Читать книгу Media Blasting & Metal Preparation - Matt Joseph - Страница 11
ОглавлениеBASIC APPROACHES TO CLEANING METAL
“Scratch, scratch.” That’s the sound of cleaning metal. Okay, you can’t always hear it, and it doesn’t always sound just like that, but abrasion is the key; it is, literally, the only way that you can clean metal.
“Sure,” you say, “that covers the old routines: wire brush, sanding disk, abrasive blasting, and the like, but what about laser, ultrasonic, and chemical cleaning, and other approaches that don’t seem to scratch anything?”
In truth, all cleaning methods involve abrasion at some level, mighty or small. Once you get beyond the obvious abrasive methods, the abrasion tends to be subtler, but it’s still abrasion. Approaches such as sanding, abrasive blasting, buffing, and polishing may keep the scratching at sub-visible levels, but they don’t eliminate it.
I’m not saying that the mean old iceberg was just trying to clean the poor Titanic’s hull, but you could look at it that way.
Take these examples: solvent and ultrasonic cleaning. In the first case, the scrubbing is done by the natural action of solvent molecules against surfaces and the things that contaminate them.
In the case of ultrasonic cleansing, the machines that do it tend to emit a slight, audible hum and hiss, but that isn’t the part of the sound spectrum that is actually doing the cleaning. In an ultrasonic cleaning tank, shock waves that are invisible to the eye and soundless to the ear are imploding water-based cleaning agents against the tank and its contents, and cavitating those surfaces that are in contact with them and the liquid cleaning agent.
Behold the most basic of automotive cleaning processes: good old solvent and a bristle brush. What it lacks in high tech shock, awe, and pizzazz, it makes up by being inexpensive, effective, and dependable. The solvent loosens and dissolves dirt and grime and the bristle brush pushes it off surfaces.
This thoroughly modern ultrasonic cleaning tank is used for cleaning aluminum engine parts at Weaver Automotive Supply in Sauk City, Wisconsin. Transducers generate ultrasonic waves in a heated chemical solution to scrub parts clean. Run cycles are timed. Cleaning is usually completed in less than an hour.
This exposes the tank and its contents to the very raw ends of very bipolar water molecules. Those molecules scratch away at surfaces at a molecular level and cleanse them of any loose materials that are adhering to them.
The same process, cavitation, sometimes afflicts the high-speed end surfaces of marine propellers. Those areas simply move through the water faster than it can fill the cavities that their rapid motion opens. You may have seen the result of this process: propeller trailing-end surfaces that look like the Loch Ness Monster had been chewing on them. In fact, this is damage at a molecular level, caused by imploding water. Of course, in an ultrasonic cleaning tank the action is tuned and calibrated to be much milder and less destructive than that.
Pumped solvent allows you to stream cleaning solvents over parts to remove contamination. In this case a stiff bristled brush is being used to add cleaning umph to the process. This approach isn’t high tech, but it is effective.
Ultrasonic cleaning is extremely effective for small parts and parts up to the range of cylinder heads and radiators. It sounds very high tech, and it is. But the basis for it is still scratch, scratch, abrasion. Always keep in mind that some form of abrasion is the basis of all metal cleaning procedures and processes.
Overspinning this bronze propeller caused water to cavitate its trailing edge. The same ultrasonic process is used to clean metal in ultrasonic cleaning tanks. Of course, in those tanks its intensity is controlled and held below levels that would damage metal.
This small ultrasonic cleaner features a timer and some serious ultrasonic cleaning power. It is ideal for cleaning items such as this delicate brass ether-filled hood shutter bellows. Warm water and detergent make the bellows almost shiny clean without endangering its integrity.
This radiator hot tank is typical of heated solvent tanks used to clean large parts. The solvent used here is water based and removes many kinds of grime from surfaces. It is terrific for getting into and cleaning complex items, such as radiators.
Liquid solvent cleaning is basic to all car shops. The old “parts bath” has been around for as long as parts and machinery have needed cleaning. The most basic solvents have been based on petroleum. Stoddard solvent, an aliphatic and alicyclic hydrocarbon concoction, has been around in various formulations since at least the 1920s, when it began to replace turpentine as the preferred mechanical shop cleaning solvent.
Carbon tetrachloride was also used widely in automotive shops, especially to cleanse brake and clutch friction surfaces, and in other areas where oily residues cannot be tolerated. In recent decades “carbon tet,” as it was commonly known, has been unavailable for this purpose because of evidence that it is carcinogenic, and particularly, implicated in causing liver cancer.
The market carries two replacements for carbon tet. The most common is a closely related chlorinated solvent, tetrachloroethylene, which looks, smells, and acts very much like carbon tet. This solvent is sold under many brand names as “brake cleaner.”
The new kid on the block in degreasing solvents is “green brake cleaner.” Again, this type of product is sold under many brand names and typically is made up of the same components often found in enamel reducers and lacquer thinners, acetone and toluene, plus a few other solvents, such as methanol, but in very different proportions from those in reducers and thinners.
