PPGI Cut Edge Corrosion: Mechanism and Mitigation

Rust along cut edges and drilled holes after a couple of years is common but not inevitable. A cut edge of prepainted steel is bare metal: the paint film stops, and the only remaining protection is the metallic coating on the substrate, usually zinc. Zinc protects the exposed steel sacrificially, but that protection is finite and geometry-dependent. Whether an edge stays sound or rusts early depends on how much zinc is available, how the edge is shaped, how the sheet is detailed, and what the local environment delivers to that edge. This article explains the mechanism and gives practical detailing and specification rules for roofing and cladding.

What Happens at a Cut Edge

Coil-coated (prepainted) steel is a layered system: steel substrate, metallic coating, pretreatment, primer, and topcoat. Shearing, slitting, sawing, or drilling removes all of the organic layers and most or all of the metallic coating along the cut plane. The result is an exposed steel face flanked by zinc.

In the presence of moisture and oxygen, zinc corrodes preferentially and protects the adjacent steel. This is sacrificial, or galvanic, protection. Two consequences follow:

• The edge is protected only while zinc remains nearby and electrically connected. Once the local zinc is consumed, or the corrosion product barrier is disrupted, steel corrosion can begin.
• Protection is local. Zinc does not travel far through a dry film or across a wide gap. It works best within a short distance of the exposed steel.

Zinc corrosion products — often white or greyish deposits — are not the same as red rust. White or dark staining at an edge usually indicates the zinc is doing its job. Red or brown staining indicates the steel substrate is corroding and the local protection has been exhausted or bypassed.

Why Edges Corrode Before Faces

The painted face is a barrier. The cut edge is not. Several factors combine to make edges the first place problems appear:

1. Bare steel is exposed

Any cut, punch, drill, or fastener hole creates an anode-cathode couple between steel and zinc. The larger the exposed steel area relative to nearby zinc, the faster the local zinc is consumed.

2. Edge geometry concentrates attack

A sheared edge is not a clean plane. It typically has a rollover, a burnished zone, a fracture zone, and a burr. The burr and fracture zone create crevices where moisture, chloride, and pollutants collect and where the electrolyte stays wet longer. Sharp burrs also hold less zinc than a smooth cut and can damage adjacent coatings during handling.

3. Water and contaminants collect there

Lap joints, end laps, cut sheet ends, and drilled holes are capillary and drainage traps. In coastal, industrial, or high-humidity environments, chloride and sulfur compounds accelerate zinc consumption. Inland rural environments are generally more forgiving.

4. The zinc reservoir is small

Zinc coating mass is expressed in grams per square metre (g/m²) or as coating thickness in micrometres. A heavier zinc coating provides more sacrificial material and generally extends the time before steel corrosion begins at an edge. Light zinc coatings, or coatings with uneven distribution, reach that point sooner.

How Zinc Mass and Substrate Choice Change the Outcome

The metallic coating is the edge’s only line of defence. Three substrate families behave differently:

• Hot-dip galvanized (GI) substrate: essentially zinc. Excellent sacrificial protection at cut edges and drilled holes. Zinc mass matters directly — a heavier coating gives more sacrificial reserve.
• Zinc-aluminium substrates (such as 55% Al-Zn): often better barrier performance on the painted face and good cut-edge behaviour, but the mechanism and edge performance differ from pure zinc. Aluminium-rich corrosion products can be less soluble and can help seal an edge in some environments, but edge protection should not be assumed to equal GI in every exposure.
• Zinc-aluminium-magnesium substrates: generally reported to give improved cut-edge performance because magnesium-containing corrosion products are less conductive and can form a more protective barrier at exposed edges. Exact performance depends on coating mass, paint system, and environment.

Whatever the substrate, confirm the specified metallic coating mass and test method in the contract, not just the generic name. A “galvanized” or “AZ” description without a coating mass figure is not a specification. For GI substrate details, see ChinaCNU GI coil; for prepainted coil, see ChinaCNU PPGI coil.

Paint system also matters at edges

Paint does not cover the cut plane, but it influences how much water reaches the edge and how well the coating resists undercutting. A thicker, well-adhered system with good edge coverage on the faces slows the ingress path. Poor adhesion, over-baking, or under-cured coatings can delaminate from the cut edge inward, widening the exposed zone.

Practical Detailing Rules for Roofing and Cladding

Most cut-edge corrosion is a detailing problem before it is a material problem. These rules reduce water and contaminant access to exposed edges.

Design and orientation

• Keep cut edges out of persistent wetting zones. Where possible, place cut ends under cover flashings rather than exposed on a weather face.
• Detail for drainage. Avoid horizontal cut edges that sit in standing water or in a lap that cannot drain. Provide a fall and a drainage path.
• Separate dissimilar metals. Avoid direct contact between prepainted steel edges and copper, stainless steel, or lead in wet conditions, which can accelerate galvanic attack on the zinc. Use isolating washers or tapes where contact is unavoidable.
• Control run-off. Water draining from copper, lead, or certain timber preservatives can be aggressive to zinc. Detail to avoid concentrated run-off onto cut edges.

Fabrication and handling

• Minimise burrs. Sharp, torn edges hold less zinc and trap moisture. Well-maintained shearing and punching tools produce cleaner edges.
• Deburr where practical. Light deburring removes the crevice-forming burr and can improve subsequent edge treatment adhesion.
• Avoid coating damage. Scratches near an edge widen the exposed area and reduce the distance over which zinc can protect. Handle sheets with clean, non-abrasive equipment.
• Drill, don’t tear. Clean drilled holes have a defined edge; torn or oversized holes expose more steel and create crevices.

