Enhancing manufacturing quality control with rinse water management

Quality management and control programs are integral to the aerospace industries. Water quality is an essential parameter in aerospace manufacturing quality and ultimately assuring the performance of critical components and assemblies.  

Quality control and management in aerospace manufacturing 

Quality management in manufacturing typically begins with ISO 9000, the recognized international standard for quality management systems that uses a process-based approach to improve process efficiencies and promote continuous improvement. The aerospace industry expands upon this standard with aerospace specific standards such as AS-9100, AS-9110 (for maintenance, repair and overhaul operations) and AS-9120 (for aerospace distributors) that add additional elements such as product safety and risk management.  

In addition to these quality management systems, the National Aerospace and Defense Contractors Accreditation Program (NADCAP) offers certification of specific critical processes such as those involved with chemical processing, coating and electronics. A common denominator between many of the specific processes that make up these categories is the use of water.

Water in aerospace manufacturing and maintenance operations

Water is integral to many aerospace manufacturing, maintenance and repair operations. Water-based or “aqueous” processes include surface finishing processes (anodizing, chromating, electroplating and cleaning), coating pretreatment processes (phosphating) and microelectronics manufacture and printed circuit board cleaning.

Why does water quality matter and how does the water quality affect these processes? Water straight from the tap contains dissolved minerals, such as calcium and magnesium (“hardness”) among others. These minerals typically exist as positively charged ions, or cations. These cations coexist along with corresponding negatively charged ions, or anions, such as bicarbonate, chloride, sulfate, etc. The totality of these components is described by the term “total dissolved solids” or TDS and is typically expressed in milligrams per liter (mg/L).

These constituents, inherent to tap water, can negatively impact many of the typical aerospace surface finishing, coating and cleaning processes and/or the final product. Some examples include:

• Impurities, such as chloride (Cl^-), in a chromate coating bath can cause poor adhesion of the coating resulting in a loss of corrosion protection while chloride and other impurities in the rinse baths can result in poor coating uniformity, loss of corrosion resistance and disrupt the adhesion of any subsequent coating layer such as paint.
• Contaminants in a cadmium cyanide bath can cause non-uniform deposit quality and other plating defects while rinses high in TDS can cause staining and/or unevenness in the cadmium deposit, cross-contamination of other baths and can negatively affect the quality of any subsequent coating such as chromate conversion coatings.
• Rinses for microelectronic components and printed circuit boards must be especially clean. Ionic components in rinses from washing populated printed circuit boards can deposit on the surface of the board and result in spotting as well as being conductive, leading to short circuits and decreased board life.

Knowing that normal water background constituents can have deleterious effects on part quality is important. Controlling the amount of impurities in water necessitates the ability to measure the extent of the impurities.

Water quality requirements and measurement

To better understand the impact of water quality in aerospace manufacturing and repair processes, it is helpful to understand some general terms that describe water quality: 

• City or well water: water from the tap with the full complement of background constituents 
• Soft water: treated to replace hardness (calcium and magnesium) with sodium 
• Reverse osmosis (RO) water: treated to remove most of the background constituents 
• Deionized (DI) water: treated to remove virtually all background constituents

Water quality may be measured in several ways; the typical water quality expressions include: 

• Total Dissolved Solids (TDS):  the totality of ionic constituents; expressed as milligrams per liter (mg/L) – the lower the number, the cleaner the water
• Conductivity:  how well the water conducts electricity; expressed as microSiemens/centimeter (µS/cm) – the lower the number, the cleaner the water 
• Resistivity:  how well the water resists the flow of electricity; expressed as megaohm-cm (MΩ-cm) – the higher the number, the cleaner the water

A comparison of water quality measurements is shown in the following table:

Water quality requirements – and tolerances – differ depending on the process in which the water is used.  A phosphate pretreatment process might be just fine using city, well or soft water while a critical circuit board cleaning application typically requires highly purified water. The water should be clean enough to not interfere with the chemistry of the process while maximizing soil-removal and minimizing the negative impact from residual water constituents. Conversely, it is possible to have water that is too pure as highly purified deionized water can be aggressive toward certain metal substrates such as copper, for example. 

Once the desired water quality for the process is known, this informs the choice of treatment technologies to achieve and maintain that quality.

Water treatment and quality maintenance

Several unit operations can be used to treat process water and wastewater for reuse and recovery. The most common primary processes are ion exchange (IX) and reverse osmosis (RO); adjunct processes to support IX and RO include carbon adsorption, filtration and ultraviolet (UV) light treatment.

Ion exchange (IX) treatment utilizes polymeric beads in a pressure vessel through which the wastewater is passed. The IX resins contain charged functional groups; these functional groups will exchange cations and/or anions between the resin and water.  In process water treatment (clean water entering the manufacturing process) and wastewater reuse and recovery applications, the resins adsorb undesirable ionic components from the water or waste water and replace the contaminant ions with something benign to the process. Water softening resin (cation exchange resin) removes calcium and magnesium (hardness) and replaces it with sodium. Deionizing resins (cation and anion exchange resins, usually as a “mixed bed”) remove hydrogen (H⁺) and hydroxide (OH^-) ions which combine to form water (H⁺ + OH^- → H₂O).