In the past three or four decades, water-based parts cleaning and degreasing solutions have become common in shops. They are beginning to replace petroleum-based solvents as the mainstays of parts cleaning. Some of these products require heating for maximum effectiveness, and many of them are most effective when they are hosed over parts. Formulas for water-based cleaning solvents vary. Some are based on citric acid; others rely primarily on aggressive detergents.
The Stoddard Solvent in this barrel was once the standard approach to cleaning metal parts. Petroleum based, it was very effective at removing many kinds of oily grime, but not paint and rust. Because it is harmful to humans and the environment, water-based solvents have largely replaced it.
Another class of cleaning and metal conditioning agents known as layered cleaning solutions, or hydroseal treatments, was once widely available for cleaning carburetors, fuel pumps, and other fuel system parts. Marketed under such names as Bendix Speedclean, Bendix Metalclean, Thyme, and Gunk, these solutions had several layers to clean and seal the porosities in metal, particularly the diecast zinc used to make major carburetor, fuel pump, and other fuel system components. Most modern versions of these cleaners lack the elaborate layering of solutions that made the older formulations both effective and environmentally problematic.
Super Agitene is a petroleum-based general-purpose parts wash that is milder than Stoddard Solvent. It is intended for pumped-stream parts wash units and even contains lanolin to reduce skin irritation from solvent drying, although you should always wear protective gloves when using any liquid solvent.
Brakleen comes in traditional and “green” varieties. The traditional concoction has some similarities to carbon tetrachloride (now banned). It contains tetrachloroethylene and is a terrific fast-drying degreaser. CRC Green Brakleen is based on non-chlorinated solvents, similar to those in lacquer thinner. It is also a good degreaser.
Citra Solv 2000 (now just called Citra Solv) was one of the earliest industrial citrus-based solvents. Made from volatile chemicals in orange rinds, it can be diluted with water in many ratios and is great for removing grease and grime quickly and thoroughly. However, it can harm some plastics and rubbers.
Oil Eater sprayable solvents are two of the many non-petroleum-based products. They do very well removing surface grease and are safe for most materials, including human skin. As with any effective solvent, it is important to keep them out of your eyes and to avoid prolonged skin contact.
These old drums of Gunk Hydro Seal II contain state-of-the-art solvent, circa 2000, for cleaning small parts, such as carburetor and throttle body castings and their internal metal parts. Stored in this dank corner, they became human and environmental hazards. Modern versions of Gunk Hydro Seal are safer, but very expensive.
Modern Gunk Carburetor Parts Cleaner is a good and relatively inexpensive immersion cleaner that is milder than the Hydro Seal II–type layered carburetor cleaners. It lacks the ability to seal porosities in die-cast metal parts, one of the features of some of the older carburetor cleaners.
To use these layered cleaners, parts were placed in metal mesh baskets and lowered into the bottoms of the drums (usually 5-gallon capacity) in which these cleaners were shipped. In a matter of hours, soils such as carbon, varnish, paint, and more were dissolved and removed. The time required depended on which product you used and how dirty the parts were. It also depended on the temperature of the solution.
After an hour to a day, the parts were pulled up through the solutions. Each layer of the solution had a different function. Cleaning layers dissolved and removed soil. Coating layers sealed porosities in the metal surfaces. The top layer was always water. Water was always the final rinse cleaner; it kept the volatiles in the chemicals below from evaporating.
These were marvelously effective cleaners, but they contained several dangerous components that caused environmental degradation. Most of them went out of manufacture decades ago. One exception is Gunk Hydro Seal II, which, while it is a terrific solvent, lacks some of the intricate and sophisticated chemistry of the earlier solvents of this type. It is also quite expensive.
I go into detail on the old layered fuel system parts cleaners because you may find references to using them in old rebuild manuals. For all practical purposes, these products no longer exist in anything like their original formulations, and it is best to find other, more modern approaches to cleaning the kinds of parts for which they were designed and/or specified. Actually, the external diecast fuel pumps and complex carburetors for which these cleaners were developed have also been absent from automotive production since the demise, decades ago, of non-submersible fuel pumps and throttle body injection systems.
Electrolytic and Chemical Processes
Electrolytic cleaning is generally considered an industrial process. It has applications in fields such as electroplating where current is run between anodes and cathodes attached to parts suspended in chemical solutions. Names for this process in plating vary. “Pickling” and “reverse plating” are among the most common monikers for it.
In electrolytic cleaning processes, parts are attached to the anode (-, in the case of reverse plating) in the system. Current flows to the cathode (+, in the case of reverse plating), which is in circuit with a conductive acid bath in which the parts are immersed. The result is to remove molecules of conductive contaminants, such as old plating, from the parts and deposit them on the metal cathode in the plating solution. For the most part, and with very few exceptions, “Don’t try this at home.”
