Weld Overlay with Nickel Alloys: Cladding Carbon Steel for Corrosion Resistance

Weld overlay - also called weld cladding or hardfacing - is a proven, code-compliant method for applying a corrosion-resistant nickel alloy layer onto a carbon steel or low-alloy steel substrate. The result: the mechanical strength and low cost of carbon steel, combined with the corrosion resistance of nickel alloys - at 40–60% lower material cost than solid nickel alloy construction. This article explains when weld overlay is the right choice, which nickel alloys are used, what standards apply, and how to specify overlay correctly.

Weld Overlay with Nickel Alloys

Weld overlay with nickel alloys is the industry-standard solution for pressure vessels, heat exchangers, and piping components that must resist corrosion from sour gas (H₂S), chlorides, acids, and high-temperature oxidation - without the cost of solid alloy fabrication.

Weld overlay is the process of depositing one or more layers of a corrosion-resistant alloy (typically nickel-based) onto the surface of a cheaper base metal (typically carbon steel or low-alloy steel) using arc welding processes. The overlay layer - typically 3–6 mm thick - acts as a barrier between the corrosive process fluid and the structural base metal.

Why Use Overlay Instead of Solid Alloy?

Table. Cost and Performance Comparison - Solid Alloy vs. Weld Overlay Construction


Parameter	Solid Nickel Alloy	Carbon Steel + Ni-Alloy Overlay	Cost Savings	Performance
Material Cost ($/kg, typical)	$35–60	$8–12 (CS) + $15–25 (overlay)	40–60% lower	Equivalent corrosion resistance
Fabrication Time	Longer (difficult to weld)	Shorter (CS welds easily)	20–30% faster	Same service life
Design Code Compliance	ASME VIII, ASME B31.3	ASME VIII, ASME B31.3	Same	Same
Typical Overlay Thickness	N/A (solid)	3–6 mm (2–3 layers)	N/A	Corrosion allowance included
Repair and Maintenance	Difficult (crack-sensitive)	Easier (overlay can be re-applied)	Lower lifecycle cost	Longer service life
Weight	Higher (density 8.4–8.9 g/cm³)	Lower (CS density 7.85 g/cm³)	5–10% lighter	Same pressure rating

Source: ASME Boiler and Pressure Vessel Code Section VIII Div. 1 (2023); ASME B31.3 Process Piping (2022); Special Metals Corporation 'Welding Products Catalog' (2023); Haynes International 'Corrosion-Resistant Alloys' Technical Data; JN Alloys internal cost analysis for 2024–2025.

For large-diameter vessels, heat exchanger shells, and piping spools where corrosion attack is limited to the internal surface, weld overlay delivers the same corrosion performance as solid alloy at 40–60% lower material cost.

Not all nickel alloys are suitable for weld overlay. The overlay material must be weldable, compatible with carbon steel dilution, and resistant to the specific corrosive environment. Two alloys dominate the industry: Alloy 625 (UNS N06625) and Alloy C-276 (UNS N10276).

Alloy Selection by Corrosion Environment

Table. Nickel Alloy Overlay Materials - Selection Guide by Corrosion Environment


Alloy (UNS)	Key Alloying Elements	Primary Corrosion Resistance	Typical Overlay Applications	Weld Process	ASME P-Number
Alloy 625 (N06625)	Ni-22Cr-9Mo-3.5Nb	Oxidation, chloride pitting, H₂S, seawater	Wellhead equipment, subsea connectors, pressure vessels	GTAW, GMAW, FCAW, SAW	P-No. 43
Alloy C-276 (N10276)	Ni-16Cr-16Mo-4W	Reducing acids, HCl, H₂SO₄, chlorine, sour gas	Chemical reactors, scrubbers, FGD systems, acid piping	GTAW, GMAW, FCAW, SAW	P-No. 44
Alloy C-22 (N06022)	Ni-22Cr-13Mo-3W	Severe mixed acids, hypochlorite, wet chlorine	Pharmaceutical, pulp & paper, waste incineration	GTAW, GMAW	P-No. 44
Alloy 825 (N08825)	Ni-22Cr-42Fe-3Mo-1Ti	Sour gas (H₂S), sulfuric/phosphoric acid	Downhole tubing, gas processing, heat exchangers	GTAW, GMAW	P-No. 45
Alloy 600 (N06600)	Ni-16Cr-8Fe	High-temperature oxidation, nitriding, caustic	Furnace components, heat treatment baskets	GTAW, GMAW	P-No. 43
Alloy X (N06002)	Ni-22Cr-18Fe-9Mo	Oxidation at 1100–1200°C, carburization	Combustion chambers, gas turbine components	GTAW, GMAW	P-No. 43
Alloy 59 (N06059)	Ni-23Cr-16Mo-1Fe	Extreme reducing acids, HCI, H₂SO₄ mixtures	Chemical process, pollution control	GTAW, GMAW	P-No. 44

