Quick answer: On most metals, corrosion speeds up noticeably above about 60 % RH, especially with salt or pollutants present. Control corrosion by keeping the enclosure’s internal RH below about 50–60 % and its surfaces above the dew point, with sealing, a cabinet dehumidifier or heater, and suitable materials and coatings in salty or polluted locations.
Corrosion inside electrical enclosures needs a film of water on metal. That film forms when surfaces drop below the dew point (condensation), and also without visible condensation when humidity stays high: salts absorb water from the air at moderate humidity. An IP rating keeps rain and dust out but does not stop humid air getting in and condensing.
Quick answer
- Keep internal RH below about 50–60 % and surfaces above the dew point.
- Salt air: salt films turn wet at moderate humidity (sea salt deposits from about 75 % RH, some salts much lower).
- IP65/66 enclosures still breathe and condense.
How corrosion starts in an enclosure
| Condition | What happens |
|---|---|
| Condensation on cold surfaces | Liquid water on roofs, doors, busbars and terminals; drips onto electronics |
| Long periods of high RH | Thin water films form on metal surfaces even without visible condensation |
| Salt or dust deposits | Absorb water from the air at moderate humidity and create conductive, corrosive films |
| Polluted air (sulphur, chlorine compounds) | Accelerates corrosion of copper, silver and solder |
| Dissimilar metals in a wet joint | Galvanic corrosion of the less noble metal |
ISO 9223 classifies atmospheric corrosivity from C1 (very low) to CX (extreme), based on time of wetness, salt and pollutant deposition (ISO 9223). Coastal, industrial and wastewater sites are often C4–C5 or worse. The Australian Stainless Steel Development Association explains galvanic (dissimilar metal) corrosion (ASSDA).
Why IP-rated enclosures still corrode
- Enclosures "breathe": heating and cooling of the air inside pumps air in and out through seals and cable glands.
- Each cycle brings in moisture; it condenses on cold walls at night and does not get out easily.
- Door openings in humid weather fill the enclosure with humid air.
See why IP-rated enclosures still condense.
Check: dew point vs coldest surface
Example: air inside a cabinet at 28 °C and 75 % RH has a dew point of about 23 °C. If the roof panel cools to 21 °C in the evening, water forms on it. Lowering the inside RH to 50 % at 28 °C drops the dew point to about 16.5 °C, giving a margin of several degrees.
Measure both air conditions and the coldest surface; see dew point sensor placement.
Control methods by risk level
| Risk | Typical site | Measures |
|---|---|---|
| Low | Indoor, heated, dry | Good sealing; cable glands sealed; normal materials |
| Medium | Unheated indoor, outdoor inland | Seal; anti-condensation heater or small cabinet dehumidifier; breather with membrane |
| High | Coastal, wastewater, chemical, mining, tropical | Seal well; active dehumidification to hold RH below about 50 %; corrosion-resistant materials (stainless, coated); conformal coating on electronics; filtered positive pressure for large panels |
Compare methods in enclosure heater vs dehumidifier and coating vs active moisture control.
Cabinet dehumidifiers for corrosion control
A cabinet dehumidifier condenses water inside the enclosure and drains it out, lowering the dew point:
| Model | Type | Capacity | Rating condition |
|---|---|---|---|
| ADS-500ML | Peltier | 500 mL/day | 35 °C / 85 % RH |
| ADS-1000ML | Peltier | 1,000 mL/day | 35 °C / 85 % RH |
| ADS-100WB-01 | Compressor | 4 L/day | 30 °C / 80 % RH |
| ADS-10D | Compressor | 10 L/day | 30 °C / 80 % RH |
See our electrical cabinet dehumidifiers and Peltier vs compressor.
Safety during inspection
Inspecting live enclosures for corrosion and moisture must follow electrical safety rules for working on or near energised parts; in the US, OSHA 1910.333 sets selection and use of work practices (OSHA).
FAQ
Frequently asked questions
What humidity causes corrosion in electrical enclosures?
Corrosion speeds up above about 60 % RH, and much more with condensation, salt or pollutants. Holding internal RH below about 50–60 % greatly reduces it.
Do IP65 enclosures prevent condensation?
No. They keep water jets and dust out, but humid air still enters through breathing and door openings and condenses on cold surfaces.
How do I stop corrosion in an outdoor electrical cabinet?
Seal the enclosure, control internal humidity with a cabinet dehumidifier or heater, use corrosion-resistant materials in salty sites, and inspect regularly.
Does conformal coating replace humidity control?
It protects coated circuit boards, but not terminals, busbars, connectors or the enclosure itself. Most high-risk sites use both.
Sources
- ISO 9223, Corrosion of metals and alloys: corrosivity of atmospheres.
- ASSDA, Galvanic (dissimilar metal) corrosion.
- OSHA 29 CFR 1910.333.
- ASHRAE, Fundamentals of psychrometrics.
