Inconel 625 or 718: weldability and corrosion against raw strength

625 is the corrosion and welding alloy; 718 is the structural one. Choosing between them is mostly a question of whether the part is loaded, welded, or exposed to chlorides.

Both alloys are nickel-chromium grades with niobium, and both are routinely specified for aerospace, marine and process equipment. The similarity ends at the strengthening mechanism: 625 gets its strength from molybdenum and niobium in solid solution plus a slow carbide reaction, while 718 is deliberately aged to precipitate a nickel-niobium phase.

	Inconel 625	Inconel 718
UNS	N06625	N07718
Nickel	≥ 58.0%	50.0 – 55.0%
Chromium	20.0 – 23.0%	17.0 – 21.0%
Molybdenum	8.0 – 10.0%	2.80 – 3.30%
Niobium	3.15 – 4.15%	4.75 – 5.50%
Iron	≤ 5.0%	11.1 – 22.5%
Strengthening	solid solution	age hardening
Corrosion	excellent, broad	good, narrower
Weldability	excellent, no post-weld treatment	good, aged after welding

Where 625 is the right answer

• Weld overlays and cladding. 625 is the standard weld-overlay alloy for carbon steel vessels because it does not need post-weld heat treatment and resists dilution well.
• Seawater and chloride process streams. The 8–10% molybdenum and higher chromium give it pitting resistance that 718 does not have.
• Bellows, expansion joints and thin-wall components that need corrosion resistance more than strength.
• Repairs on equipment whose heat treatment cannot be repeated.

Where 718 is the right answer

• Loaded structural parts: discs, shafts, landing gear, downhole tools, high-pressure housings.
• Parts where the drawing specifies a minimum yield after ageing.
• Cryogenic structures — 718 keeps toughness where many alloys go brittle.

The cost angle

625 carries more nickel and considerably more molybdenum, so it prices above 718 per kilogram — often by a third or more depending on form. Buyers sometimes try to substitute 625 where 718 was specified to "get better corrosion", and then discover that 625 annealed does not meet the strength callout. The reverse substitution is just as common and just as wrong: 718 in a chloride-heavy duty corrodes where 625 would have lasted.

If the drawing says "strength", the answer is 718. If it says "chloride" or "weld overlay", the answer is 625.

Values quoted here follow the ASTM / EN edition named in each paragraph. For a specific heat, the governing numbers are those printed on the EN 10204 certificate that ships with the material. Written by the technical desk at Shanghai Hangbo Alloy Group.