Arc Spraying and Wire Flame Spraying for Corrosion and Wear Protection

Arc spraying and wire flame spraying are the two most widely deployed thermal spray processes for large-area corrosion protection of structural steel, offshore platforms, bridges, and industrial plant — and for the dimensional restoration of worn metallic components. Both processes melt a metallic wire feedstock and project the molten droplets onto a prepared substrate, building a lamellar splat microstructure that provides either electrochemical (sacrificial) corrosion protection, barrier protection, or tribological wear resistance depending on the wire alloy selected. Together they represent the most cost-effective thermal spray technology available for field application and large-structure coverage, and are the primary methods specified in ISO 2063 and Norsok M-501 for offshore corrosion protection.

Twin-Wire Arc Spraying — Process Physics

In twin-wire arc spraying (TWAS), two metallic wires of the same or different alloy are fed continuously and converge at a contact-tip assembly where a DC arc is struck between them. The arc melts both wire tips simultaneously, and a high-velocity compressed air (or nitrogen) jet directed axially through the arc zone atomises the molten metal pool into a polydisperse droplet spray. The droplets are accelerated downstream, partially oxidise in flight, and impact the substrate at high velocity, where they flatten into thin lamellae (splats) that cool in microseconds from the molten state.

Electrical Parameters and Arc Stability

The arc voltage (typically 18–40 V DC) sets the arc length and energy density; the wire feed rate controls the welding current (100–400 A for most production guns). Constant-voltage (CV) power sources are used to maintain arc stability as wire feed speed varies with the gun’s motion over the workpiece. Atomisation air pressure (2.5–7 bar) and flow rate are the primary parameters controlling droplet size distribution and particle velocity — higher air pressure gives smaller, faster droplets and a denser, harder coating, but also increases overspray and reduces deposition efficiency.

Arc power:
  P_arc = V_arc × I   (W)

  Typical: V_arc = 28 V,  I = 200 A  →  P_arc = 5,600 W = 5.6 kW

Deposition rate (theoretical, single wire):
  DR = (I / F) × M_wire

  where F = wire feed constant (A·s/g, material and diameter dependent)

Transfer efficiency (TE):
  TE = (mass deposited) / (mass fed) × 100%
  Typical: 55–70% for arc spray (losses to overspray and fume)

Wire mass consumed per unit area at thickness t:
  m = (ρ_coating × t × A) / TE

  where:
  ρ_coating ≈ ρ_bulk × (1 – porosity fraction)
  t  = target thickness (m)
  A  = substrate area (m²)

Atomisation and Droplet Characteristics

The atomisation jet breaks the molten arc pool into droplets ranging from <10 μm to >200 μm in diameter, with a volume-median diameter typically between 40 and 100 μm depending on arc parameters and wire alloy. Larger droplets carry more thermal mass, arrive at the substrate in a more liquid state, and flatten more extensively — producing flatter, thinner splats. Smaller droplets cool rapidly in flight and may arrive partially or fully solidified, producing smaller, thicker splats with lower adhesion. The in-flight oxidation of metallic droplets (particularly aluminium and zinc) produces thin oxide shells on each droplet that become the oxide stringers visible in cross-sections of thermal spray coatings.

Substrate Impact and Splat Formation

When a molten droplet strikes the grit-blasted substrate at 100–200 m/s, it undergoes rapid spreading and solidification — the splat formation process. The flattening ratio (splat diameter / droplet diameter) is typically 3–8 for arc-sprayed metals. Solidification occurs in 1–10 μs, imposing cooling rates of 10⁶–10⁸ °C/s. This produces a fine-grained, metastable microstructure in each splat with grain sizes of 0.1–1 μm — much finer than conventionally cast or wrought material. As successive splats accumulate, the lamellar architecture develops: a stack of thin, overlapping pancake-shaped lamellae separated by thin oxide interlayers and intersplat pores.

