Getting Aluminum Clean: There’s More Than One Way
By Brendan Baker
Cleaning aluminum creates special challenges for engine builders because of its corrosiveness and soft metal properties. There are several effective ways to clean this non-ferrous metal but there is no magic solution. You will more than likely have to combine several processes to get the kind of clean that you demand. The methods that will work best will depend on the volume of aluminum you’re cleaning.
Most engines today have several components that are aluminum cast, most notably the cylinder head(s). Today’s cylinder heads have many hidden cavities and oil galleys, which makes cleaning them even more challenging. According to one prominent rebuilder, no matter what system you use to clean aluminum, it’s never easy.
First you must prepare it properly, then you need to be sure you don’t leave anything behind. So let’s examine how different cleaning methods can effectively return aluminum components to service.
Jet Spray Washers
We discussed many of the benefits of using aqueous jet washers in the September 2003 issue of Engine Builder ("Engine Cleaning Options in a Water World," page 42). Today’s jet washers have evolved from little more than a dishwasher to a complete cleaning solution. Manufacturers have improved turntables, nozzle systems, pumps and heating systems, and jet spray washers are often the starting point for cleaning aluminum in many shops. The chemical makers for their part have stepped up, too, with new lines of aluminum-safe alkaline soaps.
According to Perry Crabb, AXE Equipment, Council Grove, KS, one of the main things to remember when cleaning aluminum or any other metal in a jet spray washer is the importance of good heat. "If you’re cleaning at too low a temperature you’re wasting a lot of electricity and not cleaning as effectively as you could be," Crabb says. For this reason AXE and other manufacturers have incorporated digital thermal controllers to monitor the temperature.
"We look at cleaning a couple of different ways," says Tim Vondemkamp, Peterson Machine Tool, Merriam, KS. "Our jet spray washer not only has high pressure but we have come up with a new nozzle design as well. It’s a high impact nozzle that delivers the same high pressure all along the nozzle profile, which is different than the standard V-jet nozzle," says Vondemkamp.
Peterson’s high pressure blasting machine acts like a bead blaster but shoots out 600 psi aqueous chemical instead of glass bead. "Some shops that don’t do much aluminum, just buy the high pressure machine. It works well for final rinsing after machining but can be used for the whole cleaning process in a low volume shop," says Vondemkamp.
The manufacturers of jet spray and other aqueous equipment agree that the physical properties of cleaning are the same whether it is aluminum or cast iron. The biggest difference with cleaning aluminum is in the type of chemical you use.
"There are two things that clean the part," says Butch Boselli, Graymills, Chicago, IL. "You have the impingement of the spray nozzles on the part and then there’s the chemical itself that cleans. However, for hard, baked-on carbons, jet spray washers alone often have difficulty removing this type of soil."
Cleaning aluminum with aqueous requires using a compatible chemical that won’t etch or discolor the metal. Aluminum builds an oxide on it and that oxide has to be torn down, which has to be done chemically with caustic. Most manufacturers offer a variety of chemicals for cleaning aluminum and other metals, so you should always consult your chemical supplier to decide what is best for your shop. The type of chemical you use depends on various factors like how many parts you clean, what equipment you have (and how many units), what type of metals are cleaned primarily and how dirty are the parts.
Steve Buch from Armakleen, Princeton, NJ, says that aluminum typically turns black when exposed to caustic. "This black oxidation can be removed by a de-oxidizer – what you’ll have is a the most beautiful, uniform aluminum surface that you can get."
"The biggest selling product that we have is a moderate to high pH solution that is around 12.5pH in use, perfectly safe on aluminum engine parts. By getting it up as high in pH as we can, we can remove the heaviest of contaminants and still protect the aluminum surface. And it gives the engine builder an opportunity to clean all metals in one operation," says Buch.
Best performance from chemical can be achieved by using a high pH level with some type of caustic and then an inhibitor to protect the aluminum.
The idea is to try and squeeze out as much cleaning power as possible but still inhibit the attack on aluminum so that you can clean mixed metals without having to run two systems, explains Buch. "These are formulated to be very safe on aluminum surface so that you still get a clean, bright aluminum surface but they don’t have brighteners in them – typically, the products used to brighten aluminum are acid products. There are mixes of phosphoric and nitric acids or phosphoric and hydrofluoric acid – there’s usually an acid mix that goes beyond straight phosphoric to actually bring out brightness in aluminum."
There are three types of aqueous chemicals that an engine builder needs to be aware of: 1) high caustic products that are safe on ferrous alloys only; 2) Silicated, higher pH products that are multi-metal safe and provide the most cleaning and protection for aluminum; and 3) non-silicated products, which are used mainly in precision cleaning processes. It is aluminum compatible and thoroughly rinseable but may cause some discoloration of the aluminum.
"Probably of all the products sold to the metal cleaning market, the bulk of what is sold is a multi-metal silicated product. This product is especially suitable for engine builders," says Buch. "These types of products are built with high pH yet are still able to incorporate inhibitors, in this case silicate. Silicates inhibits the attack on aluminum during the cleaning process."
