Material comparison
Three times the molybdenum on one side, a copper addition on the other — and the cheaper alloy wins in sulphuric acid.
Both are nickel–chromium–molybdenum alloys, both appear in chemical processing and oil and gas, and 625 costs considerably more. The gap is mostly molybdenum, and whether you need to pay for it depends on how aggressive the service really is.
| Element | Incoloy 825 (N08825) | Inconel 625 (N06625) |
|---|---|---|
| Nickel | 38 – 46% | min. 58% |
| Chromium | 19.5 – 23.5% | 20 – 23% |
| Molybdenum | 2.5 – 3.5% | 8 – 10% |
| Copper | 1.5 – 3.0% | max. 0.5% |
| Titanium | 0.6 – 1.2% | max. 0.4% |
| Niobium | — | 3.15 – 4.15% |
| Iron | min. 22% | max. 5% |
| Density | 8.14 g/cm³ | 8.44 g/cm³ |
Two differences carry most of the weight. 625 has roughly three times the molybdenum, which is what buys pitting and crevice corrosion resistance. 825 has copper, which specifically improves resistance to sulphuric and phosphoric acid.
The practical test. Ask what is actually attacking the part. If it is sulphuric or phosphoric acid, 825 is both cheaper and better. If it is chloride, seawater or severe sour gas, 625 is worth the money. If it is high temperature, the question was never 825.
As a rough guide against 316L plate, 825 sits at three to four times and 625 at five to seven. Between them sit Alloy 20 at two and a half to three and a half — strong in sulphuric acid, weaker in chloride — and super duplex at under two and a half, which handles many chloride applications at a fraction of the nickel-alloy cost provided the temperature stays moderate.
A common and avoidable mistake is specifying 625 for a sulphuric acid duty. It costs roughly twice what 825 does and performs less well.
Both alloys appear in NACE MR0175 / ISO 15156 Part 3 for sour service, with different environmental limits on temperature, hydrogen sulphide partial pressure, chloride content and elemental sulphur. 625 carries the wider envelope. Where a project is governed by the standard, the specific table limits for the intended condition must be checked rather than relying on the material being listed at all.
No. In sulphuric and phosphoric acid, 825 performs better because of its copper content, and it costs less. 625's advantage is in chloride environments, severe sour service and at high temperature.
Only after reviewing the service environment. For sulphuric acid duty it is both cheaper and better. For seawater, high chloride or severe sour conditions the molybdenum difference is too large to substitute away, and on a pressure-retaining part that substitution needs engineering approval.
Around 550 °C in practice, limited by sensitisation and loss of strength rather than by oxidation. For sustained service above that, 625 or an 800-series alloy is the appropriate choice.
Both are, and the selection follows the well conditions. 825 is common in moderately sour wells; 625 is specified where hydrogen sulphide partial pressure, chloride content or temperature exceed 825's NACE envelope.
Send us the operating temperature, the medium and the pressure. We will come back with a material recommendation and the cost difference between the two options.
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