Z180 vs Z275 – Composition, Heat Treatment, Properties, and Applications

Introduction

Z180 and Z275 are common zinc coating designations applied to steel sheet and coil by the hot-dip galvanizing process. The two options present a classic selection dilemma for engineers and procurement professionals: balance lower initial cost and adequate protection for mild environments (Z180) against higher zinc mass and longer life in corrosive or outdoor applications (Z275). Typical decision contexts include building envelopes (roofing, cladding), structural components exposed to weather, automotive parts, and general sheet-metal fabrication where corrosion protection, life-cycle cost, and downstream processing must be balanced.

The principal practical difference is the zinc coating mass—Z275 carries substantially more zinc per unit area than Z180—so engineers most often compare them on corrosion-protection performance, expected lifetime, and handling/welding implications. Because both refer to coating class rather than the base-steel chemistry, substrate selection and galvanizing practice also influence final performance.

1. Standards and Designations

• Common standards and specifications that reference or use these coating classes:
• EN 10346 (continuous hot-dip coated flat products) — uses designations like Z (zinc) with coating mass classes (e.g., Z100, Z275).
• ISO 14713 / ISO 1461 — guidelines for corrosion protection by zinc coatings and hot-dip galvanizing, respectively.
• ASTM A653 / A792 — galvanized steel sheet specifications commonly using coating weight classes (often listed as G60, G90 in US practice; note: G60 ≈ Z180, G90 ≈ Z275 by mass).
• JIS H 8641, GB/T 2518 — national standards that address galvanizing and coating requirements.
• Material classification:
• The “Z” designation applies to the zinc coating; the substrate is typically a carbon/low-alloy steel (not HSLA, tool, or stainless steel). Common substrates are low-carbon cold-rolled or hot-rolled steels intended for galvanizing (DX51D, S220GD, S250GD, ASTM A1008/A1011 equivalents).

2. Chemical Composition and Alloying Strategy

Table: Typical composition context for Z coatings and typical low-carbon substrate chemistry (qualitative ranges).

Notes: - Z180 and Z275 refer to zinc mass per unit area (g/m²) specified by standards; they are not distinct alloy steels. - The steel substrate chemistry depends on the structural/formability requirements; galvanizing is applied to compatible low-carbon steels or properly prepared grades.

How alloying affects performance: - In the coating: pure zinc acts sacrificially; any intentional alloying (Zn–Al, Zn–Al–Mg) modifies corrosion behavior and appearance. Standard Z coatings rely on zinc’s electrochemical nobility relative to iron for cathodic protection. - In the substrate: carbon and microalloying control strength and hardenability; lower carbon favors weldability and formability beneath galvanized coating.

3. Microstructure and Heat Treatment Response

• Coating microstructure: a hot-dip zinc coating on steel forms several layers from the steel outward: intermetallic Fe–Zn layers (often labelled gamma, delta, zeta) adjacent to steel, capped by a top layer of nearly pure eta (η) zinc. The intermetallic layer is relatively hard and metallurgically bonded; the outer eta layer is softer, ductile zinc.
• Effect of coating mass: Z275 will generally have thicker eta and intermetallic layers than Z180 for the same substrate and process conditions; thicker layers increase sacrificial capacity and extend service life in corrosive atmospheres.
• Heat and thermal cycles: elevated temperatures (e.g., post-bake paint curing or localized welding) can cause diffusion at the zinc–steel interface and potential embrittlement of the intermetallic zone. Zinc has a low melting point; high-temperature processing must be controlled to avoid coating damage or vaporization.
• Substrate heat treatment: the zinc coating is typically applied after cold-rolling and surface preparation, not on quenched and tempered steels. Thermo-mechanical treatments of the substrate are largely independent of the coating, but any heat treatment after galvanizing can alter the coating microstructure and adhesion.

4. Mechanical Properties

Table: Comparative mechanical-effect overview (substrate properties largely unchanged by coating).

Summary: The zinc coating class (Z180 vs Z275) does not change the bulk mechanical properties of the steel substrate; differences are surface-related and influence forming, bending, and surface durability rather than tensile or yield.