Source: Special Metals Corporation 'INCONEL® Alloy 625' and 'INCONEL® Alloy 725' Data Sheets (2023); Haynes International 'HASTELLOY® C-276 Alloy' Technical Data (2023); ASME Boiler and Pressure Vessel Code Section IX (2023) QW-422 P-Number Grouping; NACE MR0175 / ISO 15156-3:2015.

Chemical Composition Comparison

Table. Chemical Composition of Common Nickel Alloy Overlay Materials (Weight %, Typical)


Alloy (UNS)	Ni	Cr	Mo	Fe	W	Nb	Ti	C (max)
Alloy 625 (N06625)	58–63	20–23	8–10	≤ 5	-	3.15–4.15	≤ 0.4	0.10
Alloy C-276 (N10276)	Balance	14.5–16.5	15–17	4–7	3–4.5	-	-	0.01
Alloy C-22 (N06022)	Balance	20–22.5	12.5–14.5	2–6	2.5–3.5	-	-	0.015
Alloy 825 (N08825)	38–46	19.5–23.5	2.5–3.5	22 min	-	-	0.6–1.2	0.05
Alloy 600 (N06600)	72 min	14–17	-	6–10	-	-	-	0.15
Alloy X (N06002)	Balance	20.5–23	8–10	17–20	0.2–1.0	-	-	0.05–0.15
Alloy 59 (N06059)	Balance	22–24	15–16.5	≤ 1.5	-	-	-	0.01

Source: ASTM B443 (Alloy 625); ASTM B575 (Alloy C-276, C-22, 59); ASTM B425 (Alloy 825); ASTM B168 (Alloy 600); ASTM B435 (Alloy X); Special Metals and Haynes International data sheets (2023).

Alloy 625 is the default choice for seawater, chloride, and sour gas (H₂S) service. Alloy C-276 is the default choice for reducing acids (HCl, H₂SO₄) and severe chemical environments. For borderline cases, consult NACE MR0175 / ISO 15156-3 and conduct corrosion testing.

The choice of welding process affects overlay quality, dilution rate, deposition rate, and cost. For critical pressure equipment, GTAW (TIG) and GMAW (MIG) are the most common choices. For large-area cladding, FCAW and SAW offer higher productivity.

GTAW and GMAW for High-Integrity Overlay

Process Comparison for Nickel Alloy Overlay

Table. Welding Processes for Nickel Alloy Overlay - Comparison by Quality and Productivity


Process	Dilution Rate	Deposition Rate	Best Application	Position	Skill Level	Cost
GTAW (TIG)	10–25%	0.5–1.5 kg/h	First layer, root pass, thin overlay, small diameter	All positions	High	High
GMAW (MIG)	20–30%	2–4 kg/h	Fill and cap layers, medium-thickness overlay	Flat, horizontal	Medium	Medium
FCAW (Flux-Cored)	20–35%	3–6 kg/h	Large-area overlay, non-critical applications	Flat, horizontal	Medium	Low
SAW (Submerged Arc)	30–50%	5–15 kg/h	Large vessels, heavy cladding, high productivity	Flat only	Medium	Lowest
PAW (Plasma Arc)	5–15%	1–3 kg/h	Precision overlay, automated systems	Flat, mechanized	High	High
Laser Cladding	5–10%	1–5 kg/h	Precision repair, thin overlay, minimal dilution	Flat, mechanized	High	Very High

Source: ASME Boiler and Pressure Vessel Code Section IX (2023); AWS D1.6/D1.6M-22 'Structural Welding Code-Stainless Steel'; Special Metals 'Welding Guidelines for Nickel Alloys' (2023); Lincoln Electric 'Nickel Alloy Welding Products' Technical Guide (2023).

