Solvent-Based Cleaning Systems: Much More Than Parts Cleaning

An Ideal Combination of Cleaning and Corrosion Protection

In metal parts production processes, surface cleanliness is among the most important factors that determine quality. Depending on the type of contamination, solvents such as halogen-free hydrocarbons and modified alcohols provide certain advantages. In addition to reliably removing metal processing fluids and metal particles, the primary benefit is that cleaning and corrosion protection can be combined in a single machine. In processing and forming operations, foreign matter inevitably adheres to parts as a result of the process. These residues—such as metal processing fluids and emulsions, polishing paste, chips and particles—typically cause problems in the workflow or affect product quality prior to shipment. This makes parts cleaning an unavoidable process step in the workflow. In metal processing, the desired cleaning is typically carried out using wet chemical methods, for example with water-based washing fluids and solvents. The dirt-dissolving power of these fluids has a decisive impact on the quality and efficiency of the washing process.

Washing Fluid: The "Like Dissolves Like" Rule

When selecting which fluid to use in the washing process, the chemical principle of "like dissolves like" must be followed. According to this rule, any contamination will dissolve easily in a substance with a similar structure. In other words, for polar (e.g., water-based) residues—salts, cooling fluids and emulsions—or solids, chips or abrasive particles, water-based washing chemicals will be used. On the other hand, non-polar (apolar) contaminants such as metal processing fluids, greases, waxes and resins are removed from the part surface with non-polar solvents such as hydrocarbons or modified alcohols. Modified alcohols are preferred because they exhibit both lipophilic and hydrophilic properties at different concentrations, allowing them to easily dissolve both polar and apolar contaminants and remove them from the part surface. Modified alcohols are generally preferred as alternatives to chlorinated solvents, hydrocarbons and water-based washings. The answer to the question of which washing fluid is most suitable for parts cleaning is provided through washing tests. For this purpose, Dürr Ecoclean's test centers worldwide are equipped with water-based washing machines, hydrocarbon washers and modified alcohol washers to address all types of contamination.

When All Conditions Are Equal, Solvents Are the First Choice

When washing results with water-based, hydrocarbon or modified alcohol washing are equal, choosing solvent-based washing (hydrocarbon or modified alcohol) over water-based washing will provide advantages. In today's vacuum-operated washing machines, even when the solvent has a high fat content, the solvent is recovered in pure form through a continuous distillation unit. As a result, the solvent can be used for extended periods without needing to be changed. This eliminates the need for periodic monitoring of washing chemical concentration, pH value and conductivity as seen in water-based washings, making process monitoring easier. In drilling and deep drawing operations, chlorine-containing oils are sometimes encountered. In this regard, solvent suppliers provide test kits, stabilizers and usage recommendations to maintain solvent quality in a suitable and reliable condition for continuous use. Additionally, solvents show high compatibility with different materials. Solvent washing does not cause oxidation, color change, dulling or damage to the part surface. The electrical energy required for the washing process and solvent heating is significantly less than the energy required for water-based washings. In addition to energy savings, another advantage is that cleaning and corrosion protection are performed in a single machine, without manual handling and in a single operation. Depending on the concentration of the corrosion inhibitor, parts can be protected from corrosion for periods ranging from a few weeks to several months or even several years. With this application, parts treated with corrosion inhibitor can be shipped to overseas countries or stored for extended periods in inventory. The choice of corrosion inhibitor to be used will depend on the machinery and equipment technology and the solvent to be used.

Integration of Corrosion Protection into Solvent-Based Washing

To integrate the corrosion prevention step into the washing process, the machine must be equipped with an additional washing tank. Inside this tank, a corrosion inhibitor is completely dissolved at the required concentration in the solvent. The corrosion inhibitor can be applied to the solvent tank manually or automatically (via a dosing unit). After the parts washing process is complete, the machine's working chamber is filled with fluid from the corrosion inhibitor tank. During the corrosion protection stage, parts are subjected to the same operations as in the washing process. This step is performed to ensure that the corrosion inhibitor can reach all surfaces of the parts. After the corrosion inhibitor is applied to the parts, the parts are dried in the working chamber using vacuum drying, while the solvent is returned to the tank containing the corrosion inhibitor, thus ensuring that the corrosion inhibitor film remains on the part surface. The corrosion inhibitor film layer remaining on the part surface is distributed homogeneously with no bare metal exposed, and the part surface is completely dry. The corrosion inhibitor film is not damaged by contact during handling and packaging.

Determining Corrosion Protection Duration and Corrosion Inhibitor Active Concentration

Information about corrosion protection duration and corrosion inhibitor active concentration is obtained from the corrosion inhibitor supplier. For example, 10 to 16 weeks of protection can be achieved with 10% concentration. Depending on the product, concentrations may vary between 0.5% and 25%. These are determined experimentally under laboratory conditions according to film weight and density. To determine film weight, the corrosion inhibitor is applied to a standard metal panel (for example, Qpanel R35) at the recommended concentration, and the panel is then weighed to determine the film weight.

Corrosion Inhibitor That Can Also Be Used as Assembly Oil

Due to increasing demand from customers, parts manufacturers are being asked to provide corrosion inhibitor oil that not only prevents corrosion but can also be used as assembly oil. To meet this requirement, the corrosion inhibitor oil must be applied according to the coating thickness specified by the customer. To ensure that coating thickness is consistently and reliably achieved, the corrosion inhibitor oil concentration in the solvent must be determined at the parts manufacturer. Different methods have been developed to determine concentration. The concentration measurement method frequently used in practice takes advantage of the fact that substances have different evaporation points. Approximately 2 grams of the solvent and corrosion inhibitor oil mixture is placed on a balance, where the solvent is evaporated by a heating plate integrated into the balance. As the solvent evaporates, the oil is separated from the mixture and the oil weight is related to density to determine the volume percent corrosion inhibitor oil content of the mixture. Another concentration measurement method is refractometry, which measures based on calculating the refractive index of substances under light. For measurement with a refractometer, a drop of pure solvent is placed on the measuring glass of the refractometer prior to mixing, and the refractometer is adjusted to the "0" point through the observation lens. Then a drop of the solvent/corrosion inhibitor oil mixture is placed on the measuring glass of the refractometer and the value is read. The value read on the refractometer is multiplied by the refractive index of the corrosion inhibitor active to calculate concentration. Since this measurement method is sensitive to temperature, refractometer measurements must be performed at 20°C, 25°C and 30°C using an internal thermometer. Additionally, precision measurement technology capable of automatic density measurements is also used. In solvent-based machines operating under closed-loop systems, washing parts to prevent corrosion and application of corrosion inhibitor oil is today widely used for long-term corrosion resistance. More than 2,000 Dürr Ecoclean solutions worldwide are used for washing and corrosion prevention purposes. Dürr Ecoclean can also perform the relevant tests in its own test centers. Ümit Tekeli / Regional Sales Manager / Dürr Ecoclean GmbH