5. Weldability

• Base-steel weldability is driven by substrate chemistry (carbon, Mn, microalloying). However, galvanized coatings introduce welding-specific issues:
• Zinc volatilizes during arc welding, producing fumes and porosity in welds if the coating is present at the weld joint.
• The presence of a thicker coat (Z275) increases the zinc mass that must be removed or vaporized locally, potentially exacerbating fume and porosity risks.
• Use of carbon equivalent formulas helps evaluate substrate weldability (coat removed). Common indices:
• $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Practical guidance:
• Remove galvanizing from the immediate weld area (mechanical grinding or chemical strip) and use appropriate ventilation and PPE to control zinc fumes.
• Use welding consumables and preheat/peaking guidelines appropriate to the substrate CE; the coating class affects the amount of local cleaning required rather than the CE itself.
• Spot welding and resistance welding of coated sheet are common in industry; thicker coatings can change contact resistance and electrode life.

6. Corrosion and Surface Protection

• Mechanism: zinc provides cathodic (sacrificial) protection to exposed steel and forms corrosion products (zinc oxide/hydroxycarbonate) that slow further attack.
• Relative corrosion performance: greater zinc mass correlates with longer protection life in atmospheric exposures. Z275 will generally outlast Z180 under similar environmental conditions, especially in industrial or marine atmospheres.
• Surface protection strategies:
• Paint over galvanizing (prepainted galvanized coil) for combined barrier and sacrificial protection; ensure compatibility and curing temperatures.
• Use additional sealants, conversion coatings, or topcoats for extended life in aggressive exposures.
• PREN is not applicable to zinc-coated carbon steels (it applies to pitting resistance evaluation of stainless steels). For stainless or Zn–Al–Mg coatings, use appropriate corrosion indices and test data.
• When zinc coatings are not sufficient, consider alloy coatings (Zn–Al–Mg) or material substitution to stainless or weathering steels.

7. Fabrication, Machinability, and Formability

• Forming and bending: galvanized coatings tolerate common forming operations, but minimum bend radii must be respected. Thicker coatings (Z275) have a larger intermetallic fraction that can cause white rust or visible cracking on tight bends.
• Cutting and punching: coated surfaces can produce burrs and coating deformation; tool life may be subtly affected with heavier coatings.
• Machining: coatings are thin relative to machining operations; standard machining practices apply but take account of particulate and fume control.
• Surface finishing: Z275 can accept paint and coil coating similarly to Z180, though surface preparation (passivation, conversion coatings) ensures adhesion.

8. Typical Applications

Table: Typical uses for each coating class.

Selection rationale: - Choose Z180 where exposure is limited, life expectancy is moderate, and lower material cost plus good formability/weldability are priorities. - Choose Z275 where long service life, outdoor exposure, or reduced maintenance intervals justify higher initial cost and slightly more careful forming/welding practice.

9. Cost and Availability

• Cost: Z275 uses more zinc and therefore has a higher raw-material cost compared to Z180. The incremental cost must be balanced against extended service life and reduced maintenance.
• Availability: Both coating classes are commonly available worldwide in coils and sheets from major steel producers and service centers. Product forms include cold-rolled galvanized coil, pre-painted galvanized coil (PPGI), and galvanized sheet/plate.
• Lead times: standard coating classes tend to be stocked; specialty alloy coatings (Zn–Al–Mg) or very high-mass coatings may have longer lead times.

10. Summary and Recommendation

Table: Quick comparison overview.

Conclusions: - Choose Z180 if: the component is used in a sheltered or indoor environment, initial cost and maximum formability are primary concerns, and expected service life is moderate. Z180 is appropriate for many light-duty structural and appliance applications where sacrificial protection is adequate. - Choose Z275 if: the component will face outdoor exposure, industrial or coastal atmospheres, or when longer maintenance intervals and service life are required. Z275 is preferred for roofing, gutters, exterior cladding, and infrastructure where thicker zinc mass materially extends lifetime.

Final practical notes: - Always specify substrate steel grade separately from the zinc coating class; ensure coating mass is stated as g/m² per EN convention and whether figures are total both sides or per side. - For welded components, plan for weld-zone coating removal, adequate ventilation, and electrode selection; consult substrate CE values for preheat/postweld recommendations. - When in doubt for exposed or long-life applications, the incremental cost of Z275 often yields lower life-cycle cost through reduced corrosion maintenance.