An alternative to chemical aqueous products is an organic compound called Recover made by Kolene Corporation, Detroit, MI, "Recover’s alkaline makeup cleans aluminum at about 250°-350° F and can be tweaked in any number of ways and custom formulated for specific applications," says Kolene’s Dennis McCardle.
Heat cleaning is an effective way to clean aluminum but you should err on the side of caution when it comes to the factors of time and temperature. Many aluminum heads are made from a composite of alloys. Unless you have access to a metallurgical lab it may be impossible to know exactly what its composition is.
The key to thermal, especially with aluminum parts, is having the ability to dry oil deposits to a dried ash state. One way to do this is by using indirect flame – in other words a non-contact, convection process.
"If you look at the kitchen appliance market, convection ovens have now accelerated cooking by more than 30 percent, simply by moving air inside a stagnant environment," says Ric Havel, consultant to Sunnen and Lincoln Electric and owner of MotoTech Engineering in West Salem, OH. "They’ve not added any more heat. You still cook a turkey at 325° F, but instead of it being done in 4 hours it takes an hour and a half."
Convection is visible in the shop environment as well, not only accelerating cleaning but offering a safer way to heat the aluminum. "We have what’s called vortex generated convection," says Randy Neal, CWT Industries, Norcross, GA. "What’s different is that we do indirect flame heating by creating a vortex inside of the cabinet to stir the air violently in the cabinet. It accelerates induction of heat into the head in a uniform pattern. If you were to use direct flame then every time the part was near the flame the temperature would spike, which is especially bad for aluminum. By having a vortex generated event the air stirs around the head or block in such a way that it’s equivalent in cycle times as direct flame method but it doesn’t have the damaging annealing effects."
There are two ways that you can anneal aluminum, according to Neal. "The first way is if the aluminum part reaches 600° F or more for at least one minute. The second way aluminum becomes annealed is if the temperature is 450° F for two hours or more. This point needs to be understood better by the industry," Neal stresses.
"You have to make sure that you don’t exceed 600° F – that’s critical," says Neal. "If you’re below 600° F then you can’t go past the two-hour envelope. In theory you could be 599° F for 59 minutes and still be safe. That all being said, we’re more cautious and we tell people not to exceed 450° F for two hours or more."
Havel agrees with Neal that the industry needs to be more cognizant of how to deal with aluminum. Recognizing which alloys are right for various automotive components is a start. "The information I got from the Aluminum Association said that 356 aluminum goes in components such as suspensions, brakes and pistons; 319 and 390 aluminum go into engine blocks, cylinder heads, rocker arms," explains Havel. "The ultimate test of whether you hurt a piece of aluminum by exposing it to heat or not is called UTS (Ultimate Tensile Strength). The aluminum that will yield without breaking is the kind of alloy that should be used on a cylinder head.
Havel says that most thermal ovens don’t reach peak temperature except for the last few minutes of a cleaning cycle so annealing shouldn’t be an issue. "When you load the oven and set the timer for 35 minutes, that part in the first 20 minutes of its cycle is nowhere near 500° F," explains Havel. "In our ovens we see 500° F for only about 8 minutes – the last 8 minutes in the 30-35 minute cycle. It’s just like with your steak: if it’s 120° F and you want it medium rare, you better take it off now. The residual heat will finish cooking it. The primary benefit of thermal is it doesn’t care what you’re cleaning. It simply dries it and turns it to ash."
For baked-on carbons, sometimes the only way to remove it is to blast it off with some sort of bead media. Bead media on aluminum can be challenging, however, because you may spend as much time cleaning the media out as you do cleaning. You have to be sure that you don’t leave anything behind in the cleaning process. Certain types of media are more resistant to lodging in a cavity than others. Glass bead and steel shot (specifically shapes that are more jagged) can become lodged in very tight, hidden areas and require extra attention to get them out. In addition, you may have to mask the area off to keep the media from getting in.
Another note of caution with using a media blaster is that you may damage the surface of the aluminum by moving the metal, affecting tolerances or changing the texture of the surface.
One form of media that does neither of these is soda media. This media is essentially made from a form of baking soda. According to Armakleen’s Delia Downes, the material used in soda blasting is called sodium bicarbonate. It is considered a "one pass" product. This means that it can only be used once and cannot be recycled. However, Downes says that even though it is one pass that doesn’t mean it’s not efficient or affordable for most shops.
"It is a particularly effective way to remove carbon without damaging the aluminum. Another benefit is that it is completely water-soluble. Therefore you don’t have to worry about blasting away in oil galleys, water jackets, boltholes or other blind areas," says Downes.
