Alloy 825 vs Alloy 625: which one for your corrosive duty?

The metallurgical difference

Alloy 825 (UNS N08825, EN 2.4858) is Ni-21Cr-3Mo with titanium added and about 30 % iron. The titanium stabilises it against intergranular attack, and the iron content lowers cost. Alloy 625 (UNS N06625, EN 2.4856) is Ni-21Cr-9Mo with niobium: nearly twice the molybdenum pushes its PREN (pitting resistance equivalent) to about 50, and Nb gives high strength without heavy heat treatment.

Where Alloy 825 wins

• Sulfuric acid at mid concentrations and moderate temperatures
• Wet-process phosphoric acid and pickling operations
• Sour oilfield service — 825 is qualified per NACE MR0175 / ISO 15156
• Budget: the ~30 % iron content makes plate and pipe noticeably cheaper than 625

Where Alloy 625 wins

• Seawater and high-chloride environments — pitting and crevice resistance is in another class (PREN ~50 vs ~30)
• Applications needing strength from the mill condition: yield ≈ 490 MPa as-welded, no ageing required
• Weld overlays and dissimilar joints — 625 filler (ERNiCrMo-3) is the industry default
• Service up to about 980 °C where oxidation resistance matters

Property	Alloy 825	Alloy 625
UNS / EN	N08825 / 2.4858	N06625 / 2.4856
PREN	~30	~50
Yield strength (typ. plate)	≈ 240 MPa	≈ 490 MPa
Product standards	B424 plate, B425 bar, B423 pipe, B163 tube	B443 plate, B446 bar, B444 pipe, B163 tube
Sweet spot	Uniform acid attack	Localized chloride attack, strength

Cost and availability

As a rough rule, 825 plate runs 15–25 % below 625 at the same size, and 825 is easier to source in heavy plate. 625 is stocked everywhere as welding wire and small-bore tubing because of its role as a filler metal. If your duty is genuinely uniform acid attack, specifying 625 is money spent on resistance you will never use.

Common mistakes

• Choosing 825 for a seawater-cooled exchanger because it is cheaper — crevice attack under deposits will end the discussion.
• Choosing 625 for hot caustic or concentrated HCl, where neither grade is the right answer.
• Assuming NACE qualification transfers automatically — 825 is MR0175-qualified only in specific hardness and condition limits.