Galvanized Sheet Metal: How Galvanization Coating Thickness Affects Quality, Durability, and Cost

Galvanized sheet metal (steel sheet coated with zinc) is widely used for fences, façades, roofing and interior ceilings to prevent rust (Galvanization – Wikipedia). The zinc layer forms a barrier that stops corrosive agents and even acts sacrificially (zinc corrodes instead of steel) (Galvanization – Wikipedia). In practice, the thickness of this zinc coating is critical: it governs how long the metal will last and how much it will cost. In this article, we explain galvanization methods (especially hot-dip galvanizing), and analyze how coating thickness impacts the quality, durability, and cost of galvanized sheet metal.

(Sheet – “Mehbud” plant)Stacks of freshly galvanized steel sheets on Mehbud’s production line. In manufacturing, hot-dip galvanizing is the most common method: steel coils or parts are dipped in molten zinc to form a metallurgically bonded zinc-iron alloy coating (Galvanization – Wikipedia) (Galvanization – Wikipedia). This process yields a thick, robust coating. For example, Mehbud’s galvanized steel sheets (often with a decorative polymer finish) are engineered to “resist corrosion” and meet stringent quality standards (Sheet – “Mehbud” plant). Continuous galvanizing (coil galvanizing per ASTM A653) instead produces thinner zinc layers (typically 6–50 µm per side) that are mostly pure zinc (ASTM A123 and G90 Specifications | American Galvanizers Association) (ASTM A123 and G90 Specifications | American Galvanizers Association). Other methods include electro-galvanizing or Sherardizing, which apply even thinner zinc films for light-duty or precision parts (Galvanization – Wikipedia). Both hot-dip and continuous galvanizing protect steel, but the amount of zinc (coating thickness) is what determines the lifespan and performance in service (Galvanization – Wikipedia) (Microsoft Word – GalvInfoNote 1-1).

Galvanization Methods for Sheet Metal

1. Hot-Dip Galvanizing (HDG): Steel parts or sheets are immersed in a bath of molten zinc. This yields a thick zinc-iron alloy coating (often 60–150 µm on structural parts). The coating is metallurgically bonded and very durable (Galvanization – Wikipedia). High-strength steel requires careful process control (to avoid hydrogen embrittlement) (Galvanization – Wikipedia).
2. Continuous (Coil) Galvanizing: Steel coils pass through a zinc bath at high speed (ASTM A653 process). By controlling speed and bath composition, the zinc layer (usually 6–25 µm per side) is uniform and ductile (ASTM A123 and G90 Specifications | American Galvanizers Association) (ASTM A123 and G90 Specifications | American Galvanizers Association). This yields thinner coatings (e.g. G60 or G90 grade).
3. Electro-Galvanizing: Zinc is electrodeposited onto steel. This produces very thin coatings (a few microns) used on automotive body panels or indoor steel. It provides corrosion protection but not as thick as hot-dip.
4. Sherardizing (Thermal Diffusion): Zinc dust vapor diffuses onto steel in a rotating drum. Suitable for small parts; yields a thin, adherent zinc layer.
5. Galvannealing: A variant of continuous galvanizing where the steel is reheated to form zinc-iron alloy on the surface, providing a matte finish (used for parts needing paint adhesion). Coating thickness is similar to normal continuous galvanizing.

Each method serves different applications. In construction, hot-dip and continuous are most relevant, with hot-dip giving the thickest coats and continuous being common for wall/fence panels. Mehbud’s products (e.g. sheet panels, fence slats, facade cassettes) typically use continuous galvanizing for coils and hot-dip for fabricated parts, balancing thickness and formability.

Why Coating Thickness Matters for Durability

Thicker zinc coatings significantly extend service life by providing more sacrificial material and barrier protection. Zinc corrodes slowly by forming a protective zinc-oxide/carbonate patina. Field data show that in normal atmosphere, “observed corrosion rates rarely exceed 0.3 mils per year” (about 7.6 µm/year) (Performance of Galvanized Steel in… | American Galvanizers Association). Thus, even a 20 µm coating lasts over two decades before full corrosion of the zinc film in benign environments.

