A crimp seal, or hemmed joint, closes two components by folding them together under high pressure so that the deformed metal itself forms the barrier. Because the sealing surface is the metal, the surface condition of the strip is the seal. That is the reason galvanized coil is a questionable choice for any joint that has to hold pressure, liquid or fine dust, even though it is cheap and readily available in coil form.
What a Crimp Seal Actually Requires
Three properties decide whether a hemmed joint seals: continuity of the surface, uniform hardness across the compressed zone, and chemical stability against the sealed medium. A zinc coating is a soft, electrochemically active layer and it compromises all three.
| Requirement | Zinc coating behaviour | Consequence |
|---|---|---|
| Continuous, dense surface | stretches, cracks, flakes under crimping | micro leak paths |
| Uniform hardness | softer than the steel substrate | wear and metal transfer at the seal face |
| Chemical stability | reacts with acids and alkalis, oxidises to white rust | seal face corrosion and looseness |
| Dimensional stability | oxide growth adds volume over time | sealing pressure drifts |
Deformation, Microcracks and Hardness Mismatch
Crimping imposes severe plastic deformation. The zinc layer is stretched, squeezed and rubbed against the tooling, and because it is far more brittle in tension than the underlying steel it develops microcracks, flaking and local peeling. A sealing surface has to stay continuous and dense across the whole contact band; these defects break that continuity and open leak paths. Zinc is also softer than the steel beneath it, so the two layers do not deform as one. On the compression face the coating can be worn away or smeared onto the mating surface, changing the effective sealing pressure; where a joint is disassembled and re-crimped, the damaged layer is usually worse on the second assembly.
Corrosion Risks
Acidic or alkaline sealing media attack zinc directly, corroding the seal face and accelerating failure.
Even with water or air, zinc oxidises over time to form white rust, and the corrosion product occupies more volume than the metal consumed, which can lift the sealing interface apart.
Where the joint is wet and the medium is conductive, galvanic effects between the coating and the steel substrate add to the damage at coating breaks.
Where Galvanized Hems Are Acceptable
There is a limited class of applications where galvanized coil hems are used purely for cost reasons: low-cost toolboxes and instrument cases, and ordinary cabinets that only need to keep out large dust particles and coarse debris. In these cases the folded edge provides shielding rather than sealing. It will not stop liquids, will not hold pressure, and will not exclude fine dust or vapours, so it should not be specified as a barrier wherever those requirements appear.
Recommended Materials for Crimp Seals
| Material | Why it works | Typical use |
|---|---|---|
| 304 / 304L stainless steel | corrosion resistance, formability and strength in one material; machines to a flat, smooth seal face | food, pharmaceutical, chemical and general industrial equipment |
| 316 / 316L stainless steel | molybdenum addition improves resistance to chlorides | marine and aggressive chemical service |
| Aluminium alloy with anodised surface | anodic oxide film is hard, wear resistant and corrosion resistant, and forms a stable seal face | lightweight assemblies |
| Carbon steel plus hard chrome plating | very hard, smooth and wear resistant surface after crimping | pressure-tight joints where stiffness is required |
| Carbon steel plus PTFE coating | combines corrosion protection with a degree of elasticity that assists sealing | chemically exposed joints at moderate pressure |
Where carbon steel has to be used for strength or cost reasons, the reliable sequence is to crimp first and apply the hard coating afterwards.
Frequently Asked Questions
Q: Can galvanized coils be used as crimp seals?
Only for non-critical, dry applications where the joint acts as a shield rather than a seal. For pressure, liquid or fine-dust tightness, stainless steel or coated carbon steel is the correct choice.
Q: What happens to the zinc coating during crimping?
The severe plastic deformation stretches and squeezes the coating, producing microcracks, flaking and peeling that break the continuity of the sealing surface.
Q: What is white rust and why does it matter at a seal?
Zinc oxide and zinc hydroxide form on the coating over time. The corrosion product expands in volume and can lift the sealing interface, reducing tightness even in water or air service.
Q: Which stainless grade is best for a crimp seal?
304 or 304L covers most industrial, food and pharmaceutical duties. Select 316 or 316L when the medium contains chlorides or the environment is marine.
Q: Can a galvanized hem be upgraded to a real seal?
Where the joint has already been crimped, the practical route is a hard chrome or polymer coating applied after forming, or replacement of the hem with a stainless or coated carbon steel component.
Q: Does crimping pressure alone compensate for a damaged coating?
No. Increased pressure deforms the substrate further but cannot restore a continuous, dense sealing surface where the coating has cracked or transferred.