1. Introduction #
Deionised (DI) water is highly purified, with most dissolved ionic compounds removed. While this makes it valuable in industrial and laboratory settings, it also creates a unique corrosion challenge. Because DI water is ion-hungry, it can leach ions from metallic surfaces, including stainless steel, potentially damaging the passive oxide film that provides corrosion resistance.
316 stainless steel (SS), containing molybdenum, is often chosen for its superior corrosion resistance—but care must be taken in DI water applications. This guide explains when 316 and 316L stainless steel, as found in the DMP 336 and DMP 331, is suitable and how system design impacts durability.
2. Key Factors Affecting Compatibility #
| Parameter | Effect on Corrosion | Recommendation |
|---|---|---|
| Water purity | Higher purity (resistivity > 1 MΩ·cm) increases aggressiveness | Limit exposure time; use electropolished 316L or higher alloys |
| Temperature | Elevated temperature (>50°C) accelerates corrosion and rouging | Keep DI systems at ≤50°C if possible |
| Stagnation | Lack of oxygen limits passive film regeneration | Maintain circulation to ensure oxygen presence |
| Surface finish | Rough or weld-contaminated surfaces corrode faster | Use polished or electropolished finishes (<20 µin Ra) |
| Weld quality | Incomplete passivation leads to local attack | Use 316L filler metals and proper post-weld cleaning |
3. Example Setups #
3.1 Laboratory Recirculating DI Water System #
- Water quality: 0.1–1 MΩ·cm (moderate DI)
- Temperature: 20–25°C
- Piping: 316L SS, orbital welded, electropolished
- Flow: Continuous circulation with a return loop
- Corrosion assessment: Very low risk. Passive film remains intact due to constant oxygenation.
- Maintenance tip: Perform routine passivation and ensure flow velocity >1 ft/sec.
Verdict: ✅ 316L stainless steel acceptable for long-term service.
3.2 Ultrapure Water System for Semiconductor Use #
- Water quality: >10 MΩ·cm (ultrapure)
- Temperature: 20–30°C
- Piping: 316L SS with electropolished finish; some sections PVDF-based
- Flow: Continuous loop, 24/7 circulation
- Corrosion assessment: Low to moderate risk. Trace rouging possible near welds or dead legs.
- Maintenance tip: Implement strict microbial and rouge control programs, and periodic chemical passivation with citric acid.
Verdict: ⚠️ Acceptable with 316L electropolished; consider PVDF or PFA for critical purity points.
3.3 Hot DI Water Storage Tank #
- Water quality: 5–10 MΩ·cm
- Temperature: 60–80°C
- Tank material: 316 SS (standard finish)
- Flow: Stagnant with intermittent drain/fill
- Corrosion assessment: High risk. Elevated temperatures and low oxygen levels promote pitting and roughing.
- Maintenance tip: Lower temperature; improve mixing; re-passivate regularly.
Verdict: 🚫 Not recommended. Use AL-6XN or titanium if heating is required.
3.4 Pharmaceutical Purified Water Loop #
- Water quality: 0.5–2 MΩ·cm (USP Purified Water)
- Temperature: 25–35°C
- Piping: 316L SS, orbital welded, electropolished (Ra ≤ 20 µin)
- Flow: Continuous loop recirculation
- Corrosion assessment: Very low risk. Systems perform reliably when hygienic design standards are met.
Verdict: ✅ 316L EP stainless steel is the industry standard.
3.5 DI Water Feed for Cooling Systems #
- Water quality: 0.05–0.5 MΩ·cm (low to moderate purity)
- Temperature: 10–30°C
- Piping: 316 SS or standard 304 SS acceptable
- Flow: Continuous
- Corrosion assessment: Minimal. Dissolved oxygen aids passivation, and moderate purity reduces aggressiveness.
Verdict: ✅ 316 stainless steel is fully acceptable.
4. Recommendations Summary #
| Application Type | Water Quality (MΩ·cm) | Temp (°C) | Flow | 316 SS Suitability |
|---|---|---|---|---|
| Lab DI water system | 0.1–1 | 20–25 | Continuous | ✅ Good |
| Ultrapure semiconductor loop | >10 | 20–30 | Continuous | ⚠️ Use 316L EP or polymers |
| Hot DI storage | 5–10 | 60–80 | Stagnant | 🚫 Poor |
| Pharma purified water | 0.5–2 | 25–35 | Continuous | ✅ Excellent |
| Cooling feed | 0.05–0.5 | 10–30 | Continuous | ✅ Excellent |
5. Design & Maintenance Best Practices #
- Prefer 316L over 316: Lower carbon minimises sensitisation.
- Polish or electropolish surfaces: Enhances passive layer.
- Ensure continuous circulation to prevent stagnation and microbial growth.
- Avoid chloride contamination: Even trace amounts of chlorides can accelerate pitting.
- Re-passivate periodically: Use citric or nitric acid solutions as a maintenance measure.
6. Conclusion #
The DMP 331 and DMP 336 pressure transmitters, which are made of 316L stainless steel, can perform reliably in DI water environments when system design, operating conditions, and maintenance are correctly managed.
For high-purity, high-temperature, or stagnant systems, consider upgrading to higher-alloy materials (such as AL-6XN, Hastelloy, or titanium) or using polymeric piping to ensure longevity and compliance with purity standards. Products such as the LMK 351, which can be manufactured with PVDF housings, FFKM seals, and high-purity ceramic sensors, are also a suitable option for these applications.