Wire Flame Spraying — Process Physics

Wire flame spraying uses an oxy-fuel flame (oxy-acetylene at ≈3,160°C, oxy-propane at ≈2,820°C, or oxy-hydrogen at ≈2,660°C) to melt the tip of a continuously fed wire. Compressed air flows coaxially around the flame and atomises the molten wire tip into a spray stream. The absence of an electrical arc makes wire flame spraying equipment simpler, lighter, and more portable than arc spray systems — a flame spray gun can be operated without any electrical supply, making it uniquely suited to remote field work, confined-space repairs, and locations where explosion-protected electrical equipment is impractical.

Comparison with Arc Spraying

The lower particle velocity in wire flame spray (40–80 m/s vs 100–200 m/s for arc spray) has two main consequences for coating properties: higher porosity (10–20% vs 5–15%) and somewhat lower adhesion strength to the substrate. For corrosion-protective TSZ and TSA coatings where the porosity is sealed anyway, these differences are less significant than for engineering coatings where dense, hard deposits are needed. Wire flame spray remains fully compliant with ISO 2063 for corrosion protection applications when applied correctly.

Table 1 — Comparative properties of arc spraying and wire flame spraying.

Lamellar Microstructure and Coating Properties

The defining structural feature of all arc-sprayed and wire flame-sprayed coatings is the lamellar (pancake) microstructure — a consequence of the rapid splat-solidification mechanism. Understanding this microstructure is essential for predicting coating performance, specifying acceptance criteria, and identifying defects in metallographic examination.

Microstructural Components

• Splats (lamellae): Individual flattened droplets, typically 20–80 μm in diameter and 1–5 μm thick. Splat boundaries represent the primary sites for intersplat porosity and oxide stringer accumulation.
• Oxide stringers: Thin layers of metal oxide (ZnO, Al₂O₃, FeO, etc.) trapped between splats from in-flight oxidation of droplets. In zinc and aluminium coatings, these constitute 1–8% of the coating volume and contribute to the coating’s barrier properties.
• Porosity: Intersplat pores (from incomplete splat contact) and intrasplat pores (from gas entrapment or shrinkage during solidification). Porosity is interconnected at lower levels of arc spray process control, allowing electrolyte ingress; sealing closes this pathway.
• Unmelted particles: Partially or wholly solid particles that arrive at the substrate without fully melting. They appear as irregular, roughly spherical particles in cross-section and represent a localised defect — high unmelted particle content indicates process problems (low current, excessive spray distance, or contaminated wire).

Coating Adhesion

Thermal spray coatings adhere to the substrate primarily by mechanical interlocking — molten splats flow into and solidify within the peaks and valleys of the grit-blasted surface profile. True metallurgical bonding is minimal or absent due to the rapid solidification and the native oxide layer that reforms on the substrate surface within minutes of grit blasting. This is why surface preparation to Sa 3 with the correct roughness profile is so critical: bond strength measured by pull-off test (ASTM C633 or ISO 4624) is directly correlated with surface roughness and cleanliness at the time of spraying.

Grit blast standard:   Sa 3 per ISO 8501-1 (visual)
Surface roughness:     Ra  6–12 µm  (profilometer)
                       Rz 40–80 µm  (ten-point height)

Grit type:            Aluminium oxide (corundum) preferred
                      Steel grit acceptable for non-critical surfaces
Grit size:            G16–G25 (coarse angular)
Blast pressure:       5–8 bar
                      
Time before spray:    ≤ 4 h in controlled environment
                      ≤ 2 h in humid / marine atmosphere
Prohibited at spray:  Any visible rust bloom, moisture, oil, or millscale

TSZ and TSA Coating Systems for Corrosion Protection

The two most important thermal spray alloys for structural corrosion protection are thermally sprayed zinc (TSZ) and thermally sprayed aluminium (TSA). Both protect steel electrochemically — by acting as sacrificial anodes whose dissolution protects any exposed steel at coating defects, holidays, or cut edges — and as physical barriers that isolate the steel surface from the corrosive electrolyte. The choice between them depends principally on service temperature and environment.