Key effects of thickness:

1. Linear Life Extension: Corrosion performance scales roughly with thickness. For example, GalvInfoNote explains that doubling the zinc coating (e.g. from G30 to G60) roughly doubles time to reach 5% rust (Microsoft Word – GalvInfoNote 1-1). Likewise, a G90 coating (~18 µm/side) is ~50% thicker than G60 and lasts ~50% longer under the same conditions (Microsoft Word – GalvInfoNote 1-1). In practical terms, this means an 18 µm layer might last 15 years in a setting where a 12 µm layer would last 10 years.
2. Higher Endurance in Harsh Environments: In aggressive settings (industrial fumes, salty air, soil), thicker zinc delays substrate corrosion. Research on soil corrosion found that steel with ~100–125 µm of zinc (3.9–5.0 mils) achieved 35–50 years in the harshest soils, and 75+ years in milder soils (Performance of Galvanized Steel in… | American Galvanizers Association). Such thick coatings are common on buried/structural steel.
3. Barrier and Cathodic Protection: A continuous zinc film blocks moisture and oxygen. If scratched, the remaining zinc around the scratch still corrodes sacrificially, protecting the steel. This cathodic protection is prolonged by more zinc.
4. Mechanical Considerations: Very thick galvanized coatings are slightly rougher and less ductile; overly heavy coatings can crack if the metal is bent sharply. For instance, pre-galvanized coils (almost pure zinc coating) are more formable than thick batch coatings (ASTM A123 and G90 Specifications | American Galvanizers Association). Mehbud optimizes coil coatings (e.g. around 18 µm/side) so sheets can be bent without cracking, while still providing strong corrosion resistance.

In summary, more zinc = more protection. Designers often specify heavier coatings for projects with longer desired lifespans or tougher conditions. The tradeoff is cost (more zinc) and weight, so one chooses an appropriate thickness based on environment.

Standards and Typical Coating Thicknesses

Industry standards define minimum zinc thicknesses for different applications. Typical guidelines include:

1. ASTM A653/A653M (Galvanized Steel Sheet) – Continuous galvanizing for coil/strip. Coatings are designated G30, G60, G90, etc. For example, G90 (0.90 oz/ft² total both sides) equates to ~18 µm of zinc per side (ASTM A123 and G90 Specifications | American Galvanizers Association). Other common grades: G60 (~15 µm/side), G120 (~24 µm). Coil products often range from 6 µm (G30) up to ~50 µm (G185) per side (ASTM A123 and G90 Specifications | American Galvanizers Association) (ASTM A123 and G90 Specifications | American Galvanizers Association).
2. ASTM A123/A123M (Hot-Dip Galvanized Coatings) – Applies to fabricated steel after forming. Specifies coating weight by steel thickness. For steel 6–12 mm thick, the minimum is about 55 µm (Class A), but coatings typically exceed 100 µm easily. A123 coatings consist of zinc-iron alloy layers and are integral to the steel (ASTM A123 and G90 Specifications | American Galvanizers Association). Most structural galvanized steel ends up with ~100–125 µm of zinc (Performance of Galvanized Steel in… | American Galvanizers Association).
3. EN ISO 1461 (Hot-Dip Galvanized Coatings) – European standard for galvanized steel. Specifies average coating thickness by steel thickness: for example, avg 55 µm for steel <6 mm, 65 µm for 6–12 mm, 85 µm for >12 mm, with all areas above a lower limit (45–70 µm) for quality. This standard is roughly aligned with typical zinc weights like Z180–Z275 (180–275 g/m² total both sides).
4. Ukrainian DSTU Standards – National standards (e.g. DSTU 2650) mirror EN ISO 1461. They similarly require 55–85 µm average for steel and correspond to common zinc mass units. Mehbud’s certified products comply with these standards (Sheet – “Mehbud” plant).

Typical Coating Ranges for Construction Sheet:

1. Light-duty (indoor): Galvanized paint-coated steel (e.g. ceilings, indoor panels) often use thin G30–G60 coatings (6–15 µm each side).
2. Standard outdoor panels: Roofing/facade panels usually use G60–G90 (15–25 µm per side, often labelled Z200–Z275 in g/m²).
3. Heavy-duty/exposed structures: Fencing posts, support beams, or outdoor steel structures often use G120–G185 (30–50 µm per side) or hot-dip coatings well over 100 µm.
4. Extremes: For very harsh service (marine installations, pipelines), coatings may go beyond 100 µm (Z350 or HDG Class 80+). In practice, >100 µm per side is rare on thin sheet due to cost and formability issues.

