Cleaning and Corrosion Protection with Solvents

It is the task of industrial parts cleaning, both during and after manufacturing, to fulfill surface cleanliness specifications for further processing or the assembly of workpieces. Depending on the contamination, non-halogenated hydrocarbons and modified alcohols demonstrate definite advantages. They not only ensure reliable, efficient and sustainable removal of oils, coolants and chips, but also enable cleaning and corrosion protection processes—even long-term protection—in one machine.

Metallic parts get dirty during manufacture and machining operations. Foreign matters, such as coolant residue, chips and particles, normally become a problem in downstream processing, such as heat treatment, welding, coating or assembly.

Contaminants may impair the quality, functionality and service life of the finished product, thus, industrial parts cleaning is an indispensable step in the production chain. The most frequently used technique is wet-chemical cleaning, which uses any liquid chemical, such as solvent or aqueous.

 

Like Dissolves Like Principle

Process effectiveness, quality, cost-efficiency and stability of the cleaning process are critically dependent on the dissolving capacity of the cleaning agent used: aqueous detergents or solvents. In selecting the right cleaning medium, the “like dissolves like” principle of chemistry, based on the polarity of a substance, applies. This principle indicates that a contaminant dissolves best in a solvent (cleaning agent) that has a similar structure to itself (see Table 1). In other words, water-based (polar) types of contamination, such as coolant and lubricant emulsions, polishing pastes, additives and salts, are typically cleaned with aqueous cleaning agents (hydrophilic).

For removing nonpolar contaminants, such as lubricating and cooling oils, greases and waxes, a solvent (lipophilic) is usually the preferred cleaning agent. Solvents are commonly classified into chlorinated hydrocarbons, non-halogenated hydrocarbons and modified alcohols, also known as polar solvents.

Because modified alcohols have lipophilic and hydrophilic properties, they effectively remove nonpolar (lipophilic) and polar (hydrophilic) contaminations. Thus, modified alcohols are used as an alternative to both hydrocarbons and aqueous detergents.

 

 

Table 1	Hydrocarbons	Chlorinated Hydrocarbons	Modified Alcohols	Aqueous Cleaners
Substance/ Substance class	C9–C13 Isoparaffins	Trichloroethylene Perchloroethylene	Modified Alcohols	Alkaline Cleaner
Polarity	non-polar	non-polar	slightly polar	polar
Organic contaminations (non-polar; e.g., oil, fat)	very good	very good	good	moderate
Organic (polar; e.g., combined contamination)	moderate	moderate - good	very good	moderate
Inorganic (polar) contamination (salts)	moderate	moderate	moderate - good	very good
Solid contamination (e.g., chips, particles, dust)	Depends on machine configuration

 

 

Hydrocarbons and Modified Alcohols

Health, safety and environmental concerns have negatively affected the reputation of industrial parts cleaning with solvents. However, if a cleaning task can be performed with an aqueous detergent or a solvent, using non-halogenated hydrocarbons or modified alcohols (also referred to as solvents or hydrocarbons) offers substantial advantages:

Good degreasing quality.

Solvent recovery via distillation. Hydrocarbons and modified alcohols are distillable, which means that oils and emulsion brought into the machine and mixed with the solvent are removed continuously and automatically, even if the oil input is high. This process ensures consistent solvent and cleaning quality. Additionally, the continuous reconditioning allows for a long solvent life span and low consumption.

No quality control necessary. In contrast to aqueous cleaning, where pH level, detergent concentration, conductivity and more have to be checked regularly, solvent cleaning does not require continuous quality control. The exception is when chlorinated oils, used in stamping and deep drawing processes, are brought into the machine. In such cases, solvent distributors offer adequate test equipment and stabilizers that might be required.

Compatible with almost all materials. Unlike aqueous detergents, solvents do not cause oxidation, discoloration, burning, dulling or other surface impairments. As a result, chemical surface effects such as etching are not possible, though these are not actual cleaning jobs.

100 percent dry parts. Solvents enable parts to dry completely, even in blind holes and hidden passageways, without the use of hot air.

Lower energy consumption and running costs. Heating up the cleaning fluid and drying the parts require less energy in a solvent cleaning process. In addition, the bath does not need to be controlled and changed regularly. The cost of cleaning agents, fresh and waste water and waste disposal are usually lower, too.

Short- and long-term corrosion protection. Modern solvent cleaning machines perform cleaning and corrosion protection steps in one system.

 

 

Vacuum Technology

The concerns about solvents in industrial parts cleaning are certainly justified regarding open-top vapor degreasing systems, which are still in use. Even “closed” systems that are equipped with cover plates and cooling coils as a condensation trap for emission reduction are not considered ideal solutions regarding HSE. Additionally, solvent consumption is much higher compared to modern, fully closed and safe vacuum cleaning machines.

These systems are equipped with a working chamber and automatic feeding system, one or more tanks for cleaning and corrosion protection and an integrated vacuum distillation system. They operate under full vacuum—cleaning, degreasing, drying and, if applicable, corrosion protection, are performed in reduced pressure atmosphere in one system. This design results in several benefits:

No hot air or waste. These systems dry parts 100 percent without energy-intensive, generated hot air. The use of a vacuum enables the hydrocarbon to dissolve from the liquid into the gaseous phase completely at a low temperature. Furthermore, it reduces the saturated vapor pressure, which enhances the capillary transport of the solvent. As a result, even the smallest blind holes and passages dry easily.

