Sheath selection is a corrosion problem wearing a thermal disguise. The temperature rating of the alloy is rarely the binding constraint in an immersion application — the chemistry is. An element that would happily run at 300 °C in air can be destroyed at 60 °C in the wrong solution.
The stainless trap
Stainless steel is not one material and "stainless" on a purchase order is not a specification. 304 is a good general-purpose water and steam sheath. Introduce chloride — from a coastal water supply, a softener regenerating on salt, or a process stream — and 304 pits preferentially along the liquid line, where the metal is wetted, oxygenated and cycled. 316 buys margin through its molybdenum content; 316L buys more where welding is involved. Above roughly 200 ppm chloride at elevated temperature, neither is a safe bet and titanium becomes the honest answer.
Carbon steel is not always the cheap option
In a closed, oxygen-scavenged, chemically treated circuit, 20# carbon steel outlasts the plant. In an open vented system it corrodes steadily. In hot caustic it is genuinely the right answer — better than several stainless grades — which is why alkali bath heaters are cataloged in carbon steel while the visually identical salt bath heater is stainless.
Where titanium stops and PTFE starts
Titanium's protection comes from a passive oxide film that reforms the moment it is damaged, which is what makes it so good in chloride and weak-acid service. Two things defeat it: hydrofluoric acid, which dissolves the film, and hot concentrated reducing acids. In those cases PTFE cladding is the only practical sheath — accepting an 80 °C ceiling and a watt density around a fifth of a metal element, which means a physically much larger heater for the same load.
常见问题
Is 316 stainless always better than 304?
In chloride-bearing service, yes — the molybdenum content gives real pitting resistance that 304 lacks. In clean treated water it is a more expensive alloy doing the same job. The question to ask is not which grade is better but whether chlorides are present, because that is the variable 316 actually addresses.
Why would anyone specify carbon steel over stainless?
Two reasons. In closed, oxygen-scavenged treated-water circuits it lasts indefinitely at a fraction of the cost. And in hot caustic alkali it genuinely outperforms several stainless grades — which is why alkali bath heaters are built in 10# steel while the geometrically identical nitrate-salt heater is stainless.
Can I use a titanium heater in a tank that occasionally reaches 120 °C?
Not safely as a routine condition. Titanium's practical ceiling in solution is about 100 °C, and excursions above it erode the margin the passive film provides. If the process genuinely reaches 120 °C, say so at inquiry — the answer is usually a different sheath or an indirect heating arrangement rather than accepting the excursion.