By referencing standards like ASTM A653 and ISO 1461 (and local DSTU norms), architects and fabricators specify the required zinc thickness. For example, a galvanized fence panel might be called out as “ASTM A653 G90 (Z275) galvanized steel,” meaning about 45 g/m² per side (~18 µm). It’s important to differentiate between total coating weight (both sides) and per-side thickness.

Coating Thickness vs. Cost

More zinc means higher material cost and longer galvanizing time. However, zinc is relatively inexpensive compared to stainless steel or heavy paints. Key points:

1. Initial Cost: Upgrading from, say, G60 to G90 adds roughly 20–30% cost (extra zinc). Large batch galvanizing jobs specify price per unit weight of steel plus a galvanizing charge. Thicker coatings typically raise that charge by 10–50%.
2. Life-Cycle Economics: Although a heavier coating costs more upfront, it often saves money long-term. The American Galvanizers Association notes that hot-dip galvanized steel can be maintenance-free for 70+ years (What is the Cost of Galvanized… | American Galvanizers Association). In contrast, painted steel may need repainting many times (incurring 2–5× the initial cost in direct maintenance (What is the Cost of Galvanized… | American Galvanizers Association)). Over a building’s lifespan, HDG is often 2–6 times more economical than alternatives (What is the Cost of Galvanized… | American Galvanizers Association).
3. Performance Tradeoff: If a slightly thicker coating doubles lifespan (per (Microsoft Word – GalvInfoNote 1-1)), it often pays off. For critical structures (bridges, industrial frames), thick coatings are cost-effective. For short-lived or indoor structures, a thinner coat may be chosen to save cost.
4. Estimated Values: As a rough example, applying G90 zinc on 100 m² of steel might cost a few hundred dollars more than G60. But if the G90 lasts 25 years longer, that cost is negligible over the project’s life.

In short, specify the minimum acceptable coating per standards and adjust only if higher corrosion resistance is needed. Many architects now do a life-cycle cost analysis, which typically favors the “Cadillac” zinc option (What is the Cost of Galvanized… | American Galvanizers Association). Mehbud’s projects emphasize quality materials so that clients avoid premature repairs.

When to Use Thinner vs. Thicker Coatings

Coating thickness should match the environment and service life requirements. Guidelines:

1. Indoor/Sheltered Use: In dry indoor spaces or mildly corrosive climates, thin coatings suffice. For example, interior suspended ceilings or dry warehouse roofs may use G30–G60 (~6–15 µm/side) or even electro-galvanized sheet.
2. Typical Outdoor Use: Residential and commercial building facades, fences, and roofs (non-coastal) typically use G60–G90 (~15–25 µm/side). This handles rain and moderate pollution without rust for decades.
3. Harsh Industrial Use: Chemical plants, heavy-industry exteriors, or urban areas with acid rain call for G90–G185 (25–50 µm/side) or hot-dip galvanizing. The extra zinc combats acid gases and high humidity.
4. Marine/Coastal Use: Salt-laden air accelerates corrosion. Coastal structures often specify ≥50 µm per side. This can be achieved by specifying Z275/Z350 coil or by hot-dip (>50–100 µm). Sometimes architects use duplex systems (galvanize + paint) for marine projects. A rule of thumb: if corrosion category is C5-M (very high, ISO 12944), use about twice the thickness of a normal atmosphere category (C3).
5. Specialty Cases: Soil burial or submerged parts follow different rules (ASTM D5906, CIS 5 classification). Here, continuous thick galvanizing (Class B, 80 µm min) is used.

Bullet Summary:

1. Indoor, light exposure → G30–G60 (6–15 µm/side).
2. Outdoor, moderate climate → G60–G90 (15–25 µm/side).
3. Industrial/urban → G90–G185 (25–50 µm/side).
4. Coastal/marine → ≥50 µm/side (Z275–Z350 or HDG).
5. Duplex (galvanize + paint) for extreme conditions.

Galvanizing guidelines (ISO 14713, AGA) provide charts for “time to first maintenance” vs. environment and zinc thickness. In practice, specifying thicker coatings for salty or polluted environments is prudent. For example, a beachfront fence might be galvanized at 60+ µm to meet a 30-year life, whereas an inland fence could use 18 µm for a 15-year life. The relationship is roughly proportional (Microsoft Word – GalvInfoNote 1-1).