Dilution Management Is Critical for Overlay Performance

Dilution occurs when the base metal (carbon steel) melts and mixes with the filler metal (nickel alloy). Excessive dilution reduces the corrosion resistance of the overlay by lowering the critical alloy content (Cr, Mo, Nb). For Alloy 625 overlay, the iron content should be kept below 10% in the first layer; for C-276, below 7%.

Table. Maximum Allowable Dilution - Iron Content Limits by Alloy


Alloy	Maximum Fe in Overlay (wt%)	Minimum Cr in Overlay (wt%)	Minimum Mo in Overlay (wt%)	Recommended Process
Alloy 625	≤ 10%	≥ 18%	≥ 7%	GTAW or GMAW for first layer; FCAW/SAW for fill
Alloy C-276	≤ 7%	≥ 14%	≥ 14%	GTAW for first layer; GMAW/FCAW for fill
Alloy C-22	≤ 6%	≥ 19%	≥ 11%	GTAW or GMAW for all layers
Alloy 825	≤ 15%	≥ 20%	≥ 2.5%	GTAW or GMAW
Alloy 600	≤ 10%	≥ 14%	N/A	GTAW or GMAW

Source: Special Metals 'Welding Guidelines for INCONEL® Alloy 625' (2023); Haynes International 'Welding and Fabrication of HASTELLOY® C-276 Alloy' (2023); ASME Section IX QW-404.12 'Filler Metal Dilution'; AWS A5.14/A5.14M:2018 'Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods.'

Use GTAW (TIG) for the first layer to minimize dilution and ensure chemistry compliance. Use GMAW, FCAW, or SAW for subsequent layers to increase productivity. Always verify overlay chemistry by PMI (Positive Material Identification) or laboratory analysis.

The minimum overlay thickness is governed by corrosion allowance, dilution depth, welding process capability, and code requirements. For most pressure vessels and piping, 3–6 mm (2–3 layers) is standard. Thinner overlays risk breakthrough; thicker overlays increase cost and may introduce residual stress issues.

Thickness Design Guidelines

Table. Overlay Thickness Design - Recommended Values by Application


Application	Corrosion Severity	Minimum Thickness (mm)	Typical Layers	Code Reference	Notes
Pressure vessels (general)	Moderate (no H₂S)	3.0	2 layers	ASME VIII Div. 1 UG-16	1.5 mm min after machining
Sour gas service (H₂S)	Severe (NACE MR0175)	4.0–5.0	2–3 layers	NACE MR0175 / ISO 15156	Verify hardness ≤ 22 HRC
Seawater heat exchangers	High Cl⁻ pitting	4.0	2–3 layers	ASME VIII + user spec	Use Alloy 625 or 825
Acid gas absorption columns	Severe (HCl, H₂SO₄)	5.0–6.0	3 layers	ASME VIII + user spec	Use Alloy C-276 or C-22
Subsea wellhead equipment	Severe + HPHT	5.0–6.0	3 layers	API 6A, API 17D	Use Alloy 625, verify by PMI
FGD (flue gas desulfurization)	Severe (wet SO₂, HCl)	5.0	3 layers	User specification	Use Alloy C-276 or C-22
Catalytic reformers	High-temperature oxidation	4.0	2–3 layers	API 560 / user spec	Use Alloy 600 or X

Source: ASME Boiler and Pressure Vessel Code Section VIII Div. 1 (2023) UG-16 'Minimum Wall Thickness'; NACE MR0175 / ISO 15156-3:2015 'Petroleum and Natural Gas Industries-Materials for Use in H₂S-Containing Environments'; API 6A (21st Ed., 2024) 'Wellhead and Tree Equipment'; API 17D (3rd Ed., 2021) 'Design and Operation of Subsea Production Systems'.

Corrosion Allowance Calculation

The overlay thickness must include: (1) corrosion allowance (typically 1.5–3.0 mm for 10–20 year service life), (2) machining allowance (0.5–1.0 mm for surface finishing), and (3) dilution zone thickness (1.0–1.5 mm for the first layer).