Thermally Sprayed Zinc (TSZ)

Zinc is anodic to steel in most neutral and mildly acidic aqueous environments (zinc has a more negative standard electrode potential, E°_Zn/Zn²⁺ = −0.76 V vs SHE, compared with −0.44 V for Fe/Fe²⁺). A TSZ coating therefore acts as a distributed sacrificial anode — current flows from zinc to steel at any exposed steel site, polarising the steel cathodically and suppressing its corrosion. This galvanic mechanism provides cathodic protection of exposed steel edges and defects that a purely barrier coating cannot achieve.

The primary limitation of zinc is the polarity reversal above approximately 60°C: in aerated, near-neutral water above this temperature, the zinc-steel galvanic couple reverses and zinc becomes cathodic. TSZ is therefore restricted to ambient-temperature service — structural steelwork, bridges, jetties, and buildings — where it is the most cost-effective large-area corrosion protection system available.

Thermally Sprayed Aluminium (TSA)

Aluminium (E°_Al/Al³⁺ = −1.66 V vs SHE) is strongly anodic to steel and remains so across a wide temperature range, including above 60°C. In marine environments, TSA forms a stable, adherent Al₂O₃ passive film that provides excellent barrier protection; any breakdown of the film generates Al³⁺ ions that form corrosion products at the defect site, progressively sealing it. TSA is the preferred coating for:

• Offshore riser pipes, process piping, and heat exchanger shells operating above 60°C
• Splash-zone and immersed structural steelwork where high galvanic activity is needed
• Components subject to elevated-temperature cycling (e.g., flare stacks, exhaust ducts)
• Fireproofed structural members where the TSA coating must survive passive fire protection (PFP) application temperatures

85/15 Zn-Al Alloy

The 85% Zn / 15% Al alloy wire represents a practical compromise: it combines the galvanic activity of zinc with the elevated-temperature stability and passive film characteristics of aluminium. It is widely used for ambient to moderate-temperature (≤120°C) structural applications and is specified in ISO 2063-1 as a standard coating alloy for C4 and C5 corrosivity categories. The alloy wire is arc-sprayed in the same manner as pure Zn or Al and produces a coating microstructure containing both Zn-rich and Al-rich phases that together provide broader electrochemical protection than either metal alone.

Coating Thickness and ISO 2063-1 Requirements

Table 2 — Minimum dry film thicknesses per ISO 2063-1:2019 for thermal spray corrosion protection. Norsok M-501 specifies 200 μm TSA minimum for Norwegian offshore structures.

Wear Protection and Dimensional Restoration

Beyond corrosion protection, arc spraying and wire flame spraying are workhorses of the repair and refurbishment industry. The ability to deposit metallic coatings on worn, undersized, or damaged components without significant heat input to the substrate makes these processes uniquely suited to dimensional restoration of precision engineering components.

Dimensional Restoration of Worn Shafts and Journals

Worn pump shafts, bearing journals, roll necks, and hydraulic cylinder rods are among the most common dimensional restoration applications. The repair sequence is:

1. Condition assessment: Dimensional survey to quantify material loss; check for fatigue cracks by MPI or dye penetrant before proceeding (thermal spray cannot bridge cracks).
2. Machining: Machine the worn surface undersize (remove minimum 0.2–0.5 mm) to create a uniform, clean surface free of oxidation, scoring, and geometric errors. The machined surface is then grit-blasted.
3. Grit blasting: Sa 3, Ra 6–10 μm, using angular Al₂O₃ grit. Spray within 1–2 h of blasting.
4. Arc spraying: Apply bond coat of Ni-Al or molybdenum wire (first 50–100 μm) for improved adhesion to the machined steel. Apply topcoat wire alloy (carbon steel 1010 for most shafts; 316 SS for corrosive service; bronze for bearing journals) to 0.5–2 mm overbuild.
5. Finish machining: Turn and/or grind the sprayed surface to final drawing dimensions. Arc-sprayed steel on steel typically machines to surface finishes of Ra 0.8–1.6 μm with standard carbide tooling.
6. Quality verification: Dimensional check, hardness survey of coating cross-section, pull-off adhesion test on a process coupon sprayed simultaneously.

Wire Alloy Selection for Engineering Coatings

Table 3 — Arc and wire flame spray wire alloys, typical as-sprayed hardness, and principal applications.

