When using aqueous part cleaning systems and parts washers, rinsing the parts with high-grade RO (Reverse Osmosis) water or DI (Deionized) water is often overlooked, but it is imperative to ensure your final cleaned parts are fully cleaned and spot free. A DI water rinse provides the highest grade of quality for your precision cleaning applications.
The cleanliness requirements and specifications of your part cleaning process will determine what type of rinse water to use, which could be municipal/city tap water, reverse osmosis or deionized water. The difference between these water types or grades is generally based on the level of Total Dissolved Solids or TDS contained.
Hard Municipal / City Water
Water is considered “hard” when it has a relatively high concentration of calcium and magnesium ions (two of the salts which make up TDS). Hard water received this name because it requires more soap to get a good lather and makes the water “hard” to work with. This same concept applies to parts cleaning and washing; more soap is required if the water is harder and has higher TDS. The main difficulty in parts cleaning does not come during the washing step, but in trying to use hard water to rinse the parts. Hard water does not rinse as clean and often leaves spots on parts from minerals and TDS in the water.
What is RO – Reverse Osmosis water purification?
Reverse Osmosis (RO) is the opposite of a natural process, osmosis, which is the movement of water molecules across a semipermeable membrane that naturally moves water from a low ion concentration to a higher ion concentration. This natural process is used by our bodies to get water to our individual cells.
Reverse osmosis works by applying pressure to the more concentrated (higher TDS) side of a semipermeable membrane. Water molecules are pushed back across the membrane to the less concentrated (lower TDS) side, resulting in more purified water. The RO process can typically remove 90-99% of contaminants.
Reverse Osmosis is often used to partially clean-up city / municipal tap water before any other purification technology such as deionization is used to remove the remaining 1-10% of contaminants.
What is DI – Deionization water purification?
Filters in a deionized water system (DI filters) can be referred to by many names: Ion Exchange, Strong Acid/Strong Base, Polishing, Nuclear Grade. Nuclear grade DI filters can remove inorganic chemicals down to very low parts per billon (PPB) levels. This makes such DI filters excellent for producing Type I, 18.2 Megohm water. Type I, 18.2 Megohm water is commonly used for precision cleaning applications.
Deionization filters function by exchanging positive hydrogen and negative hydroxyl ions for positive and negative contaminant ions in the water. Calcium and other positive contaminants exchange places with the hydrogen molecules. Iodine and other negative contaminants exchange places with the hydroxyl molecules.
Measuring DI Water Quality
The quality of DI water is measured by conductivity, rather than TDS. DI quality is expressed in microsiemens/cm, and is used to measure water with a large number of ions present. Resistivity is expressed in megohms-cm and is used in the measurement of water with few ions. Deionized water’s conductivity = 1/resistivity. So for example, at 25° C, 18.2 megohm water, which is the highest purity water obtainable, has a conductivity of 0.055 microsiemen/cm.
Over time, positive and negative contaminants in the water displace all the active hydrogen and hydroxyl molecules on the DI resin and the filter must be replaced. Regeneration of the deionization filter is possible, but only in an industrial environment setting.
Deionization is an on-demand process supplying purified water when needed. This is important because water at this extreme purity level degrades quickly. The nuclear grade deionization resin or polishing mixed bed resin removes almost all the inorganic contaminants in the water increasing the resistivity of the water to a maximum of 18.2 megohm-cm. However, deionization does not remove all types of contaminants like dissolved organic chemicals. Deionization filters are not physical filters but charge exchange filters so they do not have a pore size and cannot remove bacteria or particulates.
How to Store Deionized Water for Cleaning
Once DI water has been made with the help of a deionized water system, it has had most of its ions removed. It will attack any material to get them back. The most preferred container for DI water would be a stainless steel container or glass.
316 Stainless steel is well suited for storing DI water and having smooth, clean and passivated welds make the deionized water tanks even better. Many times Best Technology electropolishes contact surfaces to ensure even better storage compatibility. Often the systems Best Technology sells for use with DI water are 316 Stainless steel.
Since DI water has had most of its ions removed it naturally wants to come back to equilibrium and does so by leaching ionized minerals from its surrounding materials. That is why storing DI water in PVC and other polymers is not advised as non-moving, stagnant DI water will have the chance to pull ionized molecules from surrounding materials. NOTE: PVC is commonly used in DI water plumbing systems because most metallic plumbing would cause a leaching problem like copper or brass, but with PVC there is no problem because there is a continuous flow in the plumbing.
The most common cause of loss of ultra-low conductivity or deterioration of stored DI water is exposure to CO2 from air. The CO2 converts to carbonic acid in the water which causes the conductivity to increase. Adding an N2 blanket over the DI water in your storage tank can overcome this issue.
Why use deionized water for cleaning?
Using DI water for precision parts cleaning is important because:
- Since all or most of its mineral content removed, DI water is very hungry to acquire minerals from your parts which is commonly the dirt and contaminants on the surface of your parts.
- Ultrasonics are able to cavitate more effectively with an absence of particulates in the DI water.
- Since there are no minerals in DI water, it leaves no residue on your parts, so you have no water spotting after a DI water rinse.
- DI water will remove any remaining detergent, alkaline cleaning chemistry or soaps from your parts when you rinse parts in the deionized water tank.
- Alkaline cleaning solutions, soaps and detergents perform better in solution with DI water as the detergent is not wasted in converting the mineral content of the water (as explained above). All of the cleaning action of the alkaline cleaning solution, soap or detergent can be directed to the part.
- Since all contaminants have both organic and inorganic components and they typically increase the conductivity of DI water, a part cleaning system can be set up with conductivity probes to monitor the final rinse of the product in an ultrasonic tank. The final rinse process can be automatically set based on the conductivity measurements and programmed to introduce more DI water to achieve the proper desired level.