The bath has two temperature ceilings. Only one is on the drum.
Every technical data sheet gives you a working range for the chemistry. Almost none of them tell you what your parts will tolerate. Run an aluminium casting at the detergent's happy temperature and you can hand back a rainbow-stained batch that was chemically clean and commercially useless.
Why hotter is not simply better
There is a common assumption that heat always helps. Up to a point it does: warmer solution cuts viscosity, lifts soil solubility and speeds the chemical reaction. But ultrasonic cleaning does not scale with temperature the way soak cleaning does, because the cavitation doing the work behaves differently as the bath warms.
As temperature rises, cavitation bubbles form more readily — then their collapse gets weaker. The liquid's vapour pressure climbs, bubbles fill with vapour instead of collapsing violently, and the implosion loses energy. Push far enough and the effect disappears entirely: at the boiling point, cavitation stops completely.
The practical sweet spot for aqueous ultrasonic cleaning is roughly 50–65 °C — hot enough for the chemistry to work, below the point where cavitation intensity falls away. A common rule of thumb puts it near 65% of the solution's boiling point. Hotter baths also cost more to run and evaporate faster.
What the substrate will actually take
This is the half that gets left off the data sheet. These are working ceilings — the point past which you start trading surface quality for cleaning speed.
| Substrate | Working range | What goes wrong above it | Notes |
|---|---|---|---|
| Aluminium & alloys | 40–60 °C never above 60 °C |
Rainbow discolouration and frosting as the oxide layer is attacked; then etching and pitting. Cosmetically fatal on visible parts. | Amphoteric — attacked by both strong acid and strong alkali. Full aluminium guide. Keep pH 8.5–9.5. Heat speeds the attack up, so work at the lower end on thin or cosmetic parts. Never free caustic or chlorides. |
| Magnesium & alloys | 50–70 °C | Flash corrosion — in minutes, not hours. White powdery bloom if drying is slow. | pH is the control, not temperature. Needs stable high alkalinity (pH 10.5–12) to hold the protective hydroxide film. Neutral, mildly acidic, citrus and "bio" cleaners will attack it. Never basket with steel or copper parts — galvanic attack. |
| Zinc & zinc die-cast | ≤ 55 °C | Surface darkening and powdery attack; dimensional loss on thin die-cast sections. | Amphoteric like aluminium. Inhibited alkaline only, mild pH, shorter dwell. |
| Brass, copper, bronze | ≤ 60 °C | Tarnishing and pink/red staining; dezincification of brass with aggressive chemistry. | Heat accelerates tarnish. Avoid ammoniated chemistry. Rinse and dry fast — copper alloys stain on a slow dry. |
| Anodised aluminium | ≤ 60 °C | Seal degradation, dye bleed, loss of corrosion resistance — often invisible until the part is in service. | Hot alkaline strips the seal. Treat the anodise, not the aluminium, as the limiting factor. |
| Painted & coated | ≤ 55 °C or coating spec |
Softening, blistering, edge lift, gloss loss. | The coating manufacturer's limit governs. Always lower than the metal underneath. |
| Carbon steel | 60–80 °C | Not the bath — the exit. Clean bare steel flash rusts within minutes. | Temperature is rarely the constraint. Hotter parts dry faster, which helps — but you need an inhibited rinse or a rust preventative and a fast dry. |
| Stainless steel | 60–80 °C | Little, from alkaline chemistry. | The tolerant one. With acid chemistry keep temperature as low as the process allows — to protect the tank as much as the part. |
| Titanium | 60–80 °C | Temperature is not the risk — chemistry is. | Halogenated and chlorinated chemistry can cause stress corrosion cracking. This is what ASTM F-945 tests for. Check the certificate, not the thermostat. |
| Plastics & elastomers | Below HDT often ≤ 50 °C |
Softening, distortion, swelling of seals; acrylics craze (ASTM F-484). | Heat deflection temperature governs. Assemblies are limited by their softest component, not their metal. |
| Precision & optics | Process spec | Thermal expansion moves tolerances; cemented optics and bonded assemblies debond. | Bearings, gauges and lapped surfaces: consider thermal growth and the cool-down, not just the peak. |
Aluminium and magnesium figures follow published manufacturer guidance (see sources). The remainder are working ranges from industrial practice — where a drawing, coating specification or customer process sheet states a limit, that document governs, not this table.
The mixed-basket trap. A basket is limited by its most sensitive item. One aluminium bracket among steel components drops the whole load to the aluminium ceiling. One magnesium casting basketed with steel introduces galvanic attack no temperature setting will fix. Sort by substrate before you sort by soil.
Setting it in practice
- Identify every substrate in the basketIncluding plating, anodising, paint, seals and adhesives. The softest, most reactive item sets the ceiling.
- Take the lowest substrate ceilingNot an average. Not the majority material. The lowest.
- Read the chemistry's working windowFrom the technical data sheet — its range, not a single number.
- Run at the top of the chemistry window that sits under the substrate ceilingIf the windows don't overlap, change the chemistry. Do not split the difference and hope.
- Prove it on real partsRun a trial batch, inspect for discolouration and etch under good light, and record the values. The recipe is then the process — not an operator's judgement each shift.
Where this lives on the machine. On a recipe-controlled system the temperature ceiling is stored per part family, not set by whoever is on shift. That is the point of recipe control: the aluminium recipe cannot be run at the steel recipe's temperature, because nobody has to remember which is which.
Symptoms, and what they usually mean
| What you're seeing | Most likely cause |
|---|---|
| Rainbow or iridescent staining on aluminium | Bath too hot, pH too high, or dwell too long. Oxide layer attacked. Drop temperature before touching anything else. |
| Matte or frosted finish on machined aluminium | Cavitation too aggressive for the substrate, usually compounded by heat. Consider higher frequency and lower temperature. |
| White powdery bloom on magnesium | Drying too slow, or pH fell out of the protective band. Not primarily a temperature fault. |
| Pink or red patches on brass | Dezincification — chemistry too aggressive, accelerated by heat. |
| Rust within minutes of unloading steel | Working as designed — the clean removed the protective film. Needs an inhibited rinse and faster dry, not a colder wash. |
| Cleaning slowed down after the bath got hotter | Cavitation falling off as you approach boiling. Come back down. |
The short answer
50–65 °C suits most aqueous ultrasonic work on steel and stainless. Aluminium, zinc, anodised and coated parts belong nearer 40–60 °C, and aluminium never goes above 60 °C. Magnesium is a pH problem before it is a temperature problem. Plastics and assemblies are governed by their softest component.
And if a data sheet's temperature range sits entirely above your substrate's ceiling, that is the data sheet telling you it is the wrong product for the job.
Not sure what your parts will take?
Tell us the substrate, the coating or plating, and what is on the part. We will tell you the temperature to run, the chemistry to run it in and what the machine has to prove — so the temperature question is answered against your parts, not from a table.
Sources
Brulin — Complete Guide to Cleaning Magnesium & Magnesium Alloys Without Corrosion (pH 10.5–12; 50–70 °C; flash corrosion in neutral and mildly acidic chemistry).
The aluminium and substrate ceilings in the table are our own working limits, from bath testing and from trial work on customer parts. They sit at or below the figures the chemistry data sheets allow.
Cavitation falls away as a bath approaches boiling, because the bubbles fill with vapour and collapse softly instead of sharply. That is why the useful band stops well short of 100 °C.
ASTM F-484 (stress crazing of acrylics) and ASTM F-945 (titanium stress corrosion) as cited on Brulin product conformance documentation.