When a galvanized component is installed in a cold climate, in a cold store, or in low temperature process service, two materials are exposed at the same time: the carbon steel substrate and the zinc coating. Each one responds to falling temperature in its own way, and the combination decides whether the finished part keeps its ductility, its coating integrity and its corrosion protection.
Changes in the Base Steel Below Its Transition Temperature
Ferritic steel loses toughness as temperature falls. Below its ductile-brittle transition temperature (DBTT) the fracture mode changes from ductile tearing to fast cleavage: plastic deformation is almost absent, crack propagation is very rapid, and the result is a sudden structural failure without warning. Three groups of variables move the DBTT:
Chemistry and cleanliness: raised carbon, phosphorus and sulphur contents push the transition temperature up, while low-impurity, fine-grained, fully killed steel or nickel-alloyed steel pushes it down.
Rolling and heat treatment: controlled rolling with accelerated cooling, or normalising, refines the grain size and lowers the DBTT.
Thickness: the core of a thick section cools slowly during processing, so its grains are coarser and its risk of low temperature brittleness is higher than that of thin sheet of the same grade.
Because the transition is a property of the material rather than a fixed figure on a data sheet, it has to be verified by Charpy V-notch impact testing in accordance with ASTM A370 or ISO 148-1 at the lowest expected service temperature, not at room temperature.
Changes in the Zinc Coating Itself
Zinc also hardens and loses ductility when it gets cold. The coating keeps most of its strength but becomes markedly more brittle, so an impact or a deformation that would only dent the part in summer can crack or flake the coating in winter and expose bare steel. Adhesion to the substrate is governed mainly by the galvanizing process and coating quality and is not strongly temperature dependent, but a coating that has already become brittle will detach more easily under external force. Table 1 compares the coating families available for cold service.
| Coating type | Behaviour at low temperature | Typical use |
|---|---|---|
| Pure zinc, batch hot-dip galvanized to ISO 1461 or ASTM A123 | Hardens; ductility and impact toughness fall, so cracking and flaking become more likely | General outdoor structures, lattice towers, poles, frames |
| Pure zinc, continuously galvanized sheet to ASTM A653 or EN 10346 | Same trend as above; coating mass codes such as Z100 to Z275 define the protection level | Roofing, cladding, ducting, formed panels |
| Zinc-5 percent aluminium alloy coating | Retains ductility and formability better than pure zinc at low temperature | Continuously galvanized sheet that will be bent or profiled |
| Zinc-aluminium-magnesium coating | Better toughness than pure zinc and clearly better resistance to chloride-rich runoff | Coastal sites and roads treated with de-icing salt |
Corrosion Behaviour in Cold and Cold-Wet Environments
Electrochemical corrosion generally slows down as temperature falls, because ion migration is slower, oxygen solubility is lower and free water may freeze. In a dry cold climate, atmospheric corrosion of a galvanized surface is therefore usually slower than in the same climate in summer.
The exception is a temperature that repeatedly crosses the freezing point, for example between day and night or between a heated interior and a cold exterior. Repeated freezing and thawing of condensation, combined with road de-icing chloride, produces a concentrated electrolyte film on the surface and accelerates local attack such as pitting and crevice corrosion. Cold marine and cold chemical environments behave in the same way, because the corrosive medium is still present even though the temperature is low.
Cold Forming and Welding Constraints
Cold processing makes the brittleness problem worse. Cutting, punching and bending at low temperature increase the tendency of both the steel and the coating to crack, and the worked zone is the first place to fail. Main forming is best completed at room temperature or in a temperature-controlled shop.
Welding at low temperature accelerates cooling of the weld and the heat-affected zone, raises hardness and the amount of hardened structure, and greatly increases the risk of cold cracks and brittle fracture. Many construction specifications require an ambient welding temperature of at least 5 °C. Below that figure, preheating, slow cooling, low-hydrogen consumables and strict process control are required. The zinc coating should be removed locally before welding, and the welded and heat-affected areas restored afterwards with a zinc-rich repair coating, which is the recognised weak point for corrosion.
Selection Guidance by Design Metal Temperature
Low temperature toughness is a property of the substrate, so material selection comes before coating selection. Table 2 summarises the substrate families normally used.
| Design metal temperature | Substrate guidance |
|---|---|
| Down to about -20 °C | Impact-tested structural grades such as EN 10025-2 S355J2, verified at 27 J minimum at -20 °C |
| Down to about -50 °C | Normalised fine-grain structural grades such as EN 10025-3 S355NL |
| Low temperature pressure and piping service | Impact-tested pipe grades such as ASTM A333 Grade 6, specified for low temperature service |
Design measures support the material choice: avoid stress concentration from sharp notches and abrupt section changes, reduce working stress, and keep the thickness of highly stressed load-bearing parts as low as the design allows. Galvanizing quality matters as well, because a uniform, well adhered coating is less likely to fail prematurely through its own brittleness.
FAQ: Low Temperature Performance of Galvanized Steel
Q: Is galvanized steel suitable for very low temperature service?
A: Yes, provided the substrate grade is impact-tested for the lowest design metal temperature. The coating does not improve low temperature toughness, it protects the surface, and it has to be specified so that it does not crack during handling, erection or loading.
Q: Does the hot-dip galvanizing process reduce the toughness of the steel?
A: It does not change the bulk chemistry of the substrate, but it does apply a thermal cycle at a bath temperature of roughly 450 °C. Where cold service is critical, the impact test should be taken from a coupon that has seen the same galvanizing cycle when the specification requires it.
Q: At what temperature does the zinc coating crack?
A: There is no single figure. Zinc ductility falls progressively as temperature drops, so the risk of coating cracks and flaking grows in cold service, especially with thick coatings and with parts that are struck or bent after galvanizing. Keep coating mass within the appropriate standard range and avoid cold bending of galvanized plate.
Q: Why does corrosion appear to slow down in cold weather?
A: Electrochemical corrosion normally slows at low temperature because ion migration is slower, oxygen solubility is lower and free water may freeze. The exception is a temperature that repeatedly crosses 0 °C, where condensation and thaw cycles plus de-icing chloride create a concentrated electrolyte film that accelerates pitting and crevice corrosion.
Q: What welding rules apply to galvanized steel in cold weather?
A: Many construction specifications require an ambient welding temperature of at least 5 °C. Below that, preheating, slow cooling, low-hydrogen electrodes and controlled heat input are needed to avoid cold cracks. Remove the coating locally before welding and restore the area with a zinc-rich repair coating afterwards.
Q: How should cold forming and handling be managed on site?
A: Complete main cutting, punching and bending at room temperature or in a controlled-temperature shop, avoid cold bending of thick galvanized plate, and inspect the coating for cracks after handling so that any damage is repaired before the part goes into service.
Q: Which coating is the better choice for a cold, salt-exposed site?
A: For a road or coastal site where de-icing salt is present, a zinc-aluminium-magnesium coating generally performs better than pure zinc, because it combines better low temperature toughness with higher resistance to chloride-rich runoff. On heavier fabricated structures, batch hot-dip galvanizing to ISO 1461 remains the usual choice.