Table. Overlay Thickness Breakdown - Corrosion Allowance and Manufacturing Tolerances


Thickness Component	Typical Range (mm)	Purpose	Control Method	Standard Reference
Corrosion allowance	1.5–3.0	Material loss over design life	Process design spec	ASME VIII UG-25
Machining allowance	0.5–1.0	Surface finishing, NDE preparation	Machining tolerance	ASME B46.1
Dilution zone	1.0–1.5	First layer mixed with base metal	Welding procedure (PQR)	ASME IX QW-404
Minimum sound metal	0.5–1.0	Safety margin above dilution	UT or RT inspection	ASME V Article 4
Total design thickness	3.5–6.5	Sum of above components	Engineering design	Project specification

Source: ASME Boiler and Pressure Vessel Code Section VIII Div. 1 UG-25 'Corrosion Allowance'; ASME B46.1-2019 'Surface Texture (Surface Roughness, Waviness, and Lay)'; ASME Section V Article 4 'Ultrasonic Examination Methods for Welds'; ASME Section IX 'Welding, Brazing, and Fusing Qualifications'.

Conclusion: For most applications, specify a minimum overlay thickness of 3.0 mm after machining. For severe corrosion environments (sour gas, acids, seawater), specify 4.0–5.0 mm. Always include corrosion allowance in the design specification.

Because the overlay layer is thin (3–6 mm) and the corrosion resistance depends on precise chemistry, quality control is more critical than for solid alloy construction. Every overlay must be verified by Positive Material Identification (PMI) and inspected by ultrasonic testing (UT) or radiographic testing (RT).

Nickel Alloys PMI Verification and NDE

Inspection Requirements for Weld Overlay

Table. Quality Control Tests for Nickel Alloy Weld Overlay


Test	Purpose	Method	Acceptance Criteria	Frequency	Standard
PMI (Positive Material ID)	Verify overlay chemistry	XRF or OES	Cr, Mo, Nb within spec; Fe ≤ limit	100% of overlay surface	ASTM E1621
Ultrasonic Testing (UT)	Detect lack of fusion, porosity	Straight beam or angle beam	No indications > 20% DAC	100% of overlay area	ASME V Art. 4, 5
Radiographic Testing (RT)	Detect internal defects	X-ray or gamma ray	Per ASME VIII UW-51	Spot or 100% as specified	ASME V Art. 2
Liquid Penetrant (PT)	Detect surface cracks	Visible or fluorescent dye	No linear indications > 1.5 mm	100% of overlay surface	ASME V Art. 6
Hardness Test	Verify compliance with NACE MR0175	Portable hardness tester	≤ 22 HRC (sour service)	Per NACE MR0175 Table A.1	ASTM E10 / E18
Ferrite Number (if applicable)	Not required for Ni-alloy overlay	Ferrite meter	N/A (austenitic)	N/A	ASTM A342
Metallographic Examination	Verify microstructure, dilution	Cross-section, etch	No cracks, porosity, or excessive dilution	Per procedure qualification	ASTM E3, E407

Source: ASTM E1621-21 'Standard Guide for Elemental Analysis by Wavelength Dispersive X-Ray Fluorescence Spectrometry'; ASME Boiler and Pressure Vessel Code Section V (2023) Articles 2, 4, 5, 6; ASME Section VIII Div. 1 UW-51 'Radiographic Examination of Welds'; NACE MR0175 / ISO 15156-3:2015 Table A.1 'Hardness Requirements for Carbon and Low-Alloy Steels'.

Common Defects and Their Causes

Table. Common Weld Overlay Defects - Causes and Prevention


Defect	Appearance	Root Cause	Prevention	Detection Method
Lack of fusion	Linear indication at interface	Insufficient heat input, poor technique	Increase amperage, proper travel speed	UT, RT
Porosity	Round gas pockets in overlay	Contamination (oil, moisture, oxide)	Clean base metal, use dry filler, proper gas shield	RT, UT
Hot cracking	Intergranular cracks in overlay	High restraint, improper bead sequence	Control heat input, proper joint design	PT, VT, metallography
Excessive dilution	Low Cr/Mo in overlay	High heat input, improper technique	Reduce amperage, use GTAW for first layer	PMI, chemical analysis
Tungsten inclusion	Bright particles in overlay	GTAW electrode contact with weld pool	Maintain proper standoff, use proper electrode	RT, VT
Slag inclusion	Irregular dark spots	Improper interpass cleaning (FCAW, SAW)	Clean between passes, proper flux handling	RT, UT
Undercut	Groove at toe of weld	Excessive current, improper technique	Reduce amperage, proper torch angle	VT, PT

