Summary: Thermal-sprayed zinc, aluminium and their alloys (often called metallizing) are a sacrificial-anode heavy-duty corrosion control. A Sa 2½ blast-cleaned surface with a sufficient anchor profile gives mechanical key; zinc protects steel cathodically, while aluminium performs better in SO₂-bearing industrial atmospheres and at elevated temperature; a sealer/topcoat (duplex) closes the metal layer's high porosity. Designed to ISO 2063-1 and ISO 12944-5, it can be built thicker, longer-lasting and locally repairable without shutdown than a zinc-rich primer, and is used on bridges, wind, offshore and petrochemical steelwork for decades of protection.
What thermal-sprayed metal coatings are
Thermal spraying melts zinc, aluminium or a zinc-aluminium wire/powder, atomises it with compressed air and deposits it onto a blast-cleaned steel surface, where it flattens and stacks layer by layer into a metal overlay. It resists corrosion through mechanical keying to the substrate plus the sacrificial (cathodic) action of zinc/aluminium on steel, so it is an active coating and follows a different logic from a purely barrier paint system.
Failure usually comes from poor preparation (salts or a shallow anchor profile give weak keying and early blistering), unsealed porosity (the medium reaches the steel through the pores, causing pitting and swelling) and insufficient thickness or a wrong environment class, which uses up the design life early.
Key data
- Mandatory surface preparation grade Sa 2½ (near-white, ISO 8501-1); anchor profile per ISO 8503 medium-to-coarse, empirically Rz ≥ 40–70 µm, to give the metal layer a mechanical key.
- Soluble salts assessed by the ISO 8502 series (chlorides commonly held to the order of ≤ 50 mg/m²); exceedance drives early blistering and delamination.
- Typical metal-layer dry film thickness: zinc 60–200 µm, aluminium 80–250 µm (ISO 2063-1 grades the level by corrosivity environment and target life; C5/marine take the thick end).
- The sprayed metal layer is porous and must be sealed/topcoated promptly: an organic sealer or topcoat typically 40–80 µm forms a metal-plus-paint duplex system that extends life.
- Adhesion verified by pull-off (ISO 4624), systems commonly required ≥ 5 MPa; anchor profile and cleanliness are the keys to passing.
- Zn-85Al-15 (about 85% zinc / 15% aluminium) combines zinc's sacrificial action with aluminium's corrosion and heat resistance; sprayed zinc powders/wires can reference ASTM B793.
Zinc vs aluminium vs zinc-rich primer
| System | Protection mechanism | Typical thickness | Favourable environment | Main limitation |
|---|---|---|---|---|
| Arc/flame-sprayed zinc | Sacrificial anode + barrier | 60–200 µm | Long-life atmospheric/marine steelwork | High porosity, needs sealing; requires Sa 2½ |
| Sprayed aluminium | Barrier + oxide film | 80–250 µm | SO₂ industrial atmosphere, elevated temperature | Weaker sacrificial action on steel; local defects need sealing |
| Zn-85Al-15 | Sacrificial + dense synergy | Set by environment | Balances corrosion resistance and cathodic protection | Higher cost, tighter process window |
| Zinc-rich primer (epoxy/inorganic) | Sacrificial anode (zinc-dust contact) | Controlled by zinc loading and coats | Shop/field primer within a system | Limited total zinc thickness; life depends on the topcoat |
Three keys to a sound application
Blast cleaning and dew point
Degrease first, then blast to Sa 2½ and control the anchor profile (ISO 8503). During application the substrate should be held at least 3 °C above the dew point to avoid condensation and poor bonding; old coatings or salt-contaminated structures must be fully removed first.
Spraying
Arc or flame spraying builds the thickness in passes and zones to ensure bonding and density. Zinc on a clean, dry surface can be applied wet-on-wet, but sealing must be completed before rust bloom—a fresh metal surface that oxidises fast loses adhesion.
Immediate sealing and topcoating
After spraying, roll/brush/spray a sealer to fill the pores per the ISO 12944-5 sealing requirements, then build the mid and topcoats into a duplex system; the sooner the sealing, the lower the porosity and the longer the service life.
Failure modes and fixes
- Bridges and long-life steelwork: sprayed zinc + sealer + topcoat, designed for decades of protection, with local repair possible without a full shutdown.
- Wind towers and offshore: C5-M high-salt atmosphere—take the thick end of zinc/Zn-Al with a resistant topcoat and focus on tubular joints and the splash zone.
- Tank shells and hot pipe racks: where SO₂ or elevated temperature is present, choose sprayed aluminium for barrier plus heat resistance.
- Early blistering/delamination: usually salts or an insufficient anchor profile—return to Sa 2½, re-test per ISO 8502 and redo the interface.
- Pitting, rust spotting through the topcoat: unsealed porosity or under-thickness—add sealer, re-check dry film thickness and step up the grade for the environment.
FAQ
Both sprayed zinc and epoxy zinc-rich primer give cathodic protection—which to choose?
Both act as sacrificial anodes. The sprayed metal layer can be built thicker, lasts longer and can be repaired locally, suiting bridges, tunnels and marine works; a zinc-rich primer is convenient and lower-cost and is often the primer within a system. In severe environments combine sprayed metal and paint as a duplex system.
Why must it be sealed immediately after spraying?
The sprayed metal layer is inherently porous, so the medium reaches the steel through the pores, and a fresh zinc/aluminium surface oxidises quickly (flash rust), hurting adhesion. Timely sealer fills the pores, extends life and gives a reliable interface for the topcoat.
Can sprayed aluminium replace sprayed zinc?
It depends on the environment. Aluminium resists SO₂ industrial atmospheres and elevated temperature better, but its sacrificial protection of steel is weaker than zinc; where strong cathodic protection is needed (marine atmosphere, scratch self-healing), prefer zinc or Zn-85Al-15.
How are thickness and life set?
Grade the environment's corrosivity per ISO 12944 (C1–C5 / Im/In), then choose the metal and its thickness grade per ISO 2063-1; the more severe the environment and the longer the target life, the thicker the metal layer and its sealing system.
Which tests are done on site?
Common acceptance checks: Sa 2½ cleanliness and anchor profile (ISO 8501/8503), soluble salts (ISO 8502), dry film thickness, pull-off adhesion (ISO 4624), plus sealer/topcoat counts and appearance.
Can an ageing structure be repaired only locally?
Yes—this is a key advantage of thermal spraying: blast, re-spray, re-seal and re-topcoat the corroded or damaged areas to restore them, without full stripping or shutdown; well suited to bridges, towers and pipe racks in service.
Last updated: 2026-09-20
References: ISO 2063-1:2019 (thermal spraying—zinc, aluminium and their alloys—part 1: guidance for design and quality requirements); ISO 12944-5:2018 (protective paint systems, incl. metal-spraying systems); ISO 8501-1 cleanliness, ISO 8503 anchor profile, ISO 8502 soluble salts, ISO 4624 pull-off adhesion; ASTM B793 (zinc powder for thermal spraying); npj Materials Degradation 2024, 8:22, DOI 10.1038/s41529-024-00480-8.
Kexin New Materials (Guangdong) Co., Ltd.