How Zinc Coating Weight Is Specified on Cold-Rolled Galvanized Steel
Coating weight is the decisive variable in the corrosion performance of cold-rolled galvanized steel. Under EN 10346 the coating is designated by the total mass of zinc deposited on both surfaces, expressed in grams per square metre. Typical designations are Z100, Z140, Z200, Z275 and Z350. Because zinc has a density of about 7.14 g/cm³, a Z275 coating corresponds to roughly 20 µm of zinc on each face, while Z100 gives only about 7 µm per face.
ASTM A653 describes comparable coatings with a different notation, such as G90 or G185, and ASTM A924 defines the coating-mass determination method. Whichever notation is quoted on the drawing, the engineering rule does not change: the harsher the exposure and the longer the required service life, the higher the coating weight that has to be specified.
Applications That Call for a Thicker Zinc Layer
Five groups of processing and installation work consistently justify a heavier coating than the Z100 or Z120 normally used for dry indoor service.
Structures in highly corrosive environments. Building frames, bridges, gantry structures and port facilities located in coastal zones or in areas with heavy industrial pollution need a thick sacrificial layer. In a salt-laden atmosphere a Z275 coating, about 20 µm per face, keeps the annual zinc consumption at roughly 1.2 µm per year, so the steel remains protected for decades before the base metal is reached.
Parts subject to frequent friction and wear. Guide rails, sliding brackets, conveyor components and cargo-contact parts in transport equipment progressively abrade the coating. A heavier layer means that wear consumes only part of the sacrificial zinc instead of opening the substrate, so the component retains corrosion protection for a much longer fraction of its service life.
Components that require complex forming. Deep stamping, tight bending and heavy drawing strain the coating and can cause flaking or micro-cracking. Where the formed part must still be corrosion resistant after processing, a heavier and well-adhered coating provides a larger safety margin, which is why several stamped automotive parts are specified at Z200 or above.
Long-exposure installations that are difficult to maintain. Outdoor sign gantries, high-voltage transmission towers, highway furniture and similar installations cannot be repainted on a short maintenance cycle, so the coating has to be selected for the whole design life rather than for the first few years of service.
Projects with very long service-life targets. Large industrial plants, integrated pipe galleries and public infrastructure are normally ordered with a coating of at least 275 g/m² so that structural safety is maintained throughout several decades of continuous operation.
Coating Weight Selection Reference
| Coating designation | Total zinc mass, both faces | Approx. thickness per face | Typical service condition |
|---|---|---|---|
| Z100 | 100 g/m² | 7 µm | Dry indoor parts, short-life enclosures |
| Z140 | 140 g/m² | 10 µm | Sheltered inland service, light handling |
| Z200 | 200 g/m² | 14 µm | Formed parts, moderate outdoor exposure |
| Z275 | 275 g/m² | 20 µm | Coastal, industrial and wear-prone service |
| Z350 | 350 g/m² | 25 µm | Severe marine or chemical exposure, long design life |
The table is a selection guide, not a substitute for a project corrosion assessment. Where a coating rate per year has been measured for the actual site, the required coating mass can be calculated directly from the target service life and the acceptable loss of thickness.
Installation and Handling Notes
Coating weight alone does not guarantee performance; the processing and installation steps must protect the coating that has been paid for.
Cut edges are where corrosion starts. Shearing leaves a thin smeared zinc layer that still gives sacrificial protection, but torn edges and heavy burrs should be dressed or re-coated with a zinc-rich repair product.
Welding burns zinc away locally and produces zinc fume, so weld zones are normally planned during coil processing and repaired afterwards by the same repair coating.
Fasteners, clamps and support brackets must be galvanized to a comparable coating weight, otherwise the connection becomes the weak point of the whole assembly.
Storage and transport: keep coils and sheets dry, off the ground and ventilated. Condensation trapped under wrapping film causes white rust, which consumes zinc without protecting anything.
Forming lubricants should be removed after processing, because residues hold moisture against the surface and can also interfere with subsequent painting.
Frequently Asked Questions
Q: Which coating weight should be used for coastal building structures?
A: Z275, about 20 µm of zinc per face, is the usual minimum for coastal and heavily polluted sites; Z350 is selected when the design life is long or the chloride load is very high.
Q: Does a thicker zinc layer improve performance after deep drawing?
A: Yes, provided the coating adhesion is adequate. A heavier coating leaves more intact zinc after severe forming, but an unsuitable substrate or poor lubricant can still cause flaking regardless of coating weight.
Q: Does a thick coating affect welding or forming?
A: It has little effect on formability but does increase zinc fume during welding, so weld areas are usually left uncoated or are locally stripped and repaired afterwards.
Q: Is Z275 the same as G90?
A: They are close but not identical. Z275 is defined under EN 10346 and G90 under ASTM A653; the coating masses are comparable, yet the tolerance and test methods of the two standards differ.
Q: How is the coating weight verified?
A: By the coating-mass test methods referenced in ASTM A924 or the equivalent EN procedure, normally on samples taken from the coil or from a representative sheet.
Q: Can a coated sheet be used indoors without any further protection?
A: Yes. For dry indoor service a light coating such as Z100 is usually sufficient; specifying Z275 indoors simply adds cost without measurable benefit.