Source: AWS D1.6/D1.6M-22 'Structural Welding Code-Stainless Steel'; Special Metals 'Welding Guidelines for Nickel Alloys' (2023); ASME Section IX QW-190 'Examination and Repair of Welds'.

PMI verification of overlay chemistry is mandatory for every job. UT or RT inspection confirms bond integrity. For sour service, hardness testing is required by NACE MR0175. All inspection results must be documented in the material test report.

Weld overlay is not an 'alternative' method - it is explicitly recognized and governed by ASME design codes. The overlay layer can be counted as corrosion allowance but not as pressure-retaining thickness unless specifically qualified and documented.

Code Requirements for Weld Overlay

Table. ASME Code Requirements for Weld Overlay Construction


Code	Paragraph	Requirement	Overlay Treatment	Key Notes
ASME VIII Div. 1	UG-16, UW-42	Minimum thickness, weld procedure	Overlay = corrosion allowance	Overlay not included in pressure design
ASME VIII Div. 1	UCL-30 to UCL-46	Clad and weld overlay vessels	Overlay may be included if qualified	Must meet UCL-30 requirements
ASME VIII Div. 2	Part D, Article D.11	Clad and weld overlay	Similar to Div. 1	Design-by-analysis required for overlay
ASME B31.3	Para. 323.4.2	Clad and weld overlay piping	Overlay = corrosion allowance	Overlay may be used for pressure if qualified
ASME B31.3	Para. 328.5.1	Weld overlay procedure	Must be qualified per ASME IX	PQR required for each overlay process
ASME IX	QW-214, QW-283	Weld overlay procedure qualification	Overlay PQR requires chemical analysis	Must demonstrate dilution control
NACE MR0175	ISO 15156-3	Sour service materials	Overlay acceptable for CS base	Hardness ≤ 22 HRC, qualified procedure

Source: ASME Boiler and Pressure Vessel Code Section VIII Div. 1 (2023) UG-16, UW-42, UCL-30 to UCL-46; ASME Boiler and Pressure Vessel Code Section VIII Div. 2 (2023) Part D Article D.11; ASME B31.3-2022 'Process Piping' Paragraphs 323.4.2, 328.5.1; ASME Boiler and Pressure Vessel Code Section IX (2023) QW-214, QW-283; NACE MR0175 / ISO 15156-3:2015.

When Overlay Can Be Included in Design Thickness

By default, ASME codes treat weld overlay as corrosion allowance - it is not included in the pressure-retaining thickness calculation. However, if the overlay procedure is qualified to demonstrate full metallurgical bond and the design specification explicitly permits it, the overlay thickness (minus corrosion allowance) may be credited.

Table. Conditions for Including Overlay in Pressure Design


Condition	Requirement	Verification	Reference
Full metallurgical bond	No lack of fusion, complete fusion at interface	UT examination, 100% coverage	ASME VIII UCL-30
Qualified overlay procedure	PQR with chemical analysis, mechanical tests	ASME IX QW-283 qualification	ASME IX QW-214
Design specification approval	Engineer must approve overlay credit	Written specification required	ASME VIII UG-22
Corrosion allowance deducted	Only sound metal above corrosion allowance credited	Corrosion allowance per UG-25	ASME VIII UG-25

Source: ASME Boiler and Pressure Vessel Code Section VIII Div. 1 UCL-30 'Clad and Weld-Metal-Lined Vessels'; ASME Section IX QW-283 'Welding Variables for Corrosion-Resistant Overlay'; WRC Bulletin 447 'Guidelines for Assessing Weld Overlay Cladding in Pressure Equipment' (1999).

Weld overlay is fully code-compliant. By default, it is treated as corrosion allowance and not included in pressure design. If the design specification permits, overlay may be credited toward pressure thickness - but this requires qualified procedures and explicit engineering approval.