Low hydrocarbon consumption is achieved through continuous vacuum distillation and solvent treatment. Separated oil is automatically discharged from the system and collected in a waste drum for disposal after certain intervals. The regenerated solvent is led back into the tank for consecutive cleaning cycles. This design enables the hydrocarbon to be used in a closed loop without complex, time-consuming solvent tests and any disposal of used cleaning media. Fresh water, disposal of waste water and separation of chemical substances are not required.

Operator, environmental safety. Fully closed vacuum parts cleaning machines eliminate the need for additional explosion protection when working with hydrocarbons. Because of the vacuum, the concentration of the gaseous mixture of air and solvent in the machine is shifted into a nonflammable zone. Furthermore, integrated cooling systems are used to extract solvent from exhaust air.

Machine operators are not in contact with the solvent because the parts to be cleaned are fed automatically into the working chamber and the solvent is not brought to the chamber until it is closed and sealed. When hydrocarbon needs to be added, this process also takes place automatically without operator interaction.

 

Passivation vs. Rust Protection

It is not uncommon that parts made of steel and cast iron travel thousands of miles by road, air or sea before they reach their point of use. Therefore, they must be effectively protected from corrosion. A fairly common method is to put the parts in a box, sometimes lined with a foil, and pour oil over them. Though it is easy and fast, the oil only reaches external surface areas, while internal areas, especially of parts with complex geometries, are not protected and start to corrode. Some alternatives to this quick method are passivation and rust protection.

Passivation of parts. Temporary, or short-term, corrosion protection is often carried out in water-based cleaning systems with an aqueous cleaning agent (neutral) that contains amines. With this method, expect a protection time of several hours to days. With a separate passivation bath, the protection time can be prolonged to several weeks, depending on amine concentration. Using emulsified oils in water-based systems can stretch the protection time up to months. However, this method includes the risk that the emulsified oils cannot be dried on the part surface, leading to cross contamination. If  these oils are dried in a hot air dryer, they can be carried over, leading to fire hazard. Moreover, as the protective film is created out of a two-phase system (water and oil), the layer on the part surface is often not homogenous, which can lead to spots.

Rust protection of parts. For more long-term corrosion protection (protection time from weeks and months up to years, depending on the media concentration), rust preventive media such as oils, greases and waxes are used. As these media are 100 percent soluble in hydrocarbons and modified alcohols, they can be used in fully closed vacuum cleaning systems.

Solvent systems set up to perform rust protection are often equipped with a separate tank (such as a second or third food tank) that is used exclusively for this purpose. The rust protection tank contains hydrocarbon or modified alcohol, plus the required amount of rust protection media. After the parts are cleaned, the work chamber is flooded from the rust protection tank with media, and then the corrosion protection process begins. Like the cleaning process, it guarantees that all areas of the part are reached by the media. This is followed by the vacuum drying process, in which the solvent is transported back to the rust protection tank while the rust protection film remains on the part surface.

The protective film is generated from a fully soluble, salt-free, one-phase system, and the layer on the part surface is homogenous, completely closed and dry. It is not broken if the surface is touched or if it comes into contact with packaging material.

The choice of rust protection media depends on the machine technology and solvent used. The film thickness—usually between 2 and 5 microns—is influenced by the concentration of rust protection media in the solvent and the dripping off time. Dosing the rust protection media can be done manually or automatically.

 

 

Protection Time, Concentration

Information on protection time and required concentration is commonly provided by the supplier of the rust protection media. Depending on the corrosion protection product, typical concentrations are between 0.5 and 25 percent. They are based on empiric tests in the laboratory for determining the film weight and density (percent in solvent) after detecting a specific calibration curve.

To determine film weight, rust protection oil is applied on a standard sheet metal (such as Q-Panel R35) in the recommended concentration. The sheet metal is weighed before and after this process.

Film weight is one measure to determine the approximate consumption of rust protection media with the specific part geometry. The other one is film thickness.

 

Film weight [g/m²] =       mass (rust protection oil) [g]

                area (test sheet metal) [m²]

Film thickness [μm] =     film weight [g/m²] × 100

                density (rust protection oil) [g/cm³]

 

Film thickness is becoming an increasingly important scale unit because more and more customers of part manufacturers not only require the media as corrosion protection, but also as assembly oil. Therefore, the protection layer must be applied in a defined thickness.

Since the concentration should also be checked at the part manufacturer, various methods have been developed. Measurement via evaporation residue with a specific moisture analyzer is the most common method. It requires a sample of approximately 2 grams (0.07 ounces) of the solvent corrosion protection oil mixture. Inside the analyzer the solvent evaporates while the remaining oil is measured as a percentage of weight. The concentration can then be determined with a conversion chart.

Another measure method for determining concentration is the refractive index. One drop of media is placed into a manual, handheld refractometer. The refractive index is then viewed by the user through a magnifying eyepiece. As the refractive index is very temperature dependent, it is important to use a refractometer with an integrated thermometer. Measurements can be performed with media at 68°F, 77°F and 86°F. Automated and highly sensitive operating measurement instruments are also used for density measurement.

Modern, fully closed cleaning systems are an efficient and safe way of performing cleaning and provide long-term rust protection of parts in a single system. For global Tier 1–3 precision part manufacturers, this technology is well established. 

 

 

Rainer Straub is sales director and product line manager at Dürr Ecoclean GmbH, Filderstadt, Germany. For more information, call 248-560-2100 or visit durr-ecoclean.com.

 

Originally published in the March 2016 issue.