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Engineering Data
Heater Sheath Material Selection by Chemical
The sheath is the only part of the heater the liquid touches. Get it wrong and the heater fails in weeks, no matter how correct the kilowatts were.
Why this decision dominates heater life
Electric heaters almost never fail because the element wore out. They fail because the sheath was attacked by the liquid, or because the sheath ran too hot for the liquid to carry the heat away. The first is a materials problem, the second is a watt density problem, and together they account for most of the failures we see.
The complication is that compatibility is not a property of the chemical alone. It depends on concentration, temperature, whether the solution is aerated, whether chlorides are present as a contaminant, and whether the surface is running hot. A material that is fine in cold dilute acid can fail quickly in hot concentrated acid, and 316 stainless that handles a chloride solution at room temperature will pit and crack in the same solution hot.
Read the table as a starting point, not an answer
The chart below reflects common industry practice at typical concentrations and temperatures. It narrows the choice to one or two candidates. Confirming the final selection needs your actual bath analysis, concentration and operating temperature.
Sheath selection by tank liquid
| Tank liquid | Usual choice | Avoid | Note |
|---|---|---|---|
| Clean or treated water | Incoloy 800, 316 stainless | Copper (low temp only) | Incoloy resists scale-related hot spots and tolerates brief dry-fire better |
| Deionized water | 316 stainless, titanium | Copper | DI water is aggressive to copper; keep watt density modest |
| Hot water above 250°F | Incoloy 800 | 316 stainless | Standard for high temperature water and steam service |
| Salt water, brine, chlorides | Titanium | 300 series stainless | Stainless pits and stress-cracks in hot chloride service |
| Alkaline cleaners, mild caustic | 316 stainless, nickel plated | Aluminum | Common in parts washers and cleaning lines |
| Concentrated caustic soda | Nickel, Monel 400 | Stainless, aluminum | Nickel-rich alloys are the accepted choice for strong caustic |
| Nickel, zinc, copper plating baths | PTFE-coated steel, titanium | Bare stainless | PTFE coating keeps the bath chemistry off the metal entirely |
| Chromic acid, chrome plating | Titanium, PTFE-coated | Stainless, steel | Titanium performs well in oxidizing acid such as chromic and nitric |
| Nitric acid | Titanium, 316 stainless | Steel, copper | Oxidizing acid; passivates stainless but concentration matters |
| Sulfuric acid, dilute | PTFE-coated steel, quartz | Stainless, titanium | Titanium is attacked by reducing acids such as sulfuric and hydrochloric |
| Hydrochloric acid, pickling | PTFE-coated steel, quartz | Stainless, titanium | Aggressive to nearly all metals; keep the metal out of contact |
| Hydrofluoric acid | Quartz | Glass, most metals | One of the few services where quartz is the only practical answer |
| Phosphoric acid, phosphate baths | PTFE-coated steel, 316 stainless | Carbon steel | Common in pretreatment lines |
| Solvent degreasers | 316 stainless, carbon steel | Aluminum | Watch flammability and area classification, not just corrosion |
| Light oils, lube oil, hydraulic oil | Carbon steel, 316 stainless | — | Corrosion is not the issue here; low watt density is, to prevent coking |
| Heavy oil, wax, asphalt, tar | Carbon steel | — | Very low watt density, typically under 10 W/in² |
| Heat transfer fluid, thermal oil | 316 stainless, carbon steel | — | Fluid degrades above its film temperature limit, so watt density governs |
| Mixed or unknown acid | Hastelloy, quartz, PTFE-coated | — | Send a bath analysis; this is not a chart answer |
The materials themselves
| Material | Typical service | Strengths | Limitations |
|---|---|---|---|
| Copper | Clean treated water at low temperature only | Inexpensive, excellent heat transfer | Corrodes in DI water, chemicals and chlorides. Rarely the right choice in a process tank. |
| 304 stainless steel | Water, mild alkaline, some solvents | Widely available, good general resistance | Less chloride resistance than 316. No advantage over 316 in most process tanks. |
| 316 stainless steel | The general-purpose default for water and mild chemistry | Good broad resistance, moderate cost | Pits and stress-cracks in hot chlorides. Attacked by strong reducing acids. |
| Incoloy 800 / 840 | Hot water, steam, high temperature air and gas | Excellent high temperature strength and oxidation resistance | Costs more than stainless. Not a corrosion answer for acids. |
| Monel 400 | Concentrated caustic, salt solutions, some reducing conditions | Very good in strong alkaline service | Poor in oxidizing acids such as nitric. Higher cost. |
| Titanium | Chlorides, brine, chromic and nitric acid, plating baths | Outstanding in oxidizing and chloride service | Attacked by hydrochloric, sulfuric and hydrofluoric acid. Higher cost. |
| Hastelloy | Aggressive and mixed acid service | Broadest chemical resistance of the common alloys | Expensive. Specify only when a cheaper alloy genuinely will not survive. |
| PTFE-coated steel | Plating baths, dilute acids, pickling, phosphate | Chemistry never touches the metal | Coating is a wear item. Lower watt density limit, and it will not tolerate abrasion or mechanical damage. |
| Quartz | Hydrofluoric acid, strong acids where no metal survives | Nearly universal chemical resistance | Fragile. Poor mechanical durability, needs careful handling and support. |
| Carbon steel | Oils, wax, tar, asphalt, solvents | Cheap, robust, good for petroleum service | Rusts in water and is attacked by nearly all aqueous chemistry. |
Material selection questions
What sheath material should I use for a chemical tank?
It depends entirely on the chemical, its concentration and its temperature. As a general guide: 316 stainless for water and mild alkaline service, nickel or Monel for concentrated caustic, titanium for chlorides and oxidizing acids such as chromic and nitric, PTFE-coated steel for plating baths and dilute reducing acids such as sulfuric and hydrochloric, and quartz for hydrofluoric acid. Send a bath analysis to confirm rather than relying on a chart alone.
Is titanium always better than stainless steel?
No. Titanium is excellent in chlorides and in oxidizing acids, and it is the standard choice for chrome plating and brine. But it is attacked by reducing acids — hydrochloric, sulfuric and hydrofluoric — where 316 stainless or a PTFE coating does better. Paying for titanium in the wrong acid buys you a faster failure at a higher price.
When should I use a PTFE-coated heater?
When the chemistry attacks every reasonably priced metal, which describes most plating baths, pickling lines and dilute acid tanks. The coating keeps the bath off the metal completely. The tradeoffs are a lower watt density limit, because the coating adds thermal resistance, and vulnerability to mechanical damage — a scratched or chipped coating will fail at that spot.
Why did my stainless heater fail in a salt solution?
Almost certainly chloride pitting or chloride stress corrosion cracking. Hot chloride solutions attack 300 series stainless even at low concentrations, and it gets worse where the surface runs hottest. Titanium is the standard replacement for chloride and brine service.
Can chlorides in my water supply cause a problem even if I am only heating water?
Yes, and it is a common and surprising failure. Municipal water, well water and cooling tower water all carry chlorides, and they concentrate as water evaporates. On a hot tank with makeup water, chloride concentration climbs steadily. If a stainless heater is failing early in what looks like plain water service, test the water.
Does the sheath material change how many kilowatts I can use?
It changes the watt density, which changes how much heated surface area you need for those kilowatts. A PTFE-coated element carries less watt density than bare metal because the coating slows heat transfer, so the same kilowatt rating needs a physically larger element. Material and watt density have to be selected together.
Send a bath analysis, get a material recommendation
Chemical, concentration, operating temperature, and any contaminants you know about. We will specify the sheath and tell you why.