and organic matter hold moisture against edges and laps.
• No cleaning after installation. Washing removes chloride, dust and organic deposits that sustain wetting. In aggressive environments, a maintenance regime matters as much as the initial specification.

None of these are exotic requirements, but they are frequently omitted from the purchase specification because they sit between the material supplier and the installer.

Specification Decision Table

The table below is general guidance for discussion, not a substitute for project-specific engineering. Confirm required editions of any standard and the exact contract values with the parties responsible for the specification.

For the base material behind these choices, see prepainted galvanized steel coil, where coating type, paint system and coating mass are normally specified together.

Common Mistakes Buyers Make

• Specifying only a coating designation without a minimum local value. The nominal average does not protect the worst point on the coil.
• Assuming a thicker paint film solves edge corrosion. It improves face durability; it does not close a cut edge.
• Mixing suppliers and sealants without compatibility checks. An incompatible sealant can attack the paint or the coating.
• Ignoring the forming route. A good coil formed with worn tooling can arrive with damaged edges and heavy burrs.
• Buying on price per tonne alone. Coating mass, coating type, paint system and inspection basis are the cost drivers that determine edge durability; a lower-priced coil with a lighter or less uniform coating may cost more over the service life.
• Leaving site practice out of the contract. Storage, handling, cutting method and cleaning are often unassigned, and then become disputes.
• Treating cut-edge touch-up as a substitute for coating mass. Touch-up products have limited durability and are not a structural corrosion-control measure; use them only where the supplier confirms suitability.

RFQ Checklist for Cut-Edge Protection

Include the following in a written enquiry so that offers can be compared on a like-for-like basis:

1. Substrate and metallic coating type (for example, hot-dip zinc or a specified alloy coating), stated explicitly.
2. Coating mass: nominal value, minimum local value, and the inspection method and positions used to verify it.
3. Paint system: primer, topcoat, total dry film thickness, and any claimed edge-protection characteristic, with the basis for that claim.
4. Surface treatment and passivation details relevant to paint adhesion and corrosion resistance.
5. Coil dimensions, tolerances, and edge condition requirements, including any burr limit.
6. Mechanical properties and forming requirements if the coil will be roll formed.
7. Recommended sealant and fastener types for the intended environment.
8. Handling, packaging and storage instructions, including protection against moisture during transport.
9. Documentation to be supplied: mill certificates, coating mass records, and any third-party inspection reports.
10. Applicable standard and edition, with the exact acceptance values to be used in the contract.
11. Any required touch-up or repair materials, and the supplier’s stated limitations for their use.
12. Dated quotation validity, quantity, delivery basis and inspection rights.

On price: PPGI cost is driven mainly by substrate grade, metallic coating type and mass, paint system and thickness, coil width and tolerance, quantity, packaging, and inspection requirements. Because these inputs move, ask for a dated quote that states the specification basis, so a later comparison is meaningful.

Frequently Asked Questions

Can cut-edge corrosion be stopped completely?

On exposed cut edges, no. The paint film is severed and the metallic coating must protect the steel sacrificially, which means the coating is consumed over time. The realistic objective is to slow the process and delay its consequences through adequate coating mass, good detailing, sealed joints and sensible site practice. Actual performance depends on substrate, metallic coating, paint system, environment, forming and maintenance.

Does a thicker paint coating protect the cut edge?

It improves the durability of the painted face and slows edge-zone degradation, but it does not seal the cut. The metallic coating remains the primary cut-edge protection. Where edge performance is critical, specify coating mass and coating type first, then the paint system.

Is touch-up paint on cut edges a good solution?

Touch-up can be useful for small areas, hidden edges or remedial work, but its durability is generally lower than a factory-applied system, and it is not a substitute for adequate metallic coating. Confirm with the supplier whether a specific touch-up product is compatible with the paint system and where it is acceptable to use it.

How do I compare two offers with different zinc coating claims?

Ask both suppliers for the same information: coating type, nominal mass, minimum local value, measurement method and positions, and the inspection documentation provided. Without a common basis, the numbers are not comparable. If a standard is referenced, confirm the edition and the exact acceptance values in the contract.

Does the environment change the specification?

Yes. Coastal, industrial and high-rainfall environments generally justify a higher metallic coating mass and more attention to detailing, sealing and cleaning. Rural and low-pollution environments are less demanding, but the same principles apply.

What is the single most useful thing to add to an RFQ?

A minimum local coating mass requirement with a stated inspection basis. It converts a vague “galvanized” description into a verifiable value at the point where cut-edge corrosion actually starts.

Next Step

If you are specifying prepainted roofing for a project where cut edges will be exposed, the most productive first step is to define the metallic coating type, coating mass basis and paint system in writing, then ask for a dated quotation against that specification. You can send your requirements through the ChinaCNU contact page, including the environment, intended forming route, and any inspection or documentation needs, and request a quote that states the exact specification basis so it can be compared directly with other offers.

Related Products & Get a Quote

• PPGI Coil (Prepainted Galvanized)
• GI Coil (Hot-Dip Galvanized)
• Galvanized Steel Coil

Need a price? Contact ChinaCNU Steel for a fast RFQ — Send your specifications, quantity and destination port for an order-specific quotation.