"Unlike other abrasives, soda fractures into much smaller particles upon impact," says Downes. "This results in much more intense cleaning action that softens the impact on the substrates. The blasting process turns the soda particle into a fine dust that can be washed away easily with water." In fact, soda is the softest of all media: it is rated at 2.5 on the Mohs hardness scale: plastic is 3.5, glass is 5-6, sand is 7 and aluminum oxide is 9. The soda media also comes in a variety of sizes to suit your particular cleaning situation.
Downes also says that because there is no need to prewash the aluminum or do extensive rinsing as it is with other types of blast media, soda blasting can save time for the rebuilder because you can clean in one step without damaging the substrate.
Despite some of the shortcomings of glass bead, it is still used in some shops to clean aluminum. Glass bead can be recycled to use over and over before needing to be replaced therefore it is not as expensive. But you have to be careful. This media can become lodged in bind holes and galleys and you’ll have to spend extra time to be sure it is removed in the rinsing process. Shops that are using glass bead on aluminum often times will mask off areas where the bead may be trapped. Furthermore, most media blasters work best when combined with another type of cleaning process, which may be a thermal or aqueous-based system.
Airless shot blasting is another type of media that is used for cleaning aluminum. These automated systems are designed to sling either stainless or steel shot onto the aluminum part in various patterns.
"We have multiple spray patterns in the blaster so the shot can be blasted in one arc and then turn around and send it in the opposite direction," says CWT’s Neal. "This way we can go around the corners to all the areas of the part. It’s an automatic cycle that’s built into the machine so that we’re reversing the pattern approximately every 4-5 minutes."
One of the benefits of airless blasting is that you can load the parts in the cabinet and walk away. And, aluminum cleaned with stainless shot leaves a new looking finish. "The uniformity of the throwing pattern leaves the part looking aesthetically perfect," notes Neal. "The stainless shot has a great side-effect in that it makes the aluminum look very bright. Stainless shot is also good to use if you’re preparing aluminum for welding because it won’t introduce any ferrous metal, which will repel a weld."
Manufacturers recommend that you allow the aluminum to cool (but not too much, to around 200° F) and dry before putting the workpiece in the airless blaster because the shot may change the integrity of the aluminum if it is too hot. It is also very important that the piece be completely dry – no oils or water-filled boltholes – because otherwise the shot will become trapped.
For the most part, airless shot blasting is done with thermal ovens but it can be used with jet spray washers, too, as long as the aluminum part is heated and completely dried before going into the blaster. According to the experts, you must use some sort of oven to dry the aluminum and bring it up to acceptable temperature for the shot blaster.
According to various sources, ultrasonic parts washers will do as good of job cleaning the external surfaces as bead media without damaging ports and other critical surfaces. Ultrasonic cleaning also offers the convenience of being an automatic process: you can throw the part in, turn it on, and walk away to do something else.
"What we’ve seen over the last 3-4 years is more and more engine builders are going to these systems," says Frank Pedeflous, Omegasonics, Simi Valley, CA. "It’s a different way of thinking about cleaning but it is becoming more and more accepted since this type of cleaning was first introduced to the industry. It’s become huge in the industrial and medical industries."
According to manufacturers of ultrasonic systems, many engine builders are using jet spray washers to pre-wash aluminum parts to get the heavy grease off and then using the ultrasonic system to do the precision cleaning.
"One of our main selling points for any size shop is that ultrasonic systems do a great job cleaning aluminum heads and removing hard carbons," says Windy Adams, Global Sonics, Danville, IN. "Ultrasonic systems clean everything else, too, but aluminum cylinder heads have been difficult for rebuilders."
Ultrasonic cleaning works well because you’re not using a high impact force to do the cleaning, so you’re not changing the tolerances of the aluminum. It creates a vacuum bubble that when it hits the part and pops it doesn’t blow out it implodes. This is what is called cavitation. It is basically sucking dirt off the part you’re cleaning.
"If you looked at a vacuum bubble under a microscope you’d see it’s actually a vortex. It pulls everything in its path in with it. Therefore it doesn’t have an impact on the integrity of the aluminum other than to clean the surface," says Pedeflous.
Another reason ultrasonic has become more prevalent is that some shops use the system to flush out the media after some sort of blasting process. These systems will be able to remove small media particles from all of the hidden areas.
"There are many effective ways to clean aluminum. There is no one method that is absolute," says Jim Parker, LS Industries, Wichita, KS. "Issues such as your cleaning objective, parts analysis, soil analysis, part orientation and material handling, production volume requirements, plant layout and floor space, drying, and wastewater treatment must be addressed before adding new equipment to your shop."
When deciding on what equipment or chemicals to purchase the first step is education. The more you know about each type of equipment and how it is used, the better armed you’ll be to choose the right equipment. As one engine builder says, you must "know your grill." Like any other piece of equipment in your shop, you must utilize your cleaning equipment for everything it can do.
Aluminum Cleaning Directory:
Peterson Machine Tool
RMC Engine Rebuilding Equip.
Silver Seal Products
Sun Coast Industrial
Sunnen Products Co.
Viking Blast & Wash System