Weld overlay with nickel alloys is most commonly used in industries where the process environment is highly corrosive but the equipment size makes solid alloy construction economically unattractive. The oil & gas industry is the largest user, followed by chemical processing and power generation.

Nickel Alloys for Oil Gas Chemical and Power Generation

Application Examples by Industry

Table. Weld Overlay Applications by Industry and Service Environment


Industry	Equipment	Corrosion Mode	Typical Alloy Overlay	Thickness (mm)	Code/Standard
Oil & Gas (Upstream)	Wellhead, X-tree, choke valves	H₂S, CO₂, Cl⁻, seawater	Alloy 625	5–6	API 6A, API 17D, NACE MR0175
Oil & Gas (Midstream)	Pipeline, valve bodies, pump casings	H₂S, CO₂, wet gas	Alloy 625 or 825	3–5	ASME B31.3, NACE MR0175
Oil & Gas (Downstream)	Hydrocracker, HDS reactors	H₂, H₂S, high temp	Alloy 625 or 825	4–5	API 934-A, NACE RP045
Petrochemical	Reactor vessels, heat exchangers	Acids, chlorides, H₂S	Alloy C-276 or C-22	4–6	ASME VIII, user spec
Chemical Process	Acid reactors, scrubbers, piping	HCl, H₂SO₄, H₃PO₄, Cl₂	Alloy C-276 or C-22	5–6	ASME VIII, user spec
Power Generation	FGD absorbers, ducting, fans	SO₂, HCl, wet scrubber	Alloy C-276 or C-22	4–6	ASTM, user spec
Desalination	Heat exchanger tubesheets	Seawater, high temp	Alloy 625 or 825	3–4	ASTM, user spec
Marine/Offshore	Pumps, valves, sea water piping	Seawater, biofouling	Alloy 625	4–5	DNV, ABS, user spec
Pulp & Paper	Digesters, bleach washers	NaOH, ClO₂, H₂SO₄	Alloy C-276 or C-22	4–6	TAPPI, user spec

Source: API 6A (21st Ed., 2024) 'Wellhead and Tree Equipment'; API 17D (3rd Ed., 2021) 'Subsea Production Systems'; API 934-A (3rd Ed., 2019) 'Materials and Fabrication of 2¼Cr-1Mo and 3Cr-1Mo Steel Heavy Wall Pressure Vessels for High-Temperature, High-Pressure Hydrogen Service'; NACE RP045-2003 'Overlay Welding for Hydrogen Service'; ASME B31.3-2022 'Process Piping'; DNVGL-OS-F101 (2021) 'Submarine Pipeline Systems'.

Case Study: Wellhead Equipment Overlay

Subsea wellhead equipment and Christmas trees operate in some of the most aggressive corrosion environments: seawater on the outside, sour gas (H₂S) and brine on the inside, and temperatures up to 200°C. Solid Alloy 625 construction would cost 5–8× carbon steel. Weld overlay of Alloy 625 on carbon steel or low-alloy steel (e.g., AISI 4130, F22) delivers the same corrosion resistance at 40–50% lower cost.

Table. Case Study - Subsea Wellhead Hub: Alloy 625 Overlay on F22 Steel


Parameter	Solid Alloy 625	F22 + Alloy 625 Overlay	Savings	Notes
Base Material Cost	$55/kg	$4/kg (F22)	90% lower	F22 is low-alloy Cr-Mo steel
Overlay Cost	N/A	$18/kg (625 filler + labor)	N/A	GTAW + GMAW process
Total Fabricated Cost	$180,000	$95,000	47% lower	Per API 6A specification
Delivery Time	16 weeks	10 weeks	6 weeks faster	F22 welds easily, overlay is routine
Corrosion Performance	Excellent	Excellent (equivalent)	Same	Overlay qualified per API 6A
Code Compliance	API 6A, NACE MR0175	API 6A, NACE MR0175	Same	Overlay hardness ≤ 22 HRC

Source: API 6A (21st Ed., 2024) 'Wellhead and Tree Equipment'; NACE MR0175 / ISO 15156-3:2015; JN Alloys internal project data for subsea wellhead hub fabricated 2023; Special Metals 'INCONEL® Alloy 625' Data Sheet (2023).

