HTS Coatings — machining & grinding

Inconel 625

21% Cr, 3% Fe, 8% Mo, 3.5% Nb, Ni-balance

  • HVOF

Inconel 625 is the coating you reach for when corrosion is the enemy, not abrasion. It's a nickel superalloy — roughly 21% chromium, 9% molybdenum, 3.5% niobium, balance nickel — and its job is to shrug off pitting, crevice attack, chloride stress-corrosion cracking, and sour-service H2S where stainless steels give up. On the hardness chart it's soft: 250-350 HV as-sprayed. If you need it to win a gouging fight with a rock, pick WC-Co or Stellite. If you need it to live in hot chloride-laden flue gas for five years on a biomass superheater, or hold pressure in an H2S well, Inconel 625 is the answer.

At HTS we apply 625 by HVOF for 10-40 mil dense barrier coatings on downhole parts, marine shafts, and scrubber internals, and by weld overlay (GTAW, CMT) for 60-120 mil fused cladding on boiler tubes and heat-exchanger components. It holds useful strength to 1800°F per Special Metals, qualifies under AMS 5599 and NACE MR0175, and is the de-facto standard for refurbishing superheater tubes in biomass, coal, and black-liquor recovery boilers.

Got a corroded superheater bank, a sour-service wellhead, or a scrubber that's eating itself? Send us the drawing — we'll scope the right 625 process and thickness for the duty.

Technical data

Hardness
250-350 HV typical as-sprayed HVOF; some tuned parameters push 340-410 HV — this is a corrosion-resistant coating, not a hard coating
Bond strength
8,500-10,500 psi HVOF (58-72 MPa); properly prepared substrates routinely exceed the 58 MPa glue-test threshold
Max service temp
~1650-1800°F in coating form (wrought parent alloy rated to 1800°F / 982°C continuous per Special Metals; sprayed deposits somewhat lower due to oxide stringers)
Max service temp
~900-982°C; usable oxidation resistance reported to ~1093°C short-term
As-sprayed porosity
<1% HVOF (typ. 0.5-1.5%); dense enough to block pitting/crevice attack when applied to parameter
Typical thickness
10-40 mils HVOF (250-1000 µm); weld-overlay cladding 60-120 mils (1.5-3 mm) when substrate permits — Inconel 625 is best deployed as a thicker corrosion barrier, not a thin flash coat
Density
8.44

Where it earns its keep

  • Exceptional broad-spectrum corrosion resistance — pitting, crevice, stress-corrosion cracking, sour service, and mixed-acid media all in one alloy
  • Retains useful strength and oxidation resistance to 1800°F (982°C), far above WC-Co's 1000°F decarburization limit
  • Near-immune to chloride-induced SCC, which makes it the default choice for seawater and downhole service
  • Excellent weldability and proven as HVOF cladding for in-situ refurbishment of superheater and waterwall tubes
  • NACE MR0175 / ISO 15156 qualified for H2S-containing sour service — spec-acceptable for oil & gas wellhead and downhole parts

Where it doesn't

  • Soft compared with WC-Co (1200+ HV) or Stellite 6 (500+ HV) — at 250-350 HV this is a corrosion coating, not an abrasion coating
  • Expensive on a per-pound basis (nickel + molybdenum + niobium); economics favor thicker cladding over thin flash coats
  • Best deployed as a 10-40 mil HVOF barrier or 60-120 mil weld overlay — too thin a layer leaves through-porosity paths in aggressive service

Typical applications

  • Biomass and waste-to-energy boiler waterwall + superheater tube cladding against alkali-chloride fly ash
  • Coal-fired boiler tubes in aggressive fly-ash and high-sulfur environments
  • Black-liquor recovery boiler tubes and heat exchangers in kraft pulp & paper mills
  • Digester and flash-tank protection in pulp mills
  • Downhole oil & gas components in sour service (H2S/CO2/chlorides) — packers, tubing hangers, wellhead trim
  • Offshore topside piping, risers, and seawater-exposed structural components
  • Marine propeller shafts, stern gear, and propulsion hardware (chloride SCC immunity)
  • Flue-gas desulfurization (FGD) scrubber internals, spray headers, and demisters
  • Chemical-process reactor vessels, piping, and heat-exchanger tubes in mixed-acid service
  • Waste-incineration grate bars and boiler internals

Wear modes addressed

  • Hot corrosion (alkali chloride / sulfate fly-ash attack)
  • Aqueous pitting and crevice corrosion (chlorides, seawater)
  • Stress-corrosion cracking in chloride and sour-service environments
  • Erosion-corrosion (flashing liquids, wet steam, slurry impingement)
  • High-temperature oxidation and sulfidation
  • General corrosion in mixed-acid chemical process streams

Industries

  • Power generation (biomass, waste-to-energy, coal, recovery)
  • Pulp & paper (kraft recovery, digesters, evaporators)
  • Oil & gas (downhole, wellhead, subsea)
  • Offshore and marine
  • Chemical and petrochemical process
  • Waste incineration and environmental
  • Flue-gas desulfurization / emissions control
  • Aerospace (engine exhaust, turbine hot sections — coating context)

Substrates

  • Carbon steel boiler tube stock (SA178, SA210, SA213-T11/T22)
  • Low-alloy steels (4130, 4140, P91)
  • Austenitic stainless (304L, 316L, 321)
  • Martensitic stainless (410, F6NM)
  • Duplex and super-duplex stainless (with dilution control)
  • Other Ni-base substrates for repair/refurbishment (625, 825, C-276)

Which process, when?

HVOF is the default thermal-spray process for Inconel 625: dense (<1% porosity), strong bond (~9,500 psi), minimal oxide stringers, and thicknesses of 10-40 mils suitable for boiler-tube barrier cladding and downhole part refurbishment. Weld overlay (GTAW, MIG, CMT) is the right call when you need a fully fused, 60-120 mil metallurgical bond — the current standard for superheater-tube protection in biomass and recovery boilers. Plasma spray is viable but generally shows more porosity and oxide content than HVOF for this alloy; it's a fallback when part geometry prohibits HVOF line-of-sight. Cold spray is emerging for heat-sensitive substrates where you want to avoid the thermal cycle entirely. Pick HVOF for thin-barrier refurbishment; pick weld overlay for thick, fused, long-service cladding.

Sources

Data points on this page draw on the following published references. Nothing here replaces a material-specific review by our process engineers — but it's the working starting point.

Material data on this page is provided as a general reference and can vary by lot, substrate, and application. Contact HTS to confirm the right material and specification for your specific part.

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