Caustic soda is the duty that looks safe on the compatibility chart and cracks assemblies anyway. Cold sodium hydroxide and austenitic stainless are genuinely compatible — the chart is not wrong. What the chart does not say is that above roughly 60 °C, in concentrated solution, the same combination is subject to caustic stress corrosion cracking, and cracking gives no warning at all before it goes through.
What this service does to a hose
Cold caustic is easy. Dilute NaOH at ambient in 304L or 316L is an ordinary transfer duty and behaves like water with a pH.
Hot concentrated caustic is not. Caustic stress corrosion cracking needs three things together: a susceptible material, a tensile stress and the environment. Austenitic stainless supplies the first. A hose supplies the second by its nature — every convolution is a formed part with residual stress in it, and the assembly is under pressure on top of that. The process supplies the third. Where those overlap, the crack is transgranular, branching, and it runs to failure with essentially no wall loss to warn you. There is nothing to see on an inspection until there is a leak.
Concentration climbs where you did not intend it to. The classic case is a line that is steam traced or that runs hot near a jacket: caustic solution evaporating at a hot spot concentrates locally, and a 20 % bulk solution can become a 50 % film at the wall. The duty at the wall is not the duty on the datasheet.
Then there is CIP. Half the caustic in industry is a cleaning cycle rather than a process stream — hot caustic through a food or brewery line several times a week. That is hot concentrated caustic on a cycling line, which is the case above, arriving through the back door.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Cold dilute NaOH | 304L or 316L annular corrugated, braided conventionally | An ordinary duty; we will not sell you the more expensive grade for it |
| Hot or concentrated NaOH | PTFE-lined 316L, liner carrying the medium | It takes the metal out of contact with the environment that cracks it |
| Braid | Matching grade, sized on the derated pressure where the line runs hot | Hot caustic lines are usually also hot pressure lines |
| Ends | Flanged; PTFE flared over the face on lined assemblies | Threads on caustic collect and concentrate residue |
| Flushing | Built so the assembly can be flushed and fully drained | Residual caustic drying out in a valley is a concentration mechanism |
What we would not fit, and what happens
Bare stainless on hot concentrated caustic. The failure mode is cracking, which means no leak, no leak, no leak, then a hole.
An assembly that cannot be drained on a line that dries out between batches.
Interlock hose. Not pressure-tight and the seam holds residue.
A specification written from a room-temperature compatibility chart for a line that runs at 80 °C. That is how these fail.
What we need to size it
| We need | Because |
|---|---|
| Concentration, and whether it can concentrate further at a hot spot | The wall condition matters more than the bulk condition |
| Maximum temperature the assembly sees, including tracing and CIP | This is the variable that flips the answer |
| Whether the line stands full or dries out between batches | Drying is a concentration mechanism |
| Working pressure and whether the line is cycled | Cycling adds the tensile stress the mechanism needs |
| Whether the duty is process or CIP | A CIP line is hot concentrated caustic on a cycle, whatever the process is |
| End connections and whether full drainage is achievable | It decides the routing as much as the assembly |