Why Alloy 400 Handles Reducing Chemical Media
Alloy 400 is a single-phase nickel-copper alloy with a nominal nickel content of about 63% and copper as the principal balance element, supported by small controlled additions of iron, manganese, silicon and carbon. Its corrosion resistance is a property of the alloy matrix itself, not of a passive surface film. Stainless steels rely on a chromium-rich oxide layer to resist attack, and that layer cannot be sustained in oxygen-starved or strongly reducing chemistry. Alloy 400 has no such dependency, which is the single most important reason it is specified for hydrofluoric acid, chlorinated solvents and sulphur-bearing process liquors.
Two service variables dominate real corrosion behaviour, and they must always be judged together with concentration: aeration and temperature. Aeration supplies the cathodic reactant that accelerates attack on nickel-copper alloys, while temperature raises reaction kinetics. A rating that is safe in a deaerated line at ambient conditions can become unacceptable when the same stream is sparged with air or heated.
Sulphurous Acid and Pulp Mill Service
Sulphurous acid is a corrosive medium in its own right, and the dilute calcium bisulphite liquors used to prepare sulphite pulp are particularly aggressive, both in the spent cooking liquor and in the paper stock itself. Alloy 400 performs well in this chemistry and is widely used for washers, thickeners and screens in sulphite pulp mills. Sulphurous acid condensing from the flue gas of heating units and smelting processes is a closely related problem, and the same alloy is frequently chosen for the wet sections of fume washing systems, where the sulphuric acid content is low and the chemistry can shift toward the less aggressive sulphurous form.
Hydrochloric Acid: Practical Concentration Limits
Hydrochloric acid is one of the media that Alloy 400 can handle, but only inside clearly defined windows. The corrosion rate stays below 10 mpy (about 0.25 mm/y) at room temperature in a 10% solution. As a working rule, non-aerated solutions and 10% aerated solutions should not exceed roughly 20% concentration at room temperature. Where hydrochloric acid is generated by the hydrolysis of chlorides or chlorinated solvents, the levels encountered are usually below 0.5%, and the alloy serves that duty well at temperatures of about 93 °C (200 °F).
In air-saturated hydrochloric acid above room temperature, service is normally restricted to less than 3% to 4% HCl. The presence of oxidising salts accelerates corrosion sharply and should be treated as disqualifying unless their concentration can be held at a very low level.
Hydrofluoric Acid and Chlorinated Solvents
Alloy 400 is one of the best metallic choices for hydrofluoric acid. In unaerated acid it tolerates every concentration up to the boiling point. Aeration or the presence of oxidising salts raises the corrosion rate markedly, so both must be excluded from the design basis. The alloy is used in the production and filtration of hydrofluoric acid and for critical wetted components in contact with flowing acid, such as bubble caps, valves and piping. It also resists chlorinated solvents at boiling temperature, which places it in dry cleaning equipment and in solvent distillation and recovery units.
Organic Acids, Nitric Acid and Oxidising Media
Resistance to organic acids is an outstanding feature of the grade. It handles glacial acetic acid well and has been used to distil fatty acids at about 260 °C (500 °F). As with other non-oxidising acids, aeration and higher temperature increase the attack rate, and both should be factored into any rating. Nitric and nitrous acids are the opposite case: these strongly oxidising media are better served by stainless steel, and Alloy 400 is not the appropriate selection for them.
Media-by-Media Selection Table
| Medium | Typical concentration | Temperature | Behaviour of Alloy 400 |
|---|---|---|---|
| Sulphurous acid and bisulphite cooking liquor | Dilute, as spent liquor | Pulp mill process range | Excellent; standard choice for washers, thickeners and screens |
| Hydrochloric acid, non-aerated | Up to 20% | Room temperature | Rate below 10 mpy at 10% |
| Hydrochloric acid, air-saturated | 3% to 4% maximum | Above room temperature | Acceptable only within the stated limit |
| Hydrochloric acid from chloride hydrolysis | Below 0.5% | About 93 °C (200 °F) | Good service |
| Hydrofluoric acid, unaerated | All concentrations | Up to boiling point | Excellent |
| Chlorinated solvents | As used in solvent recovery | Up to boiling point | Superior resistance |
| Glacial acetic acid | 100% | Moderate | Good; aeration increases attack |
| Nitric and nitrous acid | Any | Any | Not recommended; use stainless steel |
Frequently Asked Questions
Q: Is Alloy 400 suitable for hydrochloric acid service?
Yes, inside defined limits. Keep non-aerated and 10% aerated solutions at or below about 20% concentration at room temperature, and restrict air-saturated service above room temperature to 3% to 4% HCl.
Q: How does aeration change the performance of Alloy 400?
Aeration supplies oxygen that acts as a cathodic reactant and accelerates corrosion. Many ratings quoted for this alloy assume unaerated conditions, so entrained or sparged air must be identified before a material is approved.
Q: Can Alloy 400 be used in hydrofluoric acid at the boiling point?
In unaerated acid it tolerates all concentrations up to the boiling point. Once air or oxidising salts enter the stream, the corrosion rate rises sharply and the application should be re-evaluated.
Q: What corrosion rate is expected in 10% hydrochloric acid?
Below 10 mpy, equivalent to roughly 0.25 mm per year, at room temperature in a 10% solution.
Q: Why is stainless steel preferred for nitric acid?
Nitric acid is strongly oxidising and passivates chromium-bearing stainless steels, giving them a stable protective film. Alloy 400 gains no passivation benefit in that chemistry and is the wrong choice.
Q: Where is the alloy used in pulp and paper plants?
Typical wetted parts include washers, thickeners and screens in sulphite pulp mills, together with ducting and washing sections exposed to sulphur-bearing flue gas condensate.