Measuring Galvanization Thickness

Quality control requires verifying that coatings meet specifications. Common methods include:

1. Magnetic Induction Gauges: Handheld meters measure coating thickness (in µm) on steel by magnetic pull-off. They are quick and non-destructive. Widely used in the field for spot checks.
2. Eddy Current Gauges: For coatings on non-ferrous substrates (less relevant for steel plate).
3. Coulometric Test: A small area of zinc is dissolved in acid and measured electrically (ASTM B767). It gives precise average thickness for a sample area.
4. Cross-Section Metallography: A tiny sample is cut, polished, and viewed under a microscope. This reveals the layered structure (iron-zinc alloys) and exact layer thickness. Standards like ASTM D7091/ISO 1463 cover this.
5. X-Ray Fluorescence (XRF): For some plated parts, portable XRF can estimate zinc thickness, but it is less common for thick galvanized steel.
6. Coating Certificates: Galvanizing plants provide test reports per ASTM/ISO. Inspectors may also use calibrated reference shims or test panels.

Mehbud ensures that all incoming steel and finished products meet the thickness callouts. Buyers can inspect using magnetic gauges or request lab tests. Common tolerance is ±10–20% of the specified thickness. For example, if G90 (18 µm/side) is specified, the galvanizer must meet an average ≥18 µm and no spot below about 16 µm.

Case Study: Galvanized Fencing in a Coastal Development

Consider a recent coastal resort project where fences and railings needed extreme durability. The design team specified hot-dip galvanized steel with Class 80 coating (≈80 µm of zinc). Samples tested at 78–85 µm, well above the usual 18–25 µm coil-galvanized parts. After two years of sea spray exposure, inspection showed only light patina (no red rust). This matches corrosion data: with ~80 µm of zinc, the time to 5% rust in a tropical marine atmosphere extends by decades compared to a standard 20 µm coating (Microsoft Word – GalvInfoNote 1-1) (Performance of Galvanized Steel in… | American Galvanizers Association). In contrast, an inland fence with 18 µm coating would likely show visible rust in 5–10 years under similar inspection criteria.

This example highlights the trade-off: the thicker coating cost more upfront, but maintenance is virtually eliminated. According to the American Galvanizers Association, in many categories the zinc coating requires “no maintenance for 70 years or more” (What is the Cost of Galvanized… | American Galvanizers Association). In life-cycle terms, the luxury of extra zinc paid off by avoiding repainting.

Selecting High-Quality Galvanized Products

Ultimately, the benefits of correct coating thickness depend on product quality. Mehbud, for instance, sources commercial-grade galvanized steel and rigorously inspects it. As Mehbud’s About us page notes, “We work only with quality material… conducting a thorough check at each stage” (About us – “Mehbud” plant). We apply consistent zinc coatings and polymer topcoats, and we certify our products to national and international standards (Sheet – “Mehbud” plant).

For architects and builders, choosing a supplier like Mehbud means getting the right combination of gauge, zinc weight, and finish. For example, our fencing systems use precision-cut zinc-coated profiles, and our ventilated façade panels rely on coil-galvanized steel with polymer for long life (Sheet – “Mehbud” plant) (Sheet – “Mehbud” plant).

In summary, thicker galvanization coatings improve quality by extending corrosion protection and structural durability. Specify the minimum needed thickness (per ASTM A123, EN ISO 1461, etc.) and increase it for tougher environments. Verify thickness with proper gauges. Investing in higher-zinc coatings usually pays off through longer service life and lower maintenance costs (What is the Cost of Galvanized… | American Galvanizers Association) (Microsoft Word – GalvInfoNote 1-1). For real-world examples of galvanized solutions, see Mehbud’s fencing profiles and facade systems, or learn more on our About us page.

Sources: Industry standards and studies on hot-dip galvanizing; American Galvanizers Association guidance (Galvanization – Wikipedia) (Microsoft Word – GalvInfoNote 1-1) (Performance of Galvanized Steel in… | American Galvanizers Association) (What is the Cost of Galvanized… | American Galvanizers Association); Mehbud product and company information (Sheet – “Mehbud” plant) (About us – “Mehbud” plant). These illustrate how zinc-coating thickness is chosen for optimal corrosion protection and cost-effectiveness.