For large, thick-walled equipment where corrosion attack is limited to the internal surface, weld overlay is the economically superior choice. Solid alloy should be reserved for small components (valve internals, instrument fittings) where overlay is impractical.

The filler metal specification (AWS classification) determines the overlay chemistry and corrosion resistance. For Alloy 625 overlay, use ERNiCrMo-3 or ERNiCrMo-4; for C-276 overlay, use ERNiCrMo-4. The -4 classification has higher Mo content and is preferred for severe reducing acid service.

Filler Metal Classifications for Nickel Alloy Overlay

Table. AWS Filler Metal Classifications for Nickel Alloy Weld Overlay


AWS Classification	UNS	Matching Alloy	Cr (%)	Mo (%)	Nb+Ta (%)	Primary Application
ERNiCr-3	N06082	Alloy 600, 601	18–22	-	2.0–3.0	High-temp oxidation, dissimilar welds
ERNiCrMo-3	N06625	Alloy 625	20–23	8–10	3.15–4.15	Seawater, H₂S, chlorides, general overlay
ERNiCrMo-4	N10276	Alloy C-276	14.5–16.5	15–17	-	Reducing acids, severe corrosion
ERNiCrMo-10	N06022	Alloy C-22	20–22.5	12.5–14.5	-	Mixed acids, hypochlorite
ERNiCrMo-14	N06686	Alloy 686	19–23	15–17	-	Extreme acids, seawater
ERNiFeCr-1	N08065	Alloy 825	19.5–23.5	2.5–3.5	-	Sour gas, sulfuric acid
ERNiMo-7	N10665	Alloy B-2, B-3	≤ 1.0	26–30	-	Hydrochloric acid, reducing environments

Source: AWS A5.14/A5.14M:2018 'Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods'; AWS A5.11/A5.11M:2017 'Specification for Nickel and Nickel-Alloy Welding Electrodes for Shielded Metal Arc Welding'; Special Metals 'Welding Products Catalog' (2023); Haynes International 'Welding Filler Metals' Technical Data (2023).

Filler Metal Selection by Service Environment

Table. Filler Metal Selection Guide - Nickel Alloy Overlay by Corrosion Environment


Service Environment	Corrosion Mode	Recommended Filler	Alternative	Notes
Seawater, brine	Cl⁻ pitting, crevice	ERNiCrMo-3 (625)	ERNiCrMo-4	625 is standard for seawater
Sour gas (H₂S)	SSC, HIC, SOHIC	ERNiCrMo-3 or ERNiFeCr-1	ERNiCrMo-4	Verify hardness ≤ 22 HRC
Hydrochloric acid	General corrosion	ERNiMo-7 (B-2/B-3)	ERNiCrMo-4	B-3 is preferred for HCl
Sulfuric acid	General corrosion	ERNiCrMo-4 or ERNiFeCr-1	ERNiCrMo-10	C-276 for < 50% H₂SO₄
Mixed acids (HCl + H₂SO₄)	Severe corrosion	ERNiCrMo-4 or ERNiCrMo-10	ERNiCrMo-14	C-22 or 686 for extreme service
High-temperature oxidation	Oxidation, carburization	ERNiCr-3	ERNiCrCoMo-1	Use for > 900°C service
Caustic (NaOH, KOH)	Caustic cracking	ERNiCr-3	ERNiCrMo-3	Alloy 600 filler for caustic
Flue gas desulfurization	SO₂, HCl, wet scrubber	ERNiCrMo-4	ERNiCrMo-10	C-276 is standard for FGD

Source: NACE MR0175 / ISO 15156-3:2015 'Petroleum and Natural Gas Industries-Materials for Use in H₂S-Containing Environments'; Special Metals 'Corrosion-Resistant Alloys Selection Guide' (2023); Haynes International 'Corrosion Data' Technical Brochure (2023); Outokumpu 'Corrosion Handbook for Stainless Steels and Nickel Alloys' (2021).

ERNiCrMo-3 (Alloy 625 filler) is the most versatile choice for general overlay applications. ERNiCrMo-4 (C-276 filler) is preferred for severe reducing acid service. Always verify overlay chemistry by PMI to ensure the filler metal and welding procedure deliver the required Cr, Mo, and Fe content.