Category: Technical

Ultrapure water is considered corrosive due to its extreme purity and lack of dissolved ions. Here’s why:

1. Ion Deficiency and Aggressiveness:

• Deionization: Ultrapure water has been stripped of nearly all its dissolved ions and impurities, making it highly ion-deficient. This creates a strong chemical potential to absorb ions from any material it comes into contact with.
• Aggressiveness: Because it lacks ions, ultrapure water is “hungry” for them. It will readily dissolve and absorb ions from surfaces, such as metals, plastics, and even glass, in an attempt to reach a more stable chemical state.

2. High Resistivity:

• Electrical Properties: Ultrapure water has very high electrical resistivity (around 18.2 megohm-cm at 25°C). This means it does not conduct electricity well due to the absence of free ions. Materials that would normally resist corrosion in regular water can become vulnerable when exposed to ultrapure water because the water can more easily pull ions from the material.

3. Surface Reactions:

• Surface Leaching: When ultrapure water comes into contact with a material, it can leach ions and molecules from the surface, leading to corrosion or degradation. For example, in metals, this can lead to pitting or general corrosion, and in plastics, it can lead to the leaching of additives or plasticizers.

4. Impact on Protective Layers:

• Oxide Layers: Some metals, like stainless steel, rely on a thin oxide layer for corrosion resistance. Ultrapure water can dissolve or disrupt this protective layer, making the underlying metal more susceptible to corrosion.

5. Non-Buffering Nature:

• Lack of Buffering Capacity: Ultrapure water has no buffering capacity, meaning it can easily become acidic or basic if exposed to contaminants or air. This shift in pH can further enhance its corrosive properties.

Conclusion:

Ultrapure water’s corrosive nature is not due to any chemical aggressiveness like that of acids or bases, but rather its extreme purity and strong tendency to equilibrate by absorbing ions and impurities from the materials it contacts. This makes it particularly challenging to handle and store without contamination or material degradation.

Type 1 ultrapure water is so pure that it is actually corrosive to some materials. This is because ultrapure water has an extremely high resistivity (typically 18.2 megohm-cm at 25°C) and lacks any dissolved ions, which means it has a strong tendency to absorb ions and impurities from any material it comes into contact with. This can cause corrosion or degradation in materials that aren’t specifically designed to handle such high-purity water, such as certain metals and even some types of glass or plastics. Because of its aggressive nature, ultrapure water is often used in semiconductor manufacturing, pharmaceuticals, and other applications where even the slightest contamination can have significant consequences.

Some of the recommended materials that can be used for handling and processing ultrapure water include.

1. Polytetrafluoroethylene (PTFE)

• Properties: Excellent chemical resistance, low extractables, high purity, and non-reactive.
• Applications: Tubing, seals, gaskets, and fittings.

2. Perfluoroalkoxy Alkane (PFA)

• Properties: High purity, excellent chemical resistance, and maintains clarity and flexibility.
• Applications: Tubing, fittings, and valves.

3. Polyvinylidene Fluoride (PVDF)

• Properties: Good chemical resistance, high purity, and mechanical strength.
• Applications: Piping, fittings, and valve components.

4. Polypropylene (PP)

• Properties: Good chemical resistance, low cost, and suitable for DI water at lower purity levels.
• Applications: Piping, tanks, and valve bodies.

5. High-Purity Polyethylene (HDPE)

• Properties: Good chemical resistance and high purity.
• Applications: Tubing, containers, and fittings.

6. Polyetheretherketone (PEEK)

• Properties: Excellent chemical resistance, high strength, and low extractables.
• Applications: Tubing, fittings, and pump components.

7. Quartz (Silica)

• Properties: Extremely high purity and inertness.
• Applications: Piping and containers, often in semiconductor processing.

8. Stainless Steel (316)

• Properties: High corrosion resistance, often electropolished for ultra-high purity applications.
• Applications: Piping, valves, and fittings, usually for non-corrosive DI water applications.
• Avoid using 304 grade stainless and ensure all welds have been polished and passivated.

9. Borosilicate Glass

• Properties: High resistance to chemical leaching, but less durable than plastic options.
• Applications: Laboratory containers and some piping systems

Read the next article on ‘Why is ultrapure water corrosive?‘ to find out more.

Short answer, type 1 water is purer than type 2, but there is a lot more to take into consideration before specifying one.

As the use of instruments requiring specific qualities of water increases, laboratories are moving away from the traditional Type 1, 2, and 3 specifications and toward a set of guidelines which are laid out in the Clinical and Laboratory Standards Institute (CLSI) G40-A4-AMD guidelines.

These include:

• Clinical Laboratory Reagent Water (CLRW)
• Special Reagent Water (SRW)

There are three main bodies who have had influence in establishing water standards over the years, these are CLSI with the G40-A4-AMD guidelines (mentioned above), ISO with 3696:1987, and ASTM which uses D1193-06.

All of these standards vary in some way and can cause confusion when a grade is specified without directing which organisation’s standard is being referenced. This makes it essential to ensure that your laboratories unique requirements are accounted for when specifying what level of purification you require.

Clinical Laboratory Reagent Water (CLRW)

For reference a standard Purific purification system would fall into the CLRW range with the following properties and is comparable to a traditional Type 2 in water quality. This grade water comes from the regular specifications for blood chemistry analysers requiring this high purity water.

• ≥10 MΩ/cm at 25 °C Resistivity
• <10 CFU/mL
• TOC <500 mg/g (ppb)
• 0.22 µm absolute final filtration near or at the final output stage of the purification system.

Special Reagent Water (SRW)

Special reagent water is used when different quality water to CLRW s required. In essence it is similar to CLRW but may have more or less stringent parameters added to meet the requirements of the laboratories procedures.

Some applications include

• Trace organic analysis.
• DNA and RNA testing.
• Trace metal analysis.
• Cell/tissue/organ culture and florescent antibody detection of microorganisms.
• Low CO2 water.

Feel free to call one of our friendly team to ask further questions should your laboratory require a specific grade water on 1800 573 316 or send an email to service@purific.com