Mixed bed IX can deliver highly purified water up to 18.2 megohm (MΩ-cm) in quality or containing essentially zero total dissolved solids (TDS). Whether softening or deionizing, the spent IX resins are regenerated using acid (cation exchange resin) and alkali (anion exchange resin) and returned to service; all contaminants removed by the resin are transferred to the acid and alkali regenerant streams which require treatment prior to discharge. IX is the most common deionizing recovery technology, especially for small flowrates. Xylem offers IX services, for both process water (Service Deionization or SDI) and wastewater (Wastewater Ion Exchange or WWIX) that utilize leased IX tanks and off-site regeneration thus minimizing capital outlay and minimizing onsite handling of IX resins and regeneration chemicals and residuals.

Reverse osmosis (RO) treatment uses a membrane barrier to separate contaminants from the waste stream. A pump pressurizes the waste stream and forces water through the membrane leaving most of the contaminants behind. The clean stream, or product stream, is recovered and can be used as-is or further polished with IX treatment. The dirty stream, or concentrate stream, can then be discharged or potentially reused somewhere where the higher level of contaminants will not be problematic. It is important to understand the composition of the concentrate stream to determine if it requires further treatment prior to discharge or reuse. 

Carbon adsorption is a technology used to remove organic compounds from water (typically measured at Total Organic Carbon or TOC) as well as small amounts of oil and grease (O&G). Wastewater is passed through a pressure vessel containing granular activated carbon (GAC) where the TOC and O&G adsorb to the carbon’s surface. When the carbon is spent, it is typically disposed of and replaced by fresh carbon. Both IX and RO use GAC as pretreatment to remove the TOC and O&G that can foul the IX resins and RO membranes. 

Filtration uses a physical barrier – such as bags, cartridges, or a graduated media, to remove particulates or total suspended solids (TSS) from the waste stream. Filtration is used as pretreatment for IX and RO as these technologies are adept at removing TDS but will be fouled and rendered ineffective by TSS in the water stream. 

Ultraviolet (UV) light is used on very high purity treated process water or recovered wastewater to control either biological growth or low levels of TOC. The water is treated as it passes through a chamber past a bulb generating UV light of an appropriate wavelength depending on which of those contaminants require control.  

These technologies are sometimes used alone, but more often used in tandem, to achieve whatever the desired goal is for the process or wastewater. Process water treatment, for example, could be as simple as a softener or as complex as a system utilizing softening, filtration, RO, GAC, SDI and UV treatment; typical SDI treatment trains consist of filtration, GAC and SDI mixed beds. Generating high quality water from wastewater might involve just filtration and WWIX mixed beds or it may also incorporate filtration, RO, GAC and WWIX mixed beds.; typical WWIX reuse treatment trains consist of filtration, GAC and SDI mixed beds.

Recovered water quality for both SDI and WWIX systems are typically measured semi-quantitatively using quality lights that measure water resistivity and show a specific light depending on the water quality measured – “green” if the quality is at or above the light set point or “red” if the quality is below the set point (set points typically range from 0.02 MΩ-cm to 2 MΩ-cm). Measuring water quality levels out of this range typically requires direct quantitative measurement using a conductivity meter (for quality levels less than 0.02 MΩ-cm or greater than 50 µS/cm) or a resistivity meter (for quality levels above 2 MΩ-cm).

Xylem offers both SDI (Water One^® SD) and WWIX (Water One^® WX) systems incorporating 24/7 remote monitoring. Both feed and recovered water quality are measured and predictive analytics determine how “spent” the lead IX tank is. At a certain level of remaining capacity, a tank change is automatically scheduled with the local service branch relieving customers of the responsibility of monitoring their water treatment systems. In addition to feed and recovered water quality, flowrate, volume, temperature and pressure are monitored and recorded. The data from these systems can be accessed and used, for example, to satisfy customer or NADCAP audit requirements.

There is a demonstrable link between water quality and product quality; especially for high-precision products manufactured in the aerospace industry.  It is important to understand the impact of water quality on the particular process, and plan for appropriate treatment to minimize any negative quality impact on the parts or components produced.  Working with a qualified water treatment professional can make this a straightforward process and ensure that you can concentrate on producing consistent quality components in your manufacturing process.

Author: Christopher Riley, Technical Services Director, WWIX, Xylem

Chris Riley is the Technical Services Director for the Xylem Wastewater Ion Exchange (WWIX) business and is responsible for directing technical support activities for WWIX customers as well as supporting WWIX media regeneration and quality operations.

Prior to joining Xylem, Chris held environmental engineering roles in the electroplating and environmental consulting industries where he worked in the areas of wastewater treatment and operations support, environmental auditing and process waste minimization.

Chris holds a bachelor’s degree in chemical engineering from Michigan Technological University, a master’s degree in civil (environmental) engineering from the University of Minnesota, and is a registered professional engineer