Edge treatment options

Edge treatment is optional in many specifications and mandatory in others. Where used, it must be compatible with the coil coating system and applied to a clean, dry, uncontaminated edge.

• Do nothing: acceptable where the substrate has a substantial zinc coating, edges are sheltered, and the environment is mild. This relies entirely on sacrificial protection.
• Zinc-rich repair coatings: restore sacrificial protection to the cut edge. Best applied soon after cutting, before corrosion products form. Surface preparation matters.
• Brush-applied or spray-applied repair paints: provide a barrier but usually not sacrificial protection. Useful for colour matching and appearance, less so for protecting steel.
• Edge sealants and tapes: useful in laps and concealed conditions, but they must be compatible with the coating and must not trap water.
• Touch-up after installation: repairs handling damage and fastener heads, and is often the most practical point to treat field-cut edges.

Any repair product should be evaluated for adhesion to the specific paint system, resistance to the local environment, and compatibility with the topcoat. Verify with the coating supplier rather than assuming universal compatibility.

Decision and Specification Table

The table below is general industry guidance, not a contract specification. Confirm required values, test methods, and standards editions with the buyer’s specification and the coating supplier.

RFQ Checklist for Prepainted Steel

Use this checklist to make edge-related requirements explicit before ordering. Vague descriptions such as “galvanized” or “colour coated” leave the critical variables undefined.

1. Substrate: steel grade, thickness, and tolerance.
2. Metallic coating: type (GI, Zn-Al, Zn-Al-Mg) and coating mass in g/m², with the test method and standard edition.
3. Paint system: primer and topcoat chemistry, dry film thickness per side, colour, gloss, and the standard or method used.
4. Edge condition: whether edges are sheared, slit, or sawn, and whether deburring is required.
5. Edge treatment: product type, whether sacrificial or barrier, who applies it, and at what stage.
6. Environment of use: distance from coast, industrial exposure, humidity, and expected maintenance regime.
7. Detailing requirements: drainage, lap sealing, dissimilar metal isolation, and fastener specification.
8. Inspection and documentation: coating mass report, paint thickness report, and any third-party inspection required.
9. Commercial terms: quantity, coil or sheet dimensions, packing, delivery basis, and validity period of the quotation.

Where price is discussed, note that cost is driven by substrate grade, metallic coating mass, paint chemistry and thickness, coil dimensions, order quantity, packing, and delivery basis. Because these inputs change, request a dated quotation that states the exact specification and validity period rather than comparing headline figures.

Common Mistakes

• Assuming the paint protects the cut edge. It does not cover the cut plane. Only the metallic coating protects exposed steel there.
• Specifying “galvanized” without a coating mass. Without g/m², edge performance is undefined.
• Ignoring edge geometry. Heavy burrs and torn edges create crevices and reduce available zinc.
• Treating edges after corrosion starts. Repair coatings adhere poorly to corroded or contaminated edges. Treat soon after cutting.
• Using a barrier paint where sacrificial protection is needed. A barrier coating can hide the edge without protecting the steel beneath.
• Mixing incompatible repair products. Adhesion and topcoat compatibility must be confirmed, not assumed.
• Detailing that traps water. Horizontal cut edges in non-draining laps will corrode regardless of coating mass.
• Assuming a fixed service life. Actual performance depends on substrate, metallic coating, paint system, environment, forming, and maintenance. No single figure applies to all projects.

FAQs

Is rust on PPGI cut edges after two years normal?

It is common, especially in coastal or industrial environments and where edges are exposed or poorly drained. It is not inevitable. Heavier metallic coatings, cleaner edges, better detailing, and prompt edge treatment all delay the onset of steel corrosion. White or grey staining is usually zinc corrosion and is not the same as red rust.

Does the paint coating protect the cut edge?

No. The cut plane is bare metal. Protection comes from the metallic coating on the substrate, which acts sacrificially. Paint on the adjacent faces slows water ingress but does not cover the exposed steel.

Should I specify a zinc-rich edge treatment?

It depends on exposure and contract requirements. Zinc-rich treatments restore sacrificial protection and are often specified for coastal, industrial, or highly exposed edges. In mild, sheltered conditions with a substantial metallic coating, untreated edges may be acceptable. Confirm the requirement with the project specification.

Does a heavier zinc coating solve cut-edge corrosion?

It helps by providing more sacrificial zinc, but it does not override poor detailing. Edges that sit in standing water or trapped moisture will still corrode. Coating mass, edge geometry, environment, and detailing work together.

What about drilled holes and fasteners?

Drilled holes expose steel in the same way as cut edges. Clean drilling, isolating washers where dissimilar metals are involved, and touch-up after fixing reduce the risk. Torn or oversized holes are worse than clean ones.

Which standard applies to prepainted steel?

Several regional and international standards cover coated steel and prepainted products, and they differ in scope and test methods. Confirm the required edition and the exact contract values — coating mass, paint thickness, and test methods — with the buyer and supplier rather than relying on a standard number alone.

Getting the Specification Right

Cut-edge corrosion is best managed at the specification stage, not after installation. Define the substrate and metallic coating mass, choose a paint system suited to the environment, control edge quality during fabrication, and decide who treats which edges and when. Then verify those requirements in inspection documents.

If you are preparing an enquiry for prepainted or galvanized coil, include the items in the RFQ checklist above so suppliers can quote to a defined specification. For prepainted coil options, review PPGI coil; for galvanized substrate, review GI coil. When you are ready, send your specification, quantities, dimensions, and delivery requirements through ChinaCNU’s contact page and request a dated quotation stating the exact coating mass, paint system, and validity period.

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.