Water-based or solvent-based cleaning?

Professionals in aerospace metal fabrication and machine shops have two main metal parts cleaning options: aqueous water-based cleaning and solvent-based cleaning within a vapor degreaser. Each method has advantages and drawbacks, making the decision critical to maintain employee health, regulatory compliance, and component quality.

Water-based cleaning

Aqueous cleaning is a meticulous process combining water with chemical additives to achieve pristine cleanliness in metal parts. The water is infused with compounds including detergents, surfactants, builders, emulsifiers, saponifiers, sequestering agents, chelating agents, and others to enhance cleaning performance. The cleaning process typically involves three steps.

First, the parts undergo washing in a series of machines, where high heat and ultrasonic agitation work together to break down and remove contaminants from the surfaces. A secondary rinse eliminates any residual detergent. Finally, the parts are dried using hot air knives or isopropyl alcohol, ensuring they emerge spotless and dry.

It’s crucial to note that the effectiveness of aqueous cleaning depends on the additives used, as they can vary from mild to strong, and from non-toxic to potentially hazardous. Compatibility testing is essential for all materials involved to prevent adverse reactions. Moreover, post-cleaning tests are recommended to ensure no reactive residues remain from the rinse process. The wastewater generated during this process undergoes rigorous treatment involving filtration, distillation, deionization, and osmosis preparation for safe disposal.

Aqueous cleaning is particularly advantageous when combined with other procedures, such as rust-preventative coatings or brightener applications, showcasing its versatility in the industry.

Aqueous cleaning, while effective in achieving cleanliness in metal parts, comes with significant energy consumption. These systems’ horizontal layouts and multiple tanks employ high-pressure pumps to circulate water throughout the machine, resulting in substantial power requirements. Heating is another energy-intensive aspect of aqueous cleaning, as heated water is essential to optimize cleaning performance. Furthermore, energy hungry air knives or fans are often required to dry parts to prevent flash rusting or surface spotting. And last, aqueous cleaning introduces moisture and heat to the surrounding environment, requiring air conditioning to maintain ideal air humidity and temperature. These factors combine to make aqueous cleaning an energy-intensive process, highlighting the importance of considering energy efficiency and environmental impact when choosing a cleaning method.

Solvent-based cleaning

In contrast, solvent-based cleaning is more streamlined, integrating cleaning, rinsing, and drying into a single machine, typically a vapor degreaser. In this method, metal aerospace parts are immersed in a solvent-based cleaning fluid. The choice of compounds used in the fluid and their combinations dictates the cleaning fluid’s effectiveness and compatibility with various materials.

The process starts with parts being submerged in continuously filtered and distilled cleaning fluid, dissolving and/or lifting contaminants from their surfaces. Ultrasonic agitation may be employed in some instances for enhanced cleaning power. Once removed from the cleaning fluid, parts undergo a brief vapor rinse and drying process. Then the cleaning fluid condenses and returns to the vapor degreaser for reuse, reducing the cost and environmental impact of hazardous waste disposal.

Traditionally, some popular cleaning fluids used in aerospace metal fabrication and machine shops include n-propyl Bromide (nPB), trichloroethylene (TCE), and perchloroethylene (Perc). However, many companies are transitioning to newer, more environmentally friendly cleaning fluids that maintain effectiveness while addressing worker safety and air quality concerns. These alternatives are sustainable, possessing low global warming potential (GWP) and low ozone-depleting potential (ODP), aligning with modern environmental standards.

In addition, vapor degreasing stands out as highly energy efficient. Since they use modern, nonflammable solvents to clean and dry parts, vapor degreasers eliminate the need to heat water or power blowers or air knives, significantly reducing electricity consumption. Notably, vapor degreasing continuously purifies and redistills the solvent for reuse, reducing the need for waste treatment, and it doesn’t introduce humidity into the factory environment, reducing the load on air conditioning and other environmental systems. The substantial energy savings offered by vapor degreasing make it a sustainable and cost-effective choice for metal parts cleaning.

Choosing a cleaning method

The decision between aqueous and solvent-based cleaning shouldn’t be based on cost alone. Several critical factors must be considered, ensuring the selected method aligns with the specific cleaning task and broader context of your operation.

Contaminants: The nature of contaminants on the parts plays a pivotal role. The saying “like dissolves like” is relevant here. Different cleaning fluids are suitable for polar (inorganic), non-polar (organic), or particulate contaminants. Polar contaminants, soluble in water, include salts, emulsion residue, and oxidation, making aqueous cleaning ideal. Non-polar contaminants, such as machining oils and resins, are best tackled with solvent-based cleaning. Particulate contaminants require displacement cleaning, feasible with both methods.

Part shapes: The geometry of the parts being cleaned influences the choice of method. Aqueous cleaning may struggle with intricate parts or those with tight spaces prone to water entrapment and subsequent corrosion. Solvent-based cleaning excels in these scenarios, as its lower surface tension and viscosity allow it to penetrate and evaporate from challenging areas more effectively. However, for simple parts without corrosion concerns, aqueous cleaning may be a cost-effective choice if the infrastructure is already in place.

Throughput requirements: Consider the desired throughput rate. Aqueous systems often involve longer cleaning cycles due to multiple washing, rinsing, and drying stations. They may require mechanical spraying and higher temperatures, impacting throughput. On the other hand, vapor degreasing offers rapid and consistent cleaning down to the submicron level, with minimal downtime for maintenance. This efficiency may be crucial for high-volume operations.

Environmental impact: In the quest for clean parts, sustainability shouldn’t be overlooked. Regardless of the method chosen, minimizing water pollution, controlling emissions, and reducing waste disposal are essential goals. Aqueous and solvent-based cleaning methods both impact the environment, making it crucial to work with precision cleaning experts who specialize in both methods. They can assess your specific project and recommend fluids and processes aligning with your environmental and efficiency goals.

Conclusion

Choosing the right aerospace parts cleaning method involves careful consideration of contaminants, part shapes, throughput requirements, and environmental impact. The decision should be made holistically, taking all these factors into account to ensure the best possible cleaning results while maintaining safety and sustainability in your operations.

About the author: Elizabeth Norwood is a senior chemist at MicroCare LLC, which offers precision cleaning solutions. She can be reached at 860.827.0626.