Quality Control and Inspection per ISO 2063-2

ISO 2063-2:2017 defines the execution requirements and inspection procedures for thermal spray corrosion-protection coatings. A compliant quality plan must address the following inspection points:

Surface Preparation Inspection

Visual assessment of grit blast cleanliness to Sa 3 using ISO 8501-1 photographic comparator. Surface profile measurement by replica tape (Testex) or profilometer to confirm Ra 6–12 μm. Inspection for oil, moisture, rust bloom, and salt contamination (salt test per ISO 8502-6 Bresle patch, max 30 mg/m² NaCl before spraying). Ambient conditions — temperature, relative humidity (<85%), and dew point (≥3°C above dew point) — must be recorded at start and throughout the spray shift.

Coating Thickness Measurement

Dry film thickness measured by calibrated magnetic induction gauge (Elcometer 345, PosiTector 6000, or equivalent) per ISO 2178. A minimum of 5 readings per m² (or 5 per structural element), with no individual reading below 80% of the specified minimum. Any reading below this threshold triggers local remediation by additional spraying. Readings above 125% of the nominal maximum require investigation as very thick deposits can accumulate residual stress and reduce adhesion.

Adhesion Testing

Pull-off adhesion per ISO 4624 or ASTM C633 on dedicated process coupons sprayed at the beginning of each production shift. For TSZ and TSA on steel, minimum adhesion is typically 5 MPa (ISO 4624 pull-off); for engineering/dimensional restoration coatings, a minimum of 15–25 MPa is typically specified. Adhesion below minimum requires investigation of surface preparation, spray parameters, and substrate condition.

Sealer Application

Sealers (penetrating epoxy, vinyl ester, wash primer, or zinc phosphate primer) are applied by brush, roller, or spray immediately after the thermal spray deposit — within 4 h for epoxy sealers that need to penetrate open porosity before the surface oxidises. The sealer coat is applied to achieve full wetting and pore penetration without pooling. Final DFT of the sealer coat is typically 20–50 μm, confirmed by magnetic induction gauge after cure.

Industrial Applications

Offshore Oil and Gas Structures

Offshore jacket platforms, topside structures, and flare stacks are among the most demanding applications for thermal spray corrosion protection. The combination of continuous seawater spray, UV exposure, temperature cycling, and mechanical abrasion from personnel and equipment makes long-service-life coatings essential. TSA at 200–300 μm, sealed with epoxy or vinyl ester and then topcoated with polyurethane or epoxy paint system, forms the outer corrosion barrier in Norsok M-501 System 7 — one of the most durable coating systems in the offshore industry, with demonstrated service lives of 25–40 years between major maintenance.

Bridge and Infrastructure Steelwork

Arc-sprayed zinc or 85/15 Zn-Al on structural steel bridges provides long-term corrosion protection that significantly outlasts conventional paint systems in aggressive urban and coastal atmospheres. ISO 12944-5 and many national bridge codes now recognise thermal spray zinc as a corrosion protection system with design life category >25 years (High Durability) for C4/C5 environments. Large-area bridge steelwork can be arc-sprayed in situ with mobile plant at a production rate of 10–40 m²/h depending on surface complexity.

Power Generation

Gas turbine compressor blades, boiler waterwall tubes, and heat exchanger shell sides use thermal spray coatings for combined wear and high-temperature oxidation/erosion resistance. Arc-sprayed NiCrAlY bond coats (applied by arc or HVOF) underlie thermal barrier coating (TBC) systems on turbine components. In power plant boilers, wire flame-sprayed 625 alloy (Inconel) or 316L provides erosion-corrosion protection on waterwall tubes in waste-to-energy and coal-fired boilers.

Paper and Pulp Industry

Paper machine roll surfaces (press rolls, calender rolls, dryer rolls) are among the classic applications of dimensional restoration by thermal spray. Arc-sprayed ceramic/metal composite or tungsten carbide-cobalt HVOF coatings restore worn roll surfaces to original dimensions and improve surface hardness and release properties. Wire flame-sprayed bronze is used for the anti-galling threads of digester vessels and bleach plant flange faces where seizure of stainless-on-stainless threaded connections is a